Wheat growth and physiology
E. Acevedo, P. Silva, H. Silva

Wheat is a widely adapted crop. It is grown from temperate, irrigated to dry and high-rain-fall areas and from warm, humid to dry, cold environments. Undoubtedly, this wide adaptation has been possible due to the complex nature of the plant’s genome, which provides great plasticity to the crop. Wheat is a C3 plant and as such it thrives in cool environments. Much has been written about its physiology, growth and development, which at present is reasonably well understood. This chapter will concentrate on the crop at the plant and organ levels of organization, aiming at providing physiological information that could be found useful for breeding and for agronomic purposes. The cellular level of organization will be discussed only in those cases where it is essential for the explanation of physiological behaviour at other levels.

WHEAT DEVELOPMENT

Organ differentiation defines the various stages of wheat development. Physiologically, the following stages are usually distinguished: germination, emergence, tillering, floral initiation or double ridge, terminal spikelet, first node or beginning of stem elongation, boot, spike emergence, anthesis and maturity. These stages may be grouped into: germination to emergence (E); growth stage 1 (GS1) from emergence to double ridge; growth stage 2 (GS2) from double ridge to anthesis; and growth stage 3 (GS3), which includes the grainfilling period, from anthesis to maturity (Figure 3.1). Physiological maturity is usually defined as the time when the flag leaf and spikes turn yellow (Hanft and Wych, 1982).

The time-span of each development phase essentially depends on genotype, temperature, day-length and sowing date. Table 3.1 shows typical time-lapse values for the various stages in spring- and winter-type genotypes sown in May at 34°S. Various environmental stresses, particularly heat but also water and salinity, may shorten the wheat growth phases.

Germination to emergence

The minimum water content required in the grain for wheat germination is 35 to 45 percent by weight (Evans et al., 1975). Germination may occur between 4° and 37°C, optimal temperature being from 12° to 25°C. Seed size does not alter germination but affects growth, development and yield. Bigger seeds have several advantages when compared to smaller seeds, such as faster seedling growth, higher number of fertile tillers per plant and higher grain yield (Spilde, 1989). The advantage of bigger seeds is demonstrated when the crop is grown under environmental stresses, particularly drought (Mian and Nafziger, 1994).

When crop emergence occurs, the seed embryo has three to four leaf primordia and almost half of the leaf primordia are already initiated (Baker and Gallagher, 1983a, 1983b; Hay and Kirby, 1991). During germination, the seminal roots grow first, followed by the coleoptile, which protects the emergence of the first leaf. The length of the coleoptile limits sowing depth, and its length changes with genotype, increasing only slightly when seeds are sown deeper (Kirby, 1993). Semi-dwarf wheat has shorter coleoptiles than tall wheat.

Emergence to double ridge

Wheat tillers grow from the axils of the main shoot leaves. The potential number of tillers varies with genotype, particularly among flowering types, winter types having a greater number. Semidwarf wheats usually have a high number of tillers. Bud differentiation into tillers and tiller appearance generally end just before stem elongation starts (Baker and Gallagher, 1983b). Longnecker et al. (1993), however, suggest that tillering does not end at any specific wheat development stage, but rather it is controlled by a number of genetic and environmental factors.

FIGURE 3.1
Schematic diagram of wheat growth and development stages, periods of initiation or growth of specific organs and periods of different components of grain yield

S=sowing; G=germination; E=emergence; DR=double ridge appearance; TS=terminal spikelet initiation; HD=heading; A=anthesis; BGF=beginning of grainfilling period; PM=physiological maturity; GS=growth stage

Source: Adapted from Slafer and Rawson, 1994

Not all tillers produce spikes in wheat, and many tillers abort before anthesis (Gallagher and Biscoe, 1978). The number of productive tillers depends on genotype and environment and is strongly influenced by planting density (Table 3.2). Under favourable conditions, one and one-half fertile tillers per plant is a usual number.

Tillering has great agronomic importance in cereals since it may partially or totally compensate the differences in plant number after crop establishment and may allow crop recovery from early frosts.

The duration of the vegetative stage (GS1) in wheat may vary from 60 to 150 days depending on sowing date and genotype. It depends on the rate of leaf appearance (phyllochron) and when floral differentiation occurs (double ridge), which are induced by photoperiod and vernalization.

TABLE 3.1
Days from emergence to physiological maturity in a spring and winter wheat

^aYecora, low sensitivity to vernalization and moderate sensitivity to photoperiod.
^b WW33G, high sensitivity to vernalization and moderate sensitivity to photoperiod.
Source: Adapted from Stapper and Fischer, 1990.

The phyllochron is defined as the interval between similar growth stages of two successive leaves in the same culm. It has been used extensively to understand and describe cereal development. The phyllochron is strongly dependent on temperature (Rickman and Klepper, 1991), but severe water deficits (Cutforth et al., 1992) and strong nitrogen deficiency (Longnecker et al., 1993) retard the leaf emergence rate in spring wheat. Frank and Bauer (1995) observed genetic variation (differences) in the phyllochron of genotypes of bread wheat and durum wheat.

Cereal development is normally expressed in degree-days (GDD), using 0° or 4°C as the base temperature for physiological processes in wheat, such that:

(1) GDD = [(Tmax + Tmin)/2] - Tb

where Tmax and Tmin are the maximum and minimum daily temperature and Tb is the base temperature (Cao and Moss, 1989a, 1989b). The GDD vary with growing stage and allow a rough estimation of when a given growth stage is going to occur at a particular site.

TABLE 3.2
Seeding density effects on yield components and crop growth

^a SE = standard error of the mean
^b DAS = days after sowing.
Source: Fischer et al., 1976.

Vernalization

Wheats, which are responsive to vernalization, flower after the completion of a cold period. The double ridge stage is not reached until chilling requirements are met, and the vegetative phase is prolonged generating a higher number of leaves in the main shoot; the phyllochron, however, is not affected (Mossad et al., 1995). Two major flowering types of wheat are differentiated by their response to vernalization (Flood and Halloran, 1986):

• Spring-type wheat has a very mild response or no response at all to vernalization, and frost resistance is low.

• Winter-type wheats have a strong response to vernalization and require a period of cold weather to flower. In the early stages of growth, they are very resistant to frost (-20°C), but frost resistance is gradually lost towards heading and flowering. The vernalization requirements of winter types may be fully substituted by short days at nonvernalizing temperatures between 21° and 16°C (Evans, 1987).

