Cover illustrations:

Margherita Bongiovanni
(Formerly professor of Design and Art History, and Fine Art, Ministry of Education, Rome, Italy)

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© FAO 2012

Foreword

Sustainable management and utilization of natural resources is part of the Global Goals of FAO Member Countries and essential to the mandate of FAO.

The latest FAO assessment of the state of the world’s land and water resources clearly indicated that these resources, already scarce today, will be increasingly scarce as we move into the future, threatening food security. In fact, the outstanding food demand projected for the next decades, due to the world population growth and to the anticipated shift in consumption patterns, will face very limited opportunities for further land expansion and the finite availability of fresh water resources. Such a food demand may be satisfied only if we are able to act effectively and sustainably on both sides of the food equation, i.e., production and consumption, and on the inter-linkages between these two variables, including trade, distribution and access.

Efforts are being made by FAO to address major issues on the production side, on the fairness of trade, on the consumption side (reduction of post-harvest losses and food waste; promoting nutritious and healthy diets) and other emerging challenges. Among these emerging challenges are: food price volatility, revealing the vulnerability of some countries in their dependency on imports, leading to increase production inside their national boundaries; climate change, causing greater uncertainties on rainfall patterns, thus requiring higher levels of adaptation and increased resilience of the local production systems; transboundary rivers and competing demands for land and water resources by other sectors of society and by ecosystems.

Under such circumstances, and looking into the future food demand, it is imperative that agriculture improve the efficiencies of use of the limited resources and ensure substantial productivity gains. In the case of water, scarcity is a major threat to the sustainability of food production in many areas of the world. The effective management of water in rainfed and irrigated agriculture is thus a major knowledge-based pathway to increase productivity and farmers’ income. To combine increased productivity with sustainable management of natural resources, without repeating the mistakes made in the past, will be a challenge.

With the contribution of numerous experts, professionals and scientific institutions around the world, including a few Institutes of the Consultative Group on International Agricultural Research (CGIAR), “Crop yield response to water” is published at a time of high demand for assistance by member countries in order to implement effective water management strategies and practices that are environmentally safe and climate-resilient, and enhance sustainable water productivity and yield of their farming systems, therefore alleviating the risks of food insecurity.

Preface

The FAO Land and Water Division is engaged extensively in the enhancement of global agricultural performance. A part of this effort is the production of landmark publications and guidelines that address food production and water use problems using analytical methods that often serve as standards worldwide.

In the face of growing water scarcity, declining water quality, and the uncertainties of climate change, improving the efficiency and productivity of crop water use, while simultaneously reducing negative environmental impact, is of utmost importance in responding to the increasing food demand of the growing world population. To this end, irrigated and rainfed agriculture must adopt more knowledge-intensive management solutions.

Moreover, competing demands for water from other economic sectors and for ecosystem services will continue to grow. As agriculture is by far the largest consumer of water, efficiency and productivity gains in this sector would free significant amounts of water for other uses.

Abstracting from the scientific understanding and technological advances achieved over the last few decades, and relying on a network of several scientific institutions, FAO has packaged a set of tools in this Irrigation and Drainage Paper to better assess and enhance crop yield response to water. These tools provide the means to sharpen assessment and management capacities required to: sustainably intensify crop production; close the yield-gap in many regions of the world; quantify the impact of climate variability and change on cropping systems; more efficiently use natural resources; and minimize the negative impact on the environment caused by agriculture. These tools are invaluable to various agricultural practitioners including, but not limited to: water managers and planners; extension services; consulting engineers; governmental agencies; non-governmental organizations and farmers' associations; agricultural economists and research scientists.

Representing FAO’s state-of-the-art work in water and crop productivity, it is our hope that this publication provides easy access to, and better understanding of, the complex relationships between water and food production and, in this way, help improve the management of our precious water resources.

Contents

ACKNOWLEDGMENTS

ACRONYMS OF INSTITUTIONS

LIST OF PRINCIPAL SYMBOLS AND ACRONYMS

1.

INTRODUCTION

2.

YIELD RESPONSE TO WATER: THE ORIGINAL FAO WATER PRODUCTION FUNCTION

3.

YIELD RESPONSE TO WATER OF HERBACEOUS CROPS: THE AQUACROP SIMULATION MODEL

3.1

AquaCrop: concepts, rationale and operation

3.2

AquaCrop applications

3.3

AquaCrop parameterization, calibration, and validation guide

3.4

Herbaceous Crops

Wheat

Rice

Maize

Soybean

Barley

Sorghum

Cotton

Sunflower

Sugarcane

Potato

Tomato

Sugar Beet

Alfalfa

Bambara Groundnut

Quinoa

Tef

4.

YIELD RESPONSE TO WATER OF FRUIT TREES AND VINES: GUIDELINES

4.1

Fruit Trees and Vines

Olive

Citrus

Apple

Plum

Almond

Pear

Peach

Walnut

Pistachio

Apricot

Avocado

Sweet cherry

Grapevine

Kiwifruit

5.

EPILOGUE

Acknowledgments

This publication has relied on inputs from a large number of people and institutions in various forms, including expert consultations, contributions at project workshops, experimental data and authorship.

Universities and national and international research institutions from many regions of the world have provided data and insights, which forms part of the vast amount of information and knowledge condensed in this state-of-the-art publication Crop yield response to water.

Particularly significant has been the involvement of key CGIAR Centres, specifically IRRI, ICARDA, ICRISAT, CIMMYT and CIP, and the FAO/IAEA Joint Division. Working together with the colleagues from these Centres has strengthened the institutional partnership and enhanced the synergy towards filling the gaps between scientific research and field implementation, theoretical knowledge and field practice, investigation and actual operation.

Most of these scientists, experts and colleagues are listed in this publication either as editors, authors or as scientists who have contributed with data and tests for the model Aquacrop. We are grateful to all of them for their highly valuable inputs.

We would like to thank the following experts who were involved in the initial stage of this endeavour: John Passioura (CSIRO, Canberra, Australia), Luigi Cavazza (University of Bologna, Italy), Hugh Turral (formerly IWMI, Colombo, Sri Lanka), John Annandale (University of Pretoria, South Africa), Dogo Seck (CERAAS, Dakar, Senegal), Lamourdia Thiombiano (FAO, Sub-Regional Office for Central Africa, Libreville, Gabon), Netij Ben-Mechlia (INAT, Tunis, Tunisia), Claudio Stockle (Washington State University, Pullman, United States), Florent Maraux (CIRAD, Montpellier, France), François Tardieu (INRA, Montpellier, France), Tsugihiro Watanabe (RIHN, Tokyo, Japan), Hiroshi Nakagawa (Ishikawa Prefectural University, Ishikawa, Japan), Long Nguyen (Joint FAO/IAEA Division, Vienna, Austria), Tim Wheeler (University of Reading, Reading, United Kingdom), Jan Doorenbos and Amir Kassam (formerly FAO, Land and Water Division, Rome, Italy) and Gerardo van Halsema (Wageningen University and Research Centre, Wageningen, The Netherlands). Their inputs have been determinant in correctly setting the initial condition of this work.

The support of colleagues at FAO at different stages has been indispensable for this publication. Special thanks go to Martin Smith, who started the process and later overviewed the various chapters, providing inputs and feedback to the editors to improve harmonization among the chapters and between the present and past FAO publications. Martin was ably assisted by Giovanni Munoz and Robina Wahaj. Shortly after the initial phase, an intense period of work started and we are particularly grateful to Gabriella Izzi and Lee K. Heng who managed the extensive experimental data set collected for calibration, maintained the liaison between all the collaborators, and contributed considerably to the various calibration and tests of Aquacrop. After one year of diligent effort at FAO, Lee moved to the FAO/IAEA Joint Division in Vienna, but continued to cooperate enthusiastically on model calibration and testing. Gabriella moved to the World Bank after two years of intense work on the model at FAO. More recently, support has been provided by Michela Marinelli, Patricia Mejías Moreno and Jippe Hoogeveen. We thank them for their continuous help and critical exchange in the completion of this product.

The design and accurate execution of the layout and editing of the manuscript are critical for this publication, requiring meticulous attention to the numerous details in the text, graphs, pictures, drawings, equations, symbols and citations. For their professional and immensely patient devotion to these tasks, we would like to thank the FAO collaborators Gabriele Zanolli, Paolo Mander, Rosemary Allison and Jim Morgan. A very special recognition goes to Margherita Bongiovanni for providing artistic, beautiful and accurate botanical illustrations of the crops.

Finally, we thank the former Assistant Director-General of Agriculture and Consumer Protection Department, Ms Louise Fresco, for her early endorsment and support of the iniciative, and both the Land and Water Division, Mr Parviz Koohafkan, and the Assistant Director-General of Natural Resources Management and Environment Department, Mr Alexander Müller, for their strong and continuous support of the ‘Water’ programme making possible this publication and the development of the model Aquacrop.

Without this collective and interdisciplinary effort, this outcome could not have been achieved.

Acronyms of institutions

CER:

Canale Emiliano Romagnolo, Italy

CNR:

Consiglio Nazionale delle Ricerche, Italy

CMMYT:

Centro Internacional de Mejoramiento de Maíz y Trigo, Mexico

CIP:

Centro Internacional de la Papa, Peru

CONICET:

Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina

CRA:

Consiglio per la Ricerca e la Sperimentazione in Agricoltura, Italy

CRI:

Cotton Research Institute, Uzbekistan

CSIRO:

Commonwealth Scientific and Industrial Research Organisation, Australia

DSIR:

Department of Scientific & Industrial Research, India

EEAD-CSIC:

Estación Experimental Aula Dei - Consejo Superior de Investigaciones Científicas, Spain

EMBRAPA:

Empresa Brasileira de Pesquisa Agropecuária, Brazil

FAO:

Food and Agriculture Organization of the United Nations, Italy

GRI:

Golan Research Institute, Israel

GRC:

Geisenheim Research Centre for Viticulture and Grapevine Breeding, Germany

IAM-B:

Mediterranean Agronomic Institute of Bari, Italy

IAEA:

International Atomic Energy Agency, Austria

IAS-CSIC:

Instituto de Agricultura Sostenible - Consejo Superior de Investigaciones Científicas, Spain

ICARDA:

International Center for Agricultural Research in the Dry Areas, Syria

ICREA:

Institució Catalana de Recerca i Estudis Avançats, Spain

ICRISAT:

International Crops Research Institute for the Semi-Arid Tropics, India

INIA:

Instituto Nacional de Investigaciones Agropecuarias, Chile

INRA:

Institute National de la Recherche Agronomique, France

INTA:

Instituto Nacional de Tecnologia Agropecuaria, Argentina

IRD CEFE-CNRS:

Institut de Recherche pour le Développement Centre d’Ecologie Fonctionnelle & Evolutive - Centre National de la Recherche Scientifique, France

IRRI:

International Rice Research Institute, Philippines

IRTA:

Institut de Recerca i Tecnologia Agroalimentaries, Spain

ISPA:

Istituto di Scienze delle Produzioni Alimentari, Italy

IVIA:

Instituto Valenciano de Investigaciones Agrarias, Spain

KESREF:

Kenya Sugar Research Foundation, Kenya

LARI:

Lebanese Agricultural Research Institute, Lebanon

LLNL:

Laurence Livermore National Laboratory, USA

SARDI:

South Australian Research and Development Institute, Australia

SASRI:

South African Sugarcane Research Institute, South Africa

SIA:

Servicio de Investigación Agraria, Spain

USDA-ARS:

United States Department of Agriculture – Agricultural Research Service, USA

WB:

World Bank, USA

VLIR-UOS:

Flemish Inter-University Council, Belgium

ZIS:

Zhanghe Irrigation System, China

List of principal symbols and acronyms

B

Dry biomass, of shoot for non-root crops, and of shoot plus storage root or tuber for root crops [tonne/ha or kg/m^2]

Ca

Mean atmospheric CO_2 concentration for the actual year [ppm]

Ca,o

Mean atmospheric CO_2 concentration for the year 2000 [ppm]

cco

Canopy size of the average seedling at 90% emergence [cm^2]

CC

Green canopy cover [percent or fraction]

CC*

Green canopy cover adjusted for micro advection [percent or fraction]

CCmeas

Canopy cover measured [percent or fraction]

CCsim

Canopy cover simulated [percent or fraction]

CCo

Initial canopy cover, canopy cover at 90% emergence [percent or fraction]

CCpot

Potential canopy cover [percent or fraction]

CCx

Maximum green canopy cover [percent or fraction]

CDC

Canopy decline coefficient [percent or fraction of canopy decline per unit time]

CGC

Canopy growth coefficient [percent or fraction of canopy growth per unit time]

CN

Curve number or surface runoff coefficient

[CO_2]

Carbon dioxide concentration of the atmosphere [ppm]

CWSI

Crop water stress index

d

Day

dp

Plant density [plants per unit surface]

dref

Reference plant density [plants per unit surface]

Dr

Soil water depletion of the root zone [mm]

DAE

Days after emergence [day]

DAP

Days after planting [day]

DI

Deficit irrigation

DP

Deep percolation [mm per unit time]

DU

Distribution uniformity

E

Soil evaporation [mm per unit time]

Edz

Surface evaporation from the rest of the soil surface outside the emitter wetting pattern [mm per unit time]

EStage I

Soil evaporation at Stage I (wet soil surface) [mm per unit time]

EStage II

Soil evaporation at Stage II (drying soil surface) [mm per unit time]

Ewz

Surface evaporation from the soil wetted by the emitters [mm per unit time]

ECe

Electrical conductivity of the saturated soil-paste extract [dS/m]

ECen

Electrical conductivity of the saturated soil-paste extract: lower threshold (at which soil salinity stress starts occurring) [dS/m]

ECex

Electrical conductivity of the saturated soil-paste extract: upper threshold (at which soil salinity stress has reached its maximum effect) [dS/m]

ET

Evapotranspiration [mm per unit time]

ETa

Actual evapotranspiration [mm per unit time]

ETc

Crop evapotranspiration under standard conditions [mm per unit time]

ETx

Maximum evapotranspiration [mm per unit time]

ETo

Reference evapotranspiration [mm per unit time]

fage

Reduction coefficient describing the effect of ageing on Kc,Trx [1/d]

fcc

Fraction of the orchard ground surface occupied by the cover crop

fCO2

CO_2 factor for atmospheric CO_2 normalization

fHI

Adjustment factor for HIo

FC

Field capacity

FI

Full irrigation

FDR

Frequency domain reflectometry

FTSW

fraction of transpiring soil water

gs

Stomatal conductance [m/s]

G

Ground cover fraction of the tree canopy

GDD

Growing degree days [°C d]

GIR

Gross irrigation requirement [mm or m3/ha per unit time]

GIS

Geographical information systems

HI

Harvest index [percent or fraction]

HIo

Reference harvest index [percent or fraction]

I

Infiltration [mm per unit time]

Kc

Crop coefficient

Kcb

Basal crop coefficient representing Kc for a dry soil surface having little evaporation but full transpiration

Kcc

Cover crop coefficient

Kext

Radiation extinction coefficient

Kp

Pan coefficient (for the pan evaporation method to determine ETo)

Kr,t

Empirical coefficient relating the ETc of an orchard of incomplete cover to that of a mature orchard

Ksat

Saturated hydraulic conductivity [mm per unit time]

Ks,e

Empirical soil evaporation coefficient

Ky

Yield response factor

Kc,Tr

Crop transpiration coefficient

Kc,Tr x

Crop transpiration coefficient for when the canopy fully covers the ground (CC = 1) and stresses are absent

Ke

Soil evaporation coefficient for fully wet soil surface

Kex

Soil evaporation coefficient for fully wet and non-shaded soil surface

Kr

Evaporation reduction coefficient

Ks

Stress coefficient

Ksb,c

Cold stress coefficient for biomass production

Kspol,c

Cold stress coefficient for pollination

Kspol,h

Heat stress coefficient for pollination

Ksaer

Water stress coefficient for water logging (aeration stress)

Ksexp,w

Water stress coefficient for canopy expansion

Kssen

Water stress coefficient for canopy senescence

Kssto

Water stress coefficient for stomatal closure

LAI

Leaf area index [m^2 leaf area/m^2 soil surface]

LAIref

LAI of the same crop planted at a reference density

LWP

Leaf water potential [MPa]

NIR

Net irrigation requirement [mm per unit time or m^3/ha per unit time]

p

Fractional depletion of TAW

pupper

Upper threshold of p (no water stress: Ks = 1)

plower

Lower threshold of p (maximum water stress: Ks = 0)

P

Precipitation or rainfall [mm]

PAR

Photosythetically active radiation [μmol per m^2 of surface per s]

PRD

Partial root drying

PWP

Permanent wilting point

RDI

Regulated deficit irrigation

RAW

Readily available soil water in the root zone [mm]

REW

Readily evaporable water at the top of the soil profile [mm]

RUE

Radiation use efficiency [Kg of biomass per MJ of intercepted solar radiation]

RO

Surface runoff [mm per unit time]

SDI

Sustained (or continuous) deficit irrigation

t

Time [GDD or d]

T

Air temperature [°C]

Tbase

Base temperature (below which crop development does not progress) [°C]

Tc

Canopy temperature [°C]

Tmax = Tx

Daily maximum air temperature [°C]

Tmin = Tn

Daily minimum air temperature [°C]

Tn,cold

Minimum air temperature at upper threshold for cold stress affecting pollination [°C]

Tx,heat

Maximum air temperature at lower threshold for heat stress affecting pollination [°C]

Topt

Crop optimal daily temperature [°C]

Tupper

Upper temperature (above which crop development no longer increases with an increase in air temperature) [°C]

Tr

Crop transpiration [mm per unit time]

Trcc

Cover Crop transpiration [mm per unit time]

Trx

Maximum crop transpiration (for a well watered crop) [mm per unit time]

TAW

Total Available soil Water (between FC and PWP), equivalent to the soil water holding capacity in the root zone [mm/m]

TDR

Time domain reflectometry

TE

Transpiration efficiency [Kg of biomass per unit of water transpired]

VPD

Air vapor pressure deficit [kPa]

wz

Fraction of the soil surface wetted by the emitters

Wr

Soil water content of the root zone expressed as an equivalent depth [mm]

WP

Crop biomass water productivity [tonne of biomass per ha and per mm of water transpired or kg of biomass per m^3 of water transpired]

WP*

WP normalized for ETo and air CO_2 concentration [tonne /ha or kg/m^2]

WPB/ET

WP as the ratio of biomass to ET [kg/m^3]

WPB/Tr

WP as the ratio of biomass to Tr [kg/m^3]

WPfresh Y/ET

WP as the ratio of yield measured as fresh biomass to ET [kg/m^3]

WPlint/ET

WP as the ratio of lint (of cotton) to ET [kg/m^3]

WPsucrose/ET

WP as the ratio of sucrose (for sugar cane) to ET [kg/m^3]

WPY/ET

WP as the ratio of yield (as dry matter) to ET [kg/m^3]

WPY/Tr

WP as the ratio of yield (as dry matter) to Tr [kg/m^3]

X

Irrigation depth [mm]

XR

Required irrigation depth [mm]

Y

Yield [tonne/ha or kg/ha]

Ya

Actual yield [tonne/ha or kg/ha]

Yx

Maximum yield [tonne/ha or kg/ha]

Ze

Effective rooting depth [m]

Zx

Maximum effective rooting depth [m]

Zn

Minimum effective rooting depth [m]

θ

Volumetric soil water content [m^3 of water / m^3 of soil]

Ψstem = SWP

Stem water potential [MPa]

Ψleaf = LWP

Leaf water potential [MPa]

4.1 Fruit trees and vines

Editor:

Elias Fereres
(University of Cordoba and IAS-CSIC, Cordoba, Spain)

Botanic Illustrations:

Margherita Bongiovanni
(Formerly professor of Design and Art History, and Fine Art, Ministry of Education, Rome, Italy)

Olive

LEAD AUTHORS

Riccardo Gucci,
(University of Pisa, Pisa, Italy),

Elias Fereres
(University of Cordoba and IAS-CSIC, Cordoba, Spain)

CONTRIBUTING AUTHOR

David A. Goldhamer
(formerly University of California, Davis, USA; currently Cooperative Extension Emeritus)

GENERAL DESCRIPTION

Olive (Olea europaea L.) is an evergreen tree grown primarily between 30 and 45° latitude in both hemispheres. In 2008 total harvested area was over 10 500 000 ha, 95.5 percent of which was concentrated in ten countries surrounding the Mediterranean Sea (FAO, 2011). Spain, Italy and Greece are the main producers of virgin oil followed by Tunisia, Syria, Turkey and Morocco (years 2002-2008). About 90 percent of the world production of olive fruit is for oil extraction, the remaining 10 percent for table olives. The world cultivated area of olives in 2009 was over 9.2 million ha with an average yield of 2.1 tonne/ha (FAO, 2011). Figure 1 shows the evolution of olive production over the last decades in the principal countries. European Union countries produce 78 percent and consume 68 percent of the world's olive oil.

Olive trees have been sparsely planted for centuries, without irrigation, on marginal lands in Mediterranean climate conditions because of their high resistance to drought, lime and salinity. Typical densities of traditional groves are between 50 and 100 tree/ha with trees severely pruned to stimulate vegetative growth and renewal of the fruiting surface, and the soil periodically tilled. Fruit yields are low, ranging from less than 1 up to 5 tonne/ha of olives. Although traditional groves vary in cultivar composition, tree density, training system, degree of mechanization and chemical inputs, they are still the most widespread production system and a landmark of Mediterranean landscapes. Intensive orchards have a density of between 200 and 550 tree/ha, which translates into a higher fraction of intercepted radiation that leads to higher productivity per unit land area than traditional systems, particularly during the first 10 years of production. Trees are trained to a single trunk for mechanical harvesting and the soil is often managed by temporary or permanent grass cover to reduce erosion and ease traffic in wet periods. In areas of annual rainfall higher than 600 mm, production can be maintained under rainfed conditions in soils with good water-holding capacity. However, irrigation plays an important role in the drier areas, and/or for soils with limited water storage. Elsewhere, irrigation plays an important role to stabilizing yields in the years of low rainfall. Irrigation is becoming common in the intensive orchards as it allows early onset of production (from the second to forth year after planting), high yields (averages up to 10-15 tonne/ha) under optimal conditions and less variability because of alternate bearing.

Quality considerations

Of the several categories of olive oils, defined according by the European Union legislation (Reg. EEC 2568/91, UE 702/07 and 640/08), and widely accepted internationally, the concept of quality only pertains to virgin olive oil (VOO), the main product of the olive industry. In order to qualify as VOO, it is required that oils satisfy analytical parameters and be tested and approved for their sensory characteristics by a panel of experts. Moreover, the current perception of quality is mainly based on the sensory and health-related properties, which are closely related to the concentration and composition of the phenolic and volatile fractions, respectively. Oleic acid is the most abundant fatty acid followed by palmitic acid, linoleic acid and others that do not exceed 2 percent of fatty acid composition. Fatty acid composition is cultivar dependent and changes with climatic conditions and progression of ripening. The ratio between mono-unsaturated and poly-unsaturated fatty acids first increases, then it reaches a maximum and then decreases in overripe fruit. The concentration of phenolic compounds in the fruit, and consequently in the oil, is also cultivar dependent and reaches a maximum at the beginning of ripening, when the skin (epicarp) is still partially green, to decline sharply in overripe fruit. Qualitative features of table olives are similar to those of other stone fruit used for fresh consumption and include fruit size, pulp-to-pit ratio, pulp firmness, colour and soluble carbohydrate concentration.

Most of the world's olive area is composed of the two systems described above. However, in the last 15 years very high density, hedgerow type, olive orchards (from 1 000 to 2 000 tree ha) have been developed to further reduce harvesting costs using over-the tree harvesting machines. Because of the higher ETc demand of the dense canopies and the low soil volume available for each tree, irrigation is needed. Average yields can be quite high (5-15 tonne/ha) in the first years of production (third to seventh year after planting) and may average 10-14 tonne/ha over a 10-year period, but there are questions about the sustainability of high yields in the long term, and about the adaptation of many cultivars to this production system. The area devoted to these super-intensive plantations is about 100 000 ha worldwide.

STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

Yield is the result of three main developmental processes that occur between flowering and harvest: fruit set, fruit growth and oil accumulation in the fruit pulp (mesocarp). Vegetative growth is critical in terms of olive fruit production, because flowering and fruit set originate in the axillary buds of past year’s growth. The reproductive cycle from flower bud induction to fruit ripening takes 15-18 months depending on cultivar and growing conditions, as it starts in the summer and ends in the autumn of the following year. Flowers are usually borne in inflorescences at the axil of leaves of one-year old wood, whereas the terminal bud of the shoot is almost always vegetative (Rapoport, 2008). Shoot growth starts with bud break in spring and resumes when temperatures are above 12 °C, as long as it is not inhibited by temperatures above 35 °C, soil water deficit or other environmental stresses. A second flush of shoot growth is common after the summer. Olive trees are sensitive to waterlogging and temperatures below -10 °C.

Chilling is needed for flower bud differentiation. Lack of chilling results in scarce and uneven formation of flower buds. Chilling requirements vary with the cultivar but at least 10 weeks below 12 °C are usually needed for abundant flowering. Main phenological stages for olive trees, described according to the two-digit Biologische Bundesanstalt, Bundessortenamt and Chemical industry (BBCH) scale, include bud break, bud development, leaf full expansion, beginning of flower cluster development, full elongation of flower clusters, full flowering, fruit set, fruit development, maturity and senescence. The time sequence of main developmental processes is reported in Figure 2.

Olives flower in late spring, a month or two later than to many deciduous trees; timing and duration depend on cultivar, temperature and soil water availability. Fruit set is generally low (less than 2 percent of flowers) but suboptimal conditions (temperature, rain, winds) can reduce it further. Fruit growth apparently follows the typical double-sigmoidal pattern of stone fruit, although there are some questions as to whether this pattern is inherent to olive fruit development or is a consequence of the interaction with environmental constraints in midsummer such as high temperature and low water availability. Fruit pit hardening occurs about two months after fruit set. Oil accumulation in the fruit is proportional to the intercepted solar radiation and becomes substantial in late summer for most cultivars, right after the end of massive pit lignification. At harvest, fruit contain between 10 and 25 percent oil on a fresh weight basis, depending on cultivar, crop load and growing conditions. Oil accumulation patterns in the mesocarp follow a simple sigmoid curve, but may vary with cultivar and environmental conditions.

RESPONSES TO WATER DEFICITS

Olive trees are very resistant to drought and show a high capacity to recover from prolonged drought periods. Trees can completely re-hydrate within three days of irrigation after a water deficit that reached a leaf water potential (LWP) of -4.0 MPa. Even during a severe drought that lowered the LWP down to -8.0 MPa, the trees rehydrated in less than a week following the onset of rains (Connor and Fereres, 2005). Nevertheless, as for all terrestrial plants, expansive growth of olive leaves, branches, fruit, and trunk, is sensitive to water deficits. Water stress also affects stomatal opening and photosynthesis. It is well known that olive stomata close partially during the day (Fereres, 1984) in response to increases in vapour pressure deficit, even if the trees are well watered, with corresponding decreases in CO_2 assimilation. Leaf photosynthetic rate is relatively high, of the order of 12-20 μmol CO_2 m-2 s-1, but is reduced under and following water stress because of stomatal closure. When water deficits are severe (LWP below -4 to -5 MPa), photosynthetic rate does not exceed one-third of normal values, reaching a maximum early in the morning and then declining as the day advances (Angelopulos et al., 1996).

The olive root system is extensive and vigorous and, therefore, can explore the soil profile thoroughly after the first years of tree establishment. In general, it can be assumed that most of the absorbing roots are in the first 1 m depth. However, in deep alluvial soils, roots can reach 2-3 m depth or more, whereas in marginal soils the rooting depth may be less than 0.5 m. Given the capacity of olive trees to lower their LWP to -7 MPa or less, the soil water content in parts of the root zone can easily reach values below the standard permanent wilting point. Given the shape of the moisture release curve of most soils, the amount of additional water extracted would be small. Nevertheless, such small amounts may be critical for surviving extreme droughts.

Because olives flower late, the risk of low temperature damage is significantly reduced; however, the risk is increased of being affected by water and/or high temperature stress in the Mediterranean climate, and also fruit growth is delayed into the summer, normally an extended period of water shortage. It has been observed that for rainfed trees in years when winter rainfall has been very limited, that flowering and fruit set are processes that are quite sensitive to water deficits. Thus, water stress should be avoided from floral development to fruit set in spring. However, in relatively humid climates (e.g. central and northern Italy) stress seldom develops in spring and irrigation is usually unnecessary. Once the fruit is set, it grows more or less linearly by cell division and subsequent expansion, unless water deficits and high temperatures slow its growth rate. Initial rapid fruit growth (Stage I) and the period when oil is actively accumulated in the fruit (Stage III), are also sensitive to water deficit.

Given that water deficits reduce fruit growth, irrigation that avoids water deficits increases fruit size, although the effect is mediated by the amount fruit borne on the tree. Once the fruit is set, its growth rate may be slowed down by water deficits, but growth quickly resumes upon relief from stress by rainfall or irrigation. Complete recovery of endocarp growth occurs even after several weeks of deficit, whereas recovery of mesocarp growth is less certain. The pulp-to-pit ratio, an important quality feature for both table and olive oil fruit, is increased under irrigated conditions, but it has been observed that mild water deficits during fruit development have a positive effect on the pulp-to-pit ratio (Gucci et al., 2007).

Soil water availability affects colour change and ripening of fruit. The effect of irrigation on oil content may be apparently contradictory as it affects water content and the process of oil accumulation in the fruit in different ways. It is well known that severe stress during the oil accumulation period (from mid-August through the end of October for most cultivars in the Northern Hemisphere), reduces the oil percentage on a dry weight basis at harvest. As stress becomes less intense the oil percentage increases, although at a relatively low rate. Under non-stress conditions the variability of data on the relationship between predawn LWP and oil content increases and oil concentration may be even less than for mildly stressed fruit (Figure 3).

While irrigation does not usually alter the fatty acid composition or basic parameters used for the classification of VOO (e.g. free acidity, peroxide value, spectrophotometric indexes), increasing volumes of water applied decrease the concentration of phenolic compounds in the oil, namely the concentration of secoiridoid derivatives of oils, such as 3,4-DHPEA-EDA, 3,4-DHPEA-EA and p-HPEA-EDA (Gomez-Rico et al., 2007 and Servili et al., 2007), which act as natural antioxidants during oil storage and play important functions in human diet and prevention of cardiovascular diseases. As a result, oils from irrigated orchards are usually less bitter and pungent that those from rainfed ones (Servili et al., 2007).

Assessment of tree water status

The established method of measuring tree-water status is the measurement of stem-water potential or leaf-water potential. Exposed leaves are used for the latter while shaded or covered leaves are used for SWP; in some cases, small terminal branches have been used to characterize tree water status. Predawn LWP is a reliable indicator of tree water status in mature trees, but it is inconvenient in practice because of the need for early morning measurement.

The LWP measured on exposed leaves at midday depends not only on soil water availability but also on environmental conditions that influence canopy conductance during the day. Water potential values of exposed leaves are more variable as they are affected by the degree of stomatal opening. With stomata fully open, the gradient between stem and sunlit LWP is about 0.5 MPa when water supply is adequate. Such value may increase (up to 1 MPa) as water stress increases, but it tends to decrease again as stomata close.

Stem-water potential (SWP) is considered a more reliable indicator of tree-water status than midday LWP, because it is less dependent on diurnal changes in radiation and humidity. The midday SWP of olive trees grown under adequate soil water supply, ranges between -0.5 and –1.2 MPa, depending on the evaporative demand, with a tendency to decrease even to lower values in mid- to late summer under conditions of high evapotranspirative demand (Gimenez et al., 1997). SWP values for olive are higher than -0.5 MPa have been only occasionally measured. Typical reference values for midday SWP for mature, fully-productive olive trees vary between -1.0 and -1.2 MPa for summer, sunny days with an ETo of 5-6 mm/day.

Under water deficit conditions, stress develops and SWP values decrease. Typical values of SWP for moderately-stressed trees are between –1.7 and -2.5 MPa, and become severe when values approach -3.5 to -4.0 MPa. Although olive trees have exceptional resistance to drought, and SWP or LWP values as low as –7.0 to -8.0 MPa have been measured in rainfed olive trees during severe drought periods (Fereres, 1984), these values should be considered exceptional and close to a survival state rather than acceptable for satisfactory yields.

The use of displacement sensors to monitor olive trunk growth and trunk diameter fluctuations has yielded information that confirms the high sensitivity of expansive growth to water deficits. These highly sensitive sensors could be used in young, intensive plantations where the objective is to maximize canopy growth and reach full production as early as possible. Sap flow sensors have also been used to determine changes in sap velocity, which are indicative of transpiration rate and of its changes in response to water deficits. All these instruments are still in the research and development stages and are not yet used commercially for irrigation scheduling.

WATER REQUIREMENTS

Olive trees withstand long periods of drought and can survive in very sparse plantings even in climates with only 150-200 mm annual rainfall. However, economic production requires much higher annual precipitation or irrigation. Table 1a summarizes the crop coefficient (Kc) values proposed by various authors that have been developed in different environments. The range of Kc values is quite wide, varying from less than 0.5 to about 0.75, average values varying from 0.55 to 0.65, depending on the season. A synthesis of current estimates of Kc developed recently (Fereres et al., 2011) is given at the end of Table 1b.

* In winter and spring only, the low Kc applies to dry, cold climates, while the high Kc applies to areas frequently wetted by rainfall.

TABLE 1b

Summary of recommended olive Kc values.

Climate*

Semi-arid

Arid

Spring

0.65-0.75

0.45-0.55

Summer

0.50-0.55

0.50-0.55

Fall

0.60-0.70

0.55-0.65

Winter

0.65-0.75

0.40-0.55

* Mediterranean-type climates; the one labelled semi-arid has seasonal rainfall values around 500 mm or more, mostly between autumn and spring, while the arid climate would have less than 400 mm rainfall and is more continental, with relatively cold winters. The higher Kc values of the range should be used for high rainfall situations. Kc values to be used with ETo calculated following FAO I&D Paper No. 56.

Despite the evergreen nature of the olive, the Kc is not constant throughout the year, because of tree transpiration responses to environmental and endogenous factors. Measurements have shown that relative transpiration is lowest in spring and highest in early autumn. However, the Kc also must reflect the rate of evaporation from the soil surface, which is quite high in spring in many Mediterranean environments. Thus, Kc values reflecting olive orchard water use do not differ as much between spring and autumn (see Table 1). In midsummer relatively low values of Kc are found because of partial stomatal closure in response to high vapour pressure deficit.

If more precise estimates of water use are needed, an alternative method has been proposed to calculate transpiration and evaporation independently (Orgaz et al., 2006) (See Chapter 4, Box 5). For table or canning fruit production, the higher range of Kc values is recommended. Crop coefficients should be further increased (up to about 0.8 to 1.0 in winter and early spring, depending on the type of the cover crop and its density) if the orchard floor has a permanent grass cover.

WATER PRODUCTION FUNCTION

Olive fruit yield decreases as ETc decreases below its maximum. However, it has been found that the decline in production is hardly detectable with small reductions in ETc (Figure 4). As ETc is further reduced, however, yields decline more. Thus, the response curve of yield (fruit or oil) to ETc is almost linear at low levels of consumptive water use, but levels off when water consumption is high. As a result, the overall response curves are parabolic and can be described by second order equations, such as the one presented in Figure 4. The shape of the curve implies that the water productivity (WP) increases as ET decreases and, therefore, one can find an economic optimum, in terms of ET and therefore of irrigation amount, if the price of oil and the irrigation water costs are taken into consideration. The curve shown in Figure 4 is the best fit line to a dataset obtained for the cv. Picual at Cordoba, Spain (Moriana et al., 2003). Additional data published for the cvs. Arbequina, Morisca, Mulhasan, Frantoio, Leccino, and also for the cv. Picual collected at another location, are also plotted in Figure 4. It appears that the data from other varieties/locations fall within the margins of error, on the curve originally obtained at Cordoba, with perhaps the cv. Morisca showing higher sensitivity to mild ETc deficits than the cv. Picual. More research is needed to characterize the response of the other cultivars in different environments. From the original data sets and equations in Figure 4 for Cordoba, Spain, over 700 mm of water consumed in ETc are needed to reach the maximum olive fruit yield in that particular environment. If the effective rainfall is 400 mm, at least 300 mm would have to be supplied by irrigation to achieve maximum yields in that particular location. However, because of the shape of the yield response to ETc, olive production responds very well when small irrigation amounts are applied to rainfed orchards. In different locations in southern and northeastern Spain (between 350-500 mm of annual rainfall), farmers are achieving WP values of up to 30 kg/ha/mm (fresh fruit) with seasonal irrigation applications of 100 to 150 mm in orchards that were previously rainfed. In coastal, central Italy (about 600 mm annual precipitation), less than 100 mm of irrigation water are sufficient to obtain yields that are over 80 percent of those of fully irrigated orchards (Gucci et al., 2007). Significant increases in yield were obtained in Israel using a single irrigation of 75 mm in the middle of the summer (Lavee et al., 2007).

The yield response curve to applied irrigation water is similar to Figure 4 but flatter at high irrigation levels, because the olive is capable of extracting substantial stored soil water and can compensate for reductions in applied water, as long as there is sufficient water in the root zone. The level of irrigation savings depends on the storage capacity of the soil and on the amount of rainfall needed for the sustainable replenishment of stored soil water. The response of oil production is similar to that of the response of fruit production, and the responses of oil quality are discussed below.

The yield response of table olives to a reduction in applied water (AW) is shown in Table 2 from one study (Goldhamer et al., 1994). A reduction in AW of 21 percent did not affect fruit yield or revenue. A further reduction down to 62 percent of maximum AW, decreased relative fruit yield by 10 percent, and relative revenue by 25 percent. The more drastic reduction in revenue was associated with a lower price due to the reduction in fruit size.

SUGGESTED RDI REGIMES

Several specific cases are developed below for different environments and water supply situations. Table 3 illustrates an example of the water budget approach for irrigation scheduling of a young olive orchard in central Italy. Values are reported on a monthly basis, but during normal irrigation practice water budgets are calculated weekly. Two irrigation levels are used, full irrigation and 50 percent deficit irrigation. It should be noted that the early water deficit that occurred in April was unusual for this location, where the irrigation season normally starts in June. In more arid climates it is normal that irrigation starts in spring and may extend well into the fall season. Water is preferably applied to satisfy tree daily needs by microirrigation, but in poorly-drained soils it is desirable to reduce the frequency of irrigation to 1-2 times a week. The use of longer intervals with microirrigation is often inefficient because there may be significant losses to deep percolation. When water agencies supply water at longer intervals (2-4 weeks), it is desirable to build on-farm storage facilities to irrigate as frequently as needed.

Three different RDI strategies have been successfully demonstrated for olive irrigation. In the first, the planned deficit is distributed evenly throughout the whole irrigation season (sustained deficit irrigation or SDI). In the second, the deficit is concentrated in the summer period, from pit hardening until the end of the summer (RDI_1). The third strategy (RDI_2) which is intermediate between the previous two, alternates short cycles of stress and relief during the irrigation period, maintaining the SWP or predawn LWP at variable levels which are moderately low during the fruit development period. The hypothetical seasonal course of tree water status under the three different DI strategies is represented schematically in Figure 5.

