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Physiological approaches to wheat breeding
M.P. Reynolds

While wheat breeding programmes worldwide have achieved significant genetic gains in yield potential without the aid of physiological se-lection tools (Rajaram and van Ginkel, 1996), breeders, as well as physiologists, generally agree that future successes will be realized through a greater integration of disciplinary research (Jackson et al., 1996). There are two principal reasons for this. Until the year 2020 at least, demand for wheat is expected to grow by approximately 1.6 percent/year worldwide and by 2 percent/year in developing countries (Rosegrant et al., 1995). This implies a need to almost double the world average wheat yields in that period, and albeit steady, recent rates of yield growth, as well as improvement in genetic yield potential (Sayre et al., 1997), are too low to keep pace with future demand. Thus, there is an urgent need to develop new and more efficient wheat breeding methodologies to complement existing breeding techniques, as well as to identify new traits, which will drive faster yield gains.

Secondly, several recent studies suggest that physiological selection traits have the potential to improve genetic yield gains in wheat. At the International Maize and Wheat Improvement Center (CIMMYT), research has demonstrated associations of a number of physiological traits, including leaf conductance and photosynthetic rate, with performance of a historic series of cultivars in a high-yielding environment (Fischer et al., 1998). In addition, work emphasizing genetic improvement under marginal environments has illustrated that physiological traits, including canopy temperature depression, when measured in hot selection environments in Mexico, were strongly associated with performance in yield trials at a number of warmer wheat-growing regions worldwide (Reynolds et al., 1994a). In addition, physiological selection traits for drought tolerance have been incorporated into a number of Australian wheat breeding programmes, including higher transpiration efficiency, greater early vigour and reduced tillering (Richards et al., 1996). Physiological selection techniques are now being evaluated for their role as complementary tools in wheat breeding at CIMMYT (Reynolds et al., 1998a).

This chapter will address three areas of importance in relation to the physiological basis of breeding superior wheat cultivars. Firstly, examples of physiological traits, which could potentially be applied to cultivar improvement, will be discussed. Secondly, some of the prerequisites for applied physiological research within a breeding context will be outlined. Thirdly, some specific examples of physiological approaches that may complement wheat breeding in the future will be presented.


Yield components

The most important step in improving genetic yield potential of wheat in favourable environments was the introduction of Rht alleles. The effect of the gene is to increase partitioning of assimilates to yield at the expense of non-grain biomass. It is interesting that progress in yield since the development of semidwarf lines has also been associated with increased partitioning (Austin et al., 1989; Sayre et al., 1997) and not, for example, improved radiation use efficiency (Slafer et al., 1996). Morphological traits associated with increased yield potential in CIMMYT wheat between 1962 and 1988 include grain number and harvest index (HI) (Sayre et al., 1997). While grain number can be used as a guide to visual selection, HI is less readily evaluated with the eye, and neither trait is reliably expressed in small plots or at low density in early generations. In addition, there is a theoretical limit to HI, estimated at 60 percent (Austin et al., 1980a), which would imply that unless biomass is raised, yields can increase by 20 percent at the most, using HI as a selection criterion.

Steady genetic gains in yield potential can be expected from recombining elite germ-plasm (Rasmusson, 1996) and refinement of selection methodologies. However, significant jumps in yield potential will almost certainly require introgression of genetically diverse sources (Kronstad, 1996) to permit evaluation of new yield determining genes in different backgrounds. At CIMMYT, one approach used is to cross parents with high expression for specific morphological traits, including large spikes, large grain size and large semi-erect leaves, based on the conceptual idea of improving both source (photosynthetic capacity) and sinks (grain number) simultaneously.

Physiological traits

A recent study conducted in a high-yielding environment in Mexico revealed that leaf photosynthetic rate, leaf conductance and canopy temperature depression (CTD) were all associated with yield progress in a set of eight spring bread wheat lines, representing progress in yield potential between 1962 and 1988 (Fischer et al., 1998). One important implication of this work is that such traits can be measured reasonably simply in the field, suggesting a potential methodology for screening physiologically superior lines. The idea is supported by studies in the same environment, where homozygous sisters from crosses between high- and low-CTD parents showed good association between yield and CTD (Reynolds et al., 1998b).

The physiological basis of the association of CTD and yield is unknown. However, since CTD is a direct function of evapotran-spiration rate, which itself is determined by a number of physiological and metabolic processes including stomatal conductance, photosynthetic rate, vascular capacity, etc., there are a number of alternate hypotheses. For example, high CTD may be indicative of a high demand for photo-assimilation caused by many, rapidly filling kernels (i.e. sink strength) in physiologically well-adapted lines. Alternate hypotheses are: (i) high CTD reflects an intrinsically higher metabolic capacity; (ii) high CTD is indicative of a good vascular system capable of meeting evaporative demand; and (iii) high CTD reflects a less conservative response to reduced soil water potential between irrigations. A precise understanding of the physiological basis of the association of CTD with yield will improve the likelihood of genetically improving yield potential. Canopy temperature depression, like yield, is a genetically complex trait, so selection for CTD directly is likely to be a slower approach to raising yields than selecting for the genes specifically related to current yield thresholds. Nonetheless, CTD offers the potential to discard genetically inferior lines during plant selection, adding efficiency to the breeding process. This possibility will be expanded on in section "Using canopy temperature depression to increase selection efficiency".