Flood and Halloran (1986) point out that vernalization may occur at three stages of the growing cycle of the wheat plant: during germination, during vegetative plant growth (GS1) and during seed formation in the mother plant. The effectiveness of low temperatures to accomplish vernalization decreases with increasing plant age, being almost nil after three months (Chujo, 1966; Leopold and Kriederman, 1975).

Vernalization occurs at temperatures between 0° and 12°C (Ahrens and Loomis, 1963; Trione and Metzger, 1970). Spring genotypes usually require temperatures between 7° and 18°C for 5 to 15 days for floral induction, while winter types require temperatures between 0° and 7°C for 30 to 60 days (Evans et al., 1975). Manupeerapan et al. (1992) observed that vernalization in winter genotypes stimulated cell division, overcoming an inhibitory process that occurs at high temperatures.

Photoperiod

After vernalization is completed, genotypes, which are sensitive to photoperiod, require a certain day-length to flower. Sensitivity to photoperiod differs among genotypes. Most cultivated wheats, however, are quantitative long-day plants. They flower faster as the day-length increases, but they do not require a particular length of day to induce flowering (Evans et al., 1975; Major and Kiniry, 1991).

Stefany (1993) observed a period of insensitivity to day-length in wheat, which starts with germination. During this period, the plant develops foliar primordia only. This may be considered a juvenile phase, which is longer in winter wheat.

The photoperiod is sensed by mature leaves and not by apical meristems (Barcelló et al., 1992; Bernier et al., 1993). A single leaf is usually enough to sense the photoperiod for floral induction. Once the photoperiod insensitive period ends, floral induction starts and the reproductive stage begins (double ridge). The shorter the length of the day, the longer the inductive phase is (Major, 1980; Boyd, 1986), the longer the phyllochron (Cao and Moss, 1989a, 1989b; Mossad et al., 1995) and the bigger the flag leaf (Mossad et al., 1995). On the contrary, longer days advance floral induction (Evans et al., 1975).

The development of the inflorescence after induction occurs at a rate that is also dependent on daylength in those genotypes sensitive to photoperiod (Stefany, 1993). The shorter the day, the longer the phase is from double ridge to terminal spikelet (Figure 3.2), increasing the period to terminal spikelet and the number of spikelets per spike. Changes in daylength after the terminal spikelet have no effect on floret initiation or anthesis date.

FIGURE 3.2
Development of unvernalized wheat cultivars when days were extended by 0 hours (a) and 3 hours (b)

Source: Stefany, 1993.

Wheat adaptation to a wide range of latitudes occurs at lower levels of photoperiod sensitivity such that flowering is not retarded significantly if the day-length is shorter than optimal (Santibañez, 1994).

Vernalization and photoperiod constitute the basic processes of the adaptation of wheat to various environments. Knowledge and genetic manipulation of them should continue to provide significant tools for adaptation and yield.

Double ridge to anthesis

Wheat plants have from four to eight leaves in the main shoot when the growing apex changes from the vegetative to the reproductive stage. The length of the apex at this time is approximately 0.5 mm. The glume and lemma primordium stages follow. The floret primordia are found in the axil of each lemma. Temperatures above 30°C during floret formation cause complete sterility (Owen, 1971; Saini and Aspinal, 1982). Each spikelet has from 8 to 12 floret primordia in the central part of the spike. The basal and distal spikelets have from six to eight florets. Less than half of these florets complete anthesis; the rest abort or are insufficiently developed before anthesis to be fertilized (Kirby, 1988; Kirby and Appleyard, 1987; Hay and Kirby, 1991).

Terminal spikelet

At this stage, the growing apex is 4 mm in length with 7 to 12 leaves in the main shoot. Spikelet number per spike is already determined at this stage, varying from 20 to 30 (Allison and Daynard, 1976; Kirby and Appleyard, 1984). Rahman et al. (1977) reported a positive correlation between the length of the vegetative phase and the number of spikelets per spike; lengthening the duration of the vegetative stage of the apex induces more spikelets per spike. However, the actual number of spikelets is determined by the length of the reproductive phase. Short days (eight hours) from double ridge to terminal spikelet initiation stimulate a large number of spikelets (Rawson, 1971; Rahman and Wilson, 1978).

Towards the end of this stage, the apex, which is beneath the soil surface, starts to grow faster and rise (Kirby and Appleyard, 1984). This stage is particularly sensitive to environmental stresses, especially nitrogen and water (Wuest and Cassman, 1992a). Therefore, terminal spikelet has been suggested as the stage at which the second dose of nitrogen fertilizer should be applied (Biscoe, 1988) and as an indicator of the limit for using growth regulator herbicides (Kirby et al., 1989). One problem is that this stage is not easily detected without dissection of the plant. Masle (1984) and Kirby et al. (1985) point out, however, that the terminal spikelet occurs in the field when the spike is at about 1 cm above the crown of the plant.

Spike growth

Once the terminal spikelet is formed, stem elongation starts and the spike begins to grow. Spike growth occurs from the appearance of the leaf prior to the flag leaf (penultimate leaf) up to ten days past anthesis (Kirby and Appleyard, 1984). Spike growth, slow in its early stages, increases greatly about the time the ligule of the flag leaf becomes visible (Krumm et al., 1990). Floret abortion starts in the boot stage and finishes at anthesis. Floret death occurs when the stem and peduncle are at maximum growth rate (Siddique et al., 1989). Floret death is probably, at least partially, due to competition for carbohydrates at this stage (Kirby, 1988). In the wheat crop, there is a close relation between the number of kernels per unit area and the ratio between incoming radiation to the mean temperature above 4.5°C (the photothermal quotient) calculated for the 30 days preceding anthesis (Fischer, 1985a). Higher radiation increases the amount of photosynthates available for spike growth, and lower temperatures prolong the period of spikelet growth and decrease competition for carbohydrates.

Meiosis in wheat, which originates the pollen in the anthers and the embryo sac in the carpel, coincides with the boot stage. This stage is very sensitive to environmental stresses. In wheat and barley, meiosis starts in the middle of the spike, continuing later above and below this zone (Zadoks et al., 1974).