There is no evidence to demonstrate the superiority of one RDI strategy over the others, as they all seem to give good results (see below). In areas where rainfall patterns are typically Mediterranean, and where soils have a reasonable water storage capacity (over 100 mm of extractable water), the RDI strategies are easily implemented with irrigation systems that have limited (below the ET needs) and fixed capacity (i.e. two emitters per tree). The irrigation system is run for the same period more or less throughout the entire season. In late spring, the water supply is normally adequate to meet the full ET needs, but as the summer starts and the ETo increases, supply from the drip system is insufficient and water is extracted from the soil reservoir to meet the demand.

As the season progresses, water deficits set in, to increase in severity as the summer advances, but by the end of summer ETo start to decline and the rains may arrive. At this time, the level of stress is reduced or eliminated, which is desirable during the fruit growth and maturation period. By manipulating irrigation frequency, the grower can run the system a fixed time (normally at its maximum capacity) and scheduling becomes straightforward. The RDI_2 approach permits the installation of irrigation systems designed to cope with situations of very limited water allocation, for instance in cases when water is drastically restricted during midsummer (mid-July, end of August) for conflicting urban uses (e.g. tourism in Liguria, Italy). A key factor for success with the RDI_2 approach is to start irrigating early enough to conserve the soil water reserve for when the ETo demand is high.

The sustained deficit irrigation strategy (SDI) is planned by distributing the water deficit proportionally to the monthly ET requirements. In this case, for the same amount of water, the anticipated water deficits are of less magnitude at midsummer than in RDI_1, while the stress that develops earlier and later in SDI, should be of greater magnitude than in the RDI_1. The RDI_2 strategy may be useful in soils with a high clay percentage to allow the root system not to be exposed to high humidity for long periods.

Other RDI strategies that have been sought include the concentration of the water deficit in the ‘off’ year, when the crop load is so low that vegetative growth in not affected much by the water deficits. In the ‘on’ year, the water saved in the previous year is applied to maximize fruit production with minimum water deficits. The only published test performed with this strategy (Moriana et al., 2003) did not give as good results as the previous three strategies described above. Also, the management of water supply under this strategy is not easy, and it will require commitments of water supply that exceed the current season and that, therefore, may be hard to implement by many water agencies.

Because olive irrigation is a relatively new practice, often irrigation authorities cannot deliver sufficient water to meet the full orchard requirements and are promoting deficit irrigation practices. Reasons include lack of sufficient water supply, equity considerations, the limitations imposed by the original tree spacing of the rainfed orchards, and priority for urban uses. When supply is limited to such levels, use of drip irrigation is a must, with as few emitters per tree as possible. Also, growers should irrigate as infrequently as feasible (once or twice a week) to minimize E losses and avoid prolonged exposure of olive roots to high soil-water levels in clayey soils. If supply is very limited, and a new irrigation system is going to be installed, one option would be to use subsurface drip as E losses would then be negligible.

The goals of water saving using RDI in olive orchards appear particularly interesting. Most documented evidence indicates that RDI strategies supply only 30 to 70 percent of the volume needed for fully-irrigated trees. Seasonal water volumes of as little as 50 mm are sufficient to increase yields significantly in subhumid climates, whereas about 100 mm are needed in drier climates. These amounts are definitely much less than that used in most other crops.

Today, an important issue is the use of RDI to optimize oil quality. Recent evidence shows that the concentration of phenolic compounds and volatile compounds with sensory impact can be optimized using RDI strategies rather than by applying full irrigation or under rainfed (Gomez-Rico et al., 2007; Motilva et al., 2000 and Servili et al., 2007). The beneficial effects of moderate water deficits that are imposed by RDI on olive oil composition stemmed the term ‘qualitative irrigation’, an aspect which will probably be more important. There are some reports that recommend restricting irrigation before harvest to limit or avoid trunk damage during mechanical harvesting by trunk shakers and/or problems of oil extractability during processing in the mill. Deficit irrigation appears to be beneficial for optimizing the pulp-to-pit ratio in of table olives (Gucci et al., 2009).

Additional considerations

Introducing irrigation to rainfed olive growing involves a number of potential side effects. Increasing incidence of Verticillium wilt (Verticillium dahliae) disease has been reported, especially for young orchards. There is documented evidence that inoculum density of the pathogen is higher in wet areas than in dry. Although the cause of greater incidence of Verticillium wilt in irrigated orchards is unclear, it is likely that susceptible secondary olive roots increase close to the drippers where the soil may be quite wet favouring infection, weeds survive longer and temporarily host the pathogen, and infected leaves decompose quicker under high humidity conditions (Lopez-Escudero and Blanco-Lopez, 2005). The Verticillium wilt disease is a limiting factor for irrigated orchards in some areas where localized, low-frequency irrigation is recommended as a management technique.

Further, reportedly here is an increasing damage by the olive fruit fly (Bactrocera oleae Gmel.) in irrigated orchards, but supporting evidence is not strong. The olive fruit fly preferably damages large fruit (cultivars of large fruit size and olive fruit with high water content are more susceptible than small), so its effect may be indirect because irrigation increases fruit size. However, it is likely that the fruit fly also enjoys more favourable conditions when humidity is higher because of irrigation in the olive orchard.

Problems of the low oil yield of fruit from fully-irrigated, very high density orchards of some cultivars have been reported, but they appear to be related to technological problems in oil extraction when fruit is highly hydrated rather than to less oil in the fruit itself. Because of the relatively high resistance to salinity, olive trees yield well when irrigated with saline waters (Gucci and Tattini, 1997).

REFERENCES

Angelopulos, K., Dichio, B., & Xiloyannis, C. 1996. Inhibition of photosynthesis in olive trees during water stress and rewatering. Journal of Experimental Botany 47: 1093-1100.

Caruso, G., Rapoport, H.F. & Gucci, R. 2011. Long-term evaluation of yield components of young olive trees during the onset of fruit production under different irrigation regimes. Irrigation Science (DOI: 10.1007/s00271-011-0286-0)

Connor, D.J. & Fereres, E. 2005. The physiology of adaptation and yield expression in olive. In: Darnell, R., Ferguson, I.B. & Hokanson, S.C., eds. Horticultural Reviews (31). USA, Canada, John Wiley & Sons. pp. 155-229.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Fereres, E. 1984. Variability in adaptive mechanisms to water deficits in annual and perennial crop plants. Bulletin de la Société Botanique de France. Actualités Botaniques 131: 17 32.

Fereres, E., Villalobos, FJ, Orgaz, F. & Testi, L. 2011. Water requirements and irrigation scheduling in olive. Acta Horticulturae, (ISHS), 888:31-40.

Gimenez, C., Fereres E., Ruz, C. & Orgaz. O. 1997. Water relations and gas exchange of olive trees: diurnal and seasonal patterns of leaf water potential, photosynthesis and stomatal conductance. Acta Horticulturae, 449(2):411-415.

Goldhamer, D., Dunai, & Ferguson, L. 1994. Irrigation requirements of olive trees and responses to sustained deficit irrigation. Acta Horticulturae 356: 172-176.

Gomez-Rico, A., Salvador, M.D., La Greca, M. & Fregapane, G. 2006. Phenolic and volatile compounds of extra virgin olive oil (Olea europaea L. Cv. Cornicabra) with regard to fruit ripening and irrigation management. Journal of Agricultural and Food Chemistry 54: 7130-7136.

Gomez-Rico, A., Salvador, M.D., Moriana, A., Perez D., Olmedilla, N., Ribas, F. & Fregapane, G. 2007. Influence of different irrigation strategies in a traditional Cornicabra cv. olive orchard on virgin olive oil composition and quality. Food Chemistry 100: 568-578.

Gucci, R. & Tattini, M. 1997. Salinity tolerance in olive. Horticultural Reviews 21: 177-214.

Gucci, R., Lodolini, E.M. & Rapoport, H.F. 2007. Productivity of olive trees with different water status and crop load. Journal Horticultural Science Biotechnology, 82(4): 648-656.

Gucci, R., Lodolini, E.M. & Rapoport, H.F. 2009. Water deficit-induced changes in mesocarp cellular processes and the relationship between mesocarp and endocarp during olive fruit development. Tree Physiology 29: 1575-1585

Iniesta, F., Testi L., Orgaz. F., & Villalobos, F.J. 2009. The effects of regulated and continuous deficit irrigation on the water use, growth and yield of olive trees. European Journal of Agronomy 30: 258-265.

Lavee, S., Hanoch, E., Wodner, M., & Abramowitch, H. 2007. The effect of predetermined deficit irrigation on the performance of cv. Muhasan olives (Olea europaea L.) in the eastern coastal plain of Israel. Scientia Horticulturae 112: 156-163.

Lopez-Escudero, F.J. & Blanco-Lopez, M.A. 2005. Effects of drip irrigation on population of Verticillium dahliae in olive orchards. Journal of Phytopathology 153: 238-9.

Martin-Vertedor, A., Pérez Rodríguez, J., Prieto Losada, H., & Fereres, E. 2011. Interactive responses to water deficits and crop load in olive (olea europaea L., cv. Morisca) II. – Water use, fruit and oil yield. Agricultural Water Management 98: 950-958.

Moriana, A., Orgaz, F., Pastor, M. & Fereres, E. 2003. Yield responses of a mature olive orchard to water deficits. Journal of the American Society for Horticultural Science 128: 425-431.

Motilva, M.J., Tovar, M.J., Romero, M.P., Alegre, S. & Girona, J. 2000. Influence of regulated deficit irrigation strategies applied to olive trees (Arbequina cultivar) on oil yield and oil composition during the fruit ripening period. Journal of Food Science and Agriculture 80: 2037-2043.

Orgaz, F., Testi, L., Villalobos, FJ & Fereres, E. 2006. Water requirements of olive orchards II: determination of crop coefficients for irrigation scheduling. Irrigation Science 24: 77-84.

Rapoport, H.F., 2008. Botanica y morfologia. In: Barranco, D., Fernàndez-Escobar, R., Rallo L., eds. El Cultivo del Olivo. Ed. Mundi-Prensa, Madrid, pp. 37-62.

Sanz-Cortés, F., Martínez-Calvo, J., Badenes, M.L., Bleiholder, H., Hack, H., Llacer, G. & Meier, U. 2002. Phenological growth stages of olive trees (Olea europaea). Annals Applied Biology 140: 151-157.

Servili, M., Esposto, S., Lodolini, E., Selvaggini, R., Taticchi, A., Urbani, S. Montedoro, G.F., Serravalle, M. & Gucci, R. 2007. Irrigation effects on quality, phenolic composition and selected volatiles of virgin olive oil cv. Leccino. Journal of Agricultural and Food Chemistry 55: 6609-6618.

Citrus

LEAD AUTHOR

David A. Goldhamer
(formerly University of California, Davis, USA; currently Cooperative Extension Emeritus)

CONTRIBUTING AUTHORS

Diego S. Intrigliolo
(IVIA, Moncada, Valencia, Spain)

Juan R. Castel
(IVIA, Moncada, Valencia, Spain)

Elias Fereres
(University of Cordoba and IAS-CSIC, Cordoba, Spain)

INTRODUCTION AND BACKGROUND

Citrus fruit include oranges, which account for about 70 percent of worldwide production, small citrus fruit, such as mandarins, tangerines, tangelos, clementines, satsumas, lemons, limes and grapefruit. Oranges are produced for both the fresh market and for juice — chilled single strength and concentrate. Until the middle of the twentieth century, citrus was almost exclusively cultivated locally. Speed and care when shipping the perishable fruit was of great concern. However, the development of citrus concentrate had a lasting impact on the citrus industry worldwide. Concentrating the fruit permitted the storage, transportation, and transformation of the product far from the groves. In addition to fresh fruit and juice, there are citrus byproducts such as food additives, pectin, marmalade, cattle feed (from the peel), cosmetics, essential oils, chemicals and medicines.

Because citrus is an evergreen crop sensitive to low temperatures, subtropical regions produce the bulk of the world’s citrus. Tropical cultivation is not as productive since seasonal changes in temperature favour adequate blooming patterns, fruit growth and fruit colour development during ripening. In fact, the high temperatures of the tropics induce fast development and production of large fruit that ripen quickly, remaining marketable for a very short time. In contrast, growth in the subtropical zones slows in the winter and fruit can remain mature on the tree for longer before it is harvested and marketed. Frost damages citrus fruit although the trees can withstand short periods of light frost. However, hard frosts of long duration kill trees and can be devastating.

Citrus fruit value ranks first in international fruit trade. In 2009, there were over 5.4 million ha of citrus (4.1 million ha of oranges) with an average yield of 16.3 tonne/ha. Figure 1 presents the trends since 1985 of the production of the world principal countries (FAO, 2011). As a result of trade liberalization and technological advances in fruit transport and storage, the citrus fruit industry is becoming more global. During the last decades, citrus production and trade have increased steadily; although the intensity of growth varied according to the type of fruit (it has been stronger for small fruit and juice). Citrus production is evolving in a context of highly competitive global markets. There is an apparent ever-increasing focus on the quality and the value-added aspects of the products. Citrus is often promoted for its health and nutritive properties; rich in Vitamin C, folic acid, and fibre.

Quality considerations

The goal of citrus production is to harvest as large and as high-quality crop as possible. In this case, quality refers to fruit size, peel colour and appearance, chemical constituents, and taste. Fruit size is usually the primary factor in determining fruit value. In general, larger fruit is more valuable than smaller. However, cultivars, harvest time, local preferences, and market factors can exhibit extreme influences on fruit value. Of the quality factors, peel appearance is usually most important with a smooth, deep coloured and blemish-free peel being most desirable. One limitation of citrus production in the tropics is that fruit remain green and do not change colour when mature. In terms of taste, most people prefer a balance between sweetness and tartness, as measured by the soluble solids-acid ratio (TSS/TA). Juice content is another quality factor, particularly for lemons and limes. In general, citrus fruit have a high postharvest storage potential; from weeks for mandarins and up to six months or more for lemons.

GROWTH AND DEVELOPMENT IN RELATION TO YIELD DETERMINATION

The growth of citrus trees can be divided into vegetative and reproductive stages. Vegetative growth includes the growth of roots, stems, leaves and new flushes. Reproductive growth includes flower bud initiation, differentiation, flowering, fruit set, fruit development and fruit maturity.

Early vegetative and reproductive growth

Under subtropical climate conditions in the Northern Hemisphere (California, for example), the main vegetative growth flush occurs in late February and March. There are normally additional growth flushes in the summer and autumn. Leaves are viable for about two years. Leaves are continuously replenished although leaf drop is generally highest during the spring.

Most citrus cultivars produce flowers in the spring and are self-compatible; they may be fertilized by their own pollen. In cool, coastal climates, flowers may be produced throughout the year but maximum flowering is also in the spring. Flowers develop in leaf axils on older shoots as opposed to shoots developing in growth flushes.

Citrus trees produce a large amount of flowers resulting in small fruit. However, a large percentage of these small fruit drop from the tree apparently because of a combination of physiological and environmental factors. There appears to be a hierarchy of flowers relative to fruit drop; flower in locations with a higher flower/leaf ratio are more subject to drop. Flowers that open late in the bloom period are more likely to set fruit that survives to harvest than fruit set from early flowers. Likewise, faster growing fruit is less apt to drop than slow growing fruit.

The primary factor controlling the dropping of flowers and young fruit is a weakening of tissue in a preformed abscission zone at the point of attachment at the base of the ovary to the disk or of the pedicel to the stem. The actual mechanism of the process is not well understood but is believed to be triggered by growth regulators in response to either external or internal changes. Young fruit that remain after the first period of dropping presumably are capable of developing to maturity if the weather is favourable. Nevertheless, some dropping of fruit occurs throughout the season. As the weather becomes hotter in the early summer months, there is generally a period of accelerated fruit drop, which is often referred to as the ‘June drop’ period. If the heat is severe and prolonged for several days, fruit drop can be heavy, resulting in a greatly reduced crop. This has been observed in several cultivars of the navel and Valencia types. Some cultivars, notably Valencia and mandarins, have an alternate bearing cycle; light crop years following heavy crop years.

Fruit development stage

Following early season fruit drop, and after the leaves of new flushes have fully expanded, the remaining fruit begin to develop initially by rapid cell division. At this point, a cross-section of the fruit clearly reveals its component parts:

Flavedo (epicarp) – a rough, robust and bright colour (from yellow to orange) skin or rind that covers the fruit and protects it from damage. Its glands contain the essential oils that give the fruit its typical citrus fragrance.

Albedo (mesocarp) – a white, spongy tissue that, together with the flavedo, forms the peel (pericarp) of the fruit.

Pulp (endocarp) – the internal part is comprised of individual segments or juice sacs. This is the edible part of the fruit and the source of the juice.

Fruit growth during this stage is temperature dependent; extremely high or low temperature can slow growth. Cell division is followed by cell enlargement. It is during this period that the peel reaches maximum thickness. Later, as the pulp expands, the peel becomes thinner. The fruit continues to grow as long as it is on the tree, albeit slower as temperatures fall.

The potential fruit size at harvest is highly dependent on environmental factors that exist during cell division. The conventional wisdom is that any decrease in the rate of cell division will directly translate into reduced potential fruit size at harvest. Once the fruit growth cycle passes from a predominant cell division phase to a primarily cell expansion phase, fruit size at harvest is not as susceptible to growing conditions. As for other fruit crops, there are trade-offs in citrus between fruit number and size; large numbers per tree leads to small individual size. Both very small and very large fruit fetch lower prices in most fresh markets but there is a wide range of acceptable sizes for citrus. When citrus are grown for juice, fruit size is relatively unimportant.

Fruit maturation stage

The beginning of this stage is usually characterized by the onset of fruit colour change, triggered by the cooler night temperatures in the subtropical regions. The fruit is still increasing in size but at a much slower rate than previously. During fruit maturation, the juice content of the pulp increases. The acid content of the fruit decreases as the sugar content increases. Unlike many deciduous fruit, there is no precise point that indicates maturity in citrus. In California, the basis for legal maturity of oranges is a ratio of 8-to-1 for total soluble solids to titratable acidity (TSS/TA); the so-called sugar-acid ratio. The balance between sugars, which accounts for about 75 percent of the total soluble solids, and the sourness produced by acidity is the best criterion in correlating fruit quality with consumer acceptance.

Peel colour is also used as a maturity index but this approach is not reliable since peel colour depends on temperature. Moreover, harvest timing is highly influenced by market prices and processor availability. Hormonal sprays, such as gibberellic acid, are used to prolong the viability of fruit on the tree. If left on the tree too long, the fruit is subject to drop, insect/disease damage, and breakdown of the acid and flavor components.

RESPONSES TO WATER DEFICITS

There is a large body of work on the responses of different citrus physiological processes to water stress. Many species, cultivars, and growing conditions have been studied. Since responses vary with the timing of stress during the season, the year was divided into seasons and unless otherwise noted the information focused on orange response.

Spring stress

In terms of eventual impact on yields, the flowering and fruit set period has been repeatedly identified as the most sensitive to stress for small citrus, such as clementines (Gonzalez-Altozano and Castel, 1999) and oranges (Pérez-Pérez et al., 2007). This resulted in the increased abortion of small fruit (June drop). On the other hand, there are reports indicating that early season stress significantly reduced peel creasing in a particularly vulnerable navel cv. (Frost Nucellar) without negative impacts on harvest fruit load or size (Goldhamer and Salinas, 2000).

Summer stress

During this period of cell expansion in the fruit, deficit irrigation can reduce the fruit growth rate. In fact, severe stress can result in fruit shrinkage. However, reintroduction of full irrigation can increase the fruit growth rate, such that harvest fruit size is unaffected (Goldhamer and Salinas, 2000). This has been attributed to the maintenance of dry matter accumulation in the fruit during the stress period and/or full rehydration of the fruit upon reirrigation. Under the experimental conditions on the east coast of Spain (Gonzalez-Altozano and Castel, 1999), summer stress had no impact on clementine yield as long as predawn leaf water potential (LWP) was not reduced below -1.3 MPa. However, severe stress for a prolonged duration during the summer can reduce harvest fruit size, tree growth and increase sugars (Mantell, 1977). Harvest sugar concentration can increase because of summer stress, presumably the result of fruit dehydration. Of the two primary yield components, fruit size is more susceptible to reduction than fruit load as a result of summer stress. Working with cv. Valencia, it was found that moderate stress in the summer and fall did not reduce yields in Arizona (Hilgeman and Sharp, 1970).

It should be noted that the imposition of severe stress during the summer has been used with lemons to induce an off-season bloom and resulting summer harvest fruit. This is known as the Forzatura technique or the Verdelli effect. In one study that withdrew irrigation for nine weeks starting in June, predawn LWP reached - 2.7 MPa (Barbera and Carimi, 1988). This regime yielded 30 kg/tree in each of the summer and winter harvests. They found that insufficient stress resulted in the lack of off-season flowering but that when stress was too severe, it produced excessive leaf drop, higher flower abortion, winter fruit drop and reduced winter harvest fruit quality (Barbera and Carimi, 1988).

Autumn stress

As opposed to stress-induced reductions in fruit growth being overcome upon the reintroduction of full irrigation, reducing fruit growth in the autumn usually results in smaller fruit at harvest. Irrigation of clementines at 25 and 50 percent ETc during late summer/early fall reduced harvest fruit size by 11 and 25 percent, respectively (Gonzalez-Altozano and Castel, 1999) and produced more fruit peel creasing, as has been found with navel oranges (Goldhamer and Salinas, 2000). Both studies found no influence of autumn stress on fruit load.

In addition to reducing fruit size, numerous studies have shown that autumn stress can increase both the sugar and acid content of the fruit as well as increase the peel thickness.

Winter stress

In most citrus producing areas of the world, winter soil water deficits are unlikely to occur because of rainfall. However, in Florida, water restrictions applied to Valencia oranges, during the winter were used to modify the timing of flowering, delaying bloom dates by two to four weeks (Melgar et al., 2010). The fruit were able to overcome a fruit size reduction during the following spring when irrigation was restored to full crop water needs. This was because Valencia is a very late-season cultivar and fruit had three to four months to recover to the optimum size. In this case, winter stress was used to reduce immature fruit drop for the next season’s crop during mechanical harvesting with trunk shakers.

Season-long stress

Several studies have imposed deficit irrigation throughout the season. Some of these imposed irrigation regimes based on tensiometer readings. A value of 70 kPa as a threshold was used which caused delayed bloom, flower opening, and decreased fruit set with cv. Valencia (Davies and Bower, 1994). In a study with the cv. Shamouti where tensiometers were used, with 35 percent less applied water than fully irrigated trees, flowers per tree increased by 52 percent but the flower abscission rate was high. This resulted in a 20 percent lower yield but higher sugars and acid (Moreshet et al., 1983). Another study showed that allowing 80 percent depletion of available water in the surface 1 m in the summer and winter and 60 percent depletion in spring and autumn did not reduced yields with navels (Wiegand and Swanson, 1982). The same regime reduced fruit weight in cv. Valencia. No applied water data was given and it was difficult to judge the level of stress that the depletion of soil water induced in the trees. Another approach to produce season-long stress is to irrigate at fractions of ETc. For grapefruit, applying 35 percent less than full ETc delayed maturity and reduced yield by 13 percent due to both smaller fruit and lower fruit load. The sugar and acid content was higher while there was no impact on the sugar/acid ratio (Eliades, 1994). For navels, irrigating with 22 and 46 percent less than full ETc reduced yields by 7 and 22 percent, respectively (Brych and Luedders, 1988). Finally, for clementines the application of 55 percent of tree water needs during the entire season reduced yields by only 17 percent (Gonzalez-Altozano and Castel, 1999).

Indicators of tree water status

The established indicator of tree water status is the stem-water potential (SWP) as measured with the pressure chamber. A simpler and less time-consuming approach is to measure the water potential of shaded leaves. Both Goldhamer with navels, and Castel with clementines, found the slope of regression lines very close to unity and high correlations between this measurement and SWP (slopes of 0.97 and 0.98; R^2 of 0.95 and 0.96, respectively). Midday SWP values above – 1.0 MPa indicate the absence of water stress for a typical summer day of ETo about 6-7 mm/day. Values between – 1.0 and -1.5 MPA are indicative of mild stress. Citrus can withstand more negative SWP than other fruit trees and vines. Trees show hardly any visual symptoms of stress with SWP of -2.0 to -2.5 MPa. Severely stressed orange trees that had predawn LWP around -6.5 MPa recovered their water status within a week of rewatering although they experienced severe defoliation and took several weeks more for complete functional recovery (Fereres et al., 1979).

WATER USE REQUIREMENTS

Orchard transpiration, the primary component of ETc, depends directly on stomatal conductance and scientists recognized early on that the stomatal behaviour of citrus differs significantly from that of most other crop plants. An early comparative study found that maximum stomatal conductance (Gl) of soybean, wheat, and orange under fully irrigated conditions was 1.0, 0.60, and 0.48 cm/s, respectively, and occurred at about 09:00 hours. They attributed this to differences in stomatal opening and densities (Meyer and Green, 1981). A comparison of citrus Gl against that of almond and pistachio under similar conditions in the San Joaquin Valley of California carried out by Goldhamer showed that citrus reached a Gl maximum of around 0.4 cm/s at 08:00 hours and declined steadily thereafter. By contrast, almond had its highest Gl of close to 1 cm/s at 12:00 hours, and pistachio reached an even greater Gl value of 1.15 cm/s at 10:00 hours, more than twice the maximum citrus Gl. Both pistachio and almond maintained similar high values throughout most of the day.

Since citrus is an evergreen plant, many water use studies report a single crop coefficient (Kc) value. These include 0.62 for Valencia in Sunraysia (Grieve, 1989), 0.44 for clementines in Mazagon, Spain (Villalobos et al., 2009), and 0.52 for lemons in Ventura, California (Grismer, 2000). Others have divided the season into winter and summer and suggested that the Kc was 0.70 and 0.65, respectively. They suggested increasing these values by 0.1 or 0.2 for humid and semi humid regions (Allen et al., 1998).

Many studies indicate that, compared to the summer, the citrus Kc is slightly higher in the winter and early spring and appreciably higher in the autumn. This is generally attributed to the citrus stomata being sensitive to evaporative demand (the air vapour pressure deficit, VPD), closing under dry, hot, windy conditions and opening under the opposite conditions. Thus, the Kc in the mild Mediterranean and coastal climates should be higher than those of more arid, inland valleys. In addition, in Mediterranean environments, high Kc values in winter reflect high soil evaporation rates from frequent rainfall during that part of the season. Not all studies found that the Kc was minimum in the summer. One study for cv. Valencia (Hoffman et al., 1982) and another for navels (Chartzoulakis et al., 1999) reported just the opposite.

The monthly Kc values published in six studies are summarized in Table 1. These include the cvs. Salustiana and Valencia under a variety of different climatic conditions. Spring Kc ranged from 0.49 in Valencia, Spain (navel) to 0.77 (Valencia) in Kiyú, Uruguay with a mean value of 0.63. For the summer, minimum and maximum values were 0.52 for cv. Shamouti in Rehovot, Israel and 0.87 for cv. Valencia in Kiyú, Uruguay, respectively, with a mean value of 0.66. Autumn minimum and maximum values were 0.58 for Shamouti in Rehovot, Israel and 0.85 for navels in Tempe, Arizona, respectively, with a mean value of 0.73.

Working with drip irrigated clementines in a precise weighing lysimeter in Valencia, Spain, Castel found that the annual Kc was linearly related to ground cover and reported the following relationship:

Kc = 0.006 ground cover + 0.272 (R^2 = 0.96)

He also emphasized that the Kc will depend on the frequency of wetting of the orchard floor; he found that without rain, surface evaporation for young drip irrigated trees varied between 8 to 30 percent of total ETc while after a rain, it reached 30 to 50 percent of total (Castel, 1997).

Actual annual ETc values will, of course, depend not only on the Kc but on evaporative demand and, to some degree, irrigation frequency and the amount of wetted surface area. The range of reported ETc worldwide include 820-1 200 mm in Florida and 1 080-1 500 mm in Arizona, Unites States, 1 300 mm in South Africa, and 800- 850 mm for the coastal areas of East Spain and Israel.

*

High Kc values in winter reflect high soil evaporation rates from seasonal rainfall in a Mediterranean environment.

WATER PRODUCTION FUNCTIONS

The two primary yield components of citrus are fruit load and individual fruit weight. Visual appearance and the market values of different size fruit play a large role in determining the net worth of a citrus crop, as they are in many tree and vine crops. This is illustrated with two examples from orange navels in the San Joaquin Valley, California (Goldhamer and Salinas, 2000 and Goldhamer, 2007). The mean gross tonnage from the final three years of a four-year experiment where 13 different RDI regimes of various stress timings and durations were imposed is shown versus applied water in Figure 2a. A linear expression fairly well describes the relationship (R^2=0.59) with a slope of about 2:1 in terms of reduced applied water: gross tonnage reduction. The primary difference in crop performance between the RDI regimes was that early stress significantly reduced creasing; the primary peel defect. There was relatively little impact of any of the regimes on fruit size and fruit load. The reduced creasing resulted in a higher percentage of the fruit being graded as Fancy and thus, highly marketable in the fresh market, rather than being designated as Choice that has much less value to the grower. This caused the value of the fruit in the early stress regimes to be significantly higher than the fully irrigated trees. This materially changed the relationship between gross grower revenue and applied water, as shown in Figure 2b, as compared with the yield-applied water relationship (Figure 2a).

Fruit size plays an important role in determining the value of fresh market fruit. Generally, large size fruit is preferred – very large size fruit as well as smaller fruit is normally worth less. However, the prices paid in the United States market for different fruit sizes can vary widely over the season for both grades of marketable fruit – Fancy and Choice (Table 2). For early harvest fruit where sizes tend to be smaller, the most desirable size is large (40 to 48 fruit/box). However, as the harvest period progresses and the fruit on the tree continues to grow, large fruit becomes the dominant size and the value of the scarcer smaller fruit increases. With the very late harvest varieties, fruit may became very large, which makes it almost worthless; in the example provided, the value is only US$5.00/box for the 24-36 fruit/box size compared with over US$14/box for the medium fruit size of 56-72 fruit/box (Table 2). On the other hand, this same very large size category (24-23 fruit/box) is worth twice that with the early harvest. Because of the desirability of having smaller fruit for the late harvest varieties, RDI can be an effective tool if the fruit size distribution can be shifted toward smaller sizes and there is no concomitant negative impact on other yield components.

The classical water production function relates yield to consumptive use. There are two issues with citrus that limit the usefulness of this relationship. First, crop revenue (US$/ha) depends not only on yield (kg/ha) but also on fruit value (US$/kg), which, in turn, is a function of fruit size, peel appearance, and fresh market prices. Second, most published studies of the impact of irrigation on production only report applied water, (many do not even report that), and do not attempt to quantify consumptive use (ETc), limiting the applicability of the reports.

Four crop-water-production functions are shown in Figure 3a, each for a different cultivar. Three are from California and one from Spain. Each of the data sets involved field experiments on mature trees that were conducted over at least four years. First order best fit lines for all studies show fairly strong correlations with correlation coefficients ranging from 0.68 to 0.98. The best fit linear expressions for the Frost Nucellar and the Lane Late studies in California have slopes less than one; the Parent Washington study in California has a slope of almost exactly one, and the Clementine study in Spain has a slope greater than one, suggesting that this cultivar may be more sensitive to water deficits than navel oranges.

The revenue-water-production functions (Figure 3b) generated from three of these studies (the Clementine study did not include revenue data) have a much different appearance than their companion crop-water-production functions. With the exception of the Parent Washington study, it is obvious that the relationships between gross revenue and consumptive use are not linear. The Lane Late study shows that revenues can be increased by about 80 percent with RDI regimes that reduce consumptive use by either 10 (late season stress) or 40 percent (season-long stress). On the other hand, a less successful RDI regime (midseason stress) reduced revenue by 15 percent with a 23 percent reduction in consumptive use. This large range in relative gross revenues for different RDI regimes clearly illustrates the importance of stress timing in some citrus varieties.

Both the Lane Late and Frost Nucellar orange studies show that gross revenue can be increased and/or consumptive use dramatically reduced with optimal RDI regimes. The results from these cultivars differ markedly than that of Parent Washington. The reason is that both of the former cultivars had production problems that were lessened with the successful RDI regimes — peel creasing with Frost Nucellar and excessively large fruit with Lane Late. There were no identifiable issues of peel appearance and/or fruit size with the Parent Washington. Nevertheless, the Parent Washington data show that consumptive use can be reduced by 7 percent with no impact on gross revenue or 18 percent with only a 6 percent decrease in gross revenue (Figure 3b).

SUGGESTED RDI REGIMES

Soil water instrumentation has been used for many years to monitor irrigation and impose water deficits in citrus. Hilgeman and Sharp (1970) used tensiometers in their RDI work with Valencia oranges. They recommend full irrigation from March through mid-July, then irrigating when tensiometers reach 75 cbar. This regime reduced irrigation by 66 percent relative to fully irrigated trees. Yield was not reduced, sugars were higher, and the resulting smaller trees were easier to pick. However, since fruit value depends on many factors discussed previously, it must be emphasized that a successful RDI approach on a given cultivar in a given location will not likely be optimal for other cropping situations. Additionally, it is dangerous to recommend soil-based stress thresholds because of differences in soils, placement of the instrument (depth and location), irrigation methods, and varieties/cultivars.

The easiest approach to imposing RDI regimes is to irrigate at given percentages of maximum ETc at specific periods of the season. For the Frost Nucellar cv., it is recommended to irrigate at 50 percent ETc from mid-May through early July (Goldhamer and Salinas, 2000). Goldhamer found that a late summer/early autumn stress was optimal with Lane Late in shifting the fruit size distribution toward more favourable fruit value and thus, increasing grower revenue. On the other hand, in a small citrus fruit such as Clementina de Nules, it was concluded that the best RDI strategy was irrigation at around 50 percent ETc from July (after June fruit drop) to the beginning of September when late summer autumn rainfall helped to quickly release plant water stress (Gonzalez-Altozano and Castel, 1999). The usefulness of this strategy has also been recently corroborated in commercial situations (Ballester et al., 2011).

It should be noted that the utility of ETc-based RDI regimes depends, in part, on soil conditions and the amount and storage of winter rainfall in the root zone, especially early in the season. For example, with relatively high winter rainfall, deep soils, and a full coverage irrigation system that promotes roots in the entire potential root zone, reducing applied water early in the season may not induce tree water deficits and will not have much impact on tree water status. On the other hand, with the opposite conditions — low winter rainfall, shallow soils, and localized (drip, microsprinkler) irrigation, there will likely be a rapid decline in tree water status in response to deficit irrigation soon after is imposed.

For this reason, the use of a plant-based indicator of tree water status is highly recommended when applying RDI for validating that the desired stress level is being achieved in the tree. In conclusion, given the very wide diversity of combinations of species, cultivars, market and growing conditions that exist in citrus, the need to tailor RDI strategies to specific conditions is even more pressing that for other fruit trees and makes it difficult to recommend generalized RDI programmes.

REFERENCES

Allen, R.G., S. Pereira, L., Raes, D. & Smith, M. 1998. Crop evapotranspirations-Guidelines for computing water requirements. In: FAO Irrigation and drainage paper No. 56. Rome, FAO.

Ballester, C., Castel, J., Intrigliolo, D.S. & Castel, J.R. 2011. Response of Clementina de Nules citrus trees to regulated deficit irrigation. Yield components and fruit composition. Agricultural Water Management. 98:1027-1032.

Barbera, G, & Carimi, F. 1988. Effects of different levels of water stress on yield and quality of lemon trees. In: Proceedings of the sixth international citrus congress, pp. 717-722.

Brych & Luedders. 1988. Citrus responses to different irrigation levels in South Portugal (Algarve). Proceedings Sixth International Citrus Congress 1831-1836.

Castel, J.R. 1997. Evapotranspiration of a drip-irrigated Clementine citrus tree in a weighing lysimeter. Acta Horticulturae, 449 (1), 91-98.

Castel, J. R., Bautista, I., Ramos, C. & Cruz, G. 1987. Evapotranspiration and irrigation efficiency of mature orange orchards in Valencia (Spain). Irrigation and Drainage Systems, 1(3):205-217.

Castel J.R. & Buj, A. 1989. Response of mature ‘Salustiana’ oranges to high frequency deficit irrigation. Irrigation Science 11:121-127.

Chartzoulakis, K., Michelakis, N. & Stefanoudaki, E. 1999. Water use, growth, yieldand fruit quality of ‘Bonanza’ oranges under different soil water. Advances in Horticultural Science 13:6-11.

Davies, F.S. & Bower, J. 1994. Water Stress, gas exchange and fruit set of ‘Olinda’ Valencia orange trees in Eastern Transvaal area of South Africa. Acta Horticulturae 365:121-127.

Eliades, G. 1994. Response of grapefruit to different amounts of water applied by drippers and minisprinklers. Acta Horticulturae 365:129-146.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Fereres, E., Cruz-Romero, G., Hoffman, G. L. & Rawlins, S. L. 1979. Recovery of orange trees following severe water stress. Journal of Applied Ecology 16:833-842.

García-Petillo, M. & Castel, J. R. 2007. Water balance and crop coefficient estimation of a citrus orchard in Uruguay. Spanish Journal of Agriclutural Research, 5(2):232-243.

Goldhamer, D. A. & Salinas, M. 2000. Evaluation of regulated deficit irrigation on mature orange trees grown under high evaporative demand. In: Proceedings of the International Society of Citriculture IX Congress. Orlando, FL. December 3-7, 2000: 227-231.

Goldhamer, D. A. 2007. Regulated deficit irrigation of trees and vines in Agricultural Water Management. In: Proceedings of a Workshop in Tunisia (Series: Strengthening science-based decision making in developing countries), L. Holliday, ed. National Research Council of The National Academies, Washington, DC, National Academies Press, pp. 70-80.

Gonzalez-Altozano, P. & Castel, J. R. 1999. Regulated deficit irrigation in ‘Clementina de Nules’ citrus trees. I. Yield and fruit quality effects. Journal of Horticultural Science & Biotechnology, 74(6):706-713.

Grieve, A. M. 1989. Water use efficiency, nutrient uptake and productivity of micro-irrigated citrus. Australian Journal of Experimental Agriculture, 29(1):111-118.

Grismer, M. E. 2000. Long-term evapotranspiration from coastal avocado/citrus orchard. Journal of Irrigation and Drainage Engineering, 126(1):1-7.

Hilgeman, R. H. & Sharp F. O. 1970. Response of ‘Valencia’ orange trees to four soil water schedules during 20 years. Journal of the American Society for Horticultural Science 95(6):739-745.

Hoffman, G. J., Oster, J. D. & Alves, W. J. 1982. Evapotranspiration of mature orange trees under drip irrigation in an arid climate. Transactions of the ASAE (American Society of Agricultural Engineers), 25(4): 992-996.