Additional physiological traits that may have implications on yield potential are translocation from the stems to the grain of soluble carbohydrates (stem reserves) and the ability to maintain green leaf area duration (stay-green) throughout grainfilling (Jenner and Rathjen, 1975). Both traits would be more important where a crop was assimilate limited, and physiological studies have indicated that higher yielding lines depend less on stem reserves than lower yielding ones (Stoy, 1965; Austin et al., 1980b). It has also been suggested that the two traits may be mutually exclusive, since loss of chlorophyll and stem reserve mobilization seem to be consequences of plant senescence (Blum, 1998).

Another area that has yet to be explored with respect to raising yield potential is the optimization of phasic development. The relative length of the cardinal phenological stages is a function of the interaction of environmental cues with genes determining earliness per se and sensitivity to photoperiod (Ppd) and vernalization (Vrn). The reproductive stage of development is pivotal in determining yield potential, and genetic variability for its duration relative to other phenological stages is known (Slafer and Rawson, 1994). The possibilities of manipulating this trait to improve yields will be discussed later.

Canopy-based traits

The erectophile leaf canopy has been proposed as a trait that could increase crop yield potential by improving light use efficiency in high-radiation environments. While some studies support the hypothesis, for instance in barley (Angus et al., 1972), others are less clear cut. For example, work at CIMMYT with near isogenic lines of spring wheat showed the erect leaf trait to be associated with higher grain number and increased rate of transpiration based on measurements of CTD, carbon isotope discrimination and relative water content of flag leaves, but it was not associated with yield itself (Araus et al., 1993). Based on this hypothesis, a large number of accessions from germplasm collections were screened for erect leaves at CIMMYT in the early 1970s. The trait was introgressed into the wheat germplasm base, and it is present in some of CIMMYT’s best yielding durum and bread wheat lines (Fischer, 1996).

The idea that higher yield potential could be achieved by designing a plant type that is well adapted to the commercial practice of sowing high-density monocultures was introduced 30 years ago by Donald (1968). He used the word ‘communal’ to describe the ideotype. In a more recent study, yield progress in CIM-MYT lines seemed to be associated with the communal trait, defined as the relative lack of yield response of higher yielding lines to a reduction in interplant competition; in contrast to lower yielding lines that responded considerably to removal of neighbouring plants after flag leaf emergence (Reynolds et al., 1994b). Such observations have important implications to plant breeding methodologies where individual plant selection, or even mass selection, is used on segregating generations and bulks. Competition among genotypes is likely to reduce the gene frequency of the communal trait, especially if visual selection favours more competitive plant types. Several studies have shown that selection for yield potential in early generations can be enhanced by reducing interplant competition between genotypes in bread wheat (Lungu et al., 1987), durum wheat (Mitchell et al., 1982), oat (Robertson and Frey, 1987) and rye (Kyriakou and Fasoulas, 1985). Studies have not yet shown a physiological basis for the communal trait but they seem to suggest that it can be selected for empirically. While wider spacing between plants in early generations would increase breeding costs, avoiding selection bias based on plant type in early generations may be a useful compromise. The relative success of single seed descent methodology in European winter wheat breeding would appear to back these conclusions.

Stressed environments

Two of the most important stresses of wheat are heat and drought. Wheat yields can be severely reduced in moisture-stressed environments (Morris et al., 1991), which affect at least 15 million ha of spring wheat alone in the developing world. Over 7 million ha of spring wheat are grown under continual heat stress, namely environments with mean daily temperatures of greater than 17.5°C in the coolest month (Fischer and Byerlee, 1991). In addition, terminal heat stress can be a problem in up to 40 percent of the irrigated wheat-growing areas in the developing world.

Nonetheless, wheat has been traditionally cultivated in many stressed environments, and it is not surprising that the crop is relatively stress tolerant. Wheat’s drought hardiness is apparent from the linear relationships observed between grain yield and water application when measured under moisture stress in field experiments (for example, Sayre et al., 1995), or the simple observation of a plant under severe stress, which will complete its life cycle yielding perhaps only a single viable kernel. At high temperatures, the rate of plant development is increased (Midmore et al., 1984), thus reducing the potential for biomass accumulation. Nonetheless, extensive testing of 16 spring wheat cultivars throughout the heat-stressed regions of the developing world by CIMMYT and the national programme collaborators have shown that warmer environments reduce intrinsic growth rates, as well as the length of the growth cycle, and that there is significant genetic variation in heat tolerance of modern semidwarf wheats (Reynolds et al., 1998a).

Drought-adaptive traits

As one might expect, root characteristics, such as depth and abundance, are known to be associated with performance under drought in many studies with wheat (Hurd, 1968; see also Blum, 1988). Nonetheless, decreased investment in roots in the top 30 cm of the soil has been shown to be stress adaptive, when stress occurs before flowering, and is apparently associated with a strategy that conserves stored soil moisture (Richards, 1991). It is interesting to note that no study has shown a clear effect of dwarfing genes on drought adaptation or rooting patterns, despite the fact that specific height categories may be advantageous over others under certain water-stressed environments (Richards, 1992).