Anthesis to physiological maturity

The wheat spike contains only one spikelet per rachis node. Each spikelet has between three and six potentially fertile florets (Kirby and Appleyard, 1984), which are self-pollinated in 96 percent of the cases (Martin et al., 1976). Anthesis begins in the central part of the spike and continues towards the basal and apical parts during a three- to five-day period (Peterson, 1965). The proximal florets of the central spikelet are fertilized two to four days earlier than the distal florets. These grains usually have a greater weight (Simmons and Crookston, 1979).

After floret fertilization, cellular division is rapid, during which the endosperm cells and amyloplasts are formed. This period is known as the lag phase and lasts for about 20 to 30 percent of the grainfilling period. After there is a phase of cell growth, and differentiation and starch deposition in the endosperm, which corresponds to linear grain growth and takes from 50 to 70 percent of the grainfilling period. The embryo is formed at the time of endosperm growth (Jones et al., 1985).

Quantification of wheat development

There are several scales or development codes for wheat that describe visible growth stages without the need for dissection of the plant. Among these the most widely used is Haun’s scale (Haun, 1973), which is most useful in defining vegetative growth stages. Feeke’s scale (Large, 1954) and Zadoks’ scale (Zadoks et al., 1974) provide a good description for both vegetative and reproductive stages. Comparison between scales is usually complicated but a computational programme was developed that allows conversion from one scale to the other (Harrel et al., 1993). Crop development stages are determined in representative plants in the field, avoiding the borders of plots and any interfering material.

Zadoks’ scale is the most comprehensive and easiest to use. It describes all stages of the cereal growth cycle, incorporating characteristics not considered in other scales. It is based on a decimal code (DC), which incorporates various aspects of plant development. The main growth stages are selfexplanatory and are presented in Table 3.3. A second digit, values from 0 to 9, gives more detail for each main growth stage using the position 5 as the middle value. Leaf numbers, for example, have decimal codes from 11 to 19 and the tillers in the main shoot from 21 to 29.

After emergence, all development stages are based on observations on the main shoot, usually the tallest and thickest. After stage 40 (at 39 the flag leaf ligule is just visible and at 41, the early boot stage, the spike is beginning to swell), the stages of the main shoot and tillers become similar, and the stages are determined by viewing the whole plant. Stages 70 to 93 are determined by the development stage of individual kernels or grain in the middle of average spikes (Table 3.3).

WHEAT GROWTH

The net carbon dioxide (CO₂) assimilation at the tissue level constitutes the basis for growth. Many factors affect the net assimilation of CO₂, among others, the growth and development stage of the plant and environmental characteristics, such as light, nitrogen, temperature, CO₂ and water status.

Four main basic processes are involved in photosynthesis: (i) a photochemical process determining the quantum yield and depending on light intensity; (ii) a biochemical process particularly linked to carboxylation; (iii) physico-chemical processes of CO₂ transfer from the external air to the carboxylation sites; and (iv) the photorespiration process in C₃ plants.

At optimum temperature (20° to 25°C), the maximum light saturation rates of photosynthesis (Amax) at the leaf level in bread wheat are between 15 and 25 µmol CO/m² s (25 to 40 mg CO²/dm² h). Ninety percent of the light saturation rate is reached at 1 000 µmol quanta/m² s of photosynthetically active radiation (PAR). Wild relatives of wheat, however, may have substantially higher Amax than cultivated wheat (Austin, 1990).

Much attention has been given to the question of how to increase total photo-synthetic yield. Of the two photosynthetic parameters, quantum yield (rate of photo-synthetic assimilation/incident light intensity) and Amax, a much greater improvement in canopy photosynthesis could be theoretically achieved by increasing the quantum yield. Unfortunately, the quantum yield of the photosynthetic process itself is very constant among genotypes (Austin, 1990). An improved discrimination of the enzyme ribulose 1,5 carboxylase oxygenase (rubisco) for CO₂ with respect to oxygen (O₂) would increase the quantum yield of the overall process by decreasing photorespiration (normally 25 percent of the energy produced by photosynthesis), but not much variation in the discrimination of rubisco has been found between species (Sommersville, 1986; Loomis and Amthor, 1996). Some scope appears to exist for selecting genotypes with a reduced maintenance respiration, which normally uses 2 to 3 percent of the dry weight per day (Robson, 1982), but its effect on radiation use efficiency would be low (Loomis and Amthor, 1996). Amax varies significantly among species and cultivars. In wheat, it has been known for some time that certain diploid ancestor species have higher Amax values than present advanced lines of bread and durum wheats (Dunstone et al., 1973); however, little progress has been made with respect to yield increases by this approach.

TABLE 3.3
Decimal code used to quantify the growth stages in cereals

^aWinter cereals only.

^bAn increase in the solids of the liquid endosperm is notable when crushing the seed between fingers.

^cFingernail impression held; head loosing chlorophyll.

Source: Zadoks et al., 1974.

Canopy photosynthesis

Canopy photosynthesis is closely related to the photosynthetically active (400 to 700 mm) absorbed radiation (PAR_A) by green tissue in the canopy (Fischer, 1983). The PAR_A can be calculated from the fraction of solar radiation at the top of the canopy, which is transmitted to the ground (I/I₀), such that:

(2) PAR_A = R_S * 0.5 * 0.9 * (1 - I/I₀)

where R_S refers to the total solar radiation (MJ/m² d); the factor 0.5 refers to the fraction of total solar energy, which is photosynthetically active; (1 - I/I₀) is the fraction of total solar radiation flux, which is intercepted by the crop; and 0.9 * (1 - I/I₀) is the fraction of radiation absorbed by the crop allowing for a 6 percent albedo and for inactive radiation absorption (Loomis and Amthor, 1996).

The I/I₀ essentially changes as the crop leaf area index (LAI) increases, and it is not very dependent on other factors, such as cloudiness or time of day. It is measured with a PAR sensor since the attenuation of R_S in the canopy differs from that of PAR. The relationship between I/I₀ and LAI fits a negative exponential (similar to the Beer Lambert Law), such that:

where e is the base of the natural logarithm and K is known as the canopy extinction coefficient.

The canopy extinction coefficient in wheat crops ranges from 0.3 to 0.7 and is highly dependent on leaf angle (low K for erect leaves). From equation 3, it can be calculated that 95 percent PAR interception requires a LAI as high as 7.5 for erect leaves but a LAI of only about 4.0 for more horizontal leaves.