Kalma, J. D. & Stanhill, G. 1969. Transpiration, evaporation and deep drainage losses from an orange plantation. Israel Journal of Agricultural Research, 19(1):11-24.

Mantell, A. 1977. Water use efficiency of citrus: room for improvement? In: Proceedings of the International Society Citriculture 1: 74-79.

Melgar, J.C., Dunlop, J.M., Albrigo, L.G. & Syverstsen, J.P. 2010. Winter Drought Stress Can Delay Flowering and Avoid Immature Fruit Loss during Late-season Mechanical Harvesting of 'Valencia' Oranges. Horticultural Science, 45(2) 271-276.

Meyer, W.S. & Green, G.C. 1981. Comparison of stomatal action of orange, soybean, and wheat under field conditions. Australian Journal of Plant Physiology 8:65-76

Moreshet, S., Cohen, Y. & Fuchs, M. 1983. Response of mature Shamouti orange trees to irrigation of different soil volumes at similar levels of available water. Irrigation Science 3(4): 223-236.

Pérez-Pérez, J.G., Romero, P. Navarro, J.M. & Botía, P. 2007. Response of sweet orange cv. ‘Lane late’ to deficit-irrigation strategy in two rootstocks. II: Flowering, fruit growth, yield and fruit quality. Irrigation Science 26:519-529.

Van Bavel, C. H. M., Newman, J. E. & Hilgeman, R. H. 1967. Climate and estimated water use by an orange orchard. Agricultural and Forest Meteorology 4:27-37.

Villalobos, F.J., Testi, L. & Moreno-Pérez, M.F. 2009. Evaporation and canopy conductance of citrus orchards. Agricultural Water Management, 96 (4): 565-573.

Wiegand, C.L. & Swanson, W.A. 1982. Citrus responses to irrigation. II. Fruit yield, size, and number. Journal Rio Grande Valley Horticultural Society 35: 87-96.

Apple

LEAD AUTHORS

Amos Naor
(GRI, University of Haifa, and Migal - Galilee Technology Center, Israel)

Joan Girona
(IRTA, Lleida, Spain)

CONTRIBUTING AUTHORS

Hussein Behboudian
(Massey University, Institute of Natural Resources, Palmerston North, New Zealand),

Robert G. Evans
(USDA-ARS, Sidney, Montana, USA)

Jordi Marsal
(IRTA, Lleida, Spain)

INTRODUCTION AND BACKGROUND

Apple (Malus domestica Mill.) is one of the most widely cultivated fruit crops and is produced commercially in over 80 countries around the world (FAO, 2011). Its production is the third highest fruit crop in the world after banana and grape and is ranked second behind grape for area. Most apples are grown in temperate zones because of the high chilling requirements for proper bud break in the spring. The development of apple varieties with low chilling requirement and the use of dormancy-breaking chemicals has enabled the migration of apples towards warmer regions. Production areas have increased significantly in recent decades until 2000, when China took the lead in production. Total production of apples stabilized during 2000-2010 at 60 to 70 million tonne/ year. In 2009, the harvested area exceeded 4.9 million ha with an average yield of 14.7 tonne/ha (FAO, 2011). Figure 1 shows the production trends of the principal countries. China is the largest apple-producing country (42 percent of world production) and its production is six-fold larger than the second country (United States, 7 percent).

The duration of the season from bud break to harvest varies widely among varieties, from 70 days up to 210 days (Westwood, 1993) and it is correlated with the storability duration.

Apple originated in Central Asia, where its wild ancestor is still found today. There are more than 7 500 known cultivars resulting in a wide-range of fruit characteristics. Cultivars vary in their yield, fruit size, colour, taste, and the ultimate size of the tree, even when grown on the same rootstock. Apple is grown in a wide-range of environments, in many areas of Asia and other continents. In Europe from Scandinavian countries in the north to Italy in the south, in South Africa, Australia and New Zealand, United States and Canada, and in the southern countries of South America. The optimum climatic requirements might be characterized as cool to cold winters followed by a rapid rise in temperature in spring.

Apple has two types of buds, pure vegetative and mixed buds containing a floral bud that has several vegetative buds at its base. The floral bud generates an inflorescence that usually has five flowers. Flower buds are borne terminally on shoots and spurs and, to some extent, on lateral buds of one year-old shoots. Most apple varieties have self-incompatibility (Goldway et al., 2007) thus growers usually include more than one variety within each plot, and some orchard growers use crab apples as pollinators.

Quality considerations

There are many features that affect apple fruit quality for the fresh market. External features such as size, colour, shape and appearance are very important. For many markets, fruit that is smaller than 65-70 mm in diameter have a price penalty. Depending on the variety, the background and superficial skin colours, whether green, yellow, red or red striped play an important role in fruit appearance, and therefore its fresh market value. Skin blemishes such as sunburn, russeting and other markings negatively affect fresh quality. Flesh texture and firmness are important attributes, and are affected by environmental factors, cell wall properties and calcium content. Fruit composition in terms of sugars, acids and different aromas is variety dependant and has a strong influence on fruit taste and quality. Postharvest handling in controlled atmospheres, now commonly for this fruit months after harvest, is critical in terms of maintaining fruit quality and to limit the numerous physiological disorders that may appear during storage.

Apples tend to have a biennial bearing pattern where the degree of biennial bearing varies with varieties (Lauri et al., 1996) and potential crop yield is highly affected by the timing of fruit thinning and the crop load in the previous season.

STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

Bloom date varies with varieties and climatic regions. Most varieties flower in April (Northern Hemisphere), whereas early (low chilling requirement) varieties can bloom at the end of February and early March. Apple fruit has an expo-linear growth pattern where much of the dry mass accumulation occurs in the linear phase that starts ~30 days after full bloom in some varieties while in others the linear phase starts ~60 days after full bloom (Goffinet et al., 1995) Figure 2 shows the patterns of fruit growth as affected by different irrigation treatments. The reproductive cell division phase lasts 40-50 days post bloom (Westwood, 1993).

Fruit size is a major determinant of fresh fruit quality and is highly dependent on tree water status and irrigation. However, there are many other factors that will affect the response of fruit size to irrigation including the number of cells in the fruit pericarp, crop load, the number of seeds per fruit, factors that interact with each other.

Temperatures lower than 25 °C during the reproductive cell division phase reduce apple fruit size (Tromp, 1997 and Warrington et al., 1999); on the other hand, it has been found that the number of cells in apples decreased when the trees were grown at 35/15 °C (day/night) rather than at 25/15 °C. This suggests that there is an optimum temperature for reproductive cell division, and that limitation of potential fruit size should be expected in both cold and hot climates. The previous-year crop yield (Bergh, 1985), the current-year crop level, and the timing of fruit thinning affect the number of cells and therefore potential fruit size (Goffinet, 1995 and Quinlan and Preston, 1968). The number of seeds per fruit has a dramatic effect on final fruit size as shown in Figure 3 where, for the cv. Golden Delicious, the fruit diameter when no seeds were apparent was ~52 mm, while it reached ~73 mm when ten seeds per fruit were apparent. There is the perception that water stress affects cell division, however, studies of other fruit trees (olives and pears) showed no effect of water stress on fruit cell numbers even where predawn leaf water potential (LWP) reached -4.0 MPa in olive (Rapoport et al., 2004).

Crop load in the current season affects the fulfillment of potential fruit size and it interacts with irrigation and fruit water status, which is discussed later. Information about the positive or negative effects of postharvest water deficits on the yield and quality responses of apple in the following growing season is limited, but it appears that severe stress can have negative effects on return bloom.

RESPONSES TO WATER DEFICITS

Irrigation is a major horticultural activity and is the most intensively practiced operation throughout the season. Its importance depends on the climate, and increases as one moves from temperate to drier and to arid zones. Rainfed apple orchards can survive and be productive in temperate zones without irrigation, whereas the survival of apple orchards in arid and semi-arid zones depends on the availability of water for irrigation throughout most of the growing season. The performance of apple in terms of crop yield, fruit size, fruit quality, storability, and long-term productivity are highly dependent on irrigation and irrigation management. Irrigation level and water status are known to affect yield and yield components: crop yield, fruit size and quality, growth habit, precocity, and long-term productivity. Apple fruit size is very sensitive to water stress and thus highly responsive to irrigation (Naor, 2006; Naor et al., 1995 and Girona et al., 2010). It is also highly responsive to crop load (Naor et al., 2008). Assimilate availability is thus the limiting factor for fulfilling potential fruit size (Naschitz et al., 2010). Water stress not only limits cell and fruit expansion but also reduces photosynthesis (the source for assimilates), while primarily crop load determines the demand for assimilates and to a certain extent the photosynthetic rate.

Unlike peach, apple does not have distinct stages of fruit growth and a large part of vegetative and reproductive growth overlap during the growing season. For this reason, deficit irrigation researchers normally use the terms ‘early-season’ or ‘late-season’ to describe the timing of their treatment application. Early season normally indicates the time period before flowering buds are formed for the next season fruit. For most varieties, early season will be before July in the Northern Hemisphere and before January in the Southern Hemisphere.

Early-season water stress reduces apple fruit size (Failla et al., 1992 and Rufat et al., 2003). It also reduces fruit set by dramatically increasing fruitlet drop in temperate zones; in one experiment (Powell, 1974), final fruit set decreased from 24 percent for irrigated trees to 8.7 percent for non-irrigated trees. In a semi-arid area of Israel, final fruit set of fully-irrigated trees was 15 percent decreased to 8 percent in severely stressed trees. Water storage from winter precipitation avoids rapid development of severe water stress in most temperate climatic conditions, but for containerized apples with a limited rooting zone, severe water stress may cause up to 100 percent fruitlet drop. This suggests that growers should be aware of the risk of severe water stress development early in the season especially during drought years, and/or in very shallow soils having low water-holding capacity.

Late-season water stress that occurs in the post-reproductive cell division stage affects apple depending on the degree of severity. Moderate water stress up to 102 days after full bloom reduced canopy growth (Behboudian et al., 1998), whereas water stress after this period had no effect. This indicates that shoot growth ends within three months after bloom (Forshey and Elfving, 1989). An early deficit created moderate water stress and resulted in a lower return bloom, whereas no reduction in return bloom was apparent in a late-deficit treatment (Behboudian et al., 1998). However, if water stress is severe during these later stages it may affect next year’s growth (Ebel, 1991 and Girona, 2010a). Reductions in return bloom and productivity under severe water stress have been found in apple. Fruit numbers in the following years were affected by severe stress the previous year generated by terminating irrigation in early summer. This was particularly evident in early varieties (Ebel, 1991).

The other, above-mentioned studies, where return bloom was not reduced, did not involve such a severe level of water stress (Behboudian et al., 1998). The reduction in the proportion of return bloom of apple trees that were moderately stressed early in the season could reduce the size of the bourse shoot that emerges from apple flower buds below the threshold required for its terminal bud to produce a viable flower bud (Lauri et al., 1996). It may well be that bourse shoots had already reached the threshold length prior to the start of late-deficit treatments thus no effect on return bloom during late water stress was apparent. It has been observed that excessive shading by a dense canopy can also negatively affect return bloom.

In most studies early water deficits (for about 2 months post reproductive cell division) reduced apple fruit size (Naor, 2006 and Rufat et al., 2003). In general, total crop yield increases with both irrigation level and crop load. However, as fruit size is a major attribute of fruit quality, growers are interested in larger fruit up to a certain limit, where oversize apples will fetch a lower price. The yield of large fruit is affected by both irrigation and crop load (Figure 4). At low crop loads, there is no advantage of increasing irrigation, as similar crop yields were obtained in one experiment where irrigation levels between June and harvest ranged from 46 percent to 119 percent of ETo (Naor et al., 1997).

The water deficits may reduce tree size in the following year, but do not negatively affect flower bud number or fruit load (Girona et al., 2008). As crop load increased (double number of fruit per tree), yield of large fruit increased with increasing irrigation levels. This indicates an increased limitation in assimilate availability that cannot be overcome by supplying additional water above full requirements. At the highest crop load, yield of large fruit did not respond to a seasonal irrigation level above 84 percent of ETo, which is more or less equivalent to apple ETc during the irrigation period (see below).

Fruit size increased with increasing midday stem-water potential (SWP: Figure 5) where different response curves were observed for different crop loads. The threshold of midday SWP to reach marketable size fruit shifted to higher stem-water potentials with increasing crop load. For crop loads up to medium levels, maximum fruit size can be achevied by improving tree water status. However, at extremely high crop load, marketable size fruit cannot be reached even under non-stress conditions. Therefore, there is an upper limit of crop load that would enable large fruit size, suggesting that both irrigation and fruit-thinning practices should be employed to maximize crop yield of large, marketable fruit. In some cases, summer pruning can help achieve marketable sizes, as for peach.

Water deficits and fruit quality in apple

In general, mild water deficits during fruit development advance maturity, increase total soluble solids content and firmness, and may improve red colour and decrease the background colour. It also affects volatile aroma compounds. Many past studies found higher firmness as a result of deficit irrigation. However, it was argued (Behboudian and Mills, 1997) that the increased fruit firmness of stressed trees could be an artifact because fruit size decreases as a direct result of deficit irrigation and the firmness of apples increases with decreasing fruit weight (Ebel et al., 1993). In different studies, water stress after the cell division phase increased (Mpelasoka et al., 2001), did not affect (Ebel et al., 1993), or decreased (Mills et al., 1994) apple firmness at harvest.

The dynamics of fruit firmness in cold storage, as affected by water deficits, were examined in two studies on apple (Mpelasoka et al., 2001. and Kilili et al., 1996). The difference in firmness between fruit from different water-stress treatments remained the same during 10 and 12 weeks in cold storage; they diminished after 10 weeks and reached similar levels by 17 weeks (Mpelasoka et al., 2001). During a shelf-life study (Mpelasoka et al., 2001) the higher firmness imparted by a deficit irrigation treatment was retained for six days, after which the differences diminished and disappeared. Data collected over the past decade suggest that firmness increases in response to post-cell-division water stress, but that the increase is often temporary, around 10 weeks in some studies.

Many studies found that deficit irrigation increased ethylene concentrations in apple, at harvest or during storage (Ebel et al., 1993; Kilili et al., 1996; Behboudian et al., 1998; Mpelasoka et al., 2001 and Mpelasoka et al., 2002). Background colour is an indicator of maturity in apple and it was reported to either decrease or to remain unchanged in response to deficit irrigation. These findings indicate that deficit irrigation advances maturity (and related red colour) in most cases. Studies on the effect of deficit irrigation on aroma volatiles yielded inconsistent results (Behboudian et al., 1998 and Mpelasoka et al., 2002) probably because of a dramatic rise of these compounds at a certain point together with the fact that there is no distinct definition of maturation and the advancement of maturity in response to deficit irrigation. Deficit irrigation increased total soluble solids in apple at harvest (Ebel et al., 1993; Behboudian et al., 1994; Behboudian et al., 1998; Mills et al., 1994; Kilili et al., 1996; Mpelasoka et al., 2001a and Mpelasoka et al., 2002), and the differences were retained during storage. The increased total soluble solids content was accompanied by an increased percentage of dry matter, suggesting part of the increase in soluble solids resulted from to water losses from the fruit. However, deficit irrigation elicited specific metabolic effects that were manifested in changed proportions of specific sugars by increasing fructose or sorbitol content (Mills et al., 1994) compared with unstressed treatments.

Deficit irrigation increased the red colour (Mills et al., 1994 and Kilili et al., 1996) or did not affect it in apple. Enhancement of the red colour could be an indirect effect of deficit irrigation, via a reduction in vegetative growth, which affects light regime within the canopy. It could also be associated with the advancement of maturity induced by the water deficit. More details of the effects of reduced irrigation on fruit quality for important deciduous fruit, including apple, have been recently published (Behboudian et al., 2011).

WATER REQUIREMENTS

Two key factors determining apple tree water consumption are the evaporative demand of the atmosphere and the canopy size that determines the amount of energy intercepted by the canopy. A recent lysimeter study (Girona et al., 2011) showed that the crop coefficient increases from bud break parallel with the development of canopy coverage where canopy coverage reaches a maximum ~60 days after full bloom (Figure 6). A slight increase in Kc was apparent closer to harvest followed by a sharp decline right after harvest. This decline was first probably the result of crop removal, which is known to affect transpiration, while the additional decrease thereafter was related to leaf senescence. The sharp decline in Kc right after harvest, from 1.0 to 0.6 in 2-3 weeks without evidence of leaf senescence (Figure 6) repeats every season (Girona et al., 2011) because fruit is a very important carbon sink and removal feeds back to carbon assimilation and reduces stomatal conductance and transpiration. Trees with very low crop loads therefore use less water than trees with commercial loads, which indicates that the Kc values should be adjusted downward from the values of Figure 6 for low to negligible crop loads. Excessive vegetative growth is expected in the spring in low crop loaded trees and its suppression is difficult because of the low ET in the spring, thus summer pruning should be employed.

Recommended Kc values for various locations in the world are presented in Table 1. The Kc values for apple orchards depend on the intercepted radiation and may vary with different orchard conditions (row and tree spacing, tree age and size, training system, row orientation, etc.). For intensive, hedgerows plantations, a relationship between Kc and midday light interception (Girona et al., 2011) is presented in Figure 7. Given that maximum Kc is around 1.0 (Figure 6), the relationship shown in Figure 7 may be used to determine the specific Kc values for apple orchards that have not achieved maturity.

* irrigation based on soil moisture measurements

** preharvest/postharvest

WATER PRODUCTION FUNCTIONS

Water production functions for apple are difficult to generalize because they are affected by many factors such as training system, crop load, pruning and thinning practices, and whether the target is total (as for juice production) or fresh marketable yields.

Figures 8 and 9 present the response of total and marketable yield to a decrease in ETc determined in Lleida, Spain. In both cases, it seems that relative ETc may be reduced by about 15- 20 percent without having a negative impact on final yield. Similar results were found in Israel where fruit yield >70 mm was unaffected by increasing Kc above 0.84 (Figure 10).

Water management of fresh market apple production should take into account that: 1) fruit size is highly dependent on crop load thus it should be optimized to maximize yield of marketable size fruit; 2) apples tend to have a biennial bearing pattern in response to high crop load, thus crop load should be optimized to allow stable production for each season. In one specific experiment with Golden Delicious (Figure 10), crop yield of marketable size (>70 mm) increased with both annual irrigation level and crop load, and the maximum crop yield was achieved at the highest load at maximum apple ETc (equivalent to 84 percent of the seasonal ETo). It should be emphasized that potential fruit size was high during this season and, although the threshold of maximum commercial crop yield could be higher; optimal crop load that avoids biennial bearing lies between the two highest crop loads (Figure 10) and may change with climatic conditions.

SUGGESTED RDI REGIMES

Given the growth patterns of the apple, and the sensitivity of fruit size to water deficits, it appears that only mild water deficits may be applied to this crop, when grown for fresh market, without impacting negatively on farmers’ income. However, RDI has some positive effects on quality that should be exploited, in particular in water scarcity situations. The objective of apple irrigation management under suboptimal water allocation would be to minimize damage and maximize irrigation water productivity. If a small reduction in supply is considered, it should occur preferably during the postharvest period. In common varieties that are harvested in September (Northern Hemisphere) it leaves some period of irrigation up to the start of leaf senescence, a period that will be shorter with increasing latitude. In water shortage conditions one can skip the postharvest irrigation for common varieties but that should not be done in early maturing varieties that have long postharvest period. If water shortages exceed the level equivalent to the amount used for postharvest irrigation, growers should apply the deficit on a continuous basis after the early fruit growth period (avoid stress in the first 30 to 60 days after fruit set, depending on variety), and adjust the crop load by thinning to ensure that the remaining fruit will reach commercial size. A recommendation on irrigation for different water allocations and optimal crop loads for production in warm climates is presented in Table 2.

It is important to monitor soil or tree water status when applying RDI (see Chapter 4). Climate control for apple (frost protection and/or cooling for sunburn protection or to enhance red colour in warm areas) is carried out using sprinkler irrigation and its use, in the case of cooling, may disrupt the RDI programme. Evaporative cooling reduces actual ET on the order of 20 percent, but total seasonal irrigation water requirements would be much higher than the full orchard ETc, as calculated in Chapter 4.

REFERENCES

Behboudian, M.H., Lawes, G.S. & Griffiths, K.M. 1994. Influence of water deficit on water relations, photosynthesis and fruit growth in Asian pear (Pyrus serotina Rehd.). Scientia Horticulturae. 60:89-99.

Behboudian, M.H. & Mills, T.M. 1997. Deficit irrigation in deciduous orchards. Horticultural Reviews 21:105-130.

Behboudian, M.H., Dixson, J. & Pothamshetty, K. 1998. Plant and fruit responses of lysimeter-grown ‘Braeburn’ apple to deficit irrigation. Journal of Horticultural Science and Biotechnology 73:781-785.

Behboudian, M.H., Marsal, J., Girona, J. & Lopez, G. 2011. Quality and yield responses of deciduous fruit to reduced irrigation. Horticultural Reviews 38:149-189.

Bergh, O. 1985. Effect of the previous crop on cortical cell number of Malus domestica cv. Starking Delicious apple flower primordial, flowers and fruit. South African Journal of Plant Soil. 2:191-196.

Ebel, R.C. 1991. Apple tree and fruit response to drought. Pullman, Washington State University (Ph.D. Thesis)

Ebel, R.C., Proebsting, E.L. & Patterson, M.E. 1993. Regulated deficit irrigation may alter apple maturity, quality, and storage life. Horticultural Science 28:141-143.

Failla, O., Zocchi, Z., Treccani, C. & Socucci, S. 1992. Growth, development and mineral content of apple fruit in different water status conditions. Journal of Horticultural Science 67:265-271.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Forshey, C.G. & Elfving, D.C. 1989. The relationship between vegetative growth and fruiting in apple trees. Horticultural Reviews 11:229-288.

Girona, J., Mata, M., Del Campo, J. & Marsal, J. 2008. Manejo del riego del manzano en condiciones de sequía extrema. Dossier Tècnic DAR (Geeneralitat de Catalunya) 30:5-8.

Girona, J., Del Campo, J., Bonastre, N., Paris, C., Mata, M., Arbones, A. & Marsal, J. 2010a. Evaluation of Different Irrigation Strategies on Apple (Malus domestica). Physiological and Productive Results. Acta Horticulturae (in press).

Girona, J., Behboudian, M.H., Mata, M., Del Campo, J., & Marsal, J. 2010. Exploring six reduced irrigation options under water shortage for ‘Golden Smoothee’ apple: Responses of yield components over three years. Agricultural Water Management 98:370–375.

Goffinet, M.C., Robinson, T.L. & Lakso, A.N. 1995. A comparison of Empire apple fruit size and anatomy in unthinned and hand-thinned trees. Journal of Horticultural Science 70:375-387.

Goldway, M., Sapir, G. & Stern, R.A. 2007. Molecular basis and horticultural application of the gametophytic self-incompatibility system in rosaceae tree fruit. In: J. Janick (Ed.). Plant Breeding Reviews, 28: 215-237.

Kilili, A.W., Behboudian, M.H. & Mills, T.M. 1996. Postharvest performance of ‘Braeburn’ apples in relations to withholding of irrigation at different stages of the growing season. Journal of Horticultural Science 71:693-701.

Lauri, P.E., Terouanne, E. & Lespinasse, J.M. 1996. Quantitative analysis of relationships between inflorescence size, bearing-axis size and fruit set – an apple tree case study. Annals of Botany 77:277-286.

Mills, T.M., Behboudian, M.H., Tan, P.Y. & Clothier, B.E. 1994. Plant water status and fruit quality in ‘Braeburn’ apples. Horticultural Science 29:1274-1278.

Mpelasoka, B.S., Behboudian, M.H. & Green, S.R. 2001. Water use, yield and fruit quality of lysimeter-grown apple trees: responses to deficit irrigation and crop load. Irrigation Science 20:107-113.

Mpelasoka, B.S. & Behboudian, M.H. 2002. Production of aroma volatiles in response to deficit irrigation and crop load in relation to fruit maturity for ‘Braeburn’ apple. Postharvest Biological Technology 24:1-11.

Naor, A., Klein, I. & Doron, I. 1995. Stem-water potential and apple fruit size. Journal of the American Society of Horticultural Science 120:577-582.

Naor, A., Klein, I., Doron, I., Gal, Y., Ben-David, Z. & Bravdo B. 1997. Irrigation and fruit level interactions in relations to yield and fruit size distribution. Journal of the American Society for Horticultural Science 122: 411-414.

Naor, A. 2006. Irrigation scheduling and evaluation of tree water status in deciduous orchards. Horticultural Reviews 32:111-166.

Naor, A., Naschitz, S., Peres, M., Gal, Y. 2008. Response of apple fruit size to tree water status and crop load. Tree Physiology 28:1255-1261.

Naschitz, S., Naor, A., Genish, S., Wolf, S. & Goldschmidt, E. 2010. Internal management of non-structural carbohydrate resources in apple leaves and stems under a broad range of sink and source capacity manipulations. Tree Physiology 30:715-27.

Powell, D.B.B. 1974. Some effects of water stress in late spring on apple trees. Journal of Horticultural Science 49:257-272.

Quinlan, J.D. & Preston, A.P. 1968. Effect of thinning blossom and fruitlets on growth and cropping of Sunset apple. Journal of Horticultural Science 43:373-381.

Rapoport H.F., Costagli, F. & Gucci, R. 2004. The effect of water deficit during early fruit development on olive fruit morphogenesis. Journal of the American Society for Horticultural Science 129:121-127.

Rufat, J., Mata, M., Arbonés, A., Del Campo, J., Gelly, M., Marsal, J., & Girona, J. 2003. Fruit development, yield and quality in response to irrigation and nitrogen application on Golden Delicious apples. Fifth International Conference on integrated fruit production IOBC/WPES Bulletin 24(5):359-365.

Tromp, J. 1997. Maturity of apple cv. Elstar as affected by temperature during a six-week period following bloom. Journal of Horticultural Science 72:811-819.

Warrington, I.J., Fulton, T.A., Halligan, E.A. & de Silva, H.N. 1999. Apple fruit growth and maturity are affected by early season temperatures. Journal of the American Society for Horticultural Science 124:468-477.

Westwood, M.N. 1993. Temperate-zone pomology. Portland, Oregon, USA, Timber Press, Inc.

Plum

LEAD AUTHORS

Diego S. Intrigliolo, Juan R. Castel
(IVIA, Moncada, Valencia, Spain)

CONTRIBUTING AUTHOR

Amos Naor
(GRI, University of Haifa, and Migal - Galilee Technology Center, Israel)

INTRODUCTION AND BACKGROUND

Plum species of commercial importance originated between Eastern Europe and Central Asia. Cultivated plums include two main species, European (Prunus domestica L.) or ‘prunes’ and Japanese plums (Prunus salicina L.). Both species are medium-size deciduous stone fruit-trees that differ notably in respect to their climatic requirements. European plums are cultivated in temperate climates to fulfil chilling requirements and to enable proper bud break. They are relatively late flowering, while Japanese plums grow better in temperate-warmer regions, as their chilling requirements are less. Their productive use also differs, as Japanese plums are mainly grown for fresh fruit, while dried fruit (prunes) is mainly obtained from European plum varieties. World acreage was over 2.5 million ha in 2009 with an average yield of 4.3 tonne/ha. China and Serbia are the two main world producers, followed by the United States and Romania (Figure 1). Spain occupies the eighth place with about 191 000 tonne mostly of fresh fruit, but is among the three world highest exporters. France is the main European producer of dried fruit, and Chile is now an important producer and exporter in the Southern Hemisphere (FAO, 2011).

Plum species can adapt to different soil types, although they are sensitive to water-logging, iron chlorosis and salinity. Therefore, the use of rootstocks to cope with these adverse environmental conditions is common. Plum trees generally bear fruit at an early age and the fruiting period lasts 5-35 years. Early varieties can be grown without irrigation in arid climates with rainfall as low as 300 mm/ season, and midseason varieties require at least 400-500 mm/ season. However, productivity and fruit size in these conditions are usually low and, therefore, most plantations are irrigated, especially in arid and semi-arid climates.

The quality features for fresh market include size, colour and a good balance between soluble solids and acidity, while for dry fruit production, soluble solids and size are the two most important quality parameters.

DEVELOPMENTAL STAGES IN RELATION TO YIELD DETERMINATION

Commercial plum tree varieties break dormancy and begin flowering in the Northern Hemisphere between late-February and mid-April, depending on the environmental conditions and the cultivar. Bud formation starts with the appearance of the first basal leaves and continues through June on mature trees (Westwood, 1993). Reproductive buds are in a lateral position on terminal shoots or on short shoots called spurs. Flower buds are initiated in the growing season prior to anthesis, and development continues through the dormant season until the following spring just before bud break. The proportion of spur and terminal shoots varies largely with variety and species and so does the proportion of fruit borne on spurs and shoots, which also varies with tree age. Most commercial plum cultivars are not self-pollinating and therefore the use of pollinators is required. Plum trees bloom very profusely and thinning is required, either performed manually, chemically or mechanically to obtain commercial fruit sizes. For example, Black-Gold in Mediterranean conditions may set around 10-40 percent of the flowers produced, but good commercial yields are obtained with only about 5-10 percent of fruit set (Intrigliolo and Castel, 2005). Normally, flowering is completed by late April (Northern Hemisphere) and is followed by rapid fruit expansive growth with concomitant rapid shoot growth.

The fruit with fleshy pericarp is classified as a drupe and is single-seeded. Fruit growth follows the typical double-sigmoid pattern, with rapid exponential growth during the cell division phase (Stage I, ~ 30 days in length), followed by a relatively short period of slow growth during pit hardening and embryo development (lag phase, Stage II). Finally, a second period of rapid cell and fruit enlargement prior to harvest (Stage III), when the fruit can increase in size ca. 40-60 percent, although this is linked to accumulated heat units (degree-days) after flowering, in a similar fashion to other Prunus species (DeJong and Goudriaan, 1989). Therefore, the length of each stage varies with variety and location. During the postharvest period, some shoot growth and carbohydrate storage for reserves are the primary sinks for carbon assimilation, which continues until leaf fall.

EFFECTS OF WATER DEFICITS

A distinction should be made between plums for fresh fruit production and those for dried fruit production (prunes), as dry matter accumulation is less sensitive to water stress than is the increase in fresh weight, particularly during the last stages of fruit development. In addition, lower fruit hydration rates resulting from water deficits may also offer an advantage for post-harvest fruit processing in the case of prunes for dry fruit production (Lampinen et al., 1995). Thus, prune trees are considered to be moderately resistant to water stress, as indicated by early experiments (Hendrickson and Veihmeyer, 1945) in the deep soils of California’s Sacramento Valley, where it took 4 years of no irrigation to detect decreased trunk growth, and 5 years of water deprivation to detect a significant reduction of fruit yields. This is also supported by more recent findings (Goldhamer et al., 1994) where irrigation cutoff, up to 37 days before harvest did not have any negative impact on dry fruit yields of French prune.

Water stress during fruit growth

In Japanese plums for fresh markets, water stress in the final stages of fruit growth significantly decreased fruit size, but accelerate ripening and lead to an increase in fruit sugar concentration (Naor, 2004). Under water stress, average fruit weight and yield were affected by increased tree crop load for Japanese plum (Intrigliolo and Castel, 2005; Naor, 2004) but under minimum stress conditions, the fruit size distribution was unaffected by fruit number per tree, possibly because of low crop yields, which did not introduce significant limitation of assimilates (Naor, 2004). Irrigation of previously water-stressed prune trees has been found to induce fruit-end cracking (Uriu et al., 1962); the formation of cracks was accompanied by increased osmotic potential gradients along the fruit in re-watered trees (Milad and Shackel, 1992).

Water stress during postharvest

The practice of reducing or eliminating irrigation after harvest of an early-maturing plum cultivar (P. salicina) irrigated with foggers was studied in California (Johnson et al., 1994), where completely cutting off irrigation led to partial defoliation within a few weeks and loss of yield in the subsequent year. Postharvest midday stem-water potential (SWP) reached ~-3.3 MPa with no symptoms of defoliation in Black Amber (Naor, 2004). For trees that were irrigated daily, but at half the rate of the fully irrigated control, no reduction of yield or fruit quality occurred over a 3-year period, possibly because of the contribution of stored soil water. In young orchards, postharvest water restrictions did not affect yield in the short term (Intrigliolo and Castel, 2005). However, after four seasons of deficit irrigation, there was a 10 percent reduction in yield compared with fully irrigated trees because the stressed trees were smaller. Thus, long-term deficit irrigation of young trees causes a reduction in productivity by reducing tree size. Post harvest water stress, despite its moderate detrimental effect in the long term, should be considered for commercial orchards not only in the case of water scarcity, but also as a tool for controlling vegetative growth in areas where vigorous growth may be a problem.

Plant water stress is known to potentially affect flower bud development for the next season, but there are only a few reports on a decrease in next season crop level because of bud damage (Johnson et al., 1994). Water stress during postharvest, as measured by the SWP, was also correlated with the following season’s crop yields.

In some cases, there was even an increase in return bloom leading to larger yield in prune trees where a high crop level was the target (Lampinen et al., 1995). In plum trees, water stress did not appear to be associated with the appearance of fruit disorders such as double fruit formation or fruit deep suture, as occurs in other stone fruit-trees such as peach (Johnson and Handley, 2000).

Plant water stress indicators

Midday SWP is the most useful indicator of plant water stress in plum trees, since prior to harvest it was highly correlated with tree performance (Naor, 2004; Intrigliolo and Castel, 2005; and Intrigliolo and Castel, 2006a). Figure 2 presents the results of two studies on different varieties of Japanese plums: Black-Gold plums (Intrigliolo and Castel, 2006a) and Black Amber plums (Naor, 2004) in the semi-arid climates of Valencia, Spain and Upper Galilee, Israel, respectively. In each location and season, tree-to tree variations of SWP were well correlated with the average fruit weight at harvest. However, there was no unique relationship relating SWP to fruit weight valid for all data across experiments (Figure 2). The differences in the intercept of the lines reported between seasons and locations indicate that fruit weight is not only a function of plant-water status. In addition, the different slopes of the linear relationships between locations suggest that the effect of plant water stress on fruit growth might change according to different environmental or cultural conditions. Overall these results highlight the importance of conducting local experiments when attempting to predict the effect of plant water stress on fruit weight at harvest.

Studies using other water status indicators for plum trees have also shown that daily trunk contraction, continuously measured with stem dendrometers (Intrigliolo and Castel, 2006b), is highly correlated to SWP, but other factors such as tree age and tree crop load also influence the relationship between trunk contraction and SWP (Intrigliolo and Castel, 2006b; and Intrigliolo and Castel, 2007).

WATER REQUIREMENTS

Only a few early studies quantified the consumptive water use of plum orchards. The recommended crop coefficient values for plum trees are included in the stone fruit tree section together with peach trees in the FAO I&D No. 56 publication (Allen et al., 1998). A specific study of the water use of plum trees trained to different canopy arrangements (Chootummatat et al., 1990) found that mature trees under a Tatura training system reaching full cover, used 92 percent of class-A pan evaporation in midsummer. Lower water use (82 percent of pan evaporation) was determined for trees trained as vase or palmette systems. As a first approximation, the Kc values for peach (see Peach) should be used for plum orchards.

WATER PRODUCTION FUNCTION

It seems that there are no deficit irrigation trials investigating the relationship between tree water use and yield either for Japanese plums or European prunes. However, from main results reported in the literature it is possible to derive some water productivity functions based on applied water by irrigation. Three studies on different varieties of Japanese plums were included in the analysis: Fortune plums in the humid climate of the Po Valley in Italy (Battilani, 2004), Black-Gold plums (Intrigliolo and Castel, 2006a; Intrigliolo and Castel, 2010) and Black Amber plums (Naor, 2004; Naor et al., 2004) in Valencia, Spain and Upper Galilee, Israel, respectively. In all cases curvilinear functions fit the relationships between relative yield and relative irrigation (Figure 3), but there were differences in the threshold values of relative applied irrigation for no yield reduction. The data from Spain and Israel fell on a single polynomial regression line, which fitted both data set well. In cv. Black-Gold and Black-Amber only 10 percent of reduction in applied water appears to be admissible for no yield penalty, whereas in the study in Italy with the cv. Fortune, it was possible to reduce irrigation by 20-25 percent without any yield reduction. In addition, the response of Black-Gold and Black-Amber plums showed a much sharper decrease in relative yield with irrigation deprivation relative to Fortune plums. The differences in the relationships between applied water and yield are related to the unknown contribution of stored soil water and of in-season precipitation to the crop ETc under water deficits. The similarity in the response of two different varieties in Spain and Israel probably reflect the limited contribution of soil storage in both studies (hence the sharp decline in relative yield when irrigation decreases). Additionally, the differences between the study in Italy and the other two might be the result of climatic conditions, with higher winter and growing season precipitation in the Po Valley and tree age; mature trees in Italy and younger orchards in the studies in Spain and Israel.

The patterns obtained in the above-mentioned studies are in line with general tree responses to water supply, where yield increases with increasing water application but up to a point where further increases in water application do not produce any increase in yield. Since there are no studies relating yield to ETc in plum trees, overall data reported in Figure 3 showed that for plum trees deficit irrigation could save around 10-20 percent of applied water with minimum or no yield loss.

SUGGESTED DEFICIT IRRIGATION STRATEGIES

The suggested deficit irrigation strategy may greatly vary depending on the final market product, dried fruit or fresh fruit, and on specific phenological aspects of each variety affecting bloom intensity and fruit set levels and particularly, earliness. The general strategy used to impose the water deficits for French prune was to limit water deficits during early stages of tree and crop development, imposing more severe stress during mid and late season. In this sense, in a clay loam soil in California, allowing a progressive decline in midday SWP to approximately -1.5 MPa by harvest, e.g. irrigating at about 50-60 percent ETc from spring, resulted in an effective way to reduce irrigation and maintain an economic return over a 3-year period (Lampinen et al., 2001a).

For early season fresh market varieties it can be concluded that water stress after harvest that limits the decline in SWP below -2.0 MPa, despite some possible slight detrimental effect in the long term, should be considered in commercial orchards not only for water scarcity, but also as a tool to control vegetative growth. In young orchards, postharvest deficit irrigation may be combined with closer tree spacing, a feature very common in modern fruit tree plantations where new cultivars and orchards have a short life.

For fresh market varieties water stress, if applied during fruit growth, should be concentrated during pit hardening. The length of this phase depends on the harvest date. Hence, in early and even midseason maturing cultivars there is a risk of extending the water stress into the final fruit growth stage with detrimental effects on fruit size. Recent results (Intrigliolo and Castel, 2010) suggest that some degree of water stress can be applied during the early stage of fruit growth, providing that plant-water stress is mild (SWP > -1.4 MPa) and trees return to optimum water status at least one month before harvest. The convenience of water stress applied during fruit growth would indeed depend upon price market values of different fruit size categories and fruit quality effects of water restrictions. In this sense it should be considered that deficit irrigation during fruit growth advances maturity, increases total soluble solids content and firmness, and may improve fruit colour. The effects of water restrictions on volatile aroma compounds and particularly fibre and other fruit quality components related with human health have not been extensively studied and could be of great importance for plum growers if the consumption of plums is promoted.