Traits associated with drought tolerance that are easily evaluated with the eye include rapid early ground cover by leaves, leaf glaucousness, leaf pubescence (Richards, 1996a) and erect leaf posture (Innes and Blackwell, 1983). All are associated with conserving available moisture by reducing radiation load to the leaves, or at the soil surface in the case of early ground cover. More difficult to measure, but with apparent value under drought, are abscisic acid (ABA) accumulation (Innes et al., 1984) and spike photosynthesis, which can provide over 70 percent of the assimilates for grainfilling under drought (Evans et al., 1972). Despite the difficulties of measuring spike photosynthesis, it is the awns with their very high wateruse efficiency relative to leaves or glumes that are the major contributors to spike photosynthesis under stress, and these are selected for readily with the eye.

Early escape from progressively intensifying moisture stress, through the manipulation of plant phenology, is a commonly exploited genetic strategy to ensure relatively stable yields under terminal drought conditions (for example, Richards, 1991). In order to exploit a longer growth cycle, adaptive strategies must be employed that enable physiological rather than temporal escape from moisture stress. Probably the best documented is the maintenance of leaf turgor through osmotic adjustment (OA). The benefit of OA was demonstrated by Morgan and Condon (1986) using the progeny of high by low OA crosses. In random F4 - derived sibs grown under drought, OA was shown to be associated on the one hand with yields of field plots and on the other, their increased water use, which in turn was directly related to root function through improved water extraction between 25 cm and 150 cm in the soil profile.

While OA is measured using a laboratory protocol, some of its beneficial effects can be assessed using relatively easy-to-measure traits, such as leaf rolling, which is scored visually, canopy temperature using an infrared (IR) thermometer, or stomatal conductance. In addition, there are techniques such as spectral reflectance (Araus, 1996), which can be used to estimate a range of physiological characteristics, including plant water status and leaf area index. The technique is based on the principal that certain crop characteristics are associated with the absorption of very specific wavelengths of electromagnetic radiation (e.g. water absorbs energy at 970 nm). Solar radiation reflected by the crop is measured and calibrated against light reflected from a white surface. Different coefficients can be calculated from specific bands of the crop’s absorption spectrum, giving a semi-quantitative estimate (or index) of a number of crop characteristics.

Other techniques are available that can integrate physiological processes over the whole or part of the crop cycle. For example, water-use efficiency (WUE) can be estimated using carbon isotope discrimination. The methodology is based on higher affinity of the carbon-fixing enzyme (Rubisco) for the more common 12C isotope over the less common 13C. As the internal [CO2] falls in the leaf, the 12C:13C ratio falls permitting less discrimination in favour of 12C. Lower internal [CO2] is normally associated with reduced stomatal conductance, which would increase WUE, assuming CO2 fixation is not primarily limited by other factors (e.g. thermal deactivation of photosynthesis or other metabolic processes). A lower discrimination value would be associated with higher WUE. While the trait appears to be fairly heritable, its precise association with yield under drought is yet to be fully characterized (Richards, 1996a). A probable and cheaper alternative to carbon isotope discrimination is ash analysis (Araus, 1996), based on the principal that relative ash accumulation in leaf tissue is related to evapotranspiration rate and inversely related to WUE. Relative ash content is measured after complete combustion of tissue.

One drought-adaptive trait that relates specifically to improved partitioning, though not to reproductive growth, is translocation of soluble stem carbohydrates to the grain. While time consuming to measure directly, the trait can be measured indirectly by artificially inducing some of the physiological problems attendant to drought stress through chemical desiccation of green tissue (Blum et al., 1983). Remobilization of stem reserves is associated with increased levels of ABA, which presumably is involved in the triggering of enzymes prerequisite to remobilization.

Heat-adaptive traits

Studies in controlled environments have shown genetic variability in photosynthetic rate among wheat cultivars when exposed to high temperatures (Wardlaw et al., 1980; Blum, 1986). Such differences in photosynthesis under heat stress have been shown to be associated with a loss of chlorophyll and a change in the a:b chlorophyll ratio due to premature leaf senescence (Al-Khatib and Paulsen, 1984; Harding et al., 1990). Studies at CIMMYT demonstrated genetic variability for photosynthetic rate under heat-stressed field conditions (Delgado et al., 1994). In addition, both CTD and flag leaf stomatal conductance, as well as photosynthetic rate, were all highly correlated with field performance at a number of international locations (Reynolds et al., 1994a).

Assimilates are more likely to be yield limiting under stress than in temperate environments, especially as stress typically intensifies during grainfilling. Evidence for this comes from the observation that under stress, total above-ground biomass will typically show a stronger association with yield than partitioning, i.e. harvest index (for example, Reynolds et al., 1994a), while the situation is reversed under temperate conditions (for example, Sayre et al., 1997). For these reasons, stay-green is a trait that has been promoted for heat and drought tolerance. However, as mentioned earlier, evidence suggests that the trait may in fact be a disadvantage under heat stress due to it being associated with the tendency not to translocate stem reserves to the grain (Blum, 1998).