The total canopy net photosynthesis is linearly related to PAR_A and so is crop growth rate (CGR, g/m² d), which is the net accumulation of dry weight, such that:

(4) CGR = RUE * PAR_A

where RUE is the radiation use efficiency (g/m² d).

Final yield is therefore the product of cumulative seasonal radiation absorption, RUE and the portion of total biomass that goes to yield (harvest index).

Potential radiation use efficiency in strong light depends on several factors: adequate water to allow high stomatal conductance and transport of CO₂ into leaves; leaf arrangement relatively vertical to the radiation beam; good leaf nutrition to support large photosynthetic capacity; an active Benson-Calvin cycle to incorporate CO₂; and appropriate canopy ventilation supplying CO₂ and dissipation of heat (dissipation of excess energy due to light saturation). Due to environmental constraints, a quantum requirement of 10 mol quanta/mol CO₂ under light-limited conditions may increase to 20 and 30 mol quanta/mol CO₂ under field conditions with a decrease in RUE from 8.2 to 3.7 and 2.2 g/MJ PAR (Loomis and Amthor, 1996). Practical estimates of maximum RUE by these authors were 3.8 g/MJ PAR, which would occur with long cool days and moderate radiation (20 MJ/m₂ d). Warm temperature, the small concentration of CO₂ relative to O₂ and light saturation limit attainment of a greater RUE. Measured values of RUE in a wheat crop are close to 3.0 g/MJ PAR_A when roots are included (Fischer, 1983).

The RUE varies as Amax changes. Increases in the nitrogen of the canopy increase Amax and RUE. Frost at night and temperatures below 15°C during the daytime can reduce Amax. Water stress has a small effect on RUE, but radiation intensity beyond a given value may reduce RUE. The RUE declines during grainfilling probably due to sink limitation and/or leaf senescence (Fischer, 1983). Most studies show no difference in CGR between genotypes, even when Amax varies (Austin et al., 1986), but a higher CGR at anthesis was related to higher yield in Australian modern wheat cultivars grown under water stress (Karimi and Siddique, 1991).

Potential yield

Yield potential, defined as the yield of an adapted genotype grown under optimal management and in the absence of biotic and abiotic stresses, has been found to be a very useful concept since progress in yield potential usually leads to progress in wheat yield in farmers’ fields, particularly if stresses are mild.

The yield of a wheat crop can be expressed as the product of two components, such that:

(5) GY = KNO * KW

where GY is grain yield (g/m²); KNO is the kernel number (m^-2); and KW is the kernel weight (g).

It follows from equation 5 that changes in wheat yield potential could be achieved through changes in KNO and/or KW. Strong associations with yield have been found with KNO for sets of wheat genotypes (Austin et al., 1980; Slafer et al., 1990; Slafer et al., 1996). KNO is established in the period between 20 and 30 days before flowering and ten days after anthesis. This period coincides with tiller and floret mortality, along with the active growth of the stem (peduncle) and spike. Gains in KNO, however, do not translate directly in yield potential gain due to partial compensation by decreased KW. Slafer et al. (1996) argue that the lower KW observed with increased KNO is not only due to a lower amount of assimilates per grain but is the result of an increased number of grains with a lower weight potential coming from more distal florets.

It is essential to understand kernel number variation to understand yield potential. The yield components of wheat combine with each other to give a number of kernels per unit area. The product of plants per mspikes per plant, spikelets per spike, florets per spikelet and grains per floret is KNO. However, due to the compensation effects (Table 3.2), it is difficult to isolate the effect of a given yield component on KNO.

A common observation is that the calculated grain production potential based on the potential of each yield component is much higher than what is actually realized. Theoretical estimates of 180 000 grains/m2 based on yield components end up with a realized KNO of only 18 000/m². It has been shown that competition for limited resources during the spike growth period, including light and nitrogen and hence photosynthates, is the major cause of KNO potential loss.

Spike dry weight appears to be a major determinant of KNO. Competent floret number is closely related to spike dry weight (Fischer, 1983). In turn, spike dry weight can be expressed as a fraction of the product of the spike growth period (D_s, days), the crop growth rate during the period (CGR, g/m² d) and the partitioning of assimilates to the spike during the spike growth period (F_S). Increasing any of these three components of spike dry weight should result in a higher spike dry weight and KNO (Fischer, 1985b).

The D_S is largely affected by temperature and photoperiod (see section "Wheat development"). CGR is linearly related to PAR_A (equation 4), and F_S is largely dependent on the number of competent florets in each spikelet, a factor that has a strong genotypic component. The number of kernels per competent floret is usually 1.0, but environmental stresses, such as boron deficiency and severe water stress, may induce sterility and reduce grain set.

Fischer (1985a) stated that the major environmental factors determining KNO under potential growing conditions for wheat are solar radiation (R_S) and temperature (T). These factors can be summarized in the photo-thermal quotient (PTQ, MJ/m² d C), such that:

(6) PTQ = R_S/(T - 4.5)

where 4.5°C is the base temperature for wheat growth. The numerator represents the positive association of R_S with CGR, while the denominator represents the negative effects of high temperature that shorten the duration of the spike growth period.

Indeed, Dhillon and Ortiz-Monasterio (1993) found a close positive association between the PTQ calculated for the spike growth period (from 20 days prior to heading to ten days after heading) and kernels per m² when studying three spring-type wheat genotypes grown at ten dates of planting under optimum management. They concluded that genotypes maximized their yield when the PTQ value was highest between 20 days before and ten days after heading and suggested that all genotypes should maximize their yield by flowering during the highest PTQ in the growing season. The PTQ concept should be valid for stress-free conditions and a LAI of the crop that fully intercepts the incoming solar radiation, such that the linear relation between CGR and R_S (equation 4) holds.

The potential kernel weight is mostly dependent on genotype, but it may be limited to some extent by post-anthesis assimilate supply. It should be emphasized, however, that potential grain yield is more limited by sink size (KNO) than by post-anthesis assimilate supply, i.e. higher KNO always gives higher yield. High temperatures during grainfilling may reduce the grain growth period by shortening the duration of photosynthetic tissue, and in this case the source may become limiting. It has been suggested that wheat grain yield may be increased by increasing the kernel weight (Richards, 1996) through increased grainfilling rate.