REFERENCES

Allen, R.G., Pereira, L.S., Raes, D. & Smith, M. 1998. Crop evapotranspiration. Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56. Rome, FAO pp.15-27.

Battilani, A. 2004. Regulated deficit irrigation. Effects on growth and yield of plum trees. Acta Horticulturae. 664:55-62.

Chootummatat, V., Turner, D.W. & Cripps, J.V.E. 1990. Water use of plum trees (Prunus salicina) trained to four canopy arrangements. Scientia Horticulturae, 43:255-271.

DeJong, T.M. & Goudriaan, J. 1989. Modelling peach fruit growth and carbohydrate requirements: reevaluation of the double-sigmoid pattern. Journal of the American Society for Horticultural Science, 114:800–804.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Goldhamer, D.A., Sibbett, D.S., Phene, R.C. & Katayama, D.G. 1994. Early irrigation cutoff has little effect on French prune production. California Agriculture, 48:13-17.

Hendrickson, A.H. & Veihmeyer, F.J. 1945. Some effects of irrigation on the interrelations of growth, yields and drying ratios of French prunes. Journal of the American Society for Horticultural Science, 46:187–190.

Intrigliolo, D.S. & Castel, J.R. 2005. Effects of regulated deficit irrigation on growth and yield in young Japanese plum. Journal Horticultural Science & Biotechnology, 80:177–182.

Intrigliolo, D.S. & Castel, J.R. 2006a. Performance of various water stress indicators for prediction of fruit size response to deficit irrigation in plum. Agricultural Water Management, 83:173-180.

Intrigliolo, D.S. & Castel, J.R. 2006b. Usefulness of diurnal trunk shrinkage as a water stress indicator in plum trees. Tree Physiology, 26:303-311.

Intrigliolo, D.S. & Castel, J.R. 2007. Crop load affects maximum daily trunk shrinkage of plum trees. Tree Physiology, 27:89-96.

Intrigliolo, D.S. & Castel, J.R. 2010. Response of plum trees to deficit irrigation under two crop levels. Tree growth, yield and fruit quality. Irrigation Science, 28:525-534.

Johnson, R.S, Handley, D.F. & Day, K.R. 1994. Postharvest water stress of an early maturing plum. Journal Horticultural Science, 69:1035–1041.

Johnson, R.S. & Handley, D.F. 2000. Using water stress to control vegetative growth and productivity of temperate fruit trees. Horticultural Science, 69:1048–1050.

Lampinen, B.D., Shackel, K.A., Southwick, S.M., Olson, B. & Yeager, J.T. 1995. Sensitivity of yield and fruit quality of French prune to water deprivation at different fruit growth stages. Journal of the American Society for Horticultural Science, 120:139–147.

Lampinen, B.D., Shackel, K.A., Southwick, S.M. & Olson, B. 2001. Deficit irrigation strategies using midday stem-water potential in prune. Irrigation Science, 20:47-54.

Milad, R.E. & Shackel, K.A. 1992. Water relations of fruit end cracking in French prune. Journal of the American Society for Horticultural Science,117:824–828.

Naor, A. 2004. The interactions of soil- and stem-water potentials with crop level, fruit size and stomatal conductance of field-grown ‘Black Amber’ Japanese plum. Journal Horticultural Science & Biotechnology, 79:273–280.

Naor, A., Peres, M., Greenblat, Y., Gal, Y. & Ben Arie, R. 2004. Effects of preharvest irrigation regime and crop level on yield, fruit size distribution and fruit quality of field-grown ‘Black Amber’ Japanese plum. Journal Horticultural Science & Biotechnology, 79:281–288.

Uriu, K., Hansen, C.J. & Smith, J.J. 1962. The cracking of prunes in relation to irrigation. Proceedings of the American Society for Horticultural Science, 80:211–219.

Westwood, M.N. 1993. Temperate-zone pomology: physiology and culture. Third Edition. USA, Portland, Oregon, Timber Press, pp 523.

Almond

LEAD AUTHOR

David A. Goldhamer
(formerly University of California, Davis, USA; currently Cooperative Extension Emeritus)

CONTRIBUTING AUTHOR

Joan Girona
(IRTA, Lleida, Spain)

INTRODUCTION AND BACKGROUND

Almonds are grown under both rainfed and irrigated conditions; production in semi-arid zones, such as the western United States and Spain, reflects the drought tolerance of the tree. In many areas of the Mediterranean Basin, almond trees are grown on marginal soils in areas where annual rainfall does not exceed 300 mm, being important for erosion control and to prevent desertification. As a result of the limited water supply and poor soil conditions of the rainfed areas, tree densities are quite low and yields are also low and variable from year-to-year. However, they can be much higher when the water-use requirements of the trees are fully met and, in most areas of the world, this requires irrigation. New irrigated almond plantations have expanded in recent decades in many areas and are highly productive. Nevertheless, almonds are an important crop in very diverse agricultural systems, from very marginal to highly intensive. In 2009, the cultivated area worldwide amounted to 1.8 million ha with an average yield (with shell) of 1.3 tonne/ha (FAO, 2011). Figure 1 shows the recent trends in production of the major producing countries.

Modern almond cultivation presents unique challenges to irrigation in general and regulated deficit irrigation (RDI) in particular. These include dealing with multiple cultivars in each orchard, a long period between flowering and fruit maturity the need to dry the soil prior to harvest in order to mechanically shake nuts from the trees, and a relatively late reproductive bud morphogenesis period. On the other hand, since the fruit is sold dry, many of the problems associated with fresh fruit production, including physical appearance, handling and storage are absent.

The almond flower of most varieties is self-infertile; it cannot pollinate itself. Even for cultivars that are self-compatible, production is enhanced by cross-pollination. Thus, each orchard normally contains at least two different cultivars with overlapping bloom periods to help the process of cross-pollination; the transfer of pollen from the anthers of a flower from one cultivar to the stigma of a flower from another cultivar. This transfer is facilitated by the introduction of honey bees into the orchard during flowering. To maximize pollen exchange, a common arrangement is single, alternating rows of each cultivar. The fact that two or more cultivars exist in a field complicates irrigation management in that the different harvest periods usually result in harvest-related water deprivation for one cultivar, while kernel filling is occurring in the other cultivar. Further, there is some evidence that different cultivars have different stress sensitivities.

Quality considerations

Insect damage, shrivel, kernel colour, and broken kernels are quality criteria worldwide. Additionally, marketplace differences result in cultivar-dependent crop values. Some markets also place a premium value on larger nuts; the United States, for example.

DESCRIPTION OF THE STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

Almond trees require very low chilling and thus, vegetative growth and flowering begin very early in the season relative to other deciduous tree species; although plant breeders aim to develop cultivars that bloom late to avoid chilling injuries. This earliness feature relates to the evolution of almonds in mild, subtropical climates with prolonged summer drought. The period from flowering to fruit maturation of almond is relatively long, depending on climate and cultivar; from late January-March to August-September in the Northern. Hemisphere, and the sensitivity of each of the physiological processes during this time to water deficits must be considered to assess the impact of stress on the yield and quality of the fruit at harvest. Not only current season impacts but those of subsequent seasons must be taken into account.

EARLY VEGETATIVE AND REPRODUCTIVE GROWTH

Flowering and initial leaf development occur almost simultaneously from late January in the Northern. Hemisphere for the earliest blooming cultivars until the end of March for the late blooming. Fertilization of the flower is followed by growth of the pollen tube into the ovary, which will evolve into the marketable kernel. The maximum potential fruit production is determined during this early period. It is established by the number of flowers produced (flower set) and the percentage of these that are successfully pollinated (fruit set). Early fruit development is largely the result of cell division. The early stages of fruit growth occur at the same time as most of the leaf expansion and shoot growth. This results in considerable competition for tree resources, principally carbohydrates. Thus, if flowering and fruit set are high, shoot growth may be lower. Since fruit are borne on spurs, this may reduce the number of new reproductive buds produced and, in turn, reduce the crop potential for the following year. In addition to its impact on fruiting positions, this carbohydrate competition can influence fruit set. If carbohydrate reserves from the previous year are low, the current year fruit set may be reduced (Esparza et al., 2001). This effect may enhance alternate bearing in almonds, especially under rainfed conditions.

Stages of fruit growth

Figure 2 shows the pattern of fruit growth where three stages may be defined:

Stage I is one of rapid growth of the hull, shell, and integuments. The entire fruit remains soft and reaches its maximum size. Cell division is completed in a few weeks; the major part of growth thereafter is expansion. At this point, the kernel is a white structure filled with watery, translucent tissue. The time between fertilization of the flower to the end of fruit development is about two months. The end of Stage I is marked by the attainment of the maximum external dimensions of the hull, shell, and kernel.

Stage II is characterized by shell hardening and kernel expansion. There are two types of almond varieties: hard and soft shelled. Hard shelled, which are many of the Mediterranean varieties, have shelling percentages of 25-35 percent, while soft shelled have 70 percent. They completely harden in Stage II while the soft shelled remain soft. The growth of the embryo involves clear watery tissue becoming translucent, starting at the apical end. This white, opaque embryo rapidly expands during this period. Toward the end of Stage II, kernel dry weight begins to increase.

In Stage III, the major event is the steady dry matter accumulation in the kernel. The morphological differentiation of the hull, shell and kernel are complete. Dry matter accumulation of assimilates continues at a steady rate until maturity, as long as the vascular connections remain intact. Two events signal the approach of maturity: hull split (endocarp dehiscence) and the formation of an abscission layer at the nut-peduncle connection. Complete dehiscence requires an adequate tree-water status because the sides of the hull must be turgid to separate properly. Excessive stress may cause the hulls to adhere to the shells (hull-tights), which complicates processing. Maturity is also characterized by a sharp slowing in the rate of kernel dry matter accumulation. In some areas, commercial harvests occur prior to kernel maturation to avoid insect navel orange worm (NOW) damage.

Bud development

The reproductive buds are borne on spurs and are initiated in the spring as the spurs develop. There are three subsequent stages of flower-bud development. The first is induction where the internal physiology of the growing point changes. This occurs in mid-August and the vegetative and reproductive buds are indistinguishable. Second are the morphological-anatomical changes in the internal structure, which are readily observable in September. Third is gradual growth of the reproductive parts during the autumn and winter, i.e. development of the sepals, petals, stamens and ovaries.

RESPONSES TO WATER DEFICITS

As for most crop plants, vegetative growth of almonds is very sensitive to water deficits. Avoidance of water deficits throughout the season in young trees is critical to reach full production in the shortest time period (Fereres et al., 1981). In mature plantations, responses to water deficits depend on the timing of the stress.

In areas that receive substantial winter rainfall, tree processes that occur very early in the season, such as leaf out, flowering, pollination and fruit set, will be under non-limiting soil water levels. However, as the season progresses and evaporative demand increases, shoot, spur, and fruit growth will be subjected to water deficits without irrigation or in-season rainfall. Several reports state almond vegetative growth is very sensitive and directly affected by tree-water deficits.

The research results on preharvest impacts of stress on kernel filling are seemingly contradictory and may reflect the importance of stress timing and cultivar differences. A study in Spain (Girona et al., 2005) with cv. Ferragnes, reported that kernel dry weight accumulation was not influenced during the first two seasons of an RDI regime that irrigated at 20 percent of ETc during late June through the mid-September harvest (minimum predawn leaf water potential of -1.7 MPa in August) but was lower during the final two seasons of the study, which was attributed to the cumulative impacts of stress reducing the reserves of carbohydrates available for kernel filling and to relatively low soil water levels during those years. However, a California study (Goldhamer and Viveros, 2000) with higher evaporative demand (minimum predawn leaf water potential of -3.5 MPa) and earlier stress reported significant reductions in kernel dry matter accumulation with cv. Non Pareil in all experimental years. Dry matter accumulation in the hull and shell after three successive years of preharvest stress, diverged from the fully irrigated in late April, well before there were differences in tree stress, as shown in Figure 3. This was likely because of early season competition for carbohydrates. A study (Romero et al., 2004) with cv. ‘Cartagenera’ that imposed an RDI regime that resulted in a minimum predawn leaf water potential of -2.5 MPa in late July found no reduction in dry kernel weight at the mid-August harvest.

A recent study with cv. Non Pareil in California showed that imposing water deficits primarily from early July through an early September harvest over a four-year period did not reduce kernel weight or nut load (Stewart et al., 2011). These workers attempted to maintain midday stem-water potential between -1.4 and -1.8 MPa during this period. The objective was reduced hull rot, a disease that damages the fruit (Teviotdale et al., 2001), while reducing consumptive use. Other efforts using this same philosophy have achieved positive results and this practice is now being widely adopted by California almond growers with trees afflicted by severe hull rot. However, it should be noted that detailed analysis of the fruit components (hull, shell, and kernel) suggests that the impact of preharvest stress on hull splitting may impact kernel weights. In numerous studies, California researchers found that slight reductions of kernel dry matter accumulation occurred concomitant with the onset of hull split, while at the same time, there were slight increases in the rate of dry matter accumulation in the hulls. The net result was slightly lighter kernels (generally 2-3 percent relative to full irrigation) but no difference in the dry weight of the entire nut. They hypothesized that hull split resulted in some physical disruption in assimilate transport in the pathway leading to the kernel.

It appears that there are two factors involving early season stress timing that can contribute to reduced kernel size: lower cell division and/or expansion, which is enhanced by carbohydrate competition, and the disruption in assimilate transport to the kernels because of accelerated hull split. These stress impacts may well be cultivar-dependent although comparative research studies are lacking.

Of the two primary yield components of almond, fruit load appears to be the most sensitive in terms of water stress impacts on yield. A study in Spain found that fruit loads were reduced in the final two years of a four-year RDI treatment and attributed this to the cumulative impacts of stress on the bearing surface, and thus fruiting positions, of the tree. Another study in California also reported that yield reductions associated with water deprivation in August and September (minimum midday stem-water potential of -2.5 MPa) were the result of a reduced bearing surface resulting from less shoot and spur growth. This study found that yields were reduced only after two years of stress. Other studies have found little impact of preharvest stress on fruit load.

Much less work has been done on the impacts of postharvest stress on almond production, in part, because in many parts of the world, autumn rains eliminate this possibility. However, a dramatic impact of the presence or absence of postharvest irrigation on the following season’s fruit load has been detected (Goldhamer and Viveros, 2000) (Figure 4). Even when the trees were near fully irrigated prior to harvest, postharvest water deprivation resulted in 40 percent reduction in fruit load the following season, relative to trees that received postharvest irrigation. It should be emphasized that this was with a mid-August harvest under high evaporative demand; predawn leaf water potential was below -4.0 MPa in mid September. For this same stress level at preharvest, there was near complete defoliation but after the reintroduction of full irrigation postharvest, there was vegetative bud break and new leaf growth, alleviation of the stress, and no reduction in fruit load the following season (Goldhamer and Viveros, 2000).

The dramatic impact of postharvest water deprivation on fruit load was attributed to stress impacts on reproductive bud development. Early work (Tufts and Morrow, 1925) showed that bud differentiation in almond occurred from late August through early September, and this has been confirmed by more recent work. The timing of bud development showed no clear pattern between cultivars or locations within California, spanning a distance of more than 500 km (Lamp et al., 2001). Thus, bud development can occur both after and before harvest, depending on the cultivar and geographic location. Moreover, bud development is not related to hull split: it occurred three weeks after hull split in Non Pareil but prior to hull split in ‘Butte’ and ‘Carmel.‘ Stresses that occur during flower development are likely to adversely affect flower quality to the extent that the next season’s crop load, and thus yield, would be reduced.

Indicators of tree water status

To precisely schedule irrigation, it may be necessary to monitor a given soil and/or plant parameter and make decisions according to some pre-established criteria. Also, implementing an RDI regime may have to be based on estimates of tree water status, such as the stem-water potential (SWP). The SWP values of well-irrigated almond trees in mid-summer range from -0.5 to -1.0 MPa at midday, depending on the evaporative demand and the time of the year. In one study, there was a 0.2 MPa decrease in the SWP of fully irrigated trees on different days (from -0.7 to -0.9 MPa) when the air temperature increased from 25 to 40 ºC. The SWP values decrease as stress increases but in almond, it seldom exceeds -4.0 MPa even under very severe stress (Castel and Fereres, 1982). The tree will shed its leaves before reaching the extreme dehydration levels that would induce lower water potential, as measured in other fruit tree species.

WATER REQUIREMENTS

Most of the almond water use estimates in the literature were developed using soil water balance approaches rather than from more accurate weighing lysimeters. The monthly crop coefficient values (Kc) for clean cultivated, weed free, high evaporative demand conditions published by several authors are shown in Table 1. Because almond ET has often been grouped with peach, apricot, and plum, weighing lysimeter Kc data for peach determined in California are also shown in Table 1 for comparison (Ayars, 2003). Early season crop Kc values for the peach used in their work are relatively low owing primarily to the slow canopy development of this cultivar. Maximum Kc values (July-August) for all the presented data range from 0.95 to 1.08. Recent data from California suggests that almond peak Kc values of an intensive, mature orchard irrigated with microsprinklers may reach as high as 1.17 (Goldhamer, unpublished), which is considerably higher than previously reported. Similar high Kc values have been recently reported in Australia (Stevens et al., 2011). It should be noted that when the early ET data were developed, surface irrigation (border strip) was the primary irrigation method, whereas drip or microsprinklers were used in the more recent studies. The higher Kc values are likely due to the increase in tree densities in recent plantations, larger tree canopies (there is much less annual pruning now than previously), and higher fruit loads. Also, the more frequent wetting of the orchard floor with microirrigation and thus, higher surface evaporation may be another factor for the higher Kc values.

WATER PRODUCTION FUNCTION

Relative yield versus relative applied water data derived from fourteen irrigation studies are presented in Figure 5. It was not possible to estimate ETc in many of the studies and therefore, the actual production function based on consumptive use could not be drawn. These studies were done over a wide range of evaporative demands, cultivars, and soils with various deficit irrigation regimes; different timing and magnitudes of stress. The correlation coefficients of the linear regressions for these studies ranged from 0.87 to 0.98, indicating a strong functional relationship between yield and applied water. Some of these studies had similar slopes ranging from 0.7 to 0.9, whereas others had a milder slope above 0.3. The lower yield sensitivity of these studies is likely due to a combination of deep soils, relatively low crop loads, and relatively wide tree spacing. Thus, the impact of instantaneous stresses was buffered by the high potential rate of water supply to the trees. It should be noted that these studies generally had consumptive use rates that deprived the trees of up to 30-50 percent of maximum ETc. Close inspection of Figure 5 shows that with mild deficit irrigation that would reduce relative ETc by only 10-15 percent, the impact on production is negligible.

SUGGESTED RDI REGIMES

Growers with limited water supplies must make a decision on when to stress trees. Based on research results, we believe the two most stress sensitive periods are in the Spring when the nuts are rapidly expanding and late summer/autumn when bud morphogenesis is occurring. This second stress-sensitive period is usually postharvest for early harvest cultivars but prior to harvest with later maturing cultivars. The results of an experiment (Goldhamer et al., 2006) provide useful information on the relative sensitivity of pre and postharvest stress to aid RDI decision making. It was found that the greater the preharvest water deprivation, the greater was the reduction in kernel dry weight at harvest (Figure 6a). However, minimizing preharvest stress at the expense of postharvest irrigation resulted in significantly lower fruit loads in subsequent seasons (Figure 6b). Yield, the integrator of fruit weight and fruit load, was least affected by minimizing stress after harvest (Figure 6c). These regimes also resulted in the highest irrigation water productivity (Figure 6d).

Water supply constraints may be temporary, as a result of one year drought. Single season drought RDI strategies were tested (Goldhamer and Smith, 1995) where they applied less than 40 percent (400 mm) of potential seasonal ETc with different timing regimes: irrigating at 100 percent, 75 percent, and 50 percent ETc until the 400 mm was exhausted, which occurred in early June, mid July, and late August, respectively. They found that full irrigation early in the season limited reductions in fruit size but resulted in dramatic reductions in the following seasons’ fruit load (Table 2) (Goldhamer and Smith, 1995). They attributed this to the negative impact of stress on reproductive bud differentiation. The treatment that irrigated at 50 percent ETc, which applied water longer (through August; two weeks after harvest), did not suffer any significant decrease in fruit load the following season. When they averaged the drought year and the following two fully irrigated recovery years, they found that the 50 percent ETc treatment had higher yields that the other two RDI regimes; those that applied their available water supply all preharvest. Nevertheless, none of the RDI regimes achieved complete production recovery even after two seasons of full irrigation following the single drought year, suggesting that impacts of reduced shoot and spur growth may have also been a factor (Goldhamer and Smith, 1995).

Suggested RDI regimes for five different levels of available water supply (300, 450, 600, 750, and 900 mm where full ETc is 1250 mm) expressing irrigation rates as percentages of ETc are presented in Table 3. To show how these regimes would affect applied water, we used as an example long term values of ETo from western Fresno County, California and bimonthly crop coefficients (Kc) from Goldhamer (unpublished) for ‘Non Pareil’ almonds. When the water supply was relatively high, the stress is biased to the preharvest period, saving as much water as possible for the most stress sensitive period; from mid August through the end of September. With a severely restricted water supply, the concern is about tree survival and general health in addition to maximizing stress impacts on time-averaged yields. It must be emphasized that when applying very low amounts of potential seasonal water supply, surface evaporation, and thus, the number of irrigations, should be minimized. Therefore, the duration (amount of applied water) of each irrigation should be maintained as normal but the frequency of irrigation should be changed. For example, if microsprinkler irrigation is normally operated every three days, an RDI strategy that applies 25 percent ETc would extend the frequency to every 12 days.

Since RDI reduces vegetative growth, it should not be used on young trees where the objective is to grow the canopy to full size, and thus attain maximum yields, as fast as possible. It has been confirmed (Girona et al., 2005) that RDI imposed too early in the life of the orchard can reduce potential yields.

Additional considerations

Water stress in almonds has been known to increase spider mite levels (Youngman and Barnes, 1986) and the navel orangeworm (Goldhamer, unpublished data). The latter becomes more of a problem when the onset of hull split is accelerated by preharvest stress and/or the nuts remain longer on the tree before shaking. Hull rot can be dramatically reduced by imposing water deficits during the first two weeks of July (Teviotdale et al., 2001). Their target predawn leaf water potential value was -1.6 MPa.

REFERENCES

Ayars, J.E., Johnson, R. S., Phene, C.J., Clark, D.A. & Mead, R.M. 2003. Water use by drip irrigated late season peaches. Irrigation Science 22: 187-194.

Castel, J.R. & Fereres, E. 1982. Responses of young almond trees to two drought periods in the field. Journal of Horticultural Science, 57:175-187.

Esparza, G., DeJong, T.M., Weinbaum, S.A. & Klein, I. 2001. Effects of irrigation deprivation during the harvest period on yield determinants in almond trees. Tree Physiology, 21: 1073-1079.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Fereres, E., Aldrich, T.M., Schulbach, H. & Martinich, D. A. 1981. Responses of almond trees to late season drought. California Agriculture, 42: 10-13.

Girona, J., Mata, M. & Marsal, J. 2005. Regulated deficit irrigation during the kernel-filling period and optimal irrigation rates in almond. Agricultural Water Management, 75(2):152-167.

Goldhamer, D.A. & Smith, T. 1995. Single season drought irrigation strategies influence almond production. California Agriculture, 49(1):19-22.

Goldhamer, D.A. & Viveros, M. 2000. Effects of preharvest irrigation cutoff durations and postharvest water deprivation on almond tree performance. Irrigation Science, 19:125-131.

Goldhamer, D.A., Viveros, M. & Salinas, M. 2006. Regulated deficit irrigation in almonds: effects of variations in applied water and stress timing on yield and yield components. Irrigation Science, 24(2):101-114.

Lamp, T.M., Connell, J.H., Duncan, R.A., Viveros, M. & Polito, V.S. 2001. Almond flower development: Floral initiation and organogenesis. Journal of the American Society of Horticultural Science, 126(6): 698-696.

Romero, P., Navarro, J.M., Garcia, F. & Ordaz, P.B. 2004. Effects of regulated deficit irrigation during pre-harvest period on gas exchange, leaf development and crop yield of mature almond trees. Tree Physiology, 24:303-312.

Stevens, Rob M., Caecilia M. Ewenz, Gary Grigson, Samantha M. Conner. 2011. Water use by an irrigated almond orchard. Irrigation Science. DOI 10.1007/s00271-011-0270-8.

Stewart, W., Fulton, A, Krueger, W.H., Lampinen, B. D. & Shackel, K. A. 2011. Regulated deficit irrigation reduces water use of almonds without affecting yield. California Agriculture, 65(2)90-95.

Teviotdale, B.L., Goldhamer, D.A. & Viveros, M. 2001. The effects of deficit irrigation on hull rot disease of almond trees caused by Monilinia fructicola and Rhizopus stolonifer. Plant Disease, 85(4):399-403.

Tufts, W.P. & E.B., Morrow. 1925. Fruit-bud differentiation in deciduous fruit. Hilgardia. California Agriculture Exp Sta, 1(1). 26 pp.

Youngman, R.R. & Barnes, M.M. 1986. Interaction of spider mites (Acari: Tetranychidae) and water stress on gas-exchange rates and water potential of almond leaves. Environmental Entomology, 15: 594-600.

*

Irrigation rate until allotment applied; no additional irrigation for the remainder of the season.

**

Does not include 100 mm pre-season irrigation applied each year.

***

Numbers for each year followed by a different letter are significantly different at the 5% confidence level using Duncan's New Multiple Range Test.

*

The grower would keep track of cumulative amounts to be applied with RDI scenarios. When they total 37 mm (the amount applied by the microsprinklers in 24 hrs in this example), he irrigates. Thus, there would be one irrigation in the first week of April with full water supply but with 300 mm available case, first irrigation would not be until the first week of May.

Pear

LEAD AUTHOR

Jordi Marsal
(IRTA, Lleida, Spain)

CONTRIBUTING AUTHORS

Joan Girona
(IRTA, Lleida, Spain),

Amos Naor
(GRI, University of Haifa, and Migal - Galilee Technology Center, Israel)

INTRODUCTION AND BACKGROUND

Pears, along with peaches, are the second deciduous fruit tree species in economical importance after apples. The genus Pyrus includes about 20 wild species and its primary centre of origin is Europe, and regions in temperate Asia. Two main species are cultivated. European pear (Pyrus communis L.) is grown in Europe, United States, South Africa and Oceania, and the Asian pear or Nashi (Pyrus pyrifolia Burm.) (syn. Pyrus serotina) is traditionally grown in Asia. Other species of the genus have been used as rootstocks such as Pyrus calleryana Dcne. Today the most widespread rootstock is clonal quince (Cydonia oblonga L.), though its graft compatibility is not good for all cultivars.

The cultivated pear is self-incompatible, and cross-pollination with other cultivars is required for optimum fruit production, with the exception of some varieties such as Bartlett and to some extent Conference. Pears typically bear fruit on spurs in terminal buds. Flower buds are initiated at the end of shoot development during the preceding season, and the formation of these flowers depends on the light received by spurs in the previous season. An open canopy is thus required for full fruitfulness by training branches and pruning to specific shapes.

Other factors that influence flower bud initiation are previous crop load and water stress. Water stress, to a certain extent, can be a positive stimulus for bud initiation, but a high crop load has a negative influence in the next season. For this reason pears often exhibit biennial alternate bearing. Another key issue in pear orchards is growth control to prevent decreased light penetration into the canopy. Water stress, vigour controlling rootstocks, and growth regulators are means available to reduce vigour in pear orchards. Pears are grown in a wide-range of climates, from cool to warm and from humid to arid-areas. In 2009, there were 1.58 million ha of pear orchards globally with an average yield of 14.2 tonne/ha (FAO, 2011). Figure 1 presents the production trends of the major producing countries. The major factors limiting for the expansion of pear production in warm regions are insufficient chilling temperatures during winter and the occurrence of diseases such as fireblight. Pear trees are not a drought resistant species and its commercial production in areas with dry seasons depends entirely on irrigation. As far as irrigation is concerned, pear orchards may benefit from judicial use of deficit irrigation because it can have positive effects by controlling tree vigour during current season and on flowering in the following season.

DESCRIPTION OF THE STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

The reproductive growth of pear trees can be divided into two stages based on the growth rate of the fruit (Figure 2). Stage I of pear fruit development, corresponds to the initial slow growth phase, and Stage II corresponds to the rapid growth phase (Mitchell et al., 1989) which in the cultivar Bartlett the second stage could be differentiated when growth (volume increase) surpasses the rate of 1.5 cm^3·day-1.

Early vegetative and reproductive growth; growth Stage I

Pear flowers commonly open almost synchronously with leaf appearance. Shoot growth starts after first leaf appearance and occurs concomitantly with the current season reproductive growth. Vigour-conditions will determine the extent and the timing of shoot growth enlargement. In trees grafted on vigour controlling rootstocks such as quince, shoot development occurs in one-to-two flushes during spring (April and May in the Northern Hemisphere). Under more vigorous conditions shoot growth extends into early summer throughout all Stage I. At this time, vegetative growth of the scion is a stronger sink than fruit, and fruit growth is quite slow in terms of dry mass accumulation. Root growth in spring is also relevant and occurs concomitantly with shoot development and ceases near the end of May. Root growth, however, depends on inter-organ competition and availability of carbohydrates. The end of rapid shoot growth is signalled by the appearance of a terminal bud.

Fruit growth starts right after ovary fertilization and this can be measured in the field at about one month after full bloom. Physiological fruit drop, however, lasts longer and can extend until the end of Stage I or onset of Stage II (in midseason cultivars). Stage I corresponds to the fruit main cell-division period, and takes place during the first 7 to 8 weeks after bloom (Bain, 1961). The remainder of fruit development constitutes Stage II when the major increase in cell volume occurs (Bain, 1961). Cell expansion, however, is also active during Stage I, but its effect is masked by the simultaneous occurrence of cell division.

Fruit growth during Stage II

Expansive fruit growth is the main growth event for the tree during Stage II. Stage II corresponds to the period of rapid fruit cell enlargement. Nevertheless, bud development also becomes relevant after cessation of shoot enlargement. Differentiation of buds into flower buds usually occurs at the beginning of Stage II in midseason cultivars (early to mid-June) (Elkins et al., 2007). The appearance of flower structures takes place during Stage II. However, this process is influenced and modified by climatic conditions and a number of factors that are not yet well understood. Although shoot extension growth is minimal during that time, branch thickening may occur if fruit load is low and water status is optimal.

Postharvest

Bud will continue to develop during postharvest and at a slower rate throughout winter. During this period, buds will increase in size by 25 percent, which mainly corresponds with the elongation of the carpels. During the postharvest period, there is a second peak of root growth activity, and this period is also important for reserve accumulation in roots and stems before the start of defoliation. This tends to occur somewhat sooner than in apple and other deciduous species and it is accelerated by low temperatures. Anomalous postharvest flowering can occur in autumn after a period of severe postharvest water stress, if this stress is relieved by irrigation or rainfall a month before leaf die back (Naor et al., 2006).

RESPONSES TO WATER STRESS

Although pear is not considered drought resistant, its organs and tissues can withstand a certain degree of dehydration, which surpasses the capacity of other deciduous fruit trees such as peach, plum or apple. During summer, leaf turgor loss occurs in the cv. ‘Barlett’ at values of midday stem-water potential (SWP) close to -3.1 MPa (Marsal and Girona, 1997), which is quite low relative to the other deciduous fruit trees. It has also been reported that recovery from water deficits is delayed if SWP reaches values below -3.5 MPa, suggesting this threshold as a limit for the occurrence of vascular embolism (Marsal et al., 2002b). Stomatal conductance and leaf photosynthesis decrease linearly with midday SWP in response to irrigation reductions. For European pear, nearly zero values in both stomatal conductance and leaf photosynthesis have been reported at SWP values of -2.5 MPa (Naor et al., 2000; and Marsal et al., 2002b). Trunk growth ceases at SWP below -2.2 MPa, but shoot extension growth stops sooner, at about -1.7 MPa in the case of moderately low vigour conditions. For more vigorous conditions (i.e. young, defruited trees) extension growth precedes up to -2.0 MPa of SWP. Fruit growth (fresh weight) is somewhat less sensitive to water deficits, stopping at about -2.5 MPa, either during Stage I or Stage II (Marsal et al., 2002b). The SWP values discussed above are indicative of full impairment, but the processes, whether fruit or vegetative growth, are affected by much milder water deficits. For instance, for cv. ‘Conference’ it was found that to achieve fresh market standards of fruit size for at least 50 percent of harvested fruit, SWP values below -1.1 MPa should be avoided during the Stage II of fruit growing period.

In terms of flowering, moderate water stress during the fruit-growing season (Marsal et al., 2002a) or postharvest (Naor et al., 2006) increases bloom the following season as compared to fully irrigated trees. This behaviour is attributed to the fact that moderate water stress levels hasten development of flower organs (Forshey and Elfving, 1989). However, severe water stress (SWP values below -2.8 MPa) can induce cropping deficiencies next season (Naor et al., 2006). The data in Figure 3 shows that moderate water stress was the best postharvest strategy in terms of subsequent season productivity (Naor et al., 2006). Contrary to European pear, Asian pear seems to have a differential flowering response to water deficit the previous year. In general, water stress reduced return bloom in Asian pears (Caspari et al., 1994).

In general terms it can be stated that for favourable growing conditions reference midday SWP values for unstressed pear trees oscillate between -0.65 and -0.95 MPa depending on evaporative demand; values below -1.1 MPa are indicative of water stress conditions. On the other hand, it is difficult to diagnose early waterlogging effects from SWP values.

Water stress responses during Stage I

Pear trees are highly responsive to seasonal water stress. Water stress during Stage I decreases shoot growth, fruit growth, final fruit size at harvest, and can increase fruit drop (Marsal et al., 2000; and Naor et al., 2000). Water stress during Stage I can potentially affect fruit cell division, cell enlargement or both processes (Marsal et al., 2000). Only in few cases and under moderate water stress conditions (LWP above -2.5 MPa), final fruit size was not impaired or favoured by the application of early water stress (Behboudian et al., 1994; and Mitchell et al., 1989). These authors argued that such responses were achieved by the occurrence of fruit osmotic adjustment that increased fruit growth after the early water stress. However, other interpretations of these positive effects, such as the different conditions in the timing and duration of the applied water stress, are also possible (Naor et al., 2006). Other factors such as vigour and tree-to tree shading conditions can also be added to this controversy. The early literature from Australia described the application of water deficits to cv. ‘Barlett’ pear trees in high density orchards (from 2 500 to 5 000 plant/ha) growing on largely vigorous rootstocks (Pyrus calleriana). In those experiments, fruit under RDI during Stage I sized larger than Control fruit at harvest, and water stress during Stage I helped reduce vegetative growth that was considered excessive. The growing conditions in these studies were site-specific and do not represent the typical pear-growing conditions around the world where canopy shading is optimized by the introduction of new vigour controlling rootstocks. Experiments carried out in Spain under more common growing conditions including moderate density orchards (1 100-1 600 plant/ha), and using vigour-reducing rootstocks such as clonal quince (BA-29 or M-C) indicated no significant effect on fruit size at harvest in response to RDI Stage I as compared to Control irrigated trees (Marsal et al., 2002a; and Asin et al., 2007). In one case, where trees grew in isolated large containers of 120 litre, deficit irrigation during Stage I actually reduced final fruit size at harvest (Marsal et al., 2000). Furthermore, several attempts in Spain during the nineties to use RDI during Stage I in commercial orchards aimed to increase fruit size above that of fully-irrigated trees, proved unsuccessful.

Water stress responses during Stage II

Water stress during Stage II of fruit development decreased final fruit weight and lower fruit diameter (Behboudian et al., 1994; Marsal et al., 2000; Naor, 2001; Marsal et al., 2002a; and O’Connel and Goodwin, 2007). Water stress at this time mainly reduces fruit cell size (Marsal et al., 2000) and, as a consequence, final fruit size at harvest.

Water stress responses during postharvest

There are few studies available on this topic. One study attempted to use RDI in postharvest for Spadona European pear where the elapsed time for deficit irrigation was three months (from August to the end of October) (Naor et al., 2006). The results of the experiment indicated that water could be saved, provided midday SWP did not surpass the threshold of -2.2 MPa. A positive effect related to moderate postharvest water stress (i.e. SWP >-2.2 MPa) was that, during the following season, return bloom and fruit yield increased significantly compared to fully irrigated and severely stressed trees (Naor et al., 2006).

Similar results regarding increased return bloom have been found by Marsal in Spain for the cultivar Conference. However, in his study fruit set was lower for these trees with higher bloom and this ended up reducing yield but increasing fruit size.

WATER REQUIREMENTS

There are only a few reports available on ETc information for pear trees. One study used drainage lysimeters of 105 litre capacity and conditions close to hydroponics with the soil surface covered to avoid soil evaporation (Buwalda and Lenz, 1995). The study considered three different cultivars, two training systems and presence or absence of fruit. The effect of both cultivar and training system on tree water consumption was significant; although the differences can be explained by differences in leaf area. However, the presence of fruit increased tree water consumption by 36 percent as compared to de-fruited trees, independently of leaf area (Buwalda and Lenz, 1995). These differences were probably related to increases in stomatal conductance and therefore leaf photosynthesis and transpiration which are frequently observed under higher cropping conditions of pear (Marsal et al., 2008).

The crop coefficients obtained in the lysimeter study (Buwalda and Lenz, 1995) were referred to the Priestley and Taylor ETo equation, and did not consider a soil evaporation component. Nevertheless, the reported values were quite low, with maximum values of 0.38 for cv. Conference with a leaf area index (LAI) of 2.0. Water use in this study must have been restricted by the size of the containers because a field study found a midsummer Kc of 0.9 for a pear orchard of the cv. Blanquilla with trees trained to a palmete system (Marsal et al., 2002a). This Kc value is slightly lower than the 0.95-1.0 value that has been traditionally recommended (Allen et al., 1998). A weighing lysimeter study within a pear orchard, measured the ETc of three pear trees, cv. Conference, trained to a central leader (Girona et al., 2010). Values for crop coefficients (ETo calculated according to FAO I&D Paper No. 56 Penman-Monteith equation) in midsummer were around 0.9 with a LAI of 1.4. The seasonal changes in Kc values for Conference trees are presented in Table 1.

TABLE 1

Crop coefficients relative to grass reference crop (ETo) for mature pear trees (Conference on quince) grown in a central leader training system and measured using a lysimeter located in an experimental orchard at Lleida, EEL (Spain) by Girona et al. (unpublished).