In a number of studies, conductometric measurement of solute leakage from cells was used as a methodology to estimate heat damage to the plasma membranes. Genetic variation in membrane thermostability has been inferred using conductometric measurements in various field-grown crops including spring wheat (Blum and Ebercon, 1981). Shanahan et al. (1990) obtained a significant increase in yield of spring wheat in hot locations by selection of membrane-thermostable lines, as determined by measurements on flag leaves at anthesis. Applying the membrane thermostability test on winter wheat seedlings, Saadalla et al. (1990) found a high correlation in membrane thermostability between seedlings and flag leaves at anthesis for genotypes under controlled environmental conditions. Measurements of membrane thermostability (MT) of 16 spring wheat cultivars were compared with performance at several heat-stressed locations. Variation in MT of both field, heat-acclimated flag leaves, as well as seedlings grown in controlled conditions, were associated with heat tolerance in warm wheat-growing regions (Reynolds et al., 1994a).

The physiological basis for the association of MT with heat tolerance is unknown, and in fact plasma membranes are known to be more heat tolerant than is photosynthesis for example (Berry and Bjorkman, 1980). While loss of membrane integrity is a possibility, the phenomenon of ion leakage from the cell could also be caused by thermal-induced inhibition of membrane bound enzymes, which are responsible for maintaining chemical gradients in the cell. Direct evidence for a biochemical limitation to heat tolerance in wheat comes from studies of the enzymes involved in grainfilling, specifically soluble starch synthase, which is deactivated at high temperatures (Keeling et al., 1994). If conversion of sucrose to starch is a limitation to yield under heat stress, this would explain the observation of increased levels of carbohydrates in vegetative tissue of wheat when grainfilling was limited by heat stress (Spiertz, 1978).

There are a number of other processes that are clearly affected by high temperatures, but that are not discussed in depth here since they do not lend themselves to simple screening. Respiration costs are higher with increasing temperature leading eventually to carbon starvation because assimilation cannot keep pace with respiratory losses (Levitt, 1980). However, this apparently wasteful process would seem unavoidable, at least in current germplasm, as evidenced by positive associations observed between dark respiration at high temperature and heat tolerance of sorghum lines (Gerik and Eastin, 1985) and in wheat (Reynolds et al., 1998a). Heat shock proteins are synthesized at very high rates under high-temperature stress and are thought to have a protective role under stress; nonetheless their role in determining genetic differences in heat tolerance is not established. Another trait that may have more promise as a screening trait is chlorophyll fluorescence; associations between heat tolerance and lower fluorescence signals have been reported in a number of crops including wheat (Moffat et al., 1990), though screening protocols are yet to be evaluated.

While a definitive picture of the physiological basis of reduced growth rates under heat stress is still lacking, many of the drought-adaptive traits discussed above are likely to be useful under heat stress. Examples would include leaf glaucousness to reduce the heat load, awn photosynthesis when high temperatures reduce assimilation rate of the leaves and early escape from heat stress. Heat stress is almost certainly a component of drought stress since one of the principal effects of drought is to reduce evaporative cooling from the plant surface. Nonetheless, not all traits conferring heat tolerance are also associated with genetic variability in drought tolerance, a good example being membrane thermostability (Blum, 1988). In addition, wheat germplasm that typically performs well under heat stress is not necessarily useful under drought (S. Rajaram, personal communication).

When considering deployment of selection traits it may be useful to divide them, some-what arbitrarily, into two categories: (i) simple traits associated with a particular morpho-physiological attribute such as root depth or leaf waxiness; and (ii) integrative traits, the net effect of a number of simpler traits, an example being canopy temperature. Being a function of several simpler traits, integrative traits are potentially powerful selection criteria for evaluating breeding progeny, while the simpler traits might be considered when choosing possible parental characteristics. Clearly the heritability of traits, as well as the ease with which they can be measured would modify any such rule of thumb.


Criteria for initiating research on physiological traits

When considering the adoption of physiological traits into a breeding programme, it is necessary to establish the degree of genetic variability that exists for the trait(s) of interest. However, prior to investment in trait measurement, consideration should also be given to whether selecting for specific physiological traits will improve overall efficiency of the breeding programme. While this cannot be predicted with certainty, from an economic point of view, the use of physiological criteria as part of an integrated approach to breeding must achieve results more quickly or efficiently than performance-based selection of parents or progeny.

In order to chose the best selection traits for a given breeding objective, different selection criteria can be tested empirically, based on knowledge of the physiological constraints to yield. For example, in drought environments, a number of mechanisms may be useful based on current knowledge, i.e.: (i) osmotic adjustment; (ii) accumulation and remobilization of stem reserves; (iii) superior spike photosynthesis; (iv) heat and desiccation tolerant enzymes; (v) anatomical adaptations to conserve moisture, such as leaf rolling, waxiness, etc.; and (vi) deep roots. The relatively slow progress made in breeding for moisture-stressed environments, in comparison to irrigated environments, is usually ascribed to the heterogeneity of the selection nurseries in these conditions, rendering performance-based selection unreliable. Selection for specific traits is likely to be more effective. In addition, where more than one physiological trait is involved, deliberate selection with the view to combining synergistic traits is likely to achieve results in a shorter time frame than adopting a more empirical strategy.