Nitrogen (N) is a major constituent of the photosynthetic apparatus. After water, nitrogen is the major constraint to crop growth. Around 25 kg of N are usually required as fertilizer to produce 1 tonne of wheat grain. Therefore, wheat yields are highly dependent on nitrogen. Leaf area development is responsive to N and so are CGR and RUE. Recent reports indicate that the N concentration in the spike at anthesis correlates closely with KNO and hence grain yield (Abbate et al., 1995). This finding may be related to a higher availability of carbon (C) for ear growth resulting in greater spike dry weight and floret fertility, which in turn results in greater grain number and yield (Brooking and Kirby, 1981). The highest grain yield response to N fertilizer in wheat occurs when it is applied just prior to the initiation of stem extension (DC 30). Delaying N application beyond DC 32 reduces the grain yield response such that at DC 70 (kernels recently formed) and beyond, no wheat yield response to N fertilizers is observed but an increase in grain protein only (Mossedaq and Smith, 1994). Delaying N fertilization through DC 30 not only increases grain yield but also decreases lodging and lowers fertilizer losses thus increasing N recovery (Brooking and Kirby, 1981; Wuest and Cassman, 1992a, 1992b). The benefit of split N applications in N use efficiency compared to pre-planting application has been generally confirmed (Sowers et al., 1994).

TABLE 3.4
Yield parameters in tall, semidwarf and dwarf isogenic wheat lines

^a Numbers followed by a different letter differ at P=0.05.
Source: Acevedo and Silva, unpublished data.

There are genotypic differences in the response to nitrogen. In general, genotypes with a higher yield potential have a higher N use efficiency (yield/N supply) as a result of higher N utilization efficiency (yield/N uptake). But there is a negative association between genotype yield potential and protein content of the grain, a parameter that is important in grain quality. Nitrogen applications near anthesis increase the protein content of the grain.

Prospects for increasing yield potential of wheat

It is clear that any increase in the yield potential of wheat will come from breeding. Progress in breeding for yield potential is more likely to occur if specific characteristics are targetted as has occurred in grain quality improvement and disease resistance breeding. Targetting yield potential improvement requires an understanding of the physiological processes that may be genetically modified to improve yield. Some of these are already being exploited, such as flowering time to improve adaptation to particular regions and plant height, which greatly influences yield potential (Table 3.4).

An interesting observation in several studies is that efforts to increase wheat grain yield through breeding have not resulted in an increased biomass under potential growing conditions (Evans, 1993). It should be noted that usually there are genotypic differences in biomass when stresses such as drought are present (for example, Sharma, 1992). Grain yield, however, has been substantially increased at a mean rate of 0.9 percent/year over the last 30 years (Sayre et al., 1997). The increase in potential grain yield has come essentially from an increase in harvest index and particularly from an increase in grain number per unit area (see above) rather than an increase in kernel weight. It appears that assimilate supply is not generally limiting yield potential, except for the period of rapid spike growth, thus pointing to the partitioning of carbon to the reproductive structure as the main determinant of yield potential. Trimming of the leaf area to one-half, for example, at the beginning of the rapid grain growth period did not reduce grain yield (neither KNO nor KW), and plants compensated for the reduced leaf area by increasing stomatal conductance (Richards, 1996). The wheat plant appears to have a photosynthetic system that is operating at a level below its potential. The crop can respond to an extra demand of photosynthates when required, for instance, by an increased number of grains per unit area. Indeed, yield increases from bread wheat material released in the last 30 years have been found to be related to increases in grain number (Sayre et al., 1997) and with increased stomatal conductance and photosynthetic rate as well as reduced canopy temperature (Rees et al., 1993). It appears that the driving process is increased KNO and that changes in leaf conductance, photosynthesis and temperature are a response to an increased demand for assimilates (Richards, 1996).

Grain number may be increased by: (i) reducing the size of competing organs, such as the peduncle and number of sterile tillers during spike growth; (ii) increasing the number of spikelets per spike; (iii) extending the duration of the interval between floral initiation and terminal spikelet by extending the duration of spike growth; or (iv) increasing floret survival by avoiding carbon, water and nutrient (particularly N) limitations (Abbate et al., 1995). Radiation use efficiency during the rapid spike growth period could also be increased by erect canopies with short leaves if grain demand for photosynthates is high (Araus et al., 1993).

WHEAT PHYSIOLOGY AND ABIOTIC STRESS

Abiotic stress includes any environmental conditions or combination of them that negatively affect the expression of genetic potential for growth, development and reproduction (Jones and Qualset, 1984). The main strategy used in the past to deal with environmental stress has been to alleviate the stress through irrigation, soil reclamation, fertilizer use and others. Economics, as well as ecological limitations associated with these practices, however, have prompted an interest in searching for plant genetic resistance to environmental stresses.

Abiotic environmental factors explain 71 percent of the reduction of potential yield of annual crops in the United States (Boyer, 1982). Wheat yields are depressed, among other factors, by drought, heat, low temperatures, low fertility, especially nitrogen, and soil salinity. The effect of these stresses on wheat growth, development and yield will be briefly reviewed.

Water stress

Water stress is of common and wide occurrence in nature. It occurs whenever water absorption by the crop is lower than the evaporative demand of the atmosphere. Two major processes are involved: (i) water absorption by the crop, which is controlled by root characteristics and soil physical properties; and (ii) crop evapotranspiration, which depends on atmospheric properties, notably net radiation and vapour pressure deficit (vpd), and crop characteristics, such as crop ground cover and stomatal conductance. Notwithstanding that, wheat may experience water stress in any environment. It is a typical constraint at the International Maize and Wheat Improvement Center’s (CIMMYT) mega-environment 4, which is a dry, temperate environment covering about 20 percent of the developing world area planted to wheat. The major features of this megaenvironment are presented in chapter "CIMMYT international wheat breeding".

Crop evapotranspiration (ET), and more precisely crop transpiration, is positively and linearly related to grain yield in C3 and C4 plants; therefore, water stress inevitably decreases yield. Figure 3.3 shows an ET-grain yield relation for wheat obtained from a 178 crop-year database of irrigated and dryland wheat data from Bushland, Texas, United States. An ET-grain yield relationship was determined as linear, with a regression slope of 1.22 kg grain/m3 ET above the ET threshold of 208 mm required to initiate grain yield (Musick et al., 1994).

FIGURE 3.3
Relationship of grain yield to seasonal evapotranspiration for irrigated and dryland wheat crops

Source: Musick et al., 1994.