Date

Crop coefficient (Kc)

Ground cover (%)

Apr. 1-15

0.30

23

Apr. 16-30

0.48

30

May 1-15

0.70

36

May 16-31

0.80

39

June 1-15

0.85

40

June 16-30

0.90

40

July 1-15

0.90

40

July 16-31

0.90

40

Aug. 1-15

0.90

40

Aug. 16-31

0.70

40

Sept. 1-15

0.60

40

Sept. 16-30

0.50

40

Oct. 1-15

0.40

40

Oct. 16-31

0.35

36

Nov. 1-15

0.35

25

It must be emphasized that Kc values in Table 1 are only indicative, and that they may require adjustment to each specific training system and growing conditions. In fact, in the lysimeter study (Girona et al., 2010), Kc showed ample variation over the years (see Figure 4), and there were variations even within the same season after full canopy development. It appears that, at least for the cv. Conference in a semi-arid climate, pear Kc increases under high air vapour pressure deficits (Girona et al., 2010), suggesting that transpiration (Tr) in pears is enhanced relatively more than grass Tr, which is the reference crop for ETo. This effect makes the use of a relationship between Kc and the fraction of midday crop intercepted radiation less useful in pear than in peach or apple (see both Sections). Nevertheless, the use of the midday fraction of crop intercepted radiation (or, if midday intercepted radiation data is unavailable, the percent ground cover may be used as a surrogate; see Chapter 4) provides a first approximation for adjusting the Kc values for pear trees.

A water-use study of Asian pear measured the water consumption of trees planted in 12 medium-large drainage lysimeters (9 100 litre) and with a soil surface covered with reflective net (Chalmers et al., 1992). Trees were trained to a Tatura trellis and ET was determined from pan evaporation. This and another study of Asian pears (Caspari et al., 1993; and Caspari et al., 1994), reported their Kc values on a per tree canopy area basis instead of ground area, and thus cannot be compared with the standard Kc values. Nevertheless, the conclusion of the Asian pear studies is that their water requirements are somewhat lower than the values recommended in FAO I&D Paper No. 56 for this crop.

An inherent risk of calculating orchard water requirements is irrigation overestimation. In the case of pears, over-irrigation can have a remarkable negative impact on flowering during subsequent seasons (Marsal et al., 2002a). To avoid this negative impact, accurate determination of crop-water requirements is essential in pear irrigation. One useful strategy could be to apply a mild deficit irrigation programme and monitor the level of stress with soil or plant measurements.

WATER PRODUCTION FUNCTIONS

Water production functions have been derived from four studies: Two of them for the cvs. Blanquilla and Spadona (Spadona and Blanquilla are denominations corresponding to the same cultivar but used in Italy and Spain, respectively); (Naor et al., 2000; and Marsal et al., 2002a), and two other studies on the cv. Conference; the latter dealing with postharvest deficit irrigation (Marsal et al., 2008; and Marsal et al., 2010). In these four studies it has been considered that: i) annual ETo and effective rainfall were known, ii) ETc was estimated from soil-water content variation and applied water, and iii) the effects of irrigation on fruit yield were considered for two consecutive years. Figure 5 shows the relation between relative fruit yield and relative ETc. Relative yield is unaffected by ETc deficits of 15-20 percent and then declines more or less linearly as ETc deficits become more severe.

Figure 6 shows the water production function considering relative gross revenue instead of yield as a productive parameter. The relative gross revenue penalizes fruit with cheek diameters of less than 65 mm. The prevailing market conditions are very different between these two cultivars, since Conference pears commonly receive a better price than Blanquilla. Prices may change from country-to-country. For the sake of a fair cultivar comparison and to avoid the specificity of country market effects on gross revenues, the pricing criterion of Conference in Spain was applied to all reported experiments.

Data in Figure 5 suggest that there are cultivar differences in the response to ETc deficits, cv. Conference being more sensitive than Blanquilla (or Spadona). A 30 percent reduction in ETc caused only a 12 percent yield reduction in Blanquilla-Spadona but a 22 percent decrease in Conference. However, cultivar yield sensitivity to ETc deficits was similar once they showed a response to decreasing relative ETc; their sensitivity remained similar with a slope for the yield response to ETc of 1.2 and 1.0 for Blanquilla and Conference, respectively (Figure 5).

An explanation of the differences between cultivars in the yield-response threshold to reduction in ETc may be related to a complex interaction between three factors: i) the positive effect of moderate water stress on increasing return bloom in the next season, ii) the limited use of fruit thinning as a commercial practice for pear, and iii) the possible fruit-set response to previous season water stress. In other words, changes in return bloom as a consequence of incipient ETc reductions in the previous season, may produce higher cropping next season, provided fruit set is unaffected. Under these circumstances, increases in crop load leads to the production of smaller fruit, but the smaller fruit size at harvest is often more than compensated by the positive impact of higher fruit number on yield. It is interesting to notice that this was the case for Blanquilla for RDI-SII in Spain and also for the mild irrigation reductions in the Spadona experiment in Israel (Figure 3). However, this was not found to be so Conference, because RDI-SII, besides increasing blooming return, it also reduced fruit set the next season so that competition between fruit was lowered. Specificity of cultivar yield response to ET deficits could be explained by a different sensitivity of fruit set to current bloom density and past history of water stress. On the other hand, the advantageous yield response observed in Blanquilla was lost when analysed in terms of relative revenue (Figure 6). This was because of the price penalty related to the production of smaller fruit under deficit with high cropping conditions (Figure 6). Therefore Conference and Blanquilla revenue responses were approximated by only one boundary line, which corresponded to the conditions of deficit irrigation applied during the fruit-growing season (Figure 6).

In the case of postharvest deficit irrigation for Conference, it was found that relative revenues rose above the boundary line (Figure 6). Curiously, postharvest water deficit produced yield reductions that were accompanied by reductions in fruit set and associated with increased fruit size. Accordingly, postharvest RDI fruit received a higher price and relative gross revenue was not reduced by the slight ETc reductions attained in the above-mentioned experiment (Figures 5 and 6). However, under more significant stress during Stage II, which caused a 30 percent reduction in ETc , the decrease in gross revenue of the cv. Conference reached 60 percent (Figure 6), a response that is substantially more negative than what could be predicted from the yield-ETc relationship (Figure 5).

SUGGESTED RDI REGIMES

Consistency of results across the different experiments on RDI suggests that this technique may be safely used for pear production. However, the myriad of possible combinations of pear growing conditions (cv. x rootstock x planting density x fruit load x soil type x climate) offer a wide spectrum of possibilities that have not been fully investigated in relation to RDI. Nevertheless, there is no doubt that, in climates having low rainfall during the hot season, reducing irrigation during Stage II should be avoided to guarantee maximum fruit size at harvest (Figure 6). Early water stress should also be avoided in most cases, except for high-density orchards growing under vigorous conditions. The period in which RDI could be applied to save water is postharvest, provided excessive water stress is not achieved. This risk of applying too much water stress during postharvest may depend on each specific situation. Risks increase where growing conditions are suboptimal. Bad weather can affect pollination and fruit set, and fruit drop can occur especially during late spring. Postharvest water stress has been hypothesized to reduce winter reserves in the tree (no data available on pear) and subsequently impair fruit set and yield following season.

The data available on responses to RDI make it difficult to propose a strategy that is applicable to all possible combinations of management practices. Nevertheless, the postharvest period is the safest to apply RDI, but water savings can be short if a late maturing cultivar is used. Therefore, if water shortages have to be more severe, RDI could be applied in combination with other periods. Table 2 presents various RDI strategies for different water allocations that are simulated for specific experimental and environmental conditions (Marsal et al., 2008; and Marsal et al., 2010). Water deficit in Stage I (RDI-SI), only allows a 6 percent reduction of the annual applied water. By using RDI in Stage II (RDI-SII), 33 percent of applied water can be saved, but this causes a reduction in growers' gross revenues (Table 2). A more sensible approach would be to use a combination of deficit irrigation during Stage I and postharvest, in combination with a slight reduction during the first part of Stage II fruit growth (Table 2). The latter strategy would reduce the annual water use by 33 percent (from 600 mm to 400 mm) and probably would have less negative impact on fruit growth than an RDI-SII strategy.

REFERENCES

Allen, R.G., Pereira, L.S., Raes, D. & Smith, M. 1998. Crop evapotranspiration: Guidelines for computing crop water requirements. Irrigation and & Drainage Paper 56. Rome, FAO.

Asin, L., Alegre, S. & Montserrat, R. 2007. Effect of paclobutrazol, prohexadione-Ca, deficit irrigation, summer pruning and root pruning on shoot growth, yield, and return bloom, in a 'Blanquilla' pear orchard. Scientia Horticulturae, 113:142-148.

Bain, J.M. 1961. Some morphological, anatomical, and physiological changes in the pear fruit (Pyrus communis var. Williams Bon Chrétien) during development and following harvest. Australian Journal Botany, 9(2):99-123.

Behboudian, M.H., Lawes, G.S. & Griffiths, K.M. 1994. The influence of water deficit on water relations, photosynthesis and fruit growth in Asian pear (Pyrus serotina Rehd). Scientia Horticulturae, 60:89-99.

Buwalda, J.G. & Lenz, E. 1995. Water use by European pear trees growing in drainage lysimeters. Journal of Horticultural Science, 70:531-540.

Caspari, H.W., Behboudian, M.H., Chalmers, D.J. & Renquist, A.R. 1993. Pattern of seasonal water use of asian pears determined by lysimeters and the heat pulse technique. Journal of the American Society for Horticultural Science, 118(5):562-569.

Caspari, H.W., Behboudian, M.H. & Chalmers, D.J. 1994. Water use, growth and fruit yield of ‘Hossui’ asian pear under deficit irrigation. Journal of the American Society for Horticultural Science, 119:383-388.

Caspari, H.W., Behboudian, M.H. & Chalmers, D.J. 1994. Water use, growth and fruit yield of ‘Hossui’ asian pear under deficit irrigation. Journal of the American Society for Horticultural Science, 119:383-388.

Chalmers, D.J., Andrews, P.K., Harris, K.M., Cameron, E.A., & Caspari, H.W. 1992. Performance of drainage lysimeters for the evaluation of water use by Asian pears. Horticultural Science, 27: 263-265.

Elkins, R.B., Van den End, B. & Beutel, J. 2007. Vegetative growth and development. In: Mitcham, E.J. & Elkins, R.B., eds. Pear Production and Handling Manual. University of California Agriculture and Natural Resources, Publication 3483. 199 pp.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Forshey, C.G. & Elfving, D.C., 1989. The relationship between vegetative growth and fruiting in apple trees. Horticultural Reviews, 11:229-288.

Girona, J., del Campo, J., Mata, M., Lopez, G. & Marsal, J. 2010. A comparative study of apple and pear tree water consumption measured with two weighing lysimeters. Irrigation Science. Online publication DOI 10.1007/s00271-010-02175-5.

Marsal, J. & Girona, J. 1997. Effects of water stress cycles on turgor maintenance process in pear leaves (Pyrus communis L.). Tree Physiology, 17:327-333.

Marsal, J., Rapoport, H.F., Manrique, T. & Girona, J. 2000. Pear fruit growth under regulated deficit irrigation in container-grown trees. Scientia Horticulturae, 85: 243-259.

Marsal, J., Mata, M., Arbones, A., Rufat, J. & Girona, J. 2002a. Regulated deficit irrigation and rectification of irrigation scheduling in young pear trees: an evaluation based on vegetative and productive response. European Journal of Agronomy, 17: 111-122.

Marsal, J., Mata, M., Arbonès, A., Rufat, J. & J. Girona. 2002b. Water stress limits for vegetative and reproductive growth of ‘Barlett’ pears. Acta Hortaculturae, 596:659-663.

Marsal, J., Arbones, A., Mata, M., del Campo, J., Girona, J. & Lopez, G. 2008. Factors involved in alleviating water stress by partial crop removal in pear trees. Tree Physiology, 28:1375-1382.

Marsal, J., Behboudian, M.H., Mata, M., del Campo, J., Basile, B., Girona, J. & Lopez, G. 2010. Fruit thinning in ‘Conference’ pear grown under deficit irrigation to optimize yield and to improve tree water status. Journal of Horticultural Science & Biotechnology, 85:125-130.

Mitchell, P.D., Van Den Ende, B., Jerie, P.H. & Chalmers, D.J. 1989. Responses of 'Barlett' pear irrigation to witholding irrigation, regulated deficit irrigation and tree spacing. Journal of the American Society for Horticultural Science,109:604-606.

Naor, A., Peres, M., Greenblat, Y., Doron, Y., Gal, Y. & Stern, R.A. 2000. Irrigation and crop load interactions in relation to pear yield and fruit-size distribution. Horticultural Science & Biotechnology, 75:555-561.

Naor A., 2001. Irrigation and crop load influence fruit size and water relations in field-grown ‘Spadona’ pear. Journal of the American Society for Horticultural Science, 126:252-255.

Naor, A., Stern, R., Flaishman, M., Gal, Y. & Peres, M. 2006. Effects of postharvest water stress on autumnal bloom and subsequent-season productivity in midseason ‘Spadona’ pear. Horticultural Science & Biotechnology, 81:365-370.

O’Connell, M. & Goodwin, I. 2007. Water stress and reduced fruit size in micro-irrigated pear trees under deficit partial rootzone drying. Australian Journal of Agricultural Research, 58:670-679.

Peach

LEAD AUTHORS

Joan Girona
(IRTA, Lleida, Spain),

Elias Fereres
(University of Cordoba and IAS-CSIC, Cordoba, Spain)

CONTRIBUTING AUTHORS

Jordi Marsal
(IRTA, Lleida, Spain),

David A. Goldhamer
(formerly University of California, Davis, USA; currently Cooperative Extension Emeritus),

Amos Naor
(GRI, University of Haifa, and Migal - Galilee Technology Center, Israel),

M. Auxiliadora Soriano
(University of Cordoba, Cordoba, Spain)

INTRODUCTION AND BACKGROUND

Peach (Prunus Persica L.), was originally from China, but in ancient Greece and Rome it was thought to have originated in Persia, is a stone fruit tree that exhibits ample diversity in terms of fruit types: freestone or cling; round or flat shape; hairy or smooth skins; flesh that is either firm or soft and white or yellow. There is also a wide-range of maturity dates for the peach cultivars; from very early where the fruit matures at the end of spring, to very late that reach maturity at the end of summer, as much as four months after the earliest varieties arrive at the market. The tree is vigourous and can reach more than 5 m in height, but peach production is limited worldwide by its relatively narrow range of climatic adaptation. On the one hand, it flowers early and is quite sensitive to frost–particularly at flowering- but on the other, it has chilling requirements that are not met in some of the frost-free areas of the temperate zones and the subtropics.

The evolution of peach production in selected countries in the last ten years is shown in Figure 1. In 2009 there were over 1.5 million ha of peach and nectarine globally with an average yield of 13.0 tonne/ha (FAO, 2011). The main producing country is China, which represents 50 percent of the world peach production. Production in China rose spectacularly over the last decades from 380 000 tonne in 1970 and an average yield of 3.6 tonne/ ha to over 10 million tonne in 2009 with an average yield of 14.4 tonne/ ha, followed by Italy (FAO, 2011). Other major commercial production areas are located in southern Europe (Spain, Greece, and France), United States (California, Georgia), Chile, and Australia. Highest yields are obtained in United States with almost 20 tonne/ha.

The fruit is usually consumed fresh and, because consumers in many world areas prefer large-size fruit, peach is grown mostly under irrigation even in many subhumid areas. There, the most important role of irrigation is to stabilize production in years of below-normal rainfall, and to guarantee adequate soil moisture during the critical fruit enlarging period, just prior to harvest. In more arid areas where rainfall is only a fraction of ETc, irrigation is essential for commercial production, as the period of fruit growth can span all summer in the late maturing varieties. Most peach production systems are quite intensive, with orchards planted at high densities (from 400 up to 1 000 tree/ha); the highest densities are normally for peach production used in industry (canning and food processing). A wide-variety of training systems are used, designed to maximize the distribution of solar radiation to all tree parts in order to achieve good fruit colour, and to promote fruiting branches throughout the tree canopy. Sometimes, trellises are used with the trees shaped in a horizontal plane, or in a V shape. For the very intensive plantations, low tree vigour is favoured to reduce mutual shading and harvest costs. Some dwarf tree cultivars have been bred in the past, but have had little commercial success.

Fruit quality

There are many factors determining peach fruit quality and some of them have a significant influence in the crop value, particularly, fruit size. Fresh market peaches are valued for their size, and large sizes fetch premium prices in many markets. Fruit size depends on tree fruit numbers and is affected by environmental factors such as water deficits. Colour and lack of visual defects are also important quality aspects; the colour is determined by light exposure during fruit growth that, in turn depends, on tree configuration, degree of vegetative growth and fruit position on the tree. Other quality factors include firmness, concentrations of total soluble solids (TSS), soluble sugars, titratable acidity (TA), sugar to acid ratio, aroma volatiles, enhanced maturity and better storability (shelf-life). All of these parameters respond to variations in the tree water supply that affect tree water status.

DESCRIPTION OF THE STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

Vegetative and reproductive growth

As with other stone fruit, peach flowering is immediately followed by vegetative growth in early spring. Peach chilling requirements, usually computed during dormancy as hours above 7 ºC, vary widely among varieties, but in some may be substantial. During the canopy development period, fruit set and initial fruit growth take place simultaneously. Figure 2a depicts the patterns of fruit growth for three cultivars differing in maturity and the relative rate of vegetative growth for the late-maturing cultivar. Although early maturing cultivars bloom earlier than the late, initial fruit growth is very similar for the three types. In the early cultivar, such initial growth is directly followed by a fast fruit enlargement phase (Figure 2a) that ends with fruit ripening. In the other cultivars, there is a slowdown in growth rate of the fruit, coinciding with the acceleration of vegetative growth (Figure 2a). Vegetative growth, measured as seasonal shoot length or the increase in trunk diameter, has similar trends for the different cultivars. Extension of primary shoots occurs first followed by the growth of secondary shoots. In the early cultivars there are two peaks of rapid extension growth, the second one taking place after fruit harvest. In the medium and late-maturing cultivars, there is normally only one peak of fast shoot growth, but there may be another one after harvest in some cultivars and environments. At some point in the season, shoot growth slows and the rapid fruit expansion rate period begins (Figure 2b). This last fruit enlargement phase extends until the fruit matures prior to harvest, and the longer this period, the greater is the accumulation of dry matter in the fruit and the larger is the final potential fruit size (Figures 2a and 2b).

Because bud emergence usually occurs with a fully charged soil profile, water deficits affecting the early growth stages of fruit and canopies are uncommon. When they occur, tree leaf area and final fruit size will be reduced, because in the latter case, initial growth is mostly caused at cell division stage that sets the final number of cells that a fruit will have.

While flower buds are quite resistant to freezing temperatures above -7 to -10 ºC during dormancy, flowers and recently formed fruit are very sensitive to mild frosts, with minimum temperatures below -2 to -3 ºC. Thus, peaches are very sensitive to spring frosts that can completely wipe out fruit production.

Stages of fruit growth

As discussed above, fruit growth, measured by either increase in fruit volume or in dry weight, follows a double sigmoid curve in peach. From Figure 2 it can be seen that there are three apparent stages of fruit growth, although in early varieties, it is difficult to detect more than two from periodic measurements of fruit volume. The first stage, defined as Stage I, starts soon after pollination and is a period of active cell division that ends around pit hardening, when the fruit has reached about 20 to 25 percent of its final size. Fruit growth slows in the second stage and this coincides with a period of active shoot extension and leaf development (Figure 2b). The duration of this second phase (Stage II) varies; in early varieties, it is hardly detectable, while in very late varieties it can lasts for more than 40 days. Following Stage II, fruit enlargement resumes at a very high rate in what is called Stage III, proceeding more or less in an exponential fashion until harvest (Figure 2b). Dry matter accumulation in the fruit lags behind fruit enlargement, but also follows a double sigmoid pattern, less marked than for the accumulation of fresh weight, and more evident in late varieties but hardly detectable in medium and early varieties. The most relevant difference among cultivars varying in season length, from early to late maturity, is the duration of Stage II (Figure 2a).

Final fruit size is determined primarily by fruit load (number of fruit per tree), but tree size, canopy configuration and pruning, water and nutrient status are also important factors. The relationship between final fruit size and fruit load is cultivar dependent as shown on Figure 3 (Johnson and Handley, 1989) where these relationships are shown for several early and midseason cultivars. Both final fruit size (Figure 3a) and total yield per tree (Figure 3b) are fruit load dependent. Fruit size increases as fruit load decreases (Figure 3a), while final yield increases with increases in fruit load (Figure 3b) For each cultivar, and depending on market prices, the total number of fruit per tree that are left after thinning should be chosen to optimize profits by obtaining a maximum yield with a minimum fruit load. Final fruit size depends also on the timing of hand thinning. Early hand thinning reduces fruit competition and allows larger fruit.

Bud development

The terminal peach bud at the end of a shoot is always vegetative and produces a leafy shoot. Auxiliary buds develop during the summer at the base of leaves on the current season's shoots and can be either leaf or flower buds. A flower bud produces a single flower that can set one fruit. Each node on a vegetative shoot may have from zero to three buds. The buds that generate vegetative growth are small and pointed while flower buds are larger, rounder, and more hairy. Many of the nodes on the lower two-thirds of a shoot have two or three buds arranged side by side. Most often a leaf bud is flanked by flower buds. The number and distribution of flower buds on a shoot varies with tree vigour, cultivar, and the radiation environment that the shoot experiences. Short shoots generally have the most fruit buds per unit length. Moderately vigorous shoots have a high proportion of nodes with two flower buds. The leaf buds at most nodes develop into lateral shoots that may be fruitful in subsequent years. In the very vigorous current season's shoots, a number of auxiliary buds produce secondary shoots that are not desirable because fruit buds do not develop at many of their nodes. Ideal shoots (between 30 and 50 cm long) have enough growth to produce sufficient fruit buds for the following season but do not have secondary shoots.

The postharvest period is important for peach trees, as it is during this period when next season's flower buds are initiated (usually around August) and when these buds are clearly differentiate from vegetative buds they start to develop floral organs (Handley and Johnson, 2000). Another important feature of the period between harvest and leaf fall is the accumulation of carbohydrate reserves, needed for continuous bud development processes until bloom, and also because fruit set is highly dependent upon carbohydrate availability (Arbeloa and Herrero, 1991). Heat and water stress during post harvest enhances the formation of abnormal fruit (Naor et al., 2005).

RESPONSES TO WATER DEFICITS

Peach water relations have been studied in more detail than most other deciduous fruit tree species. As for most plants, vegetative growth is extremely sensitive to water deficits, and several studies have shown that leaf and young shoot expansive growth are slowed by mild water deficits that are difficult to detect.

Peach trees are mostly grown to produce fresh fruit, where both size and some quality characteristics are important. However, while there is a premium paid for large fruit size in most markets (or a penalty for the small sizes), quality features, other than size, do not generally influence growers’ revenue, although many consumers are well aware of the important differences in quality among and within peach and nectarine varieties.

Final fruit size depends directly on the number of cells and the average cell size of the mesocarp. The number of cells is primarily determined during Stage I and several experiments (some in container-grown trees) have demonstrated that this is a very sensitive period for water deficits in terms of final yield and revenue. One field study (Girona et al., 2004) with mild to moderate stress at Stage I demonstrated that fruit dry matter was affected at high fruit loads, but that fresh weight could recover if the water supply during Stages II and III was adequate. Nevertheless, risks of inducing damaging stress levels at Stage I are low because the initial soil profile is usually full, evaporative demand is low and the canopy development process has not been completed (and thus the crop coefficient, Kc, is below the maximum that will be achieved when canopy growth is near completion). Thus, peach transpiration (Tr) is relatively low at this time, which also coincides with seasonal spring rains in many peach growing regions. It is therefore not difficult to avoid water deficits in Stage I, even inadvertently. Nevertheless, in shallow soils and/or very dry environments, or in drought years when the soil profile is dry, it is possible to induce significant levels of water stress that will impact negatively on fruit size and yield (Girona et al., 2004).

As the fruit continue to grow and the pit starts to harden, the rate of vegetative growth accelerates and fruit growth slows at the onset of Stage II. The duration of this phase varies from only a few days in early varieties (and thus is almost impossible to detect) to about 60 days in the very late varieties. Water deficits during Stage II affect primarily lateral shoot expansion and trunk growth while having minimal or no impact on fruit growth. Figure 2b shows the impact of water deficits in shoot extension growth and the negligible influence that it has on fruit growth and final fruit size (Girona et al., 2003). This differential sensitivity between vegetative and fruit growth formed the basis for the successful application of water stress in Stage II in peach first described by Mitchell and Chalmers (1982). These authors and several others since that time have found that Stage II is not sensitive to water deficits in terms of negatively impacting yield. It has been shown (Girona et al., 2003) that significant water deficits applied during Stage II may induce some dehydration of the fruit, but that subsequent recovery of fruit growth is usually complete after the water stress is relieved at the onset of Stage III, and that Stage II water deficits have no impact on final yield.

Even though there has been an initial report showing (Chalmers et al., 1981) increased fruit size, relative to fully irrigated controls, when applying RDI in Stage II, no other published papers reported such results, with one exception (Girona et al., 2003) for a single season in a three-year study, where low temperatures at blooming time damaged many fruit and the final fruit load was very low. A comprehensive analysis of the effect of fruit load on the response to RDI at Stage II (Girona et al., 2004), detected larger fruit in the RDI treatment compared with an unstressed control only with low fruit loads, and as the fruit load increased, no effects were detected and in some cases, even a reduction in fruit size was observed. Presumably the RDI in Stage II enhances fruit growth relative to unstressed controls by directing more carbohydrates to fruit growth, but this phenomenon apparently occurs only with low fruit loads (Girona et al., 2004). There have been reports of less fruit drop before harvest under RDI (Girona et al., 2003), and this could explain the few observations where water deficits during Stage II had positive effects on yield, relative to fully-irrigated treatments.

Vigorous fruit expansion takes place during Stage III when the rate of fruit expansion is highest and most sensitive to water deficits. Fruit water content is more sensitive to water deficits than fruit dry weight during this period. A reduction of 25 percent in fruit water content occurred with Stage III water deficits in a medium peach cultivar (Girona et al., 2004). Water deficits that affect fruit dry matter accumulation must be quite severe because, not only must they decrease photosynthesis but they must also counterbalance the tendency of many fruit trees, including peach, where assimilate allocation to fruit has higher priority relative to its distribution to other tree parts (DeJong et al., 1987). Leaf photosynthesis and tree transpiration in peach are not affected by water deficits until more than 50 percent of the available water in the root zone is depleted (Girona et al., 2002). When water deficits occur under these conditions, the peak of daily Tr moves from a plateau between noon and 14:00 hours towards the morning hours, and by the time Tr was reduced by 70 percent, the maximum Tr rate occurred at 9:00 am hours (Girona et al., 2002).

Indicators of peach tree water status are used to quantify the water stress levels. A comparative study among different indicators (Goldhamer et al., 1999) found that indices derived from micrometric measurements of trunk diameter fluctuations were the most sensitive for water stress detection, followed by stem-water potential. Other indicators such as stomatal conductance, leaf photosynthesis, and leaf temperature were less sensitive (Goldhamer et al., 1999).

Water deficits may have a negative impact on fruit appearance in the next season. An increased frequency of fruit doubles and deep sutures have been observed in water-stressed peach trees (Johnson and Phene, 2008). These problems have been overcome by relieving the water stress shortly before and during carpel differentiation (Johnson et al., 1992). With early-season cultivars, this stress-sensitive period is in August and September and suggests avoidance of water deficits during these months (Johnson and Phene, 2008). For a midseason cultivar, the increase in occurrence of double and deep suture fruit is highly correlated with the midday stem-water potential in August of the previous year, i.e. during the initial stages of flower bud development (Naor et al., 2005). The occurrence of double fruit was observed to increase sharply as the midday stem-water potentials fell below -2.0 MPa, suggesting that a midday stem-water potential of -2.0 MPa could serve as threshold for postharvest irrigation scheduling (Naor et al., 2005).

Fruit set can also be influenced by postharvest stress. Both early season (Johnson and Phene, 2008) and midseason (Goodwin and Bruce, 2011) cultivars found that fruit set was moderately sensitive to the degree of water stress during the previous season’s postharvest period. In late season-cultivars, fruit set was highly affected by the level of water stress during postharvest, as shown by the strong correlation between the average leaf water potential during the postharvest period and the fruit set (Girona et al., 2004) (Figure 4). The negative impact of water deficits on fruit set in the next year may not be important as thinning is a common practice in peach, but severe impacts on fruit set cannot be corrected by thinning (Goodwin and Bruce, 2011).

Moderate water deficits applied during Stage II improved fruit quality (firmness, colour, improved TSS) without affecting yield (Gelly et al., 2003 and Gelly et al., 2004). Moderate water stress in Stage III also improves fruit quality, but a negative impact on fruit size and yield is very likely. The trade-offs between quality and size must be resolved bearing in mind the market where the produce will be sold.

BOX

Detecting water stress in peach

Although the established method of detecting tree water stress in peach is the leaf or stem-water potential (Goldhamer et al., 1999), visual indicators may be used to estimate stem-water potential when it is not possible to take actual SWP measurements.

In a normal summer day, typical stem-water potential patterns are shown in the figure below.

Visually, it is possible to differentiate a leaf that has a water potential of -0.9 MPa from one that has a value of -1.9 MPa. The first is fully expanded and usually oriented towards the sun (Photo D), while the second is partially rolled and droops (Photo A).

For the optimal RDI regime that applies stress on Stage II, it is good practise to arrive at midday SWP values close to -1.5 MPa. At that SWP level, some leaf-rolling symptoms may be observed, but without leaf drop or yellowing, which will indicate excessive water stress. In the morning, growers should observe expanded leaves. A lead rolling symptom of water stress in the morning will indicate excessive stress, while no symptoms at midday will indicate lack of the desired level of stress during Stage II.

WATER REQUIREMENTS

The water use rates of peach trees are similar to other Prunus species, such as nectarines or plums. Table 1 lists the crop coefficients for mature peach trees, and an estimation of the water use at Lleida, Spain. The crop coefficients were obtained from lysimeter studies (Ayars et al., 2003). In one study, the ETc of a peach tree in a weighing lysimeter was followed for several years since its planting until it reached maturity. This study has provided data on the evolution of Kc for a peach orchard, as the canopy expands (Ayars et al., 2003), and the results are shown in Figure 5. The lysimeter studies have shown that Tr in peach does not decrease much from peak values until soon before leaf fall, unless the water deficits imposed by restricting irrigation in the postharvest period induced stomatal control of Tr and early leaf senescence.

WATER PRODUCTION FUNCTIONS

A number of experiments have been conducted to quantify the relation between yield and applied irrigation water for peach (Girona et al., 2002). A summary of some of the experiments concluded that applied water may be reduced by 10- 20 percent below the maximum needs without a negative impact on yield. However, the analyses have very seldom included revenue considerations, relative to the price differential that different sized peach fruit fetches in the market. The response to applied water depends on the water storage capacity of the soil, and thus cannot be generalized. Figure 6 shows the relation between yield and ETc in relative terms for different irrigations strategies, full irrigation (Control), RDI, and sustained DI (SDI). For the optimal RDI regime, it is possible to reduce the ETc by 15-20 percent without a detrimental impact on yield. However, when the water deficits were imposed on sensitive stages or throughout the season (SDI), a reduction in ETc was accompanied by a yield reduction (Figure 6). The differences in the responses to the various DI regimes illustrate the benefits of stress management when planning deficit irrigation programmes.

TABLE 1

Crop coefficients relative to grass reference crop (ETo) for mature peach trees (Phenological stages for midseason cultivar at Lleida, Spain).

Date

Crop Coefficient (Kc)

Growth Stage

Mar. 1-15

0.25

Mar.16-31

0.30

Bloom

Apr. 1-15

0.45

Apr. 16-30

0.60

May 1-15

0.70

May 16-31

0.80

End of FGS I *

June 1-15

0.90

June 16-30

0.95

Beginning of FGS III *

July 1-15

1.05

July 16-31

1.05

Aug. 1-15

1.05

Harvest

Aug. 16-31

1.00**

Sept. 1-15

1.00**

Sept. 16-30

1.00**

Oct. 1-15

0.75

Oct. 16-31

0.55

Nov. 1-15

0.45

* FGS = Fruit growth stage

** Management reductions in postharvest irrigation may lower these Kc values

SUGGESTED RDI REGIMES

Based on the results shown above, it can be concluded that RDI strategies applied during Stage II in peach, especially when a fast recovery at the beginning of Stage III can be achieved, has proved to be very effective in controlling excessive vegetative growth and improving fruit quality without a yield penalty. For early season cultivars, RDI that concentrates the water deficits in the postharvest period is an effective strategy, provided severe water stress is avoided (Tables 2 and 3). Given the impact that different root zone water storage capacities have to the response to RDI, Tables 2 and 3 present different RDI schedules for two types of soils (shallow and deep), also giving indications of the minimum stem-water potential trees can withstand without having a detrimental effect on yield.

* To prevent next year crop failure

In applying RDI strategies an important factor is fruit load, and to manage the degree of plant water stress according to the number of fruit per tree. As has been discussed previously, the fruit load per se has a strong influence on the final size of the fruit (Naor et al., 1999). If fruit numbers are very high and water is limited, there is a risk that imposing an RDI regime would induce a fruit size reduction. In that case it would be better to reduce fruit load by thinning to achieve the fruit size distribution of the crop that economically provides the highest net profit to the grower. If fruit load is low, RDI may even increase fruit size above that of full irrigation and will decrease vegetative growth and summer pruning costs. With medium fruit loads, optimal RDI that reduce ETc by 15-20 percent would not have a negative impact on yield in most cases.

REFERENCES

Arbeloa, A. & Herrero, M. 1991. Development of the ovular structures in peach (Prunus persica (L.) Batsch). New Phytologist 118: 527–534.

Ayars, J.E., Johnson, R.S., Phene, C.J., Trout, T.J., Clark, D.A. & Mead, R.M. 2003. Water use by drip-irrigated late-season peaches. Irrigation Science 22:187-194.

Chalmers, D.J., Mitchell, P.D. & Van Heek, L. 1981. Control of peach tree growth and productivity by regulated water supply, tree density and summer pruning. Journal of the American Society for Horticultural Science 106:307-312.

DeJong, T.M., Doyle, J.F., & Day, K.R. 1987. Seasonal patterns of reproductive and vegetative sink activity in early and late maturing peach (Prunus persica) cultivars. Physiologia Plantarum 71:83-88.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Gelly, M., Recasens, I., Mata, M., Arbones, A., Rufat, J., Girona, J. & Marsal, J. 2003. Effects of water deficit during stage II of peach fruit development and postharvest on fruit quality and ethylene production. Journal of Horticultural Science & Biotechnology 78:324-330.

Gelly, M., Recasens, I., Girona, J., Mata, M., Arbones, A., Rufat, J. & Marsal, J. 2004. Effects of stage II and postharvest deficit irrigation peach quality during maturation and after cold storage. Journal of the Science of Food and Agriculture 84:561-568.

Girona, J., Mata, M., Fereres, E., Goldhamer, D.A. & Cohen, M. 2002. Evapotranspiration and soil wáter dynamics of peach tres under wáter déficits. Agricultural Water Management 54:107-122.

Girona, J., Mata, M., Arbones, A., Alegre, S., Rufat, J. & Marsal, J. 2003. Peach tree response to single and combined regulated deficit irrigation regimes under shallow soils. Journal of the American Society for Horticultural Science 128:432-440.

Girona, J., Marsal, J., Mata, J., Arbones, J. & DeJong, T.M. 2004. A comparison of the combined effect of water stress and crop load on fruit growth during different phenological stages in young peach trees. Journal of Horticultural Science & Biotechnology 79:308-315.

Girona, J., Gelly, M., Mata, M., Arbones, A., Rufat, J. & Marsal, J. 2005. Peach tree response to single and combined deficit irrigation regimes in deep soils. Agricultural Water Management 72:97-108.

Goldhamer, D.A., Fereres, E., Mata, M., Girona, J. & Cohen, M. 1999. Sensitivity of continuous and discrete plant and soil water status monitoring in peach trees subjected to deficit irrigation. Journal of the American Society for Horticultural Science 124:437-444.

Goodwin, I. & Bruce, R. 2011. Postharvest deficit irrigation decreases subsequent fruit number in T204 peach. Acta Horticulturae 889: 205-212.

Handley, D.F. & Johnson, R.S. 2000. Late summer irrigation of water-stressed peach trees reduces fruit doubles and deep sutures. Horticultural Science, 35(4):771.

Johnson, R.S. & Handley, D.F. 1989. Thinning response of early, mid-, and late-season peaches. Journal of the American Society for Horticultural Science, 114(6):5-12.

Johnson, R.S., Handley, D.F. & DeJong, T.M. 1992. Long-term response of early maturing peach trees to postharvest water deficits. Journal of the American Society for Horticultural Science, 117(6):881-886.

Johnson, R.S. & Handley, D.F. 2000. Using water stress to control vegetative growth and productivity of temperate fruit trees. Horticultural Science, 35(6):1048-1050.

Johnson, R.S. & Phene, B.C. 2008. Fruit quality disorders in an early maturing peach cultivar caused by postharvest water stress. Acta Horticulturae 792:385-390.

Mitchell, P.D. & Chalmers, D.J. 1982. The effect of reduced water supply on peach tree growth and yields. Journal of the American Society for Horticultural Science 107:853-856.

Naor, A., Klein, I., Hupert, H., Grinblat, Y., Peres M. & Kaufman, A. 1999. Water stress and crop level interactions in relation to nectarine yield, fruit size distribution, and water potentials. Journal of the American Society for Horticultural Science 124:189-193.

Naor, A., Stern R., Peres M., Greenblat Y., Gal, Y. & Flaishman, M.A. 2005. Timing and severity of postharvest water stress affect following-year productivity and fruit quality of field-grown Snow Queen nectarine. Journal of the American Society for Horticultural Science 130: 806-812.

Walnut

LEAD AUTHOR

David A. Goldhamer
(formerly University of California, Davis, USA; currently Cooperative Extension Emeritus)

INTRODUCTION AND BACKGROUND

Walnuts (Juglans regia L.) are large trees that are cultivated in temperate climates for their nuts, rich in oil, and for their wood. Plantations are relatively widely spaced as this species does not tolerate mutual shading well. Common spacing of vigorous varieties varies between 8 x 8 and 10 x 10 m, while the less vigorous cultivars may be planted at 7 x 7 m spacing. Experiments that have increased tree density above the spacing mentioned have resulted in higher yields during the first years of the orchard but this may be at the expense of reduced orchard longevity. In 2009, world acreage was 843 000 ha and average global yield (with shell) was 2.7 tonne/ha (FAO, 2011). Figure 1 presents the production trends of the main producing countries. China and the United States are the two main world producers, followed by Iran, Turkey, and Ukraine. France is the main European producer and Chile is now an important producer in the Southern Hemisphere.