While the identification of stress-adaptive physiological mechanisms may be time consuming and costly, once the initial investment is made, the information is permanently available. The information can be used at different stages of the breeding process, depending on the degree of resources available. In a relatively low-investment scenario, information on important physiological traits can be collected on potential parental lines. For example, it might be worth screening an entire crossing block, or a subset of commonly used parents, to produce a catalogue of useful physiological traits. The information can be used strategically in designing crosses, thereby increasing the likelihood of transgressive segregation events, which bring together desirable traits. In a scenario where more resources are available to screen for physiological traits, the same selection criteria could be applied to segregating generations in yield trials, or any intermediate stage, depending on where genetic gains from selection are optimal.

Experimental evaluation of physiological selection traits

Before designing a research programme, it is important to define the physical and agricultural characteristics of the target environment. This information is necessary to: (i) facilitate experimental design through identification of appropriate research sites (i.e. temperature profile, daily radiation, rainfall, latitude, soil type, etc.) and experimental treatments (i.e. sowing dates, crop management, etc.); and (ii) ensure that results are reasonably representative of the target environment as a whole and not just the site where the research was conducted. In many instances, it will be advisable to replicate trials across a number of locations within the target environment.

Careful choice of appropriate germplasm for physiological research is extremely important since the conclusions and recommendations generated by the study depend on it being representative of current breeding objectives. Breeders should be involved in identifying suitable materials from crossing blocks and among their advanced material. Similarly, material from germplasm collections may provide useful genetic diversity, especially where lines originate in the same target environment, or one where the constraints to yield are similar. Ideally, germplasm should have a number of common characteristics that will facilitate experimental work and interpretation of the results. Ideal characteristics for germplasm used in physiological breeding experiments are (Reynolds et al., 2001a):

Differences in height, maturity, adaptation, disease susceptibility, etc. are all potentially confounding factors to the trait under study, and variation in these increase experimental error.

One very important additional point that should be checked is the current status of germplasm coming out of ongoing selection programmes with respect to the trait of interest. If that material already shows high expression for the trait of interest with no significant genetic diversity, then current methodologies are already effective in making genetic gains for the trait. Work in developing physiological selection methodologies would only be worthwhile if they are likely to prove to be more efficient than existing approaches. Otherwise, the only further role for physiological measurements may be to identify new and better genetic sources of the trait.

To decide on how many lines to include in a study, a good strategy is to start with a broad range of genetic diversity for the trait. This could run into many hundreds of lines depending on the complexity of the trait. Once diversity has been established from preliminary observations, numbers can be reduced drastically, including perhaps the best 20 to 30 lines that encompass the full range of genetic diversity, so that detailed observations can be made in subsequent cycles.

The efficiency of physiological trait selection will be related to how well a trait is expressed and measured. Experimentation must take place to establish how and when measurements should be made to maximize genetic resolution of trait expression. There are three groups of factors that can potentially affect the expression of a trait: (i) macro-environment, i.e. temperature, radiation, irrigation status, nutritional status and soil type; (ii) micro-environment, i.e. small daily fluctuations in temperature and radiation, etc., as well as small environmental differences among plots or between plants caused, for example, by soil heterogeneity, weeds, pests, etc.; and (iii) physiological, i.e. age of plant or its organs, diurnal rhythms of plants, small amounts of genetic diversity that may exist within socalled fixed lines. A trait such as leaf chlorophyll, for example, is relatively simple in its expression in that it is not affected by diurnal changes. However, its expression may vary, for instance, under different nutritional regimes. Chlorophyll will also be a function of leaf age, and some standardization in measurement will be necessary to take this into account. On the other hand, traits such as leaf conductance or canopy temperature depression are strongly affected by temperature and relative humidity and have a diurnal function. Studies at CIMMYT have shown that CTD is best expressed on warm sunny, cloudless afternoons, with well-watered plots (Amani et al., 1996). In addition, the trait is affected by phenology, and while pre-heading readings are usually higher, readings made during grain-filling are best associated with yield potential (Reynolds et al., 1998a).

Once experimental protocols have been refined and data have been collected in at least two or three environments (these may be different representative sites and/or years), data must be assessed for two things: (i) significant and consistent expression of the trait of interest; and (ii) an association of the trait with performance among genotypes. Assuming statistical significance, the results should be checked for biological and economic significance by graphing the association between the trait and performance. Multiple regression techniques may also be used to examine the influence of a number of traits on genotype performance.

Interpretation of data from unrelated fixed lines is highly speculative, since the association between traits and performance may be confounded by other genetic factors, such as differences in phenology, plant type, etc. For this reason, assuming the above criteria are met, it is necessary to enter a second phase of experimentation aimed at demonstrating a definitive genetic linkage between the trait and performance among homozygous progeny. Genetic gains resulting from selection and measured as improved performance can then be estimated. After that, other issues can be addressed, such as in which generation to apply selection and what breeding methodologies would be most effective.

Incorporating physiological criteria into a selection programme

Where a trait shows a strong association with performance in unrelated fixed lines, as well as in homozygous sister lines, it is probably worth applying selection pressure for the trait in preliminary trials. However, the nature of the association should also be examined, i.e. the distribution of the values of the trait in relation to the performance of lines. Where the distribution is clustered towards the positive end of performance, selection for the trait may only be effective in eliminating the poorest material. If values of the trait are clustered towards the negative end of performance, selection for superior performance is more likely.