In order to show the physiological effects of water stress on wheat, the major developmental phases described earlier (Figure 3.1) will be used. Water stress may occur in any of these phases according to the environment in which the crop is grown. The most critical phase for water deficit is GS2, when KNO is being determined.

Germination to emergence

Rainfed, arid environments may present early drought in the growing season affecting wheat germination and crop establishment. Decreased seed reserves, low germination and high soil mechanical impedance may hamper crop establishment (Bouaziz and Hicks, 1990).

Seed size, protein content and initial root and aerial biomass are correlated in wheat (Ries and Everson, 1973). Several authors have pointed out the positive effect of larger seed size on wheat germination and establishment (Singh, 1970; Ries and Everson, 1973; Hampton, 1981; Kalakanavar et al., 1989). The negative effects of early drought are also lower if wheat seeds are bigger in size (Mian and Nafziger, 1994). The larger root mass in seedlings from bigger seeds may help to maintain a better water balance under early water stress if water is available deeper in the soil profile.

Another seedling trait useful to improve crop establishment under current variable rainfall is coleoptile length. The major variation in the coleoptile length is genetic (ICARDA, 1987). Genotypes with a long coleoptile allow sowings at greater soil depth avoiding a ‘false start’ by rainfall that is not of sufficient magnitude or frequent enough to assure the establishment of the crop. Early autumn planting has demonstrated clear advantages in rainfed Mediterranean environments (Acevedo et al.,1991a); the penalty in terms of crop yield associated with delayed planting is in the order of 1 percent yield loss per day (Acevedo et al., 1998a). Deep early sowing is required in this case to avoid a ‘false start’. In soil that is dry at the beginning of the season, seeds should be planted at a depth that would not allow germination unless significant rainfall has occurred to wet the first 10 cm of soil.

Emergence to double ridge

Water stress during GS1 may increase the phyllochron of bread and durum wheat (Krenzer et al., 1991; Simane et al., 1993), but leaf expansion is most sensitive to water stress (Acevedo et al., 1971); and leaf growth can be drastically reduced at leaf water potentials of -0.7 to -1.2 MPa (Eastham et al., 1984). Tillering is also very sensitive to water stress, being almost halved if conditions are dry enough (Peterson et al., 1984; Rickman et al., 1983). As a result, leaf area index development is the most affected physiological process during this stage. Water deficit just before flower initiation may also decrease the number of spikelet primordia at this stage (Oosterhuis and Cartwright, 1983).

TABLE 3.5
Effects of water stress on leaf area index, yield components and water-use efficiency of wheat various stages of growth

^aWUE = water-use efficiency; ET = evapotranspiration.
Source: Hochman, 1982

Double ridge to anthesis

Wheat plant growth (roots, leaves, stems and ears) continues up to approximately ten days after anthesis. GS2 is therefore a period of very active plant growth. It follows that mild to moderate water deficits during this period will decrease cell growth and leaf area with a consequent decrease of photosynthesis per unit area. If the water deficit is more intense, net photosynthesis will decrease even more due to partial stomata closure (Acevedo, 1991a). Stomata start to close in wheat at leaf water potentials of -1.5 MPa (Kobata et al., 1992; Palta et al., 1994). Decreased leaf internal CO₂ (Ci) has the effect of reducing electron transport. Continued over-excitation of the light-harvesting system with no electron transport causes photoinhibition, thus damaging the system (Long et al., 1994). Maintenance of the plant’s water status and open stomata is therefore important not only for cooling but also for maintaining a high conductance for CO₂, which keeps photo-synthetic dark reactions going and electron transport functioning (Loomis and Amthor, 1996). Chlorophyll fluorescence is observed when light harvesting exceeds the capacity of the dark reactions; consequently, fluorescence measurements are now used widely for detection of stress effects on crops (Seaton and Walker, 1990).

Grain number decreases sharply when water stress occurs during the spike growth period (Hochman, 1982). Yield reduction is at a maximum when water stress develops from ten days before spike emergence. Water stress during this stage also decreases the spikelets per spike of fertile tillers (Hochman, 1982; Moustafa et al., 1996) (Table 3.5) and causes death of the distal and basal florets of the spikes (Oosterhuis and Cartwright, 1983). As mentioned earlier, carbon and nitrogen availability for spike growth are critical at this stage of development; both are decreased by water stress.

Anthesis to maturity

Water deficit close to anthesis accelerates development (Simane et al., 1993); the accumulation of soluble carbohydrates in the stem occurring between anthesis and the linear phase of grain growth is decreased (Nicholas and Turner, 1993). The remobilization of pre-anthesis assimilates to the grain becomes important as photosynthesis is decreased by water stress and total non-structural carbohydrates from wheat leaves and stems (particularly fructans and sucrose) significantly contribute to grain growth (Bidinger et al., 1977; Richards and Townley-Smith, 1987; Kiniry, 1993; Palta et al., 1994).

Water stress during grainfilling does not affect the number of fertile tillers nor KNO; grain weight is, however, reduced (Hochman, 1982; Kobata et al., 1992) due to a shortening of the grainfilling period resulting from accelerated senescence. Foliar application of dilute solutions of potassium orthophosphate (KH2POat a rate of 10 kg/ha may delay high temperature and terminal drought-induced senescence thus increasing the yield of wheat (Benbella and Paulsen, 1998).

It has been consistently found that barley and tall bread wheats have higher drought resistance while semidwarf wheats are intermediate and durum wheats are most susceptible (Fischer and Maurer, 1978; Sojka et al., 1981). A similar yield trend under drought was found by Acevedo et al. (1990), who also pointed out that in their nurseries the two-row barleys had higher yields than the six-row barleys.

Water deficit and grain yield

The yield of a dryland crop can be expressed as the product of transpiration (T), transpiration efficiency (TE) and harvest index (HI) (Passioura, 1977), such that:

(7) GY = T * TE * HI

where GY is grain yield.

The identity in equation 7 has been widely used to identify traits that would increase the grain yield of winter cereals under drought stress (Acevedo, 1987; Richards, 1987). Grain yield increases with transpiration. Those genotypes that keep their stomata open during water stress have higher yield under stress (Venora and Calcagno, 1991; Acevedo and Fereres, 1993).

