handbook on seeds of dry-zone acacias

Cover:
Shade provided by an acacia at Kargi, Kenya.

A guide for collecting, extracting, cleaning, and storing
the seed and for treatment to promote germination of
dry-zone acacias

by
J.C. Doran, J.W. Turnbull,
D.J. Boland and B.V. Gunn

Division of Forest Research, CSIRO,
PO Box 4008, Canberra A C T 2600,
AUSTRALIA

FOREWORD

The importance of conserving and utilizing existing genetic variation is recognized as fundamental in most tree species used in large-scale industrial plantations. However little or no information is yet available on intra-specific variation in a large number of tropical species which today are receiving increased attention as providers of goods and services for rural communities.

Following the recommendations made by the FAO Panel of Experts on Forest Gene Resources and with financial support from the International Board for Plant Genetic Resources (IBPGR), FAO's Forestry Department initiated in 1979 a project on the conservation and better utilization of genetic resources of arboreal species for the improvement of rural living. Based on a list of species drawn up by the FAO Panel and in accordance with the wishes expressed by the future cooperators, priority has been initially given to a few selected species mainly in the genera Acacia and Prosopis.

The general objectives of the project are the conservation and improved utilization of genetic resources of multi-purpose arboreal species growing in arid and semi-arid areas. The means of achieving these objectives are exploration, collection of reproductive material (mainly seed) and evaluation of genetic variability and adaptability to varying environmental conditions of the species included. These activities will enable appropriate action to be taken in conservation in situ and ex situ and in planting well-adapted and otherwise suitable species and provenances in village woodlots, firewood plantations, for food and fodder, and for shade, shelter and land amelioration.

As the species included in the project have not in the past received much attention, little information and experience are available on fundamentally important aspects such as taxonomy and seed collection, handling, storage and treatment. Where such information exists, it is often scattered and difficult to obtain. Yet, the species present a number of serious problems in these specific fields: their taxonomy is often confused and a number of interbreeding species complexes are thought to exist, making proper identification difficult and predictability of performance in subsequent generations impossible; seed collection and handling are difficult because of the scattered, often remote stands in which the trees occur, irregularity of good seed years, difficulty of extraction and uncertainties on safe but efficient methods of breaking the seed dormancy. Last, but not least there are the problems caused by insects in all stages of development and storage of the seeds.

In order to remedy at least in part these identified information gaps it was decided to prepare a series of handbooks within the framework of the FAO/IBPGR project. Such handbooks, aimed at professional staff involved in actual field operations, have been prepared on taxonomy; seed collection, handling, treatment and storage; and seed insects of the two main genera, Acacia and Prosopis. The handbooks are published in English, French and Spanish to help bridge language barriers between countries and continents.

We are aware of the fact that much more work is still needed in this field: the species covered are only a small fraction of those which merit urgent attention, the topics covered in these handbooks are but a few of the numerous ones which need to be systematically tackled. However, through this and related work we hope to catalyze action elsewhere in the world, to show one of the ways of going about the problems which we all should be determined to solve: the conservation of our heritage of genetic resources and the utilization of these resources for the betterment of life particularly of rural communities dependent on these resources.

Preface

1. This handbook has been prepared to serve as a guide to operational personnel involved in the collection, handling and storage of seeds of dry-zone acacias. Particular reference is made to Acacia albida, A. aneura, A. caven, A. nilotica, A. senegal and A. tortilis, six species identified for intensive testing in the FAO/IBPGR⁽¹⁾ Forestry Project (Conservation of Genetic Resources of Arboreal Species for the Improvement of Rural Living in Arid and Semi-Arid Zones), but many of the techniques can be applied to the majority of Acacia species. An attempt has been made to provide a systematic synthesis of published information on acacias and of experience of the Division of Forest Research CSIRO in the collection and processing of seed of A. aneura and other Acacia species in Australia.

2. The handbook begins with a brief account of the habit of acacias, the state of acacia taxonomy, the origin and occurrence of the genus and some aspects of acacia ecology. This information is intended to provide an understanding of the general background of acacias which will assist those concerned principally with the more practical operations of seed collection, handling and storage. Descriptions of the morphology of pods, seeds and other organs will aid the field identification of species and the use of botanical keys.

3. The aim of Chapter 3 is to help seed collectors understand the reproductive biology of the genus so that the planning, timing and execution of seed collection operations may be undertaken efficiently. With these objectives in mind, details are given on the reproductive processes that occur during seed development such as floral development and sexuality, pollination, breeding system, hybridization and seed and fruit development. Information is provided on the phenology of flowering and fruiting, seed productivity, and predation of the pod and seed. Flowering and seed collection times together with seed crop and germination information for the six species of special interest are given in Table 1.

4. Chapter 4 has been arranged as a series of sequential steps that should be followed, or, at least considered, when collection is undertaken. A discussion of the importance of provenance is followed by an outline of the many factors that should be considered during the forward planning phase. Methods and equipment used in the collection of dry-zone acacias in Australia are described. The need for the careful labelling of seedlots at all stages of collection is stressed and factors to consider in the transport of pods and cleaned seed are discussed.

5. Chapter 5 covers the principles and methods of drying, extracting and cleaning acacia seeds. The methods described are mainly applicable to the processing of relatively small research seedlots where contamination between lots and wastage of seed often inherent in bulk processing is unacceptable. Extraction and cleaning methods which have been successfully applied in the processing of a wide range of acacia seedlots by Division of Forest Research CSIRO are described in detail.

6. Storage conditions including discussion of seed longevity, methods of treating seed before storage to kill insect pests, containers for storage, and an example of a comprehensive seed store record system are given in Chapter 6. Complete and accurate documentation of seedlots is basic to species and provenance testing, and requires special attention.

7. Most Acacia species have seed dormancy due to a seedcoat which is impervious to water. Chapter 7 describes presowing treatments which may be applied to ensure rapid and complete germination after sowing. The storage of treated seed is discussed.

8. References are given in Chapter 8 which is followed by the appendices including a glossary of terms used in the handbook.

(1) FAO: Food and Agriculture Organization of the United Nations
IBPGR: International Board for Plant Genetic Resources

Acknowledgements

The authors acknowledge with thanks the assistance of the staff of the CSIRO Tree Seed Centre, Canberra and in particular Mr. E. G. Cole and Mr. D. A. Kleinig for technical support; Mr. A. G. Brown, Mr. K. W. Cremer, Dr. K. G. Eldridge, Mr. N. Hall, Mr. B. R. Maslin and Dr. J. H. Ross for valuable comments; Mr. P. J. Hay, Ms. W. L. Parsons, Dr. J. O'Connell and Mr. B. van Aken for assistance with photographs; and the CSIRO Division of Plant Industry in providing plans of the CSIRO 15 cm flailing thresher.

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
Rome, 1983

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1. INTRODUCTION

2. HABIT, TAXONOMY, OCCURRENCE AND ECOLOGY

2.1 Habit

2.2 An introduction to Acacia taxonomy for seed collectors

2.21 Classification - past to present

2.22 Botanical keys to acacias

2.23 Morphology of some acacia plant organs

2.231 Leaves and phyllodes

2.232 Spines and prickles

2.233 Flowers

2.234 Inflorescences

2.235 Pods