If a trait is heritable, it is more efficient from a breeding point of view to select lines as early as possible in the breeding process. In addition, if shuttle breeding is being exploited, deployment of generations should permit the trait to be evaluated in the environment most suitable for its expression. In either case, it is necessary to establish the realized heritability of the trait at that generation so that genetic gains can be evaluated. This can be estimated by measuring trait expression in a population of lines or bulks and, using an arbitrary selection intensity, dividing the population into high and low groups. These are advanced one generation, and the trait measured again, heritability being a function of the consistency of the difference between high and low groups in the subsequent generation.

Where a trait shows high heritability and a good association with performance, it might lend itself to early generation selection (EGS), instead of or in addition to selection in yield trials (Table 7.1). Selection pressure might be applied in F3 plants or F3:4 plots, etc. depending on the sensitivity of trait expression to planting method. Even for a trait that is relatively weakly associated with performance, but highly heritable, early generation selection may be a useful tool for eliminating the poorest material (Table 7.1). In summary, the advantages of EGS over later generation selection are: (i) resources may be saved by eliminating physiologically inferior material from the programme; and (ii) the likelihood f losing favourable genetic diversity is decreased. The potential disadvantages are: (i) without close collaboration between disciplines, time may be wasted measuring traits on, or even promoting, agronomically unsuitable material; and (ii) in early generations, large numbers of plants must be tested, and some currently available physiological tools are either too expensive or cannot be applied quickly enough.

Criteria for applying physiological traits in a breeding programme

Trait heritability

Association of trait with performance




Selection in yield trials

No application


Early and/or late generation selection

Negative selection in early generations

Source: Reynolds et al., 2001a.


Using canopy temperature depression to increase selection efficiency

As discussed earlier, experimental data have shown a clear association of CTD with yield in both warm and temperate environments. Since an integrated CTD value can be measured almost instantaneously on scores of plants in a small breeding plot, thus reducing error normally associated with traits measured on individual plants, work has been conducted to evaluate its potential as an indirect selection criterion for genetic gains in yield. While CTD is affected by many physiological factors making it a powerful integrative trait, its use may be limited by its sensitivity to environmental factors (Figure 7.1).

Factors affecting expression of CTD

Leaf temperatures are depressed below air temperature when water evaporates from their surface, and one of the factors determining evapotranspiration is stomatal conductance, which itself is regulated by the rate of carbon fixation. To maintain high rates of photosynthesis, a genotype must have an effective vascular system for transpiration of water, as well as for transport of nutrients and assimilates. Since CTD is directly or indirectly affected by a number of physiological processes, it is a good indicator of a genotype’s fitness in a given environment. Canopy temperature depression also seems to be affected by the ability of a genotype to partition assimilates to yield, indicated by the fact that CTD frequently shows a better association with yield and grain number than it does with total above-ground biomass (Table 7.2).

Figure 7.1
Factors affecting canopy temperature depression (CTD) in plants

Source: Reynolds et al., 2001b.

For a given genotype, CTD is a function of a number of environmental factors (Figure 7.1), principally soil water status, air temperature, relative humidity and incident radiation. The trait is best expressed at high vapour pressure deficit (Amani et al., 1996), conditions associated with low relative humidity (RH) and warm air temperature. For these reasons, CTD will not be a useful selection trait in generally cool and/or humid conditions, and it is quite sensitive to environmental fluxes. Therefore, it is important to measure the trait when it is best expressed, that is on warm, relatively still and cloudless days. Some environmental flux during the period of measurement is inevitable, but correcting data against reference plots, spatial designs, use of replication and repetition of data collection during the crop cycle can compensate for this.

Association of CTD with performance

Measurements of CTD made on 16 wheat lines at CIMMYT’s subtropical experiment station (Tlaltizapan, Mexico) were compared with performance of the same lines at a number of hot wheat-growing regions internationally (Figure 7.2). In some cases, CTD was associated with over 50 percent of the variability in yield of the same lines at sites in Brazil, Sudan, India and Egypt (Table 7.3), giving a clear indication that CTD was a potentially promising indirect selection criterion for yield. In subsequent work, crosses were made between parents contrasting in CTD to generate homozygous sister lines. These were evaluated for both CTD and yield in warm and temperate environments. Populations of random derived F5 sister lines from two crosses indicated a clear association of CTD with yield potential in both warm (Figure 7.3) and temperate environments (Table 7.4), with CTD explaining up to 50 percent of variation in yield.

Association of CTD with traits of 60 advanced lines, March sown, Ciudad Obregon, Mexico, 1995


Correlation coefficient with CTDa





Harvest index


Kernel weight








Days to maturity


Days to flowering




aSignificant at P£ 0.05 represented by * and significant at P £ 0.01 represented by **.

Source: Reynolds et at., 1998b.

The relationship of mean grain yield CTD for 23 genotypes, average over two sowings, Tlaltizapan, Mexico, 1992/93.

Source: Amani et al., 1996.