Gas exchange measurements indicate differences in TE. The difficulty of integrating instantaneous gas exchange measurements temporally and spatially has limited its use in crop improvement programmes (Menendez and Hall, 1996). The TE can be estimated at the plant level using carbon isotopic discrimination. The magnitude of the discrimination against the heavier atmospheric carbon (¹³C) in the photosynthetic process is linearly and negatively related to TE in wheat (Farquhar and Richards, 1984; Masle and Farquhar, 1988; Condon and Richards, 1993) and positively and linearly correlated to grain yield (Condon et al., 1987; Sayre et al., 1995). The ¹³C discrimination provides an indirect measure of TE and it can be used in genetic improvement programmes for limited rainfall environments (Farquhar and Richards, 1984; Austin et al., 1990; Acevedo, 1993). There is genetic variability in ¹³C discrimination in wheat, hence high-yielding, high-TE lines can be found. It should be noted that TE is largely dependent on vpd; therefore, whenever comparisons are made, they should be expressed on a vpd basis.

In a crop situation where rains are frequent, water evaporation from the soil surface can be substantial, particularly if the crop cover is not complete and there is a fraction of soil exposed to solar radiation. Values of soil water evaporation of 50 percent of ET or more are common; therefore, much can be done to increase grain yield in dryland crops by increasing water-use efficiency through appropriate crop management practices (Harris et al., 1991; Acevedo et al., 1999).

In dry environments, harvest index is determined by the water transpired during grainfilling and the prevailing TE during that period (Passioura, 1977). If no water is left for transpiration after anthesis, the best attainable yield is in the order of 10 to 15 percent of the biomass at anthesis.

Drought resistance

Drought resistance is usually quantified by grain yield under drought. Wheat grain yield under drought, however, depends on yield potential as well as the phenology of the genotype (Acevedo, 1991b).

The quantification of drought resistance has also been approached by a yield stability index across environments (Finlay and Wilkinson, 1963; Eberhart and Russell, 1966), as well as by drought susceptibility indexes (Fischer and Maurer, 1978). These indexes are highly dependent on yield potential and crop phenology, which are characters with a high genotype x environment interaction (Acevedo, 1991b). To avoid these effects, Bidinger et al.(1987a, 1987b) proposed a drought resistance index (DRI) equivalent to the residual effect of yield under stress once the effects of yield potential, phenology and experimental error had been removed. The DRI is a criterion to be used to select drought resistant genotypes or geno-typic traits related to drought resistance that could be manipulated as independent genetic characters (Acevedo and Ceccarelli, 1989).

Physiological and morphological characters that confer drought resistance can be classed according to their association to water absorption or water loss by the crop. Morphological and physiological traits related to an increase in water absorption include: root growth, osmotic adjustment and related solutes and membrane stability (Acevedo et al., 1998a). Morphological and physiological traits related to a decrease in transpiration include: leaf colour (van Oosterom and Acevedo, 1992), leaf movements, epicuticular waxes and trichomes on leaf surfaces (Upadhayaya and Furnes, 1994), stomatal behaviour (Venora and Calcagno, 1991), transpiration efficiency (Farquhar and Richards, 1984; Austin et al., 1990; Acevedo, 1993) and air to canopy temperature difference (Blum, 1988; Rees et al., 1993).

Morgan and Condon (1986) demonstrated that segregating lines of bread wheat and durum wheat with a high capacity for osmotic adjustment had a yield advantage (11 to 17 percent in bread wheat and 7 percent in durum wheat) when compared to near isogenic lines without this character.

Heat stress

High temperatures severely limit wheat yield. They accelerate plant development and specifically affect the floral organs, fruit formation and the functioning of the photosynthetic apparatus. Although recognizing the fundamental linkage between water and heat stresses in plants, attention here will focus on one of them, heat stress, and assume that the wheat plants do not suffer water shortages. For breeding purposes, however, resistance to these two stresses usually has to be combined.

Transpiration, a mechanism of heat avoidance, is the primary agent for energy dissipation. A crop that maintains transpirational cooling may be a good heat avoider. The temperature of plant organs in the field may differ from air temperature by several degrees. This difference increases with a greater rate of transpiration. For wheat with no shortage of soil water, the leaf to air temperature difference increases linearly with vapour pressure difference (Idso et al., 1984). If water shortage arises and stomata begin to close, leaf temperature rises and may exceed air temperature.

Leaf photosynthesis is negatively affected as leaf temperature rises above 25°C in cool-grown wheat leaves, but leaves acclimated to warm temperature start to show a similar decline as temperatures exceed 35°C. At 45°C leaf photosynthesis may be halved.

Heat stress decreases total above-ground biomass and grain yield in wheat. For the analysis purposes of heat stress effects, the development phases described earlier (Figure 3.1) will be used. Temperature has a differential effect on each of these phases (Shpiler and Blum, 1986; O’Toole and Stockle, 1991). The most thermosensitive stage of wheat grain yield is GS2, when KNO is being determined.

Germination to emergence

From sowing to emergence, seedling mortality, and hence crop establishment, is a problem when soil temperatures are high. Plant emergence and population establishment are the starting points of crop growth. In hot environments, however, the maximum soil temperature in the top centimetres may exceed maximum air temperature by 10° to 15°C if the soil surface is bare and dry and radiation intensity is high. Under such conditions, maximum soil temperature may reach 40° to 45°C with serious effects on seedling emergence. The initial plant population may fall below 100 plants/mconsidered to be deleterious to crop yield. Table 3.6 shows the average number of plants established in a nursery of bread wheat genotypes planted at increasing soil temperatures in the field.

TABLE 3.6
Number of wheat plans established at various soil temperatures

^aThe equivalent to 100 kg seed/ha were planted at a depth of 3 to 4 cm.
^bTemperature measured in the field at a depth of 5 cm.
^cNumbers followed by a different letter differ at P = 0.05.

Source: Acevedo et al., 1991b.

Significant differences in crop establishment, genotypes and genotype x environment interactions were found under heat stress by Acevedo et al. (1991b). Angus et al. (1981) also found that the effect of temperature on emergence varied among wheat genotypes. If seedlings emerge satisfactorily, brief exposures to extreme soil temperatures may inhibit crown root growth and tiller initiation (Fischer, 1985b).