VEGETATIVE AND REPRODUCTIVE DEVELOPMENT

Commercial varieties of walnut trees break dormancy and begin to leaf out in the Northern Hemisphere between mid-March and mid-April, depending on the environmental conditions and the cultivar. This is followed by flowering, both the emergence of pollen-producing male flowers and the female flowers that evolve into the nut after pollination. Normally, flowering is completed by late April and is followed by rapid fruit expansive growth with concomitant rapid shoot growth. Bud formation occurs from leaf out through June on mature trees. By early June, the fruit has reached its full size and this is followed immediately by the internal development of the nut. At this time, shoot growth slows. The internal nut development sequence begins with shell expansion and hardening and dry matter accumulation in the kernel that continues through harvest. The indicator for physiological maturity is the development of ‘packing tissue brown’ which occurs before hull split. This can be identified by cracking open the nuts and observing the colour of the tissue surrounding the kernel inside the shell. If this tissue is white, the nuts have not reached maturity and there will likely be continued dry matter accumulation in the kernel. A brown colour indicates the nuts have matured and kernel development has ceased. Hull split generally follows closely after walnuts have reached physiological maturity. After splitting, the hulls break down rapidly. During the postharvest period, some shoot growth and carbohydrate storage are the primary sinks of photosynthesis products.

EFFECTS OF WATER DEFICITS

Water stress can decrease nut size and quality (kernel colour and shrivel). Stress-related reductions in shoot growth can reduce fruiting wood for the following season(s). There is some evidence that not only are the number of reproductive buds less because of lower vegetative growth but that some flower buds are not viable, resulting in fewer flowers and ultimately less fruit. Further, the reduction of shoot growth causes higher fruit temperatures as a result of both more sunlight penetration into the canopy and thus, more direct solar radiation on a higher percentage of the fruit, and higher canopy temperatures because of less transpiration, that can darken kernel colour, reducing crop value. This temperature effect is very cultivar dependent.

As walnut fruit load is very dependent on the previous year’s shoot growth, the impact of water deficits is much more severe in the season following the imposition of water deficits. The primary impact in the year that stress is imposed is on fruit size and quality while in the following season, the impact is on fruit load, regardless of the irrigation regime used in the following season. One California study found that hedgerow walnuts (cv. Chico) irrigated at 33 and 66 percent ETc suffered marketable nut yield reductions of 32 and 50 percent, respectively, after three years because of reduced nut size, fruit load, and crop quality (Goldhamer, 1997). Upon returning these trees to full irrigation, tree growth and gas exchange immediately recovered but yields were little changed the first recovery year, even though shoot growth dramatically increased. It wasn’t until the second recovery year that harvest yields completely recovered as a result of the fruiting positions created by the first recovery year’s shoot growth. Similar stress impacts and recovery results have been obtained from other studies in California (cv. Chandler) (Lampinen et al., 2004). The rapid production recovery from severe water stress was possible because of the absence of stress-induced disease or insect pressures. Trunk diseases such as deep bark canker that often occur in water stressed orchards were not evident in these studies.

WATER USE

Walnut orchards have high water use rates as because of the high leaf, tall tree stature, and near full ground cover when the trees are fully mature. Table 1 provides the crop coefficients for mature walnut orchards obtained from studies in California (Goldhamer, 1997).

DEFICIT IRRIGATION STRATEGIES

The general strategy followed experimentally for reducing irrigation in walnut orchards has been to limit water deficits during early stages of tree and crop development in favour of imposing them during mid and late season. One study in northern California used midday stem-water potential to impose the ‘low’ and ‘moderate’ stress treatments. The target midday stem-water potential values were -0.5 to -0.7 MPa and -1.2 to -1.4 MPa during the bulk of the season for these two regimes with corresponding reductions in applied water of 30 and 50 percent of potential ETc, respectively (Fulton et al., 2002). It should be noted that the water potential values of fully irrigated walnut trees are much less negative then the two primary nut crops, almond and pistachio. Walnut predawn leaf water potential values for fully irrigated trees range between -0.15 and -0.2 MPa and midday stem-water potential between -0.40 to -0.60 MPa. After three seasons, yields in these stress treatments had declined by 26 and 40 percent, respectively, relative to fully irrigated trees (Lampinen et al., 2004). Full recovery was achieved after two years of full irrigation. A companion study was conducted on deeper soils with older trees with a lower tree density. Yield reductions were appreciably lower at this site, which was attributed to the stress development being relatively slow because of the larger soil moisture reservoir and possibly the larger carbohydrate reserves of the bigger trees.

To simulate a one-year drought with a water supply of 400 mm where potential ETc was 1 100 mm, a team in California applied 85 percent of ETc through April to mature cv. Chico trees and then progressively lower percentages of ETc as the season progressed (25 percent was the minimum from early July through the early September harvest) and no postharvest irrigation (Goldhamer et al., 1989). Fruit yields in the drought year were about 10 percent lower than the fully irrigated control (not statistically significant). However in the following recovery year, when full irrigation was applied to all trees, the drought year trees had about 80 percent lower yields almost entirely the result of a lower fruit load. Yields returned to near full levels during the second recovery year (Goldhamer et al., 1990).

TABLE 1

Crop coefficients for mature walnut trees (Goldhamer, 1997).

Date

Crop coefficient (Kc)

Mar. 16-31

0.12

Apr. 1-15

0.53

Apr. 16-30

0.68

May 1-15

0.79

May 16-31

0.86

June 1-15

0.93

June 16-30

1.00

July 1-15

1.14

July 16-31

1.14

Aug. 1-15

1.14

Aug. 16-31

1.14

Sept. 1-15

1.08

Sept. 16-30

0.97

Oct. 1-15

0.88

Oct. 16-31

0.51

Nov. 1-15

0.28

It appears that walnut trees do not respond well to water deficits, regardless of the deficit irrigation strategy, in terms of nut yield. This is probably because of the high sensitivity of shoot growth to water deficits and, in turn, to the heavy dependence of fruit load on the shoot growth of the previous year in walnuts.

REFERENCES

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Fulton, A., Buchner, R., Little, C., Gilles, C., Grant, J., Lampinen, B., Schackel, K., Schwankl, L., Prichard, T. & Rivers, D. 2002. Relationships between midday stem-water potential, soil moisture measurements, and walnut shoot growth. In: Walnut Research Reports. Sacramento, CA, USA, Walnut Marketing Board, pp. 135-143.

Goldhamer, D.A., Beede, R., Sibbett, S., Fereres, E., DeJong, T.M., Ramos, D., Katayama, D., Doyle, J. & Day, K. 1989. First year effects of controlled deficit irrigation on walnut tree performance. In: Walnut Research Reports. Sacramento, CA, USA, Walnut Marketing Board, pp. 91-100.

Goldhamer, D. A., Beede, R., Sibbett, S., Ramos, D. & Van Brocklin, F. 1990. First year recovery following a simulated drought in walnut. In: Walnut Research Reports. Sacramento, CA, USA, Walnut Marketing Board, pp. 66-72.

Goldhamer, D. A. 1997. Irrigation scheduling for walnut orchards. In: Walnut Orchard Management. D. E. Ramos, ed. University of California Publication No. 3373, Oakland, CA pp. 159-166; p. 328.

Lampinen, B., Buchner, R., Fulton, A., Frant, J., Mills, N., Prichard, T., Schwankl, L., Shackel, K., Gilles, C., Little, C., Metcalf, S., Rivers, D. & Gamble, V. 2004. Irrigation management in walnut using evapotranspiration, soil and plant based data. In: Walnut Research Reports. Sacramento, CA, USA, Walnut Marketing Board, pp. 113-136.

Pistachio

LEAD AUTHOR

David A. Goldhamer
(formerly University of California, Davis, USA; currently Cooperative Extension Emeritus)

CONTRIBUTING AUTHOR

Riza Kanber
(Cukurova University, Adana, Turkey)

INTRODUCTION AND BACKGROUND

Pistachio (Pistacia vera L.), is native to the Near East, primarily Syria and Iran, with large areas planted just recently in the United States. In 2009, there were 586 000 ha globally with an average yield of 1.1 tonne/ha (FAO, 2011). Figure 1 presents the production trends of the main producing countries since 1985. The bulk of Near East production is dryland as the pistachio tree is very drought tolerant. Most of the production in the United States is in California, which is irrigated. There is a huge difference in productivity between dryland and irrigated trees. For example, the average dryland yield in Turkey is only 1.4 kg per tree compared with 16-18 kg per tree under irrigation in California (Tekin et al., 1990). While the value of irrigation for pistachio production is currently unchallenged, there are still some growers in rainfed areas who have the misconception that irrigation is harmful (Kanber et al., 1993). Much of the Near East production is on marginal soils because the tree is perceived to be drought tolerant and good soils are scarce. This is not the case in California where irrigated orchards have been planted on productive valley soils.

The pistachio tree is dioecious; the male flowers are borne on one tree and female flowers on another. The male trees do not produce nuts. However, a certain percentage of the orchard, generally around 4 percent in commercial orchards, must be planted with male trees to ensure adequate pollination.

Among fruit and nut trees, pistachio has one of the highest degrees of alternate bearing. Crop yields can show up to a 90 percent year-to-year reduction. The physiological mechanisms of alternate bearing in pistachio are not well understood. It is likely to involve carbohydrate levels and/or competition with hormonal activity also being a possible factor. Alternate bearing is first manifested during nut filling in early July when the fruit buds (for next year) die and abscise. The heavier the crop, the greater is the bud abscission. The alternate bearing cycle is expressed not only for individual trees but for entire growing regions. It is thought that low production resulting from poor weather in a given year puts the entire region on the same alternate bearing cycle. Excessive alternate bearing in a region can cause a marketing problem for the industry. The current state of the art control of alternate bearing is pruning; heavily prior to an ‘on’ year and minimally going into an ‘off’ alternate bearing year. The fact that pistachio fruit are borne on year-old wood dictate the location and severity of pruning practices designed to mitigate alternate bearing. Moreover, pistachio shoot growth can be characterized as either preformed or neoformed, which is based on when the tissue was differentiated. Virtually, all the crop is borne on the preformed growth and this has implications for irrigation management.

Quality considerations

Pistachio is distinguished by having more quality components than other nut crops. These include not only the alternate bearing feature that affects nut size but also embryo abortion; nuts that have full size hulls and shells but where the kernels die prematurely or don’t fill at all. Also, endocarp dehiscence (shell splitting) is required to produce the highest value nuts. Closed nuts shell at harvest cannot be marketed as snack food, which is the largest market for pistachios. Another type of fruit that is not commercially viable are early splits; nuts that split well before the onset of normal shell splitting. Not only are these nuts worthless but they are prone to fungal disease infection that can lead to the formation of Aflatoxin. Finally, the percentage removal of filled nuts by mechanical shaking also impacts harvest yields.

Although pistachios have been cultivated for centuries in countries of the Near East and West Asia, the industry is relatively young in California compared with the other nut crops grown there. The California pistachio industry is dominated by one variety, Kerman, and large growers and processors have readily embraced advanced practices, including drip or microsprinkler irrigation. In the main producing countries of the Near East, there is a wide-range of varieties and irrigation is being introduced, even though much production is still rainfed. California pistachio growers were very quick to adopt useful research results on water requirements and the impact of water stress on yield and crop quality partly because much of the acreage is located in high water cost and/or low availability areas.

DESCRIPTION OF THE STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

The reproductive growth of pistachio trees can be divided into three stages based upon the development of the nut component parts: the hull+shell and the kernel. The development patterns of the nut components are shown in Figure 2. The hull+shell grows rapidly from late April through mid-May (Northern Hemisphere), after which full size is attained. This period is referred to as Stage I. However, the feniculous (embryo), which will eventually evolve into the kernel, normally does not begin to grow until early July. From mid-May through early July, the nut's primary growth activity is thickening of the shell. This period, characterized by a relatively low rate of dry matter accumulation in the nut, is known as Stage II. Rapid growth of the kernel begins in early July and remains so as harvest is approached. The biofix for this period, which is known as Stage III, is the appearance of a distinct green colour in the feniculous. Research (Spann et al., 2009) has identified the concomitant vegetative growth associated with these stages and their eventual importance as locations for fruiting positions.

Early vegetative and reproductive growth; growth Stage 1

Shoot growth occurs simultaneously with the current season reproductive growth (swelling buds that will form the crop) as well as with embryonic (inflorescence primordia) bud development for the following season’s crop from late April through mid May. Lateral inflorescences in the leaf axils are borne on shoots with, generally, a single apical vegetative bud. Buds differentiate in April, May and June, remain quiescent from July to September, and resume differentiation in October.

There are two types of shoot growth; the above-mentioned preformed or neoformed. All components of a preformed shoot are differentiated in the dormant bud whereas in neoformed growth, some differentiation of its component parts can occur during the growing season. Most of the buds found on preformed growth are reproductive; there are very few lateral vegetative buds on preformed shoots. Most of the vegetative growth occurs from terminal buds. Preformed shoots tend to be short compared with neoformed shoots which are longer. This longer shoot growth is undesirable because it tends to be weak and hangs down in the orchard rows, making management and harvest difficult. For these reasons, growers typically remove these shoots on mature trees by pruning during the dormant season. However, long shoot growth may be desirable in young, developing trees to ensure the most rapid development of the tree canopy.

Reproductive bud swelling begins in March. By mid-April, there are 100-300 flowers per rachis. Pollination and fruit set occur at this time. There are generally 20-25 developing fruit per rachis and they grow rapidly, with the hull+shell attaining full size by about mid-May. This event also coincides with a hardening of the shell.

Lag phase of reproductive growth: growth Stage II

From mid-May through early July, the primary activity in the nut is thickening and hardening of the shells, a process called lignification. However, dry matter accumulation in the fruit during this growth phase is low relative to the preceding (Stage I) and succeeding (Stage III) periods. There may also be some additional shoot growth in late May. Reproductive buds that will form the following season’s fruit continue to differentiate through June. Sometimes there is an additional vegetative flush of growth in late June.

Rapid kernel development: growth Stage III

This phase is characterized by the resumption of a high rate of dry matter accumulation in the nut almost entirely results from the rapid growth of the kernel. Within a matter of a few weeks, the kernel will entirely fill the nut cavity and begin to exert pressure on the shell. Shell splitting is primarily because of this expansion of the kernel (Polito and Pinney, 1999). Shell splitting generally begins in early August. At this time, the hull begins to breakdown, changing from turgid tissue that is tightly bound to the shell with a papery, loosely connected covering that can easily be peeled from the shell. During Stage III, leaves on the same shoot as developing fruit sometimes become yellow and defoliate. This is thought to be the consequence of translocation of resources from the leaves to the fruit.

A certain percentage of the nuts, generally from 10 to 30 percent, do not fill. These are known as ‘blanks’ or ‘aborted’ nuts. With the former, there is no evidence of any development of the embryo whereas with the latter, the embryo development is aborted. The term ‘blanking’ is sometimes used to describe both phenomena. The hulls of these nuts do not breakdown as with the filled nuts. Also they are much more difficult to remove from the tree with mechanical shaking at harvest, resulting in a high percentage remaining on the tree.

Harvest is generally from late August to mid-September. Where it is done mechanically by shaking machines, similar to those used for almonds, which remove the nuts. In addition to the shaker, a companion machine, the receiver, is located on the opposite side of the tree and is used to collect the nuts, which are not allowed to drop to the ground. Drying is accomplished at the processing plant rather than on the ground as for almonds. One primary reason for this is that pistachio nuts need to be dried quickly, otherwise the shell can become stained, making them less attractive to buyers.

Postharvest

From harvest to the onset of defoliation, there is very little outward appearance of tree activity. Following the removal of fruit, reproductive bud differentiation resumes and continues through October. Trees generally defoliate in mid to late November because of leaf senescence, which is accelerated by low temperatures.

RESPONSES TO WATER DEFICITS

Pistachio has a well-deserved reputation as being drought tolerant. Measurable photosynthetic activity in the leaf has been measured even when leaf water potential (LWP) was in excess of -5 MPa (Behboudian et al., 1986). This was attributed to the fact that pistachio trees had a turgor pressure of about 3 MPa even when the LWP was -6 MPa; a higher value that for even other xerophytes. Pistachio can maintain high turgor even with high soil salinity levels (Walter et al., 1988), and high photosynthesis and stomatal conductance were found in different pistachio species under severe stress (Steduto et al., 2002). Researchers ascribed this primarily to an extensive rooting system rather than xerophytic morphologic characteristics. Unirrigated trees of P. atlantica had transpiration rates about three times higher than P. terebinthus (Germana, 1997).’ This suggests that pistachio trees can transpire at rates far higher than those normally found in mesophytes and that carbon assimilation with limited water supplies can be much higher than in other fruit crops, such as apple, peach, plum, cherry, citrus and almond. Pistachio exhibited a strong photosynthetic response to high N and water supply, although the rates of unirrigated trees were also quite high (Steduto et al., 2002 and Aydın, 2004). With respect to water, pistachio is somewhat of a paradox; it transpires at an extremely rapid rate, in partly because the fact that its leaves are isolaterals meaning the upper and lower sides are similarly structured with almost identical stomatal density and conductance but, at the same time, it is also extremely drought tolerant.

Effects during Stage I

Spann and others tested RDI regimes that imposed water deficits of about -1.6 MPa midday shaded LWP on mature trees of Kerman on PG1, Atlantica, and UCB rootstocks during Stage I and both Stage I and Stage II. They found that especially for the ‘short’ shoots, those that are characterized as preformed growth, full elongation occurred by about the third week of April; well before the onset of Stage II. There were generally no reductions in this short shoot length because of these early season water deficits (Figure 3). However, stress during both Stages I and II significantly reduced the growth of the ‘long’ shoots; the neoformed growth (Figure 3). This did not decrease the number of fruiting positions since they are located mostly on the short shoots. Indeed, they found no differences in fruit load, fruit size and yield between these RDI regimes and the fully irrigated control. This was attributed to the fact that most fruit was borne on the preformed growth and that reducing neoformed growth was actually beneficial in commercial production since it must be pruned. Water and pruning costs are about 30 percent of the total for California pistachio growers and thus, the reductions in consumptive use and pruning attributed to the early season stress would likely increase grower profit (Beede et al., 2004). Recent research with the UCB rootstock found some reduction in the total number of growing shoots per tree with both Stage I and Stage II stress that could reduce future yield if it continued for a number of years. This confirmed earlier work (Goldhamer et al., 1987) that stress-related reductions in fruit load were primarily because of the reduction in the initiation of short shoots rather than potential or actual number of nuts per rachis.

Goldhamer and Beede imposed dryland conditions during Stage I with Kerman on Atlantica rootstock. They found that nut size was reduced by 6.1 percent relative to fully irrigated trees but that shell splitting was increased by 14.0 percent. They theorized that the stress impacted shell growth more than kernel growth, resulting in a greater splitting percentage. Since no other yield components were significantly affected, they reported slightly better total kernel yield of marketable product (split nuts) with Stage I stress (Goldhamer and Beede, 2004). More recent research has confirmed that shell splitting can be increased with Stage I stress but at the expense of nut size (Goldhamer et al., 2005). Thus, the decision to use this strategy would depend on whether the grower had a severe problem with the production of closed shell nuts. Closed shell nuts can be as low as 5 percent of the harvested nut load and as high as 60 percent. Further, Stage I stress not only increased shell splitting but it increased the shell opening; the distance between shell halves at the distal end of the nut. This can result in the shell detaching from the kernel during commercial nut processing and the loose kernels can decrease the harvest value.

Effects during Stage II

Goldhamer and Beede evaluated an array of RDI treatments that imposed either dryland, applied water at 25 percent ETc, or applied water at 50 percent ETc during Stage II on mature Kerman on Atlantica rootstock under the high evaporative demand conditions of the western San Joaquin Valley in California (Goldhamer and Beede, 2004). These Stage II deficit irrigation treatments were coupled with different postharverst water regimes. They found that none of the Stage II stresses significantly reduced individual nut weight although there was a trend toward lighter nuts when Stage II irrigation was totally eliminated. One of these Stage II dryland treatments, when coupled with irrigation at 25 percent ETc postharvest, significantly reduced the yield of split nuts. They concluded that Stage II was, indeed, a stress tolerant period, as has been found for other double sigmoid development fruit crops, such as peach, plum, and nectarine, and recommended an RDI regime that irrigated at 50 percent ETc during Stage II (Goldhamer and Beede, 2004).

A June deficit irrigation schedule of 20 percent less than full irrigation doubled early splits, while a July deficit of 35 percent increased early splits by 30 percent (Sedaghati and Alipour, 2006). Early splits are nuts that split well before the onset of normal shell splitting. These nuts are not commercially viable. Moreover, they are susceptible to fungal diseases that can eventually result in Aflatoxin contamination. Doster and Michailides (1995) recommended that water stress in mid-May be avoided to decrease the incidence of early splits.

Effects during Stage III

Stress imposed during Stage III can have a dramatically negative impact on virtually all the yield components of pistachio. When a dryland treatment was imposed during Stage III, it was found that this reduced individual kernel weight by 10.6 percent, increased the sum of blanking and kernel abortion in the total tree nut load by 22.7 percent, and increased the production of closed shell nuts by 175 percent. Somewhat remarkably, the Stage III dryland treatment had no affect on total tree nut load. However, the yield of split nuts was reduced by 62.6 percent (Goldhamer and Beede, 2004).

Earlier work indicated that withholding irrigation during the first half of Stage III, which reduced consumptive use by 320 mm, had no significant impact on shell splitting but increased the number of filled nuts left in the trees after mechanical shaking by 119 percent. On the other hand, dryland conditions during the last half of Stage III (a 200 mm reduction in consumptive use) both increased the production of closed shell nuts at harvest by 31.6 percent and the number of filled nuts retained on the tree after mechanical shaking by 50 percent. It was concluded that Stage III was the most stress sensitive period of the season for pistachio (Goldhamer et al., 1991).

Of the numerous pistachio yield components, it is remarkable that tree nut load was unaffected by any of the nine deficit irrigation treatments imposed, including dryland conditions during Stage I, Stage II, Stage III, and postharvest and the various Stage II and postharvest stress combinations (Goldhamer and Beede, 2004). When averaged over the last two years of their four-year study, tree nut load ranged from 10 900 to 12 300 with the fully irrigated trees averaging 11 500 nuts per tree (Goldhamer and Beede, 2004). This suggests that there was enough preformed shoot growth very early in the season, even with Stage I dryland conditions, to produce the number of nodes (fruiting positions) necessary to support a full crop and that stored winter rainfall (200 mm per year) was sufficient to support this growth. This ability of the pistachio tree to produce equal fruit loads under a variety of stress regimes highlights the importance of the preformed shoot growth from mid-April to mid-May; a period when trees would normally rely on stored winter rainfall rather than irrigation. Indeed, the early work of Spiegel-Roy et al. (1977) found that 54 to 163 mm of annual precipitation was sufficient for dryland trees to differentiate enough flower buds to obtain appreciable yields.

Effects on alternate bearing

Kanber and others observed that a long duration of water stress aggravated alternate bearing and suggested that irrigation could alter periodicity, presumably by making more carbohydrates available during peak carbon demand periods (Kermani and Salehi, 2006). Goldhamer found that Stage I stress during an ‘on’ year (shown as 2004 in Figure 4) resulted in more than a three-fold increase in fruit load the following season (the subsequent ‘off’ year) relative to fully irrigated trees. The mechanisms of why this happened are unknown and it should be emphasized that the early season stress was possible only because winter rainfall was abnormally low. The following season, the winter rainfall eliminated any Stage I stress but the fruit loads of this RDI regime were 25 percent lower than the fully irrigated trees (Figure 4). This pattern continued in the succeeding season when this RDI regime had a fruit load 25 percent higher than those under full irrigation. It appears that regardless of why there are higher yields in a normally ‘off’ year, the one time higher yields can alter the alternate bearing pattern for the following years.

Crop load also influences the impact of deficit irrigation on the various yield components of pistachio. This was observed when dryland conditions with Kerman on Atlantica rootstock were imposed in both ‘off’ and ‘on’ alternate bearing years (Figure 5). All the yield components, with the exception of harvestability, were more negatively impacted in the ‘on’ year. Harvestability was higher in the ‘on’ year only because entire rachises, rather than individual nuts, were removed from the tree with the mechanical shaking. Thus, growers can anticipate greater negative impacts of serious droughts during ‘on’ versus ‘off’ alternate bearing years. In fact, a possible management strategy, with very limited water supplies under microirrigation, would be to cutoff irrigation to the trees with low fruit loads (those in the ‘off’ year), making that water available for the trees in the orchard with the high fruit loads.

Indicators of tree water status

The established method to quantify water stress for pistachio is to measure water potential with a pressure chamber. Although the standard method is to measure stem-water potential (see Chapter 4), as in other species, there is a good correlation between midday shaded LWP (faster to measure) and stem-water potential (Goldhamer et al., 2005).

One factor that complicates taking LWP measurement on pistachio leaves is that at the onset of gas injection into the chamber, exudates, presumably from the phloem, appear at the cut end of the petiole. These can interfere with identifying the instant xylem fluids appear. One approach to eliminating this problem is to use a cotton swab to soak up these exudates prior to the appearance of the xylem fluid. Another approach to eliminating this problem uses blotting paper positioned at the cut end of the petiole that absorbs only xylem fluid but excludes the other interfering fluids. A third approach is not to use individual leaves but small, interior shaded spurs that may have one to four leaves. The procedure involves covering the spur with a damp cloth just prior to excision. A few millimiters of bark is removed at the cut end with either a small knife or a thumbnail. The entire spur is placed in the chamber after the cloth is removed and the reading is taken. It is quite easy to identify the appearance of the xylem since there is no interference of phloem exudates and the cross-sectional area of view is larger than the leaf petiole. Goldhamer also found a good correlation between spur water potential and midday shaded LWP. The slope of the relationship was about unity but the intercept indicated that the spur water potential differs from the shaded LWP reading by about -0.7 MPa.

WATER REQUIREMENTS

Relatively few studies have quantified pistachio ETc. Research in Iran with Ohadi on Badami Zarand rootstock (Kermani and Salehi, 2006), concluded that 600 and 1 200 mm per season should be applied with drip and flood irrigation, respectively, although 910 mm was reported as a ‘previously determined’ irrigation amount for mature pistachio trees (Kermani and Salehi, 2006). Early studies found that trees irrigated with a Kp (pan evaporation) value of 0.50 produced equally as well as those irrigated with a Kp of 0.75 for Larnaka on P. integerrima rootstock (Monstra et al., 1995). In Southeast Turkey, the Kc values for Antep and Uzun varieties rose from 0.49 in May to 0.80 in August and continued at this magnitude through the first week of September when they declined to 0.32 during October because of leaf senescence (Kanber et al., 1993). However, it was noted that while all irrigation regimes began the season with a nearly full soil water profile, they all ended with it nearly depleted. The researchers suspected that there was insufficient irrigation to meet ETc for their most heavily irrigated trees (Kanber et al., 1993).

A soil water balance approach with arrays of neutron probe access tubes to a depth of 3 m and ETo estimates from a nearby weather station was used to calculate bimonthly Kc values for mature Kerman on Atlantica rootstock (Table 1) (Goldhamer et al., 1985). A unique aspect of this approach was to make use of soil hydraulic conductivity data obtained in a separate experiment to eliminate one of the shortcomings of the water balance approach to determine ETc: deep percolation below the deepest depth monitored.

WATER PRODUCTION FUNCTION

The current recommended optimal RDI regime applies stress during Stage II and postharvest to achieve the same production as fully irrigated trees while reducing the consumptive use of water (Goldhamer and Beede, 2004). These authors tested this approach in numerous field trials (Goldhamer et al., 1984) in the San Joaquin Valley of California. The results of these experiments are summarized in the production function shown in Figure 6. While there is appreciable scatter in the data, it suggests that a plateau in the yield of marketable product is achieved with 10 to 20 percent less consumptive use than potential ETc. Thus, reducing consumptive use by up to 20 percent can generally be achieved without a negative impact on the yield of marketable product. This occurred with both Altantica and PG1 rootstocks.

It should be emphasized that that a 10 to 20 percent reduction in ETc would translate into a higher percentage reduction in applied water. For example, if ETc was 1 100 mm of which 300 mm was effective rainfall, then applied irrigation water would have been 800 mm. A 15 percent reduction in ETc would reduce consumptive use by 165 mm; that is equivalent to about a 21 percent reduction in applied water. Percentage reductions of applied water would increase as effective rainfall increased.

TABLE 1

Crop coefficients relative to grass reference crop (ETo) for mature pistachio trees ('Kerman' on P. atlantica) measured using on soil water balance approach in western Kings Co., CA (Goldhamer et al., 1985).

Date

Crop Coefficient (Kc)

Apr. 1-15

0.07

Apr. 16-30

0.43

May 1-15

0.68

May 16-31

0.93

June 1-15

1.09

June 16-30

1.17

July 1-15

1.19

July 16-31

1.19

Aug. 1-15

1.19

Aug. 16-31

1.12

Sept. 1-15

0.99

Sept. 16-30

0.87

Oct. 1-15

0.67

Oct. 16-31

0.50

Nov. 1-15

0.35

Nov. 16 -30

0.28

SUGGESTED RDI REGIMES

Based on the assumption that Stage II and postharvest are the most stress-tolerant periods, Stage I has intermediate tolerance, and Stage III is least tolerant, one can develop an array of drought irrigation strategies based on meeting certain percentages of ETc during these periods (Table 2). The percentage amounts for each period vary depending on the available water supply. It should be pointed out that these recommendations are based on experimental results of applied water amounts generally above about 750 mm (about 65-70 percent ETc); tests of RDI regimes below this amount have not been published. Thus, the suggested regimes here for water supplies below 750 mm are our best estimate of what would result in optimal tree performance, again based on the stress sensitivities of each growth stage.

Since the contribution of soil moisture to ETc is difficult to determine, especially early in the season, the RDI management strategy of only irrigating at certain percentages of ETc is problematic. An alternative is to use a plant-based indicator of tree water stress, such as leaf/spur water potential with the pressure chamber. In the studies cited above, midday shaded LWP during Stage I, Stage II, and postharvest did not exceed -1.8 to -2.0 MPa. Recommended values for fully irrigated, mature pistachio trees grown under high evaporative demand conditions should have midday shaded LWP values for Stage I, Stage II, Stage III, and postharvest of -0.7 to -0.8 MPa, -0.8 to -1.0 MPa, -1.0 to -1.1 MPa, and -1.0 to -1.2 MPa, respectively.

One pistachio grower in California has observed that there is little need to irrigate male trees at full ETc since their only role is to supply pollen very early in the season. He suggests that male tree irrigation can be eliminated or substantially reduced after Stage I with no negative impact on the subsequent season’s pollen formation. This can be accomplished relatively easily with microirrigation systems. However, given that male trees usually make up only about 4 percent of all trees, the reduction in irrigation would be small.

Typical microsprinkler application:

Tree spacing

289 ft^2

Application rate

1.47 ft^3/hr

Depth application rate per plant

0.01 ft/hr

(0.06 in/hr)

Amount per irrigation

1.47 in/24 hr

(37.2 mm/24 hr)

The grower would keep track of cumulative amounts to be applied with RDI scenarios. When the total is 37 mm, he irrigates. Thus, there would be one irrigation in April 16-30 with full water supply but with 300 mm available case, first irrigation would not be until third week of June.

REFERENCES

Aydın, Y. 2004. The effects of different irrigation water and nitrogen levels on yield and alternate bearing on pistachio. Dep. of Agric. Struc. and Irrig. Basic and Applied Sciences, University of Çukurova, Adana, 145 pp. (Ph.D. Thesis)

Beede, R.H, C.E. Kallsen, M.W. Freeman, L. Ferguson, B.A. Holtz, K.M. Klonskey, & DeMoura, R.L. 2004. Sample costs to establish and produce pistachios. University of California Coop. Extn. Publ. PI-SJ-04. 25 p.

Behboudian, M.H, Walker, R.R. & Torokfalvy, E. 1986. Effects of water stress and salinity on photosynthesis of pistachio. Scientia Horticulturae 29:251-261.

Doster, M.A. & Michailides, T.J. 1995. The development of early split pistachio nuts and their contamination by molds, aflatoxins, and insects. Acta Horticulturae 419:359-364.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Germana, C. 1997. The response of pistachio trees to water stress as affected by two different rootstocks. Acta Horticulturae 449:513-520.

Goldhamer, D.A., Kjelgren, R., Beede, R., Moore, J.M., Lane, J., Menezes Jr., J., & Weinberger, G. 1984. Physiological response of pistachio to severe water stress. In: Annual Report. Crop Year 1983-1984. Fresno, CA, California Pistachio Industry, pp. 44-47.

Goldhamer, D.A., Kjelgren, R.K., Williams, L., & Beede, R. 1985. Water use requirements of pistachio trees and response to water stress. In: Proceedings of the National Conference on Advances in Evapotranspiration. American Society of Agricultural and Biological Engineers, Chicago, IL, USA, December 16-17 216-223.

Goldhamer, D.A., Kjelgren, R.K., Beede, R., Williams, L., Moore, J.M., Lane, J., Weinberger, G., & J. Menezes, Jr. 1985. Water use requirements of pistachio trees and response to water stress. In: Annual Report. Crop Year 1984-1985. California Pistachio Industry, Fresno, CA, pp. 85-92.

Goldhamer, D.A., Phene, B.C., Beede, R., Sherlin, L., Mahan, S., & Rose, D. 1987. Effects of sustained deficit irrigation on pistachio tree performance. In: Annual Report. Crop Year 1986-1987. Fresno, CA, California Pistachio Industry, pp. 61-66.

Goldhamer, D.A., Beede, R. H., & Fusi, S. 1991. Effects of deficit irrigation timing on pistachio tree productivity. In: Annual Report, crop year 1990-1991. California Pistachio Industry, Fresno, CA, pp. 84-87.

Goldhamer, D.A., & Beede, R. H. 2004. Regulated deficit irrigation effects on yield, nut quality and water-use efficiency of mature pistachio trees. Journal of Horticultural Science & Biotechnology, 79(4):538-545.

Goldhamer, D.A., Beede, R.H., Michailides, T.J., Salinas, M. & Doster, M. 2005. Effects of regulated deficit irrigation on shell splitting and nut quality at harvest (second year report). In: Annual Report, crop year 2004-2005. Fresno, CA, California Pistachio Industry, pp. 104-106.

Kanber, R., Yazar, A., Onder, S. & Koksal, H. 1993. Irrigation response of pistachio (Pistacia vera L.). Irrigation Science 14:7-14.

Kermani, M.M. & Salesi, F. 2006. Determination of pistachio crop evapotranspiration (ETc). Acta Horticulturae 726:441-448.

Monstra, F., Avanzato, D., Martelli, S. & Dascanio, D. 1995. Pistachio trial under different volumes of irrigation in Italy. Acta Horticulturae 419:249-252.

Polito, V.S. & Pinney, K. 1999. Endocarp dehiscence in pistachio (Pistacia vera L.). International Journal of Plant Sciences 160:827-835.

Sedaghati, N. & Alipour, H. 2006. The effect of different time of irrigation on occurrence of early split (ES) of pistachio nuts. Acta Horticulturae 726:583-586.

Spann, T.M, Beede, R.H. & DeJong, T.M. 2009. Contributions of short- and long-shoots to yield of ‘Kerman’ pistachio (Pistaca vera L.) Scientia Horticulturae, 121(4):495-500.

Spiegel-Roy, P., Mazigh, D. & Evenari, M. 1977. Response of pistachio to low soil moisture conditions. Journal of the American Society for Horticultural Science, 102(4):470-473.

Steduto, P., Kanber, R., Yükçeken, Y., Unlu, M., Özekici, B., Çetinkökü, Ö. & Others. 2002. Growth, yield and periodicity of pistachio under different water and nutritional levels: Investigation in the Southeastern Anatolia Project Region (GAP). Final Report of WASIA PROJECT. CIHAEM-IAM/Bari, 112 pp.

Tekin, H., Genc, C., Kuru, C. & Akkok, F. 1990. Determining the nutrition levels and suitable time for leaf sampling of pistachio. In: Proceedings of the First Pistachio Symposium, Gazeiantep, Turkey, pp. 120-138.

Walter, R.R., Torokfalvy, E., & Behboudian, M.H. 1988. Photosynthetic rates and solute partitioning in relation to growth of salt-treated pistachio plants. Australian Journal of Plant Physiology, 15(6):787-798.

Apricot

LEAD AUTHORS

Cristos Xiloyannis
(Università degli studi della Basilicata, Potenza, Italy),

Bartolomeo Dichio
(Università degli studi della Basilicata, Potenza, Italy)

CONTRIBUTING AUTHORS

Elias Fereres
(University of Cordoba and IAS-CSIC, Cordoba, Spain),

Giuseppe Montanaro
(Università degli studi della Basilicata, Potenza, Italy)

INTRODUCTION AND BACKGROUND

Throughout the world, 90 percent of commercially grown apricots are derived from the Prunus armeniaca (L.) specie, a few cultivars are from P. mume or P. sibirica, or more recently, originate from apricot × plum (and vice versa) hybrids. Apricots are small-to-medium sized trees with spreading canopies (usually kept under 3.5 m), cultivated for fresh or processed fruit (dried, jam, juice), and for their oil extracted from the kernel. Apricot grows well in temperate regions; however, it is also able to tolerate very low temperatures during winter. Particularly, P. sibirica can tolerate air temperatures of about -35 °C, and soil temperatures down to -13 °C at the 40 cm depth did not damage its roots (Kramarenko, 2010). Total global production in 2009 was 3.73 million tonne on 504 000 ha (FAO, 2011). Figure 1 presents the evolution of production since 1985. Turkey is the main producer, followed by Iran; Italy is the first producer in the European Union. Most cultivars mature between the end of April and end of June (Northern Hemisphere). Over the last five years, new cultivars with a much later maturity date (August-September) have been bred and introduced in some areas.

Normal plantation density is about 400-500 tree/ha, using some training systems (e.g. transverse Y) density could reach 1 200–1 500 tree/ha. In this case careful canopy management (e.g. summer pruning) is required to minimize excessive shading that reduces water-use efficiency at the leaf level (Figure 2), the size, sugar content and colour of the fruit, bud induction and flower quality for next year yield and the level of carbohydrate stored in the buds, flowers and shoots (Nuzzo et al., 1999 and Xiloyannis et al., 2000).

STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

Floral bud induction begins in late spring or summer. The chilling requirements for flowering (No.of hours < 7 °C), range from 300 to 1 200, depending on the cultivar. The minimum bloom temperature (namely the GDH heat units required after rest (Ruiz et al., 2007)) is relatively low, causing apricots to bloom (and leaf out) early in most locations, thus apricot flowers and new shoots tend to suffer frost injury in early spring. Apricot trees break dormancy and begin to bloom in the Northern Hemisphere by mid-February, depending on the environmental conditions and cultivar. Usually, flowering is completed within 20 days; thereafter fruit grows rapidly and attains its maximum size by mid-May (Northern Hemisphere). At this time, about 90 percent of total fruit dry matter has been gained (in early cultivars). In temperate regions, harvest starts by the end of May and lasts until the end of July; Northern Hemisphere).

Leaf emergence takes place at the end of February followed by fast shoot growth rates. About 80 percent of full leaf area is completed by the end of May. Thereafter, about 95 percent of final leaf area is achieved by the end of June (Figure 3). Values of leaf area index (LAI) range from about 2 in orchards with normal plantation density (∼400 plant/ha) up to 4.5 in orchards planted in high density systems (∼1 100 plant/ha) (Figure 3).