While heritability of CTD has not been thoroughly evaluated, preliminary data indicate moderate values of heritability for the trait. When comparing traits measured on F2 (five bulks) with subsequent yields in F5 (seven lines), performance was better predicted by CTD than it was by yield when both were measured on bulks (Reynolds et al., 1997).

Combining selection for CTD and leaf conductance

Since CTD and leaf conductance show an association with each other and with yield (Amani et al., 1996), the possibility of combining selection for both traits is an attractive proposition. At CIMMYT, work is in progress to evaluate the application of this strategy for yield potential. The objective is to evaluate families of F3 (four lines) for CTD. Having established which lines show high expression for CTD, individual plants can be assessed for leaf conductance using a viscous flow porometer being newly marketed (Thermoline & CSIRO, Australia). This instrument can give a relative measure of stomatal conductance in a few seconds (Rebetzke et al., 1996), permitting the possibility of identifying physiologically superior genotypes from within bulks that have already been selected for CTD and other important selection criteria (Figure 7.4). Supporting evidence of the utility of this approach has already been collected under warm conditions in Mexico (Gutiérrez-Rodriguez et al., 2000).

Correlation coefficients between yield, averaged over two cycles (1990-1992) at five locations of the IHSGEa, and CTD of 16 wheat lines measured at different stages of development with December and February sowings, Tlaltizapan, Mexico, 1992-1993


Correlation coefficient of CTD with yieldb

CTD December

CTD February



































Tlaltizapan, Mexico







Average correlation







aIHSGE = International Heat Stress Genotype Experiment.
bSignificant at P £ 0.05 represented by * and significant at P £ 0.01 represented by **.
Source: Reynolds et al., 1994a.

Regression of yield on CTD measured after heading for 40 recombinant inbrided lines from a cross between lines contrasting in heat tolerance (Seri 82/Siete Cerros 66), Tlaltizapan, Mexico, 1995/96.

Source: Reynolds et al., 1998a.

Association of CTD with yield potential of homozygous sister lines from two crosses, sown in warm (1995/96) and temperate environments (1996/97)


Correlation coefficient of CTD with yielda

Cross 1
Seri 82/Siete Cerros 66

Cross 2
Seri 82/Fang 60

Tlaltizapan, Mexico (warm)



Obregon, Mexico (warm)



Obregon, Mexico (temperate)



aSignificant at P £0.05 represented by * and significant at P £0.01 represented by **.
Source: Reynolds et al, 1998b.

Using CTD and leaf conductance in early generation selection

Source: Reynolds et al., 2001b.

CTD as an efficient means of evaluating advanced lines

In addition to the work on early and intermediate generation breeding lines, experiments were also conducted with advanced lines to assess the power of CTD as a predictive tool of performance. Sixty advanced lines (ALs) of diverse genetic backgrounds were selected for superior performance under hot conditions using late sowings in Obregon, Mexico. The 60 ALs were multi-plied and grown as replicated yield trials in the 1995/1996 spring wheat cycle at 15 warm environments: four in Mexico, four in Sudan, three in Bangladesh, three in India and one in Nigeria. Physiological traits were measured on yield plots and on three-row plots in the selection environment, i.e. a March sown trial in Obregon. Yield and CTD in the selection environment were compared with performance of ALs averaged across the 15 environments. Canopy temperature depression measured in the selection environment explained at least as much of the variability in performance across all warm sites as yield itself (Table 7.5). In this study, a number of other physiological., 1997, 1998a). Data also indicated that CTD measured on three-row plots was an equally good predictor of yield as those measured in yield plots, suggesting that the technique would be amenable to selections in smaller plots.

Phenotypic correlations between mean yield of 60 advanced lines at international sites and CTD and yield measured at Ciudad Obregon, Mexico, March sown, 1995/96

Traits measured in selection environment

Average yield in target environmentsa

11 international sitesb

15 international sites




CTD-three row plot



CTD-five row plot



aSignificant at P (< 0.01) represented by **.

bEleven locations with the least GxE determined by cluster analysis for crossover interaction (J. Crossa, personal communication, 1997).

Source: Reynolds et al, 1997.

Data indicate that CTD has the potential to complement selection for heat tolerance at different stages of a breeding programme (Figure 7.5). Work is being conducted to confirm its value in temperate, irrigated environments.

Manipulating the duration of spike growth

At a recent workshop held at CIMMYT on raising yield potential in wheat (Reynolds et al., 1996), opinion concurred on the notion that yield gains are most likely to be achieved by simultaneously increasing both source (photo-synthetic rate) as well as sink (partitioning to grain) strengths (Slafer et al., 1996; Richards, 1996b). The reproductive stages of development, from initiation of floral development to anthesis, are pivotal in determining yield potential. During this period, grain number per spike is determined, a factor which determines subsequent partitioning of assimilates to yield, as well as heavily influencing the assimilation rate of the photosynthetic apparatus during grainfilling.

The duration of spike growth relative to other phenological stages is variable (Slafer and Rawson, 1994), and the genetic basis for this variation is associated with sensitivities to photoperiod and vernalizing cold temperatures, and to developmental rate independent of these stimuli (i.e. earliness per se). Slafer et al. (1996) hypothesized that the possibility exists of improving final grain number and yield potential by manipulating the genes associated with sensitivity to environment. To exploit this possibility, research will have to define more precisely how the major genetic factors that influence adaptability and development in spring wheat interact with environment to influence the source (photosynthesis)-sink (partitioning to spike) relationship during reproductive growth.

Research in this area would provide the following kinds of information that could be applied directly to crop improvement pro-grammes: (i) physiological markers, in the form of definable spike development patterns, indicating presence of yield optimizing genes and alleles; (ii) provision of direct evidence of physiological links between duration of reproductive phases and final grain number; and (iii) information to enable more strategic deployment of major genes Ppd and Vrn to optimize yield potential of cultivars in irrigated spring wheat environments.

Potential use of CTD in a breeding programme

Source: Reynolds et al., 2001b.

Other ideas for improving the physiological ideotype

In addition to the above hypothesis, a number of other suggestions were made in relation to physiological approaches to raising wheat’s yield potential in the 1996 consultancy. Many were related to the development of a conceptual model for a physiological plant type. The physiological ideotype would encompass a combination of traits that experimental data suggest would improve yield potential. Traits may include morphological and physiological traits already discussed, such as larger spikes, higher harvest index, high photosynthetic rate, CTD, etc. Other traits were also proposed and some are presented below.

Breeding less ‘conservative’ wheat

Wheat evolved, and was subsequently selected by man, under relatively low-yielding conditions. Physiological traits conferring survival were strongly favoured for most of the crop’s evolution. Examples of stress-adaptive traits are: extensive root systems, tall plant stature and high tillering capacity facilitating competition for physical space as well as increasing potential for uptake of nutrients and water. A less obvious example of a ‘conservative’ trait is root signalling (Davies and Zhang, 1991), which, in response to reduced soil water potential, causes reduced stomatal conductance well in advance of leaf water deficit. The trait probably evolved to increase the likelihood of completing the life cycle in dry conditions. Reducing stomatal conductance reduces evaporative cooling, hence raising plant temperature and developmental rate, and results in a more conservative use of available soil moisture. Obviously these types of traits are not advantageous when a single genotype is grown at high density in none water or nutrienr limited situation. The introduction of dwarfing genes, as well as empirical selection for yield potential, has probably removed most of these traits from modern wheat. Nonetheless, research at CIM-MYT suggests that advanced lines still show genetic variability in competitive ability, a trait associated with lower yield potential (see section "Canopy-based traits"), and the practice of selecting individual plants in early generations may be a factor contributing to continued genetic variability for these kinds of traits. Blum (1996) suggests that subtle expression of these and other ‘conservative’ traits may still hold back yield potential in modern wheat. He argues that any degree of competition for assimilates from alternate sinks, for example root growth, osmotic adjustment, carbohydrate reserves in stems, etc., will reduce partitioning of assimilates to yield. Given the importance of assimilate availability during the rapid spike growth phase in determining yield (Fischer, 1985), Blum’s ideas merit serious consideration. The physiological ideotype may require alternate competing sinks to be minimized, especially during critical stages of development, such as the rapid spike growth phase.

Rapid early light interception

As an example of a trait that has yet to be explored in wheat, Richards (1996b) discusses the relatively rapid canopy development of barley. The leaf area of barley immediately after emergence is double that of wheat and has a 40 percent greater dry weight. Barley achieves greater early leaf area for two main reasons: one being that the embryo size is twice that of wheat, and secondly that barley has a greater specific leaf area in early growth (López-Castañeda et al., 1995). Richards’ group has succeeded in identifying wheat lines with both traits and incorporating them into a common wheat background. While agronomic manipulations of early light interception have no influence on yield, as researchers push forward yield barriers by removing current yield-limiting factors, early assimilation may become an important trait for further increasing yield potential.

Increasing grain weight potential

Genetic progress in yield potential is strongly associated with increases in grain number, while grain weight has, if anything, gone down (for example, Sayre et al., 1997). An important question is whether grain weight potential can be increased independently of increases in grain number. Simplistically, it can be argued that this inverse relationship is a necessary trade-off when more grains are competing for limited assimilates. However, Slafer et al. (1996) argue that there is limited evidence that assimilate limitation during grainfilling causes reduced grain weight. They present evidence for a colimitation of grain weight determined by both source and sinks. This may occur, for instance, when grain weight potential is determined at an earlier phenological stage, such as spike growth, resulting in different potential sizes at different spike positions. The conclusion is that one avenue for increasing yield potential may be to exploit genes that increase kernel weight potential, especially at spike positions that currently have low kernel size. Richards (1996b) suggests that while smaller kernel size may be a pleiotropic effect of the dwarfing genes, final kernel size may be increased genetically by increasing the number of endosperm cells, for example.


Focused research on the physiological basis of yield can potentially complement traditional breeding in the following three ways: (i) identifying traits that may serve as indirect selection criteria for yield; (ii) developing selection methodologies that increase the efficiency of parental and progeny selection; and (iii) providing insights into the physiological and genetic basis of raising yield potential. The Jackson et al. (1996) survey indicates optimism among crop scientists for a greater integration of disciplines in future breeding efforts. This scientific challenge will hope-fully be realized in light of the need to meet projected demands for cereal production in the near future.


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