In Mediterranean climates, vegetative shoot growth of apricot trees continues (especially in young orchards) for several months after fruit have been picked, until October. During this postharvest period, carbohydrates and mineral elements stored in the different plant organs are of primary importance. Thus, it is important to provide sufficient mineral nutrition and water supply from irrigation to protect the trees from abiotic stress, during the postharvest period. In this way leaf photosynthetic activity is maintained and extended until natural leaf senescence, and in turn, storage of reserves in shoots, buds, branches and main roots is maximized.

RESPONSES TO WATER DEFICITS

In areas of winter and spring rainfall, water stress conditions rarely occur before harvest in early cultivars, particularly when soils are deep and with high water-holding capacity. The effects of reduced soil water availability, as well as the level of stress experienced by plants, depend on the intensity and duration of the water deficit, and on the plant phenological stage. Generally in June, July and August (months with high evaporative demand) the effects of water deficit are more evident.

Apricots, like most stone fruit trees, are sensitive to water shortages during the entire fruit development period. The early stages of fruit growth are of great significance not only for fruit size but also for the accumulation of some phloem-immobile nutrients (e.g. calcium, Ca). About 85 percent of fruit Ca content at harvest is gained within the early four weeks of development (Montanaro et al., 2010). Hence optimal soil water supply during these weeks is essential to avoid reduction of water (and nutrients) uptake.

Water deficits during the later stages of fruit growth lead to smaller fruit at harvest. However, it has been reported that for the cv. Búlida, recovery from water stress (-1.0 MPa predawn leaf water potential (LWP)) during Stage II of fruit growth, induced a compensatory fruit growth rate during the final stages, which allowed the fruit to reach a similar diameter as fruit from fully irrigated plants (Torrecillas et al., 2000). In the same experiment, water deficits applied during Stages I and II that imposed mild to moderate stress from mid-March to mid-May (predawn LWP of -1.1 MPa) caused a yield decline of about 15 percent in the last three years of a four-year experiment. Surprisingly, this difference was not statistically significant from the yield of a fully irrigated control (predawn LWP of -0.4 MPa) (Torrecillas et al., 2000).

For mature trees water deficits (-1.0 MPa predawn LWP) negatively affected trunk growth during the drought period (Perez-Pastor et al., 2009). However, upon recovery of optimal soil water condition trunk circumference may easily recover (Torrecillas et al., 2000).

Water stress (-1.5 to -2.2 MPa of predawn LWP) occurring during the early postharvest period (~30 days after harvest), could have detrimental effects on the potential yield of the following year, particularly for early cultivars. This is because water deficits at this time negatively affect bud induction and the floral differentiation process, which happen in the early postharvest period.

WATER REQUIREMENTS

The water requirements for irrigation depend primarily on the annual water deficit of the environment, than on cultivar and yield target. For example, the amount of irrigation water needed to produce 1 kg of fruit in southern Italy, where the seasonal water deficit (ETo-rainfall) is around 850 mm/year, is about 160 litre (~30 tonne/ha yield). For the same cultivar, this value decreases to about 40 litre/kg in northern Italy (44°08’ N; 12°44’ E) where the seasonal water deficit is only around 160 mm/year.

There have been very few measurements of the consumptive use (ETc) of apricot trees. Crop coefficients (Kc) for apricot orchards are similar to those of other stone fruit such as plum or peach. However, because fruit is harvested quite early, limited post harvest irrigation is common, which affects the reported Kc values, sometimes much lower than the values that may be observed in orchards where the water supply is not limiting transpiration after harvest. Table 1 provides adjusted Kc values for mature drip-irrigated apricot orchards based on an experiment in southeastern Spain (37°52’ N; 1°25’ W) (Abrisqueta et al., 2001).

WATER PRODUCTION FUNCTION

Based on results from published experiments relating yield to different irrigation regimes for apricots (Torrecillas et al., 2000 and Perez-Pastor et al., 2009) and from our own experience, an SDI programme has been outlined in Table 2 and an RDI strategy in Figure 5. This programme reduces the seasonal applied water by 20 percent relative to a fully irrigated orchard, will decrease irrigation water use without impacting negatively on yield.

SUGGESTED RDI REGIMES

As described above, apricot trees have some positive characteristics that help them face water restrictions and these can be used in RDI strategies. Moreover, shoot and fruit growth are separated (Figure 4) in late cultivars. As for peach, this is highly relevant when adopting deficit irrigation strategies devoted to the control of vegetation without affecting fruit growth. However, the opportunity to reduce water application in Stage II is limited for apricots, in particular in early cultivars where the duration of Stage II is quite limited.

A regulated deficit irrigation strategy should avoid water deficits during the critical period of high sensitivity to water stress (i.e. the whole fruit growth and the early postharvest period, around 30 days after harvest). After this period, based on the amount of water available in the soil volume explored by roots, irrigation could start being reduced just after harvest. In this way, the trees deplete water from the deep soil layers and gradually adjust their water status without affecting bud induction and differentiation processes. It is recommended that in the early 30-day period after harvest, the predawn Ψleaf should not be below -0.7 MPa. After this period, a reduction of about 50 percent of the ETc is a practicable deficit irrigation strategy (Figure 5). To avoid excessive tree water stress it is desirable to monitor tree water status (minimum predawn Ψleaf should not be below a threshold of -1.30 MPa, which corresponds to a midday value varying between -2.5 to 3.0 MPa). The percentage of ETc reduction should be carefully evaluated according to the available water in the root zones, which is affected by soil hydraulic characteristics and rootstocks. Table 2 presents recommended Kc values for an SDI strategy tested in Southern Italy. This strategy was tested for three years in a drip irrigated apricot orchard (cv. S.Castrese, Palmette 740 plant/ha) grown in an area with 980 mm ETo from April to September. About 5 600 m^3/ha were supplied during the whole season. Long-term application of the SDI strategy should be evaluated locally and season-by-season. The sustainability of the orchard under the RDI/SDI regime in the long run depends on how effectively winter rainfall refills the soil volume explored by roots.

REFERENCES

Abrisqueta J.M., Ruiz A. & Franco, J.A. 2001. Water balance of apricot trees (Prunus armeniaca L. cv. Búlida) under drip irrigation. Agricultural Water Management 50:211-227.

Dichio B., Montanaro G., Nuzzo V., Xiloyannis C. & Giandomenico I., 1999. Leaf area evolution, light interception, yield and quality of apricot trees (cultivar Tyrinthos) trained to transverse Y and Vase. Acta Horticulturae, 488: 527-532.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Kramarenko, L.A. 2010. Withstanding of Moscow apricots under extremely low (winter 2006) and extremely high (winter 2006-07) temperatures. Acta Horticulturae (ISHS) 862:453-458

Montanaro, G., Dichio, B. & Xiloyannis, C. 2010. Significance of fruit transpiration on calcium nutrition in developing apricot fruit. Journal of Plant Nutrition Soil Science, 173: 618–622. doi: 10.1002/jpln.200900376.

Nuzzo, V., Biasi, R., Dichio, B., Montanaro, G., Xiloyannis, C. & Lanzieri, A. 1999. Influence of different light availability on flower bud quality in cultivar Tyrinthos (Prunus armeniaca L.). Acta Horticulturae, 488: 477-482.

Perez-Pastor, A., Domingo, R., Torrecillas, A. & Ruiz-Sanchez, M. 2009. Response of apricot trees to deficit irrigation strategies. Irrigation Science 27:231-242.

Ruiz, D., Campoy, J.A. & Egea, J. 2007. Chilling and heat requirements of apricot cultivars for flowering. Environmental and Experimental Botany 61 (2007) 254–263.

Torrecillas, A., Domingo, R., Galego, R. & Ruiz-Sanchez, M.C. 2000. Apricot tree response to withholding irrigation in different phenological periods. Scientia Horticulturae 85:201-215.

Xiloyannis, C., Nuzzo, V., Dichio, B., Celano, G. & Montanaro, G. 2000. Characterization of training system in relation to water use efficiency in apricot and kiwifruit plants. Acta Horticulturae, 537: 207-213.

Avocado

LEAD AUTHOR

Raúl Ferreyra and Gabriel Selles
(INIA, Santiago, Chile)

CONTRIBUTING AUTHOR

Elias Fereres
(University of Cordoba and IAS-CSIC, Cordoba, Spain)

INTRODUCTION AND BACKGROUND

Avocado (Persea americana Mill) is a tree that has been known for centuries in areas of Central and South America, but only recently has become a commercial crop. In 2009, there were over 430 000 ha of commercial plantings with a world average yield of 8.8 tonne/ha, with Mexico (100 000 ha), Chile and the United States as the main producing countries. Other countries with significant exports are South Africa, Spain, and Israel (FAO, 2011). Figure 1 presents the production trends of the main producing countries. Avocado fruit yields are comparatively low relative to those of other fruit trees because of the high energy requirements of producing fruit, because of both its large seed size and its composition, rich in oil (Wolstenholme, 1986). Average yields of the variety Hass, one of the most popular commercial cultivars, are around 12 tonne/ha, but may reach 25 tonne/ha in very good years, with the fruit containing up to 20 percent oil. There are three avocado races: Mexican, Guatemalan and West Indies, with different sensitivity in their responses to the environment. Avocadoes have evolved in volcanic soils that have very low bulk density, acid pH, and very high pore volume. It is therefore not surprising that this species is extremely sensitive to waterlogging and does not do well in heavy soils with aeration problems. Planting on berms or ridges is customary when feasible to improve drainage around the areas close to the trunk base. For this reason, sandy rather than heavy soils are preferred for planting avocados. Avocados are also very sensitive to low temperatures, and even light frosts (temperatures below -1 to -2 ºC) may cause significant damage. Among the three races, the Mexican is most tolerant to cold temperatures, as it originated in the cool highlands of Mexico.

GROWTH AND DEVELOPMENT IN RELATION TO YIELD DETERMINATION.

Vegetative growth occurs in two flushes; a strong one in spring and a weaker one in the autumn. Flowering occurs in spring (between early October and mid-November in the Southern Hemisphere) and is followed by fruit set. Heavy fruit drop takes place during the first 3-4 weeks after fruit set, at the end of spring, leading to a first adjustment in fruit number, which is further adjusted with an additional fruit drop period, which takes place around the end of summer, when fruit size is between 10-40 percent of mature size. Figure 2 depicts the developmental stages of the cultivar Hass in Central Chile showing also two root growth periods occurring in early summer and at the beginning of fall.

The most critical developmental period for avocado take place between late spring and early summer. At this time, there is vigorous shoot and root growth, the fruit are set and their final size is defined. Environmental stress could negatively affect fruit set, final fruit numbers and fruit size. It may induce some fruit internal defects as well. During that period, evaporative demand is relatively low and variable, and thus it is possible this inadequate irrigation practices cause water deficit or excess. Even though fruit expansion rates are quite high during a short time period, as shown in Figure 3, fruit growth and development takes a long time, from spring to fall. Fruit shape is also affected by temperature; it becomes more elongated with lower average temperatures.

RESPONSES TO WATER DEFICITS

Avocado trees are quite sensitive to water deficits; it has been determined that their inflorescences are more sensitive to water deficits than the surrounding leaves. The sensitivity of fruit set to water deficits is also quite high, and fruit drop may occur at any time between fruit set and about 50 percent of final size, if water stress is induced by lack of irrigation. Fruit size is affected by water deficits during its growth period that lasts about four months after fruit set. Water stress during fruit development in the later stages of fruit growth negatively affects the quality of mature fruit. When trees are frequently irrigated calcium concentration in the fruit increases and this seems to prevent several fruit physiological disorders.

Stem-water potential (SWP) values at midday of well-watered trees on a typical summer day oscillate between 0.5 MPa and -0.6 MPa. These values should be the reference threshold levels of SWP. Mild water deficits induce midday SWP values between -0.6 and -1.0 MPa. More severe water deficits are indicated by SWP levels below -1.0 MPa reaching down to -2.0 to -3.0 MPa. Stomatal conductance values are somewhat above those of citrus leaves but below the values observed in deciduous orchards. Average leaf conductance values around 0.3 cm/s have been measured for well-watered trees (Ferreyra et al., 2007). Excess water in poorly drained soils does affect avocado tree water relations and, therefore, its overall performance. Low oxygen levels in the soil reduce leaf expansion rates as well as root growth and, if prolonged, may cause root necrosis and leaf abscission. Several studies measuring the oxygen diffusion rate have shown that avocado roots are extremely sensitive to anaerobic conditions. While most species vegetate well in soils, where just 10 percent of the pore volume is air filled, avocado roots seem to require a minimum of about 30 percent pore volume air filled for optimum performance (Ferreyra and Selles, 2007). In addition to the aeration problem, there is a pathogen, Phytophthora cinamomi, which thrives in waterlogged soils and can kill avocado trees upon infection (Stolzy et al., 1967).

Avocado trees are also quite sensitive to salinity, the Mexican race being the most sensitive and the West Indian, the least. While sodium is excluded by the roots up to some level, chloride moves freely along with the transpirational stream and causes tip burn and leaf abscission.

WATER REQUIREMENTS

Avocado is an evergreen tree that follows the evaporative demand of the environment. Studies in Chile and in California indicate that an average crop coefficient (Kc) between 0.7 and 0.72 throughout the year adequately represents the ETc of avocado. One additional study in California has suggested a somewhat lower Kc of 0.64. However, there are trade-offs between irrigation amounts, canopy size and production efficiency (Faber et al., 1995) Figure 4 shows that the canopy size increases and yield per unit canopy volume drops as the fraction of ETc increases from 60 to 115 percent of the requirements. Canopy size is therefore the determining factor of actual ETc , but yields were maximized in the study when a Kc of 0.64 was used throughout the year (Faber et al., 1995).

Part of the avocado water requirements are met by rainfall; in central Chile, annual gross irrigation needs vary between 800-900 mm, while in the drier areas of Southern California it may reach values of over 1 000 mm. To determine the actual irrigation needs in an area, there would be a need to carry out a water budget that takes into account the effective rainfall and the actual ETc.

IRRIGATION SCHEDULING AND DEFICIT IRRIGATION

The extreme sensitivity to water deficits and water excess indicate that irrigation scheduling in avocado must focus on maintaining adequate aeration at the same time that tree water deficits are avoided (Lahav and Kalmar, 1983). In this situation, irrigation frequency is an important issue; in coarse-textured, well-drained soils, daily applications using drip or micro-sprinklers are appropriate. However, in heavier-textured soils that could suffer anaerobic conditions, irrigating every 2-3 days is more desirable. Experiments in Chile have shown that allowing 50 to 60 percent soil water depletion between irrigation applications (every 5-6 days) did not affect yield and fruit size as compared to more frequent applications (Ferreyra, et al., 2006). Irrigation frequencies that deplete about 25-30 percent of the tree-water reservoir are adequate for most soils as a compromise to maintain both adequate water and oxygen supply to the avocado root system. Under drip irrigation, it is important to be able to leach the excess salts out of the potential root zone, and to wet enough soil volume particularly in shallow, coarse-textured soils. Fruit oil content is an important quality feature that is negatively affected by inadequate irrigation.

All experimental evidence so far indicates that RDI is not a recommendable practice for irrigation of avocadoes, because of the high sensitivity of commercial yields to water deficits during most of the irrigation season. On the other hand, excess irrigation is highly detrimental, given the sensitivity to water logging and the high risks of fungal disease infection. Best irrigation practices for avocado should be based on supplying ETc at optimal intervals that both prevent tree water deficits and supply adequate oxygen to the root system.

REFERENCES

Faber, B., Apaia, M. & M. Yates. 1995. Irrigation management of avocado in a California coastal environment. In: Proceedings of the World Avocado Congress III, pp. 189 – 195.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Ferreyra, R., Selles, G., Maldonado, P., Celedon, J. & Torres, A. 2006. Efecto de la macroporosidad y atmósfera del suelo en el estado hídrico del palto. V Congreso Internacional de Ingeniería agrícola, 9 – 12 mayo 2006, Chillan, Chile 208 p.

Ferreyra, R., Selles, G., Maldonado, P., Celedón, J. & Gil, P. 2007. Efecto del Clima, de las Características de la Hoja y de la Metodología de Medición en el Potencial Hídrico Xilemático en Palto (Persea americana Mill.) Agricultura Técnica (Chile) 67(2):182-188

Ferreyra, R. & Selles, G. 2007. Manejo del suelo y riego en palto. La Cruz Chile. Instituto de Investigaciones Agropecuarias. Boletín 160 p. 120.

Lahav, E. & Kalmar, D. 1983. Determination of irrigation regimes for an avocado plantation in spring and autumn. Australian Journal of Agricultural Research 34, 717–724.

Stolzy, L., Zentmyer , G., Klotz, A. & Labanauskas, C. 1967. Oxygen diffusion, water, and Phytophthora cinnamomi in root decay and nutrition of avocados. American Society for Horticultural Science. 90:67-76.

Wolstenholme, B. 1986. Energy costs of fruiting as a yield limiting factor. Acta Horticulturae 175: 121-126.

Sweet cherry

LEAD AUTHOR

Jordi Marsal
(IRTA, Lleida, Spain)

INTRODUCTION AND BACKGROUND

Sweet cherry (Prunus avium L.), contribute modestly to the global economy of deciduous fruit tree species. However cherry production can be crucial at local level in regions specialized for cherry growing, while for other areas it can become a good alternative whenever the market for the main fruit trees, such as apple, peach, and pears slows down. In 2009, there were 381 000 ha with an average world yield of 5.8 tonne/ha (FAO, 2011). Figure 1 presents the production trends for the principal countries.

Foliage development of cherries follows a pattern similar to that described in the peach chapter. Cherry flowers develop in clusters from individual buds. Each bud bears two-to-five flowers. These buds can be borne laterally in individual buds or can be grouped in short spurs on two-year old twigs. Although cherry flowers are monopistil, in very hot summers many form two pistils that result in double fruit. It has been argued that water stress could have a role in helping the formation of undesired double fruit, as it occurs in peaches, but there is little evidence for this in sweet cherries (Beppu and Kataoka, 1999). In addition, air temperatures also affect fruit shape, becoming more irregular at high air temperatures. Canopies can be cooled by using overhead sprinkler irrigation. The majority of commercial sweet cherry cultivars are self-sterile and thus they require the use of pollinizers. Cherry trees are very vigorous and annual shoot extension rates can surpass 1 m when they are young. Cherry trees have an upright growth habit and may need the use of adequate rootstocks to help controlling vigour and induce early appearance of reproductive buds. Vigour control is commonly managed with growth regulators.

STAGES OF DEVELOPMENT

Cherry flower buds, initiate just before the end of the shoot enlargement phase, and will continue to form and develop throughout postharvest (Flore, 1994). Next spring cherry flowers will open before leaf appearance. The reproductive growth can be divided into approximately three growth stages; similar to the case of early maturing peach fruit. Stage I comprises the first month after full bloom and it is characterized by a rapid increase of fruit volume which is mainly produced by cell division (Figure 2). The extent of the activity in cell division will have a major contribution to fruit final size. Stage II corresponds to the pit hardening phase and it can coincide with a slowing of the fruit growth rate (Figure 2). Finally, Stage III takes place around 20 days before maturation and leads to a rapid increase in fruit size (Figure 2). This is a phase of rapid expansive growth. All three stages may take place in less than three months depending on the cultivar used and the temperature regime (growing degree days) of the site.

RESPONSES TO WATER DEFICITS

The timing of water stress is important for cherry. Stages I and III are short and very sensitive to water stress. Stage II can be even shorter and usually overlaps with Stages I and III; for this reason water stress should not be imposed on any of the fruit growth stages. However, irrigation can be reduced under certain conditions i.e. deep soils and low evaporimetric demand, without inducing plant water stress. The postharvest phase is the period for accumulation of reserves. Fruit set in the following spring will be affected by the level of carbohydrate reserves, which have been accumulated during the previous growing season, and more so if flower buds developed earlier than vegetative buds as for cherries. Sweet cherry trees do not grow well without irrigation in areas having dry and warm seasons (Proebsting et al., 1981). Providing accurate information on how to irrigate cherry trees requires a good assessment of plant water status. The standard method of assessing plant water status in cherry orchards is to measure midday stem-water potential (SWP). Air vapour pressure deficit (VPD) has a definite impact on the measures of SWP in cherry trees and VPD reference lines need to be developed to account for this effect. In cherry, Ψstem values below -1.0 MPa during midseason are likely to be indicative of water stress conditions (Marsal, 2009 and Marsal, 2010). Therefore, the level of water stress in a tree having a SWP lower than -1.0 MPa would depend on the prevailing VPD conditions. For instance, early season (from late April until the end of May, Northern Hemisphere), with typically low VPD (< 1.5 kPa) and a canopy still under development, a non-water stressed cherry tree would have a Ψstem less than -0.7 MPa (Marsal, 2009 and Marsal, 2010). The level of SWP is related to tree transpiration because, as water stress increases leaf conductance to water vapour decreases. The response of leaf conductance to SWP for cherry trees follows a standard exponential function (Figure 3); however the response of tree transpiration to midday SWP has not yet been described. Incipient leaf wilting in cherry trees can be observed in the field at a SWP of -1.8 MPa. At this value, stomata are mostly closed and vegetative growth hastened (Figure 3 – midseason conditions). However, leaf wilting can be more clearly observed at SWP values below -2.2 MPa.

Under certain growing conditions, stomata remain slightly closed at the end of the postharvest season, even when the trees are not water stressed (Figure 3 – September conditions). This has been found in cherry orchards growing under warm Mediterranean conditions. Since the cherry market in these regions sets pressure for the use of early ripening cultivars, the postharvest period may last more than four months. During this period, little growth is being accomplished because tree growth is checked by the application of growth regulators. After harvest, leaf net photosynthetic rates tend to decrease with time (Figure 4). Reductions in photosynthesis usually come with reductions in stomata aperture and consequently, the transpiration also declines with time. This is referred in the physiology literature as photosynthetic down-regulation.

WATER USE

Although some preliminary work on modelling cherry tree transpiration has been done recently (Antunez, 2006), specific reports on measurements of cherry ETc or Kc values are lacking in the literature. Cherry irrigation requirements could be approximated by using the information developed for peach trees (see Peach Section) where the Kc is related to tree intercepted radiation at midday. An example of the seasonal evolution of Kc is provided in Figure 5 where cherry tree intercepted radiation was measured every two weeks until mid-August. A steady decrease in Kc is assumed after mid-August. This Kc decline is due to a leaf die-back process, but also to the previously referred effect of the down-regulation of photosynthesis, which may already apparent by late July (Figure 5). Two different tree vigour conditions are considered in Figure 5, and the effects on crop intercepted radiation and estimated Kc is evident. Low vigour conditions had noticeable lower Kc, resulting in a 18 percent reduction in annual water requirements for the conditions in the Ebro basin, Spain (Marsal, 2010). This emphasizes the need to adjust the Kc values to the specific orchard conditions.

SUGGESTED RDI REGIMES

The application of RDI is currently widespread in some regions along with the use of growth regulators. However RDI before harvest is rarely used because besides reducing fruit growth and fruit final size (Werenfels, 1967), it can also increase cracking if stress is relieved during ripening (Sekse, 1995). RDI is more commonly applied after harvest. The reason for the grower acceptance of using postharvest RDI is because it decreases tree internal shading and controls excessive vigour. However, deficit irrigation is often applied after harvest in the absence of research-based recommendations. For instance, in certain areas irrigation is commonly reduced until visual leaf wilting without being aware of possible carryover effects during the following season. From the few research reports published it can be inferred that, under certain conditions, postharvest water deficits can negatively affect cherry quality the following season, with excessive water stress exacerbating this problem. A study on postharvest RDI in New Star sweet cherry grown in the semi-arid climate of Catalonia, Spain found a significant linear relationship between reduction in cherry firmness and soluble solids with the average midday stem-water potential experienced the previous postharvest season (Marsal, 2009). Another issue related to the use of postharvest RDI is the possibility of applying excessive water stress and negatively influencing fruit set and crop load in the next season, as it has been reported for peach and almond. In a recent study on Summit cherry, a postharvest RDI treatment, receiving 50 percent of the water given to a Control treatment, reduced fruit set and crop load in the following season (Marsal, 2010). However, the realization of possible yield reductions in the next season after a RDI postharvest treatment will depend on whether cherry thinning is being used to maintain fruit size (Marsal, 2010). Nevertheless, if water stress during postharvest is maintained above -1.5 MPa in midday stem-water potential, large savings of up to 40 percent of the water used in a fully irrigated control during postharvest can be achieved without noticeable negative impact on fruit yield and quality. The avoidance of water stress, more severe than a certain level (i.e. -1.5 MPa), implies that irrigation reduction must be adjusted with time and a fixed irrigation rate should only be maintained for a certain period to avoid surpassing such tree water status threshold. For this reason significant irrigation reductions during postharvest have to be applied cautiously. Irrigation could also be reduced during postharvest at lower rates (i.e. 80 percent full irrigation). The use of 80 percent full irrigation during postharvest produced water savings up to 15 percent of annual applied water with no detrimental effects on fruit yield and quality (Table 1) (Marsal, 2009 and Marsal, 2010) although the research indicated that cherry quality and yield responses to RDI are cultivar dependent. Therefore more research is needed before reliable assessments can be made for each specific growing condition.

REFERENCES

Antunez A., Stockle, C. & Whiting, M.D. 2006. A gravimetric lysimeter for modelling transpiration in young cherry trees. Acta Horticulture 707:127-133.

Beppu, K. & Kataoka, I. 1999. High temperature rather than drought stress is responsible for the occurrence of double pistils in ‘Satohnishiki’ sweet cherry. Scientia Horticulturae 81:125-134.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Flore, J.A. 1994. Stone fruit. In: Schaffer, B.& Andersen, P.C., eds. Handbook of environmental physiology of fruit crops. Boca Raton, FL, USA, CRC Press, Inc. pp. 233-270

Marsal, J., Lopez, G., Arbones, A., Mata, M., Vallverdu, X. & Girona, J. 2009. Influence of postharvest deficit irrigation and pre-harvest fruit thinning on sweet cherry (cv. New Star) fruit firmness and quality. Journal of Horticultural Science and Biotechnology 84:273-278.

Marsal, J., Lopez, G., J. del Campo, Mata, M., Arbones, A. & Girona, J. 2010. Postharvest regulated deficit irrigation in Summit sweet cherry: fruit yield and quality in the following season. Irrigation Science 28: 181-189

Proebsting, E.L., Middleton, J.E. & Mahan, M.O. 1981. Performance of bearing cherry and prune trees under very low irrigation rates. Journal of the American Society for Horticultural Science 106: 243-246.

Sekse, L. 1995. Cuticular fracturing in fruit of sweet cherry (Prunus avium L.) resulting from changing soil water contents. Journal of Horticultural Science 63:135-141.

Werenfels, L.F., Uriu, K., Paul, H. & Charles, F. 1967. Effects of preharvest irrigation on cherry fruit size. California Agriculture 21(8):15-16.

Grapevine

LEAD AUTHORS

Victor O. Sadras
(SARDI Waite Campus, Australia)

Heinz R. Schultz
(GRC, Geisenheim, Germany)

CONTRIBUTING AUTHORS

Joan Girona, Jordi Marsal
(IRTA, Lleida, Spain)

ACKNOWLEDGEMENTS

VS research in grapevine physiology is funded by the River Murray Improvement Programme and the Australian Grapes and Wine Research and Development Corporation

INTRODUCTION AND BACKGROUND

Grapevine is a long-lived deciduous crop traditionally grown in a latitudinal range between 30° and 50°. The geographical range of wine grapes includes the traditional European countries Italy, France and Spain that account for most of world production, and other countries where the industry has achieved different degrees of maturity (Figure 1). The crop is expanding into new areas in countries with an incipient industry; in 2000, Denmark was accepted as a commercial wine producing nation within the European Union and the Association of Danish Winegrowers had 1 400 members in 2009 (Bentzen and Smith, 2009). Tonietto and Carbonneau (2004) characterized worldwide macroclimates for viticulture using three indices: soil water balance over the growing cycle, solar radiation and temperature conditions, and night temperature during maturation relative to variety requirements, vintage and wine quality.

Profitability of the wine industry is related to both production volume and value per unit volume. The relative contribution of these two factors ranges from enterprises specializing in a high-volume approach to those targeting low-volume, high-value product. Trade-offs between high yield and berry traits related to wine quality are not universal but are common and may constrain the dual maximization of volume and value per unit volume of production. The trade-off between yield and quality underlies regulations in some European countries where no irrigation is allowed for quality wine production. Accepting that wines attracting higher prices are often from vines producing low to moderate yields, the critical question from an irrigation viewpoint is how to manage irrigation to capture the benefit of high yield while achieving a level of quality that maximizes economic returns.

Thus, whereas the core of the grapevine crop remains in the temperate latitudinal band, there is an increasing diversity of environments that, together with diverse production objectives and potential trade-offs dictate contrasting water-management practices in the vineyard. Additionally, the grape and wine industries operate in a global context of competing agricultural and non-agricultural uses of scarce resources – chiefly land, water, and energy – increasing environmental concerns, and shifts in climates and markets.

STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

Overview

The annual cycle of grapevine in temperate and cool environments includes a dormancy phase and a phase of active vegetative and reproductive development and growth. In tropical environments, vine physiological activity is continuous during the year. Figure 2 shows a scale accounting for phenological stages in temperate environments and Table 1 illustrates the range of key vegetative and reproductive components of grapevines in vineyards with contrasting yield targets.

After an overwintering period, when vegetative and reproductive buds remain dormant, visible leaf tissue marks the beginning of budburst (Stage IV in Figure 2). Early shoot growth depends on plant reserves and is initially slow; it then accelerates in late spring. Parallel to root and vegetative shoot growth, two important reproductive processes take place: (a) inflorescence primordia initiated in the previous season resume growth, branching, branch elongation and flower formation, and (b) a new set of reproductive buds is induced and starts differentiation that will be completed in the following season (Dunn, 2005). Bunch number is generally the largest source of seasonal and site-related variation in grapevine yield (Tables 1 and 3).

Current season inflorescences become visible several weeks after budburst. Flower cap fall and stamen release marks the full bloom stage (Stage 23 in Figure 2). Berries per bunch, the second yield component, depend on number of flowers and berry set, which may be particularly affected in some particular combinations of sites, seasons and cultivars, e.g. Chardonnay in cool conditions. As in most flowering plants, however, only a fraction of flowers set fruit, typically for grapevine this is 20-50 percent.

TABLE 1

Key vegetative and reproductive components in low and high-yielding vineyards (Pearce and Coombe, 2005).

Component

Low

High

Yield (kg m-2)

0.20

5.0

Equivalent volume of table wine (litre m-2)

0.12

3.0

Pruning weight (kg m-2)

0.03

1.0

Leaf area index (m^2 m-2)

0.50

5.0

Number of nodes (m-2)

3

30

Number of shoots (m-2)

2.5

25

Number of bunches (m-2)

5

50

Berry growth has a characteristic double-sigmoidal pattern; it is dominated by cell division in the first two weeks after flowering and by cell expansion afterwards. The first sigmoidal trajectory reaches a plateau in synchrony with full seed size in seeded varieties. After an intervening lag-phase, the onset of the second sigmoidal phase is characterized by berry softening, accumulation of sugars, decline in acid concentration and accumulation of pigments in the skin of coloured varieties. This stage is called veraison (Stage 35 in Figure 2) and is very responsive to environmental factors. For example in physically constrained berries, the threshold cell turgor pressure ≈ 0.1 MPa associated with veraison under the experimental conditions of Matthews et al. (2009) was delayed by two weeks in relation to controls, and a similar delay was recorded for the onset of sugar accumulation. The second sigmoidal stage ends in a plateau corresponding to variety- and environment-specific maximum berry size. Varieties like Shiraz, which often exhibit substantial berry dehydration late in the season are characterized by a decline in fresh weight rather than a plateau at the end of the second phase (Sadras and McCarthy, 2007). This decline is also observed when harvest is delayed to enhance berry traits associated with wine quality at the expense of fruit weight and yield. Harvest maturity (Stage 38 in Figure 2) is defined by winemaking criteria for fruit composition; it is often specified in terms of sugar concentration or sugar: acid ratio in cooler climates, but colour and flavour criteria complement these simple definitions. Environmental variables including temperature, radiation and water availability during berry growth and ripening can have substantial impact on berry composition and hence on wine attributes. This viticulturally important aspect of berry biology is beyond the scope of this section, but readers are referred to reviews by Coombe and Iland (2005), Conde et al. (2007), and Dai et al. (2010). We focus on the effects of water deficits on berry and wine attributes in a further on in this section.

Varietal differences in development

The developmental plan outlined in the previous section applies to all grape varieties. However, the actual timing of each critical stage and the resultant season length are genetically controlled and modulated by the environment, chiefly temperature. Some phenological stages may also be responsive to management practices, e.g. timing of pruning may shift the timing of budburst, and manipulation of canopy-to-fruit ratio by defoliation or bunch removal may delay or advance ripening in some conditions.

There are two distinct aspects related to the variety-dependent phenological pattern. One is the average time to reach a certain stage; for example Gladstones (1992) classified varieties in eight maturity groups (Table 2). The other aspect is the plasticity of phenological development, defined as the degree of response in phenology to environmental conditions. Cabernet franc and Riesling, for example, have comparable maturity requirements ~ 1 200-1 250 °Cd (Table 2), but under South Australian conditions Cabernet franc is phenologically more plastic than Riesling (Figure 3), as its flowering date varies more in different environments than that of Riesling. The actual timing of occurrence of critical phenological stages, total season length and phenological plasticity are critical traits in the quest to match varieties and environments including the fine-tuning of irrigation management.

TABLE 2

Maturity groups and biologically effective thermal time* from October 1st (Southern hemisphere) or April 1st (Northern Hemisphere) to ripeness of grapevines according to Gladstones (1992).

Maturity group (°Cd to maturity)

Red wine

White or rosé wine

1 (1 050)

Madeleine, Madeleine-Sylvaner

2 (1 100)

Blue Portuguese

Chasselas, Müller-Thurgau, Siegerrebe, Bacchus, Pinot Gris, Muscat Ottonel, Red Veltliner, Pinot Noir, Meunier

3 (1 150)

Pinot Noir, Meunier, Gamay, Dolcetto, Bastardo, Tinta Carvalla, Tinta Amarella

Traminer, Sylvaner, Scheurebe, Elbling, Morio-Muskat, Kerner, Green Veltliner, Cardonnay, Aligoté, Melon, Sauvignon Blanc, Frontignac, Pedro Ximenes, Verdelho, Sultana

4 (1 200)

Malbec, Durif, Zinfandel, Schiava, Tempranillo, Tinta Madeira, Pinotage

Sémillon, Muscadelle, Riesling, Welschriesling, Furmint, Leanyka, Harslevelu, Sercial, Malvasia Bianca, Cabernet Franc

5 (1 250)

Merlot, Cabernet Franc, Shiraz, Cinsaut, Barbera, Sangiovese, Touriga,

Chenin Blanc, Folle Blanche, Crouchen, Roussanne, Marsanne, Viognier, Taminga, Cabernet Sauvignon

6 (1 300)

Cabernet Sauvignon, Ruby Cabernet, Mondeuse, Tannat, Kadarka, Corvina, Nebbiolo, Ramisco, Alvarelhão, Mourisco Tinto, Valdiguié

Colombard, Palomino, Dona Branca, Rabigato, Grenache

7 (1 350)

Aramon, Petit Verdot, Mataro, Carignan, Grenache, Freisa, Negrara, Grignolino, Souzão, Graciano, Monastrell

Muscat Gordo Blanco, Trebbiano, Montils

8 (1 400)

Tarrango, Terret Noir

Clairette, Grenache Blanc, Doradillo, Biancone

* calculated using a base temperature of 10 °C, and a cutoff in the monthly average temperature at 19 °C.

Grapevine development and warming trends

Phenology is temperature driven; therefore warming trends recorded since the middle of the twentieth century are reflected in grapevine phenological shifts of great significance for vine management and winemaking (Duchene et al., 2010). Several studies have assessed the rates of change associated with phenological variables in both the northern and Southern Hemisphere (Wolfe et al., 2005 and Duchene and Schneider, 2005). Not surprisingly, vines develop faster in warmer conditions but the actual rates need consideration. Two important aspects of these responses are the differential sensitivity of particular phenological phases, and the potential decoupling of berry attributes. For example faster sugar accumulation that is not fully compensated by early harvest means higher sugar content in berries and higher alcohol potential, as suggested for Riesling in Alsace and for Cabernet Sauvignon and Shiraz in Australia (Petrie and Sadras, 2008 and Duchene and Schneider, 2005).

RESPONSE TO WATER DEFICITS

Overview: rainfall patterns and development of water deficit

Rainfall pattern and soil-water storage capacity are major drivers of the temporal pattern of water supply and water deficit in rainfed systems, as illustrated by comparison of winter- and summer-rainfall viticultural regions. Aschmann (1973) highlighted the concentration of rainfall in the winter half-year as the most distinctive element of the Mediterranean climate, and proposed 65 percent of annual rainfall in this period as a boundary in his definition. Grapevines are grown in Mediterranean-type climates in southern Europe, California, and parts of South Africa, Chile and Australia. Winter rainfall often ensures soil water storage that allows for early growth, whereas a pattern of terminal drought is typical of rainfed vines in Mediterranean environments. Temporary water deficits are common in temperate, summer rainfall regions of western and central Europe where vineyards are established in shallow soils or soils with low water-holding capacity. In these environments, rainfall pulses and limited buffering capacity of soils drive marked wet-dry cycles. The frequency, duration and severity of the dry spells in these cycles are largely unpredictable. Owing to drying trends and reliability of quality wine supply required by globalized markets, supplemental irrigation is likely to increase even in these areas that have been traditionally rainfed.

Figure 4 compares the plant water status of rainfed vines from contrasting climates and soils within a given region; examples from irrigated vines are also included. It is clear that irrigation stabilizes predawn leaf water potential at a level rarely found naturally over the growing season in these areas, including the very cool climate regions of the Loire Valley in France and the Rheingau in Germany. Figure 4 also shows that the differences in water status between vineyards within each of the regions could be larger than the differences in general water status between different climate zones. It is also clear that variation in water status during a particular season increases from warm and dry climates to summer rainfall because of the irregularity of the frequency and intensity of summer precipitation in the latter.

Growth and yield

Crop responses to water deficit depend on the intensity, duration and timing of stress. Intra-specific variation has been reported for major traits related to the development, water and carbon economy of grapevine including phenology, susceptibility to embolism, stomatal density and stomatal conductance in response to both soil water content and vapour pressure deficit, biomass per unit transpiration, dry mater partitioning, and rootstock response to water deficit, salinity and soil-borne diseases.

In common with most crops, tissue expansion in grapevine is more sensitive to water deficit than stomatal regulation and gas exchange (Figure 5a-d). The effects of intensity and duration of water stress have been integrated in stress indices based, for example, on soil water status (Figure 5a-d), plant-water status or canopy temperature (Figure 6). Box 2 summarizes techniques used to monitor water status of vines, and below analyses yield response to water deficit from the perspective of production functions.

Owing to the developmental programme of the plant and the definition of yield over two consecutive seasons, we need to consider the effects of water deficits in the previous season on the growth and yield in the current season. All possible responses have been reported, namely effects of water deficit in season 1 had negative, neutral or positive effect on reproductive outcome in season 2 (Williams and Matthews, 1990). This diversity of responses is partially the result of differences in varietal sensitivity, timing, intensity and duration of water deficit, interactions with other factors, and in same cases to poorly designed experiments. In a well-designed factorial experiment looking at the combined effects of pruning and post-veraison irrigation on Shiraz, Petrie et al. (2004) measured statistically significant reductions in shoot number, bunch number and yield in season 2 in response to reduction in irrigation rate in season 1 (Figure 7). In an equally well designed experiment with Tempranillo, Intrigliolo and Castel (2010) found no carry over effect of irrigation regime on bud fertility. Nevertheless, grape growers do have some capacity to regulate yield components by pruning and bunch thinning (Table 1, Figure 8).

Many studies measured the effect of in-season water supply on yield and its components, as illustrated in Table 3 for three contrasting production systems. The combination of cultivar, environment and management resulted in yield of fully irrigated vines from 10 kg/vine for Bobal in Requena and Chardonnay in Niagara to 20 kg per vine for Shiraz in Riverland. Comparison of rainfed and fully irrigated crops shows a large (up to twofold) benefit of irrigation in the drier environments (Riverland, Requena) in comparison to yield gains of only 10-25 percent in the cooler, humid environment (Niagara). Bunch number shows large variation among production systems, but is relatively stable in response to in-season water supply, as expected from the reproductive cycle of vines. In-season water deficit therefore reduces yield by reducing bunch weight, and the relative importance of its components, namely berry number and size, depends on the timing of water deficits. Water deficit around anthesis and berry set has the potential to reduce berry number and size, whereas water deficit at later stages only reduces berry size. The post-veraison period is particularly critical because of the trade-off between maintenance of water supply to ensure berry growth and the requirements of berry composition, which may benefit from controlled water deficit.

Fewer studies characterized the long-term effect of water deficit on grapevine yield. For Riesling on a steep slope vineyard in Germany, the combination of in-season and across-season temporary water deficits reduced long-term production from an average 7.6 tonne/ha in vines with small amounts of supplementary irrigation (29 litre/m^2 per season) to 5.0 tonne/ha for rainfed vines (Table 4).

Availability of water after harvest has at least two effects. First, it may influence canopy activity, build up of reserves and hence the performance of the crop in the following growing cycle. Second, irrigation between harvest and leaf fall may alter phenology under some conditions; for instance reduced irrigation after harvest may advance budburst in the following season (Williams et al., 1991), potentially increasing frost risk in some environments.

TABLE 4

Yield, fruit sugar concentration and sugar yield of irrigated and rainfed Riesling on a steep slope vineyard close to Geisenheim (50 °N), Germany. Values are mean ± standard deviation for eight consecutive years since 2002. Combination of an irrigation threshold of -0.3 MPa predawn water potential and weekly irrigation decision interval resulted in 7.4 ± 3.4 irrigation events per season, and applied water 29.3 ± 12.2 litre/m^2 (Gruber and Schultz, unpublished).

Response variable

Irrigated

Rainfed

Yield (tonne/ha)

7.6 ± 3.22

5.0 ± 2.16

sugar conc. (g/litre)

212 ± 19.5

204 ± 29.7

sugar yield (kg/ha)

1 182 ± 460.1

728 ± 280.1

Berry and wine attributes

Wine quality is an elusive concept and attempts to quantify it are bound to be controversial (Box 1). Quantitative assessments of berry and wine attributes in response to water deficit are, however, essential for irrigation scheduling. Indeed, regulation of grapevine water relations is an important tool for quality management in irrigated viticulture. There is a significant body of literature dealing with the effects of water relations on the composition of red grapes, especially on phenolic compounds; information regarding the effects of plant water status on the composition of white varieties is scarce.

Red varieties

Figure 9 illustrates a typical, although not universal, relationship between wine quality and irrigation for red varieties. The negative association between water supply and wine quality is partially mediated by an apparent trade-off between yield and quality attributes of berries (Figure 10, Figure 11). The negative associations between rate of accumulation of sugar and anthocyanins and yield components in Figure 10b probably reflect a high fruit-to-canopy ratio, rather than high yield per se. If this hypothesis is correct, manipulation of this ratio by irrigation, pruning, thinning, and canopy management maybe important to achieve both high yield and high quality.

BOX 1 Wine quality

The elusiveness of ‘wine quality’ stems from the complexity of wine attributes compounded by the complexity and variability of human smell and taste sensitivity. Temporal and regional variation in wine quality has been assessed with price and vintage ratings (Cicchetti and Cicchetti, 2009 and Almenberg and Dreber, 2009). The drawbacks of each of these approaches are many, including marketing factors influencing price beyond specific quality parameters, and vintage scores derived from expert, albeit subjective evaluations (Sadras et al., 2007). Views on vintage scores range from “…controversial, potentially misleading and essentially impossible to get consistently correct…” (Fuller and Walsh, 1999) to the proposal of ratings that “…express the likelihood of what might reasonably be expected from a wine of a given year…” (Stevenson, 2005). Individual attributes of berries and wine such as colour or content of many critical compounds, on the other hand, can be measured with accuracy. The challenge to this approach is however, the integration of individual measurements into a complete measure of quality. There is no doubt that wine quality is a controversial concept, and there is no doubt either that, however imperfect, quantification of key berry and wine attributes is essential to irrigation management.

Water deficit generally increases the concentration of phenolic compounds, but has a differential effect on individual groups of phenols depending on timing and severity of the stress. Tannin biosynthesis can be negatively affected by severe stress after anthesis (Ojeda, 2002), but later deficits often increase tannin concentration (Roby et al., 2004). In most cases, anthocyanin concentration responds positively to water shortage after veraison but less frequently to pre-veraison deficits (Matthews and Anderson, 1988); although the expression of genes involved in anthocyanin biosynthesis can be increased by water stress pre-veraison (Castellarin et al., 2007). Aside from phenolic compounds, aroma attributes may also be affected (Chapman et al., 2005). Differences in the response of varieties are likely but not well documented (see: Suggested RDI regimes below for examples). In addition to the effects on amount and proportion of key compounds, water deficit can cause more subtle but relevant effects. For example, water deficit after veraison has been shown to increase the structural complexity (degree of polymerisation) and to reduce the extractability of phenolic compounds in berries of several red varieties (Ojeda et al., 2002; Sivilotti et al., 2005). The effects of water deficit on berry attributes are partially related to reductions in berry size, although size-independent effects have also been reported. Allometric analysis is required (i) to separate size-dependent effects of water deficit on a particular component, e.g. sugar, and (ii) to compare the relative responsiveness of different berry components to water deficit (Sadras et al., 2007 and Sadras and McCarthy, 2007). For example, allometric analysis revealed that water deficit accelerated the rate of accumulation of anthocyanins with respect to sugar of Cabernet Sauvignon in a warm environment (Figure 12). Water management is therefore important to ensure a certain coupling of key berry components during ripening; this will eventually affect wine balance.

White varieties

Compared to red varieties, white varieties are generally more sensitive to stress periods and can show negative compositional changes (Christoph et al., 1998 and Peyrot des Gachons et al., 2005). Phenolic compounds are judged less desirable in white grapes, since sensory attributes such as astringency or bitterness, associated with both flavonoid, i.e. flavan-3-ols and proanthocyanidins (Singleton and Noble, 1976 and Brossaud et al., 2001) and nonflavonoid phenols, i.e. hydroxycinnamic acids or their esters (Arnold et al., 1980 and Hufnagel and Hofmann, 2008) are incompatible with the current type of white wine popular with the consumer. Flavonoids undergo oxidative polymerisation, thus lowering their flavour threshold during wine ageing (Schneider, 1995) and can negatively affect the volatility of flavour compounds (Aronson and Ebeler, 2004). Depending on the variety, water stress and associated reduction in nitrogen uptake, have been implicated in alterations of flavour-quality of various white varieties such as Chasselas, Silvaner, Sauvignon blanc and Riesling (Peyrot des Gachons et al., 2005 and Linsenmeier, 2008). In one example, the development of negatively judged wine attributes after only a short period of bottle ageing was retarded by irrigation (Table 5). However, there has been at least one report showing an increase in glycosidically bound monoterpenes contributing to the flavour profile of White Riesling under water deficit (McCarthy and Coombe, 1999), but even in that particular trial, sensory attributes changed differentially over time.

WATER REQUIREMENTS

Crop evapotranspiration increases with vine age from establishment until the canopy and root system reach their full capacity to capture radiation and water (Figure 13). For established vines, seasonal ETc in semi-arid to arid environments, e.g. central California (United States), and Riverina (Australia) ranges from approximately 500 to 800 mm (Williams and Matthews, 1990). Figure 14 shows the seasonal dynamics of Kc as affected by crop age and environment (temperate vs tropical). The seasonal dynamics of crop coefficients comprises two phases with increasing Kc from onset of active growth to peak canopy size, and decreasing Kc during leaf senescence. Assuming linearity, the average rate of increase of Kc was 0.005 d-1 for young vines, 0.007 d-1 for older vines in Washington, and 0.013 d-1 in the tropical São Francisco region. In the declining stage, the rate of change in Kc was 0.006, 0.011, and 0.042 d-1, respectively. In addition to differences in rate of change in Kc, non-zero Kc at the onset of the irrigation season in tropical environments reflects the lack of dormancy and continuous physiological activity during the year in tropical environments.

More refined crop coefficients could be obtained on the grounds of a direct, often non-linear association with canopy light interception or related variables such as leaf area index. Owing to large variation in canopy structure with pruning and training systems, however, the relationship between crop coefficient and leaf area index is not unique. The relationship between crop coefficient and leaf area index may also show hysteresis, i.e. the relationship is different for increasing or decreasing Kc (Netzer et al., 2009). To deal with these problems, Williams and Ayars (2005) proposed an approach to characterize crop coefficients on the basis of canopy light interception, and demonstrated the robustness of a practical grid-method to measure the amount of shade cast on the ground by table grapes.

WATER PRODUCTION FUNCTIONS

The diversity of production systems targeting different combinations of fruit volume and quality contribute to the large scatter in the relationship between yield and water use. Furthermore, scarcity of data means the actual shape of the function remains speculative in particular for relative ETc below 0.4 (Figure 15). In common with other tree crops yield is maintained until relative ETc approximates 0.8, and a consistent almost linear decline is observed in the range of relative ETc from 0.8 to 0.4. The data for Tempranillo wine grapes suggest a more sensitive response to relative ET deficits in contrast with the response of table grapes (Figure 15). This is probably because of a limitation in the maximum ETc imposed by pruning and other cultural techniques in the case of wine grapes, while such limitation is not normally imposed in table grapes. The variations in the scarce data in wine grapes of Figure 15 suggest that the production function is probably variety and cropping system dependent.

Qualitatively, the relationship between gross revenue and ETc in wine grapes is expected to feature an optimum resulting from the mismatch between the ETc required to maximize yield and the ETc required to maximize quality and value per unit volume (Box 19 in Chapter 4). The actual parameters of this function, however, are likely to be specific for local production systems with their own water and grape prices.

SUGGESTED RDI REGIMES

For wine grapes, where the quality objective is critical to profit, the recommended irrigation regime must account for many more factors than in most other crops. To achieve the triple aim of high yield, quality and irrigation water productivity, irrigation of red grape varieties in temperate environments needs to ensure vegetative growth and early reproductive growth in spring, and allow for progressive water deficit to develop towards maturity. For white grapes, water management should seek to avoid both severe water deficit and excess water supply. Management of irrigation after harvest accounts for the need to build up carbohydrate reserves in vines. Owing to the many factors involved, we selected five case studies to illustrate this general pattern, and the variations particular to the target production system. These examples also highlight the application of different monitoring methods (Box 2).

Box 2 Crop and soil measurements to schedule grapevine irrigation

Plant, crop and soil indices of water status have been tested in vines grown in diverse environments. The table below presents some examples, and: Suggested RDI regimes outlines the application of some of these methods in irrigation scheduling. Measurement of leaf water potential is time consuming and requires considerable expertise, but seems more consistent than faster measurements including trunk diameter and stomatal conductance.

Under the conditions of the study of Girona et al. (2006), midday leaf water potential outperformed soil water balance as a trigger for irrigation, i.e. leaf water potential captured spatial variability better, and crops managed using this plant-based index had less variability in yield and berry composition, that could potentially improve the homogeneity of grape juice. Thermal, visible and hyperspectral imagery are attracting increasing attention (Moller et al., 2007; Rodriguez-Perez et al., 2007). These technologies, coupled with GIS, allow for effective account of spatial variation at relevant scales from region to fields. There is a large variation in cost and expertise required for the implementation of these approaches, form relatively cheap, easy-to-use hand-held infra-red thermometers to tractor-mounted, air-borne or satellite imagery across wide spectral ranges. Soil water status can be assessed indirectly through predawn leaf water potential, and directly through measurements with a range of instruments including neutron probes, time domain reflectometry and pressure transducer tensiometers (Chapter 4). A water balance model is often a practical alternative to direct measurements of soil water status (Pellegrino et al., 2006).

Suitable indices for irrigation management need to combine flexibility, and a reasonable accuracy-to-acquisition cost ratio in terms of time, resources and skills. In addition to trade-offs between accuracy and cost, there are also trade-offs between the multiple effects of irrigation on yield, quality, reserves and diseases (Pellegrino et al., 2006). Pellegrino et al. (2006) developed an elegant method that combines a soil water model and simple, empirical response function that allow for the changes in crop responsiveness to water deficit through the growing season (Section 5.1).

Shiraz, Grenache and Mourvèdre in southern France

Figure 5 outlines a scheme for water management derived for Shiraz, Grenache and Mourvèdre in vineyards of southern France (43 °N) with contrasting soil types (Pellegrino et al., 2006). This scheme is based on eight classes of water deficit and aims at quality wine production. It highlights the need for good water supply, i.e. fraction of transpirable soil water (FTSW) above 0.6, early in the season. This allows for the establishment of a balanced canopy and fruit load, and proper development of inflorescence buds that would largely determine next season's yield. A mild water deficit from flowering to veraison, (FTSW between 0.6 and 0.4) leads to a drier finish to account for disease and berry composition at harvest. High water supply, particularly during ripening, may lead to a combination of undesirable indirect (e.g. disease) and direct effects on berry composition and wine quality. The goal of this RDI regime is to achieve a dry finish that often leads to higher concentration of colour and flavour compounds in berries.

Cabernet Sauvignon in California, United States

Prichard (2009) derived an RDI regime based on extensive experimentation with mature Cabernet Sauvignon at Lodi (38 °N). The regime aims at the best yield/quality relationship and is conceptually similar to the general pattern outlined above, namely ensure good water supply at the beginning of the growing season, and progressively reduce water supply towards ripening. After harvest, full watering is recommended to encourage root growth and accumulation of plant reserves in an environment notably warmer and with greater evaporative demand than in the previous case study.

Pinot Noir and Tempranillo in Lleida, Spain

Girona et al. (2006) used mid-day leaf water potential to schedule irrigation of 12-year-old Pinot Noir vines at Raïmat (42 °N) during three consecutive seasons. They used the same general principle of ensuring water supply early in the season and allowing for a deficit at late reproductive stages using the thresholds summarized in Table 6. The RDI in this study reduced yield by 14-43 percent, increased irrigation water productivity by 28-46 percent and improved concentration of anthocyanins and polyphenols in berries by 10-19 percent (Table 6).

Working with Tempranillo in the same environment, Girona et al. (2009) measured the effect of timing of water deficit on must attributes including soluble solids content, polyphenol, and anthocyanin concentration. They found negative impact of water deficit between fruit set and veraison and positive effects of mild water deficit after veraison. They proposed that irrigation management should aim to avoid severe water deficits for Tempranillo before veraison and that RDI should target the window between veraison and harvest.

Sauvignon Blanc in Marlborough, New Zealand

Irrigation in this cool climate needs to account for the high evaporative demand in mid-summer (ETo > 7 mm/d) and the risk of excess irrigation with negative effects in terms of both wine quality and environmental deterioration associated with leaching of nutrients and pesticides. Greven et al. (2005) established an RDI study on 5 ha of 9-year old Sauvignon Blanc. They combined measurements of vine water use, assessments of vegetative and berry growth, and modelling to calculate transpiration. Before veraison, vines were irrigated when predawn leaf water potential was below –0.2 MPa and the threshold was –0.4 MPa between veraison and harvest. Preliminary conclusions for this particular production system are that yield and juice attributes were unaffected by reductions in seasonal irrigation up to 40 percent from fully irrigated vines receiving 360-690 mm.

a fraction of control; control ~ 378 mm

b fraction of control; control ~ 10.8 kg/vine

c fraction of control; control ~ 54 kg/ha per mm

d fraction of control; control ~ 556 mg/kg

e fraction of control; control ~ 13.0 mg/kg

Riesling in the Rheingau area, Germany

The region is characterized by vineyards on slopes, some of them very steep with shallow soils (depth < 0.8-1 m). Midsummer reference evapotranspiration is between 3 and 6 mm d-1 and annual precipitation is 534 mm. Due to the strong variability in cloud cover, temperature and vapour pressure deficit (also in the absence of precipitation), leaf or stem-water potential are not stable enough to schedule irrigation. The irrigation threshold used is a predawn water potential of -0.3 to -0.4 MPa throughout berry development, with the exception of the first berry growth phase, where no irrigation is applied. Very small amounts of water are given at each irrigation event (on average 4 litre/m^2) to minimize the risk of excess water when precipitation occurs shortly after an irrigation. Over a period of 8 years, vines were irrigated 7.4 times per year on average with a total of 29.3 mm per year (Table 4), which represents about 5.5 percent of annual precipitation. Despite these small amounts, yield of the irrigated vines was about 50 percent higher at similar sugar concentrations (Table 4).

REFERENCES

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Kiwifruit

LEAD AUTHORS

Cristos Xiloyannis
(Università degli studi della Basilicata, Potenza, Italy),

Giuseppe Montanaro
(Università degli studi della Basilicata, Potenza, Italy)

CONTRIBUTING AUTHOR

Bartolomeo Dichio
(Università degli studi della Basilicata, Potenza, Italy)

INTRODUCTION AND BACKGROUND

Globally, the green kiwifruit (Actinidia deliciosa [A.Chev.] C.F. Liang and A.R. Ferguson), represents about 95 percent of the commercial kiwifruit, all produced with just one variety, Hayward. Other species, such as Actinidia arguta (known as baby kiwi) is grown for a niche market and is also recognized for ornamental purposes. Only recently, some yellow fleshed varieties that originated in in New Zealand and Italy (Actinidia chinensis Planch.) have appeared on the international markets.

The main training systems adopted for the kiwifruit are the T-bar and the Pergola, with plantation densities ranging from 400-600 (Pergola) up to 720 plant/ha (T-bar). Canopy management should focus on determining the appropriate bud load (150 000 -200 000 bud/ha) in winter and on maximizing the carbon budget during the growing season by reducing the amount of shaded leaves by summer pruning (Xiloyannis et al., 1999). This in turn enhances light availability within the canopy improving fruit growth and some fruit quality traits (e.g. calcium content, Montanaro et al., 2006).

As for pistachio, kiwifruit needs male plants to produce pollen for the female. The standard male to female plant ratio adopted is 1:6. Distribution of male plants is important to ensure pollination and adequate fruit size and yield. The use of bee hives or artificial pollen distribution during bloom is recommended.

Total global production in 2008 was 1.31 tonne, on 82 547 ha production area (FAO, 2011). Italy is the first producing country in the world (36 percent) followed by New Zealand (28 percent) and Chile (13 percent). However, these statistics do not include China, whose production has been estimated at 403 000 tonne of fruit in 2004 (about 65 000 ha producing area). Figure 1 presents the production trends of the main countries since 1985.

In addition to the green cultivar Hayward, a recent review describes the new cultivars of A. deliciosa and A. chinensis (yellow fleshed), which have been recently selected and released (Testolin and Ferguson, 2009).

Quality considerations

Particular attention should be paid to kiwifruit calcium (Ca) nutrition because of its involvement in determining tissue mechanical strength and tolerance to biotic and abiotic stresses. Cessation of Ca import into fruit has been linked to a number of morpho-anatomical changes of fruit properties related to reductions in fruit water loss, as transpiration from the fruit is the only mechanism responsible for Ca import. Therefore, it is desirable that canopy and irrigation management ensure adequate light distribution and windspeed within the canopy to enhance fruit transpiration and Ca accumulation. (Montanaro et al., 2006; Xiloyannis et al., 2008).

STAGES OF DEVELOPMENT IN RELATION TO YIELD DETERMINATION

Bud development starts in spring and is largely completed by midsummer. Some buds are vegetative in that they give rise to vegetative shoots, while others are mixed, generating both leaves and flowers that develop from the same bud. The induction of the reproductive development of the bud occurs during late-summer to autumn followed by inflorescence differentiation, which will be completed by the next spring.

The chilling requirement to break dormancy is about 700-800 Richardson units for the Hayward cultivar (Linsley-Noakes, 1989), while for the yellow-fleshed cultivars chilling requirements are lower (500-600 units). The growing-degree-hours for bud break range from about 9 000 to 16 000 (Wall et al., 2008). The buds break in spring which takes 10-15 days, thereafter shoots elongate quickly reaching 20-30 cm after 3 weeks. Leaves expand very rapidly during the early 30 days of growth, and the full leaf area is reached within three months (Figure 2). Values of LAI are around 2.5-3 in orchards trained to T-bar (~400 vine/ha), and up to 4-5 in orchards trained to the pergola system (~700 vine/ha). Different vigour rootstocks could greatly affect the LAI (Figure 3) particularly during the early years after planting. The evolution of LAI during the first years of planting is shown in Figure 4. Because the fraction of shaded leaves may represent up to 60 percent of LAI in pergola trained vines (Figure 5), it is important to control growth by summer pruning so that the proportion of leaves with high water use efficiency increases (Figure 6) and some fruit quality traits (e.g. calcium content) related to light availability are enhanced (Montanaro et al., 2006).

After fruit set, the development of a fruit entails an initial rapid growth (predominant cell division stage) which spans about 50 days and is followed by the cell enlargement stage that slows down late in the season. The pattern of fruit growth, described as changes in fruit length or surface area, is shown in Figure 7.

RESPONSES TO WATER DEFICITS

Kiwifruit is quite sensitive to water deficit; vines do not survive stress levels associated with predawn leaf water potential values (LWP) below -1.5 MPa. Even mild water deficits determine rapid stomatal closure and increase in leaf temperature, which results initially in leaf tip burn and later leads to necrosis of the entire lamina. Restriction of water supply over the summer easily reduces fruit size at harvest. Mild water stress (about -0.5 MPa predawn LWP) occurring during the early growth of fruit (cell division stage) or later during fruit growth, causes a reduction in fruit size (Figure 8). However, in the case of deep soils with high water-holding capacity, an initial irrigation deficit may be tolerated without affecting fruit size (Reid et al., 1996) as long as the soil water content is high enough to avoid mild water stress. Further, the evaporative demand and vine water consumption are low at this time.

Achieving a specific fruit dry matter (DM) target has been identified by the industry as a key component in the ongoing sustainability of kiwifruit production. Moreover, the minimum DM content for marketable fruit has been established in some countries (e.g. 15 percent DM is the minimum threshold for European markets (EC, 2004). The accumulation of DM in the fruit depends on maintaining high canopy photosynthesis, which is related to the training system, the light distribution inside the canopy and the level of orchard management (irrigation, fertilization, protection from biotic stress). Photosynthetic rates recover within about 10 days upon re-watering in severely (-1.0 MPa predawn LWP) stressed vines (Montanaro et al., 2007) but apparently the loss in dry matter accumulation during a stressed period is never recovered.

Kiwifruit is quite sensitive to water stress throughout the whole growing season, hence soil water content should not decline below 70 percent of the water available in the root zone (Miller et al., 1998). Knowledge of the effective soil volume explored by roots is of great significance when designing and managing irrigation of both mature and young orchards (Figure 8). Because of the peculiar kiwifruit root system, which has low dominance of root apex and a high number of lateral roots (Figure 9 and Photo 1), the kiwifruit has an overall high rooting density in the explored soil volume (~ 0.9 cm per cm^3) (Miller et al., 1998) in comparison to other fruit tree species, but the lack of dominance of a tap root slows down and limits the exploration of the subsoil by kiwi roots. This pattern of root exploration may be partly responsible for the high sensitivity of kiwi to water deficits.

WATER REQUIREMENTS AND IRRIGATION MANAGEMENT RECOMMENDATION

On a midsummer day, a Mediterranean kiwifruit orchard consumes ~ 60-70 m^3 of water per ha, and seasonally, around 300-350 litre of water per kg of fruit are supplied (for a yield of 35 tonne/ha). Because of its high water demand and the sensitivity to dry environments, kiwifruit grown in areas of high evaporative demand must be irrigated by microsprinklers in order to maximize the soil surface area that is wetted. Volumetric soil water content should remain close to field capacity at all times (never reaching values below 30 percent of the root zone water storage capacity), hence the need for frequent irrigation applications. The recommended crop coefficients for kiwi are reported in Table 1, and Figure 10 shows an example of the seasonal variation of soil water content in a Hayward kiwifruit orchard (southern Italy 40°08’ N; 16°38’ E).

In conclusion, because of the sensitivity of kiwifruit to water deficits, RDI is not feasible in this species, and full water supply to meet the crop water requirements must be ensured for sustainable kiwifruit production.

REFERENCES

Clearwater, M.J., Lowe, R.G., Hofstee, B.J., Barclay, C., Mandemaker, A.J. & Blattmann, P. 2004. Hydraulic conductance and rootstock effects in grafted vines of kiwifruit. Journal of Experimental Botany, Vol. 55, No. 401, pp. 1371-1382.

EC. 2004. European Commission Regulation (EC) No. 1673/2004 laying down the marketing standard applicable to kiwifruit. Official Journal L 300, 25/09/2004.

FAO. 2011. FAOSTAT online database, available at link http://faostat.fao.org/. Accessed on December 2011.

Linsley-Noakes, C.G. 1989. Improving flowering of kiwifruit in climatically marginal areas using hydrogen cynamide. Scientia Horticulturae 38: 247-259.

Miller, S.A., Smith, G.S., Boldingh, H.L. & Johansson, A. 1998. Effects of water stress on fruit quality attributes of kiwifruit. Annals of Botany 81: 73– 81.

Montanaro, G., Dichio, B., Xiloyannis, C., G. & Celano. 2006. Light influences transpiration and calcium accumulation in fruit of kiwifruit plants (Actinidia deliciosa var. deliciosa). Plant Science 170: 520-527.

Montanaro, G., Dichio, B. & Xiloyannis, C. 2007. Response of photosynthetic machinery of field-grown kiwifruit under Mediterranean conditions during drought and rewatering. Photosynthetica 45(4): 533-540.

Reid, J. B., Brash, D. W., Sorensen, I. B. & Bycroft, B. 1996. Improvement in kiwifruit storage life caused by withholding early-season irrigation. New Zeal. Journal of Crop and Horticultural Science 24: 21—28.

Testolin, R. & Ferguson, A.R. 2009. Kiwifruit (Actinidia spp.) production and marketing in Italy. New Zealand Journal of Crop and Horticultural Science 37: 1–32.

Wall, C., Dozier, W., Ebel, R.C., Wilkins, B., Woods, F. & Foshee, W.III. 2008. Vegetative and floral chilling requirements of four new kiwi cultivars of Actinidia chinensis and A. deliciosa. Horticultural Science 43: 644-647.

Xiloyannis, C., Massai, R., Piccotino, D., Baroni, G. & Bovo, M. 1993. Method and technique of irrigation in relation to root system characteristics in fruit growing. Acta Horticulturae 335:505-510.

Xiloyannis, C., Dichio, B., Montanaro, G., Biasi, R. & Nuzzo, V. 1999. Water use efficiency of pergola-trained kiwifruit plants. Acta Horticulturae 498: 151-158.

Xiloyannis, C., Dichio B., Montanaro, G., Lang A., Celano, G. & Mazzeo, M. 2008. Fruit morphological and physiological traits influence calcium transport and accumulation in kiwifruit. Acta Horticulturae, 767:369-378

5. Epilogue

This publication, Irrigation and Drainage Paper No. 66, follows the tradition of FAO in producing papers addressing pressing land and water issues and providing guidelines to improve agricultural resources management.

With this new I&D paper, FAO is providing an enhanced set of tools, methods and knowledge, for the analysis and assessment of crop production in relation to climate, to water supply and shortage, to advance management strategies for improving crop productivity and water saving. The targeted potential users are agricultural practitioners, including farmer associations, irrigation districts, extension services, consulting engineers, governmental and non-governmental agencies, research scientists, agricultural and natural resource economists.

In the face of the increased threats of water scarcity worldwide, this publication comes at a time of great demand for maximizing the efficiency and productive water use. There is a need to sustainably intensify agricultural production in the face of incessant acceleration of competition for finite water resources, along with the uncertainties arising from climate change.

The tools and methods presented are effective as long as the user is fully aware of the strengths and limitations of their applications. The user is urged to carefully read the various Chapters, so not to fall into simplistic and inappropriate assessments.

Chapter 2 discusses the original FAO water production function method (I&D No. 33) of assessing yield response to water. This empirical approach is mostly suitable when a quick and first approximation of yield reduction related to water limitation is needed. This method has found many applications, particularly in various interdisciplinary studies at regional or even national scales, where generalized crop conditions prevaile and rapid assessments of yield reductions under limited water supply are required. Caution is necessary as the simplification introduced limits its applicability for accurate estimates of yield responses to water.

The use of AquaCrop (Chapter 3) is confined to herbaceous crops only, but with a wide spectrum of applications from the plot to field level and can be scaled up to watershed or regional level through aggregation. It can be used for yield gap assessment, to benchmark irrigation performance, and can assist in making informed water-related operational management decisions ranging from tactical to strategic. It can be used to optimize the cropping system in terms of crop and cultivar choice, planting time, and irrigation schedule for a given soil-climate combination where water is limiting. AquaCrop is also particularly suitable for the analysis of the impact of climate change scenarios on crop productivity, water requirements and consumption. Caution is required in the use of AquaCrop as its performance mostly depends on the accuracy of the input information and how well the crop has been calibrated. A well-calibrated crop and accurate local parameters, particularly the weather data, soil water characteristics, and phenology of the cultivar, are prerequisite for high-quality simulation results. Therefore the users are recommended to not rely solely on the crop file for the parameters of a particular crop, but check the AquaCrop web-page (www.fao.org/nr/water/aquacrop.html) for update of the crop parameters, and pay special attention to rough estimations or approximations involved in determining the local parameters. Questions and request for assistance regarding the model should be directed to the AquaCrop help-desk at its dedicated e-mail address (AquaCrop@fao.org).

The yield response to water of tree crops and vines is tackled through Guidelines for Trees and Vines (Chapter 4), as the current level of knowledge and the complexity of perennial crops prevented the development of a simulation model such as AquaCrop. Irrigation management of fruit and vine production must be based on accurate estimates of crop water requirements and on the crop-specific responses to water deficits, with emphasis on fruit quality. General concepts and applications are discussed first, followed by sections on specific crops where yield responses to water supply are generalized in the form of production functions. For many of these perennial crop plants, there are trade-offs between fruit quality and yield, as the maximum net economic return is achieved at irrigation levels below those needed for maximum yield. One outstanding example of this response is described in the section on grapes for wine production. Given such trade-offs and the increased water scarcity of many world areas, guidelines on the use of regulated deficit irrigation are included in every chapter, under certain assumptions on the restriction levels of water supply. The focus is on providing guidelines to optimize the use of a limited water supply, taking into account the sensitivity of certain growth stages to water deficits.

The perennial crops tackled in Chapter 4 are mostly grown in the temperate zones because insufficient information prevented inclusion of some important tropical tree crops. Given the growing demand for fruit, this subject matter is now receiving increased attention in research programmes. Guidelines for additional perennial crops will be introduced in future versions of this publication. The synthesis responses of fruit trees and vines to water presented here offers guidelines not only for improving the efficiency of water use in plantations but, also, for dealing with situations of water scarcity, which are a major threat to the viability and sustainability of fruit production in many areas of the world.

Included in this new publication is a CD containing a copy of each of the following: (i) this whole publication (I&D Paper No. 66), (ii) the original FAO water production function (I&D Paper No. 33), (iii) the FAO guideline on crop evapotranspiration (I&D Paper No. 56), and (iv) a document listing potentially useful free-ware software and the internet link.

As next steps, the maintenance and development of the model AquaCrop has three aspects. (1) One is the continuous improvement of the model and refinement of the parameters for the crops already calibrated. It is anticipated that a new version of the model, prompted by feedbacks from users and testing with more extensive experimental data sets, may be released on the AquaCrop web-page as and when warranted. Regarding the crop parameters, as indicated on the AquaCrop website, the thoroughness of the parameterization varies from crop-to-crop. As more experimental data becomes available for calibration and testing, refinement in the parameter values are expected, and they will be posted on the website.(2) Additional herbaceous crops are and will continue to be calibrated. These may include common bean, millet, cassava, sweet potato, chickpea, forage crops and leafy vegetables. Some of these are among the so-called locally-important crops, to which particular attention will be paid. New crop sections, as well as the calibrated crop files with the calibrated parameters, will be provided on the AquaCrop web-page as they are finalized. (3) As part of the ongoing development of the software, special versions will be made available for use with various operating systems (e.g., Linux, Unix, etc.) and platforms for spatial analysis (e.g. GIS, DSS, etc.). For update, the user is recommended to visit the AquaCrop web-page periodically.

Special attention will also be given to the community of AquaCrop users and data and information providers. An AquaCrop Network is established with the purpose of sharing experience and scaling-up significant results. Particularly relevant is the scaling-up and propagation of ‘training’ in the use and application of AquaCrop and of the Guidelines for trees and vines, as well as of other related tools. Face-to-face workshops, training-of-trainers approaches, distance-and e-learning methods and other capacity development activities are being developed to respond to this need.

The overall goal is to reach out and engage a large community of users and potential users, and researchers with good experimental data, to enhance and expand the utilization of these tools, methods, and knowledge, and to improve them based on community input and feedback. This will ultimately lead to an augmented capacity of the users to sustainably manage water and crops while enhancing their productivity.

FAO IRRIGATION AND DRAINAGE PAPERS

1

Irrigation practice and water management, 1972 (Ar* E* F* S*)

1 Rev.1

Irrigation practice and water management, 1984 (E)

2

Irrigation canal lining, 1971 (New edition, 1977, available in E, F and S in the FAO Land and Water Development Series, No. 1)

3

Design criteria for basin irrigation systems, 1971 (E*)

4

Village irrigation programmes - a new approach in water economy, 1971 (F E*)

5

Automated irrigation, 1971 (E* F* S*)

6

Drainage of heavy soils, 1971 (E* F* S*)

7

Salinity seminar, Baghdad, 1971 (F E*)

8

Water and the environment, 1971 (E* F* S*)

9

Drainage materials, 1972 (E* F* S*)

10

Integrated farm water management, 1971 (E* F* S*)

11

Planning methodology seminar, Bucharest, 1972 (E* F*)

12

Farm water management seminar, Manila, 1972 (E*)

13

Water use seminar, Damascus, 1972 (F* E*)

14

Trickle irrigation, 1973 (E* F* S*)

15

Drainage machinery, 1973 (F* E*)

16

Drainage of salty soils, 1973 (C* E* F* S*)

17

Man’s influence on the hydrological cycle, 1973 (E* F* S*)

18

Groundwater seminar, Granada, 1973 (E* F S*)

19

Mathematical models in hydrology, 1973 (E)

20/1

Water laws in Moslem countries - Vol. 1, 1973 (F* E*)

20/2

Water laws in Moslem countries - Vol. 2, 1978 (F E)

21

Groundwater models, 1973 (E)

22

Water for agriculture - index, 1973 (E/F/S*)

23

Simulation methods in water development, 1974 (E* F* S*)

24

Crop water requirements, (rev.) 1977 (C* E F S)

25

Effective rainfall, 1974 (C* E* F* S*)

26/1

Small hydraulic structures – Vol. 1, 1975 (E* F* S*)

26/2

Small hydraulic structures – Vol. 2, 1975 (E* F* S*)

27

Agro-meteorological field stations, 1976 (E* F* S*)

28

Drainage testing, 1976 (E* F* S*)

29

Water quality for agriculture, 1976 (E* F* S*)

29

Rev.1 Water quality for agriculture, 1985 (C** E* F* S*)

30

Self help wells, 1977 (E)*

31

Groundwater pollution, 1979 (C* E* S)

32

Deterministic models in hydrology, 1979 (E)*

33

Yield response to water, 1979 (C* E F S)

34

Corrosion and encrustation in water wells, 1980 (E)

35

Mechanized sprinkler irrigation, 1982 (C E* F S*)

36

Localized irrigation, 1980 (Ar C E* F S*)

37

Arid zone hydrology, 1981 (C E)

38

Drainage design factors, 1980 (Ar C E F S)

39

Lysimeters, 1982 (C E* F* S*)

40

Organization, operation and maintenance of irrigation schemes, 1982 (C E* F S*)

41

Environmental management for vector control in rice fields, 1984 (E* F* S*)

42

Consultation on irrigation in Africa, 1987 (F E)

43

Water lifting devices, 1986 (F E)

44

Design and optimization of irrigation distribution networks, 1988 (F E)

45

Guidelines for designing and evaluating surface irrigation systems, 1989 (E)

46

CROPWAT – a computer program for irrigation planning and management, 1992 (E F* S*)

47

Wastewater treatment and use in agriculture, 1992 (E)

48

The use of saline waters for crop production, 1993 (E)

49

CLIMWAT for CROPWAT, 1993 (E)

50

Le pompage éolien, 1994 (F)

51

Prospects for the drainage of clay soils, 1995 (E)

52

Reforming water resources policy, 1995 (E)

53

Environmental impact assessment of irrigation and drainage projects, 1995 (E)

54

Crues et apports, 1996 (F)

55

Control of water pollution from agriculture, 1996 (E* S)

56

Crop evapotranspiration, 2006 (E S)

57

Soil salinity assessment, 1999 (E)

58

Transfer of irrigation management services: guidelines, 1999 (E F R S)

59

Performance analysis of on-demand pressurized irrigation systems, 2000 (E)

60 Rev.1

Materials for subsurface land drainage systems, 2005 (E)

61

Agricultural drainage water management in arid and semi-arid areas, 2002 (E)

62

Guidelines and computer programs for the planning and design of land drainage systems, 2007 (E)

63

Modernizing Irrigation Management – the MASSCOTE approach. Mapping System and Services for Canal Operation Techniques, 2007 (E)

64

Manual on small earth dams. A guide to siting, design and construction, 2010 (E)

65

Guidelines on spate irrigation, 2010 (E)

66

Crop yield response to water, 2012 (E)

Ar

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Arabic

C

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Chinese

E

–

English

F

–

French

P

–

Portuguese

R

–

Russian

S

–

Spanish

Multil

–

Multilingual

*

Out of print

**

In preparation