Emergence to double ridge

Sensitivity to high temperature increases as vegetative growth develops and tillering proceeds towards the end of GS1 (O’Toole and Stockle, 1991). The sensitivity to high temperature during this phase is expressed as a decreased duration of GS1 (Shpiler and Blum, 1986) and reduced leaf area and growth. A reduction in total number of leaves and spike-bearing tillers is also an effect of high temperature during this phase (Mid-more et al., 1984). Table 3.7 from Acevedo et al. (1991b) exemplifies these effects. The phyllochron increases when the growth temperature increases (Cao and Moss, 1994), reducing the number of leaves.

TABLE 3.7
Leaf area index, duration of GS1 and plant height as related to growing temperature

^aNumbers followed by a different letter differ at P = 0.05.
Source: Acevedo et al., 1991b.

Double ridge to anthesis

The presence of double ridges marks the beginning of the GS2 growth stage. By the end of this stage, the potential number of grains, KNO, has been determined. The GS2 stage is therefore critical in setting the extent to which the grain yield potential is realized.

The main effect of heat stress after floral initiation is observed on KNO. The number of kernels per unit area decreases at a rate of 4 percent for each degree increase in mean temperature during the 30 days preceding anthesis (Fischer, 1985b). A similar value can be calculated from the data of Acevedo et al. (1991b) as shown in Table 3.8 for a mean seasonal temperature range from 12.2° to 27.5°C.

TABLE 3.8
Some yield components and duration of GS2 at various mean seasonal growing temperatures

^aNumbers followed by a different letter differ at P = 0.05.
Source: Acevedo et al.,1991b.

The effect of temperature on grains per unit area may be attributed to a decreased number of fertile spikes or to fewer grains per ear. In a controlled experiment, Warrington et al. (1977) showed that wheat grown at 25°C during GS2 had only 40 percent of the kernel number in the main spike when compared with plants grown at 15°C during this period. Table 3.8 shows that spike number is also drastically reduced over this range of temperature.

The decrease in duration of GS2 at high temperatures is affected by genotypic variation in photoperiod sensitivity, vernalization response and optimum temperature for spikelet formation (Blum, 1988). If genotypes are able to maintain high carbon exchange rates at high temperatures, the decrease in GS2 duration and spike weight is smaller (Blum, 1986). High temperatures affect the capacity of the chloroplast membranes for electron transport (Berry and Rawson, 1981). An increase in chlorophyll fluorescence at temperatures where CO2 fixation begins to be affected indicates heat damage of photosystem II (Seeman et al., 1984). It appears that selection for an improved photosynthetic process is possible when exposing wheat germplasm to heat in the field (Acevedo, unpublished data).

Anthesis to maturity

Heat stress during GS3 mainly affects assimilate availability, translocation of photo-synthates to the grain and starch synthesis and deposition in the developing grain. The net result is a lower kernel weight. Over the range of 12° to 26°C increase in mean temperature during grainfilling, grain weight is reduced at a rate of 4 to 8 percent/°C (Wardlaw et al., 1980; Wiegand and Cuellar, 1981). Acevedo et al. (1991b) reported a mean reduction of 4 percent in grain weight per degree increase in mean temperature during grainfilling (Table 3.9). Shortened grainfilling duration is partially offset by increased grainfilling rate (Sofield et al., 1977), but the effects are much more complex. Hastened senescence, on the other hand, reduces assimilate supply to the grain. Also high temperature reduces final cell number in the endosperm, reducing grain weight. The results, however, are usually confounded with water stress.

Heat stress tolerance has been related to membrane stability, increased compatible solutes, increased protein stability and the synthesis of heat shock proteins (HSP). Plant response to heat shock is characterized by a rapid production of a specific set of proteins for supra-optimal temperatures. This happens when plant cells are abruptly exposed to temperatures that are about 5° to 10°C above their normal physiological growth temperature.

TABLE 3.9
Grainfilling duration and final grain weight of wheat grown at four temperaturesa

^aMean of 24 genotypes.
^bNumbers followed by a different letter differ at P = 0.05.
Source: Acevedo et al., 1991b.

The HSP synthesis is also induced by other stresses, such as drought and salinity (El Madidi and Zivy, 1993). These proteins are presumably involved in repairing and/or protecting structures that have been damaged by an increase in temperature or other stress. The presence of denatured proteins inside the cell is enough to induce the synthesis of HSPs, which may have a ‘chaperone’ role interacting with other proteins protecting their structure and conformation (Ellis, 1990). Genotypes having higher heat tolerance appear to synthesize HSPs in a higher concentration.

Cold and freezing

Most cultivated plants are sensitive to low temperatures, showing negative effects in yield at around 12°C (Lyons, 1973). Light saturation at lower photosynthetic rates and photoinhibition are commonly observed phenomena at low temperatures (Powles et al., 1983) along with increased chlorophyll ‘a’ fluorescence (Van Hasselt and Van Berlo, 1980; Greer et al., 1986). Prolonged exposure to light at low temperatures may produce severe and irreversible photoinhibition, followed by chlorophyll destruction from photo-oxidation and finally death of the tissue (Bongi and Long, 1987). The major effect of cold damage is the decrease in photosynthesis. Furthermore, the export of C from the leaf decreases and soluble carbohydrates accumulate (Pollock et al., 1983; Pollock, 1984). An advantage of sugar accumulation is that it protects the cells exposed to low temperatures (Koster and Lynch, 1992). After exposure to low temperatures, it has been found that the rubisco activity increases (Leegood and Edwards, 1996).

Mild frosts do not usually affect green area, but severe frosts can cause leaf necrosis and death. The more advanced the stage of development, the more susceptible the plants are and the greater the effect can be on yield. After stem elongation begins, the growing stems and spikes can be damaged and the culm killed, the only recovery being the production of new lower yielding tillers.

Frost can be particularly damaging between flag leaf emergence and ten days after anthesis. The damage appears as an erratic occurrence of aborted spikelets at the base, centre or tip of the spikes. It is also manifested as sterile florets in parts of the entire spike. This is due to an initial supercooling of plant tissues and a later erratic spread of the freezing front through stems and ears. The damage during this period occurs at minimum screen temperatures below 0°C and tissue temperatures of around -4°C (Harding et al., 1990) and is associated with radiative cooling in calm clear nights. No genetic resistance for this type of damage is usually found, and the only way to deal with the problem is through escape at the flowering stage, either by earlier or later flowering. Earlier flowering may be a better strategy in environments where terminal heat stress and drought are common stresses.

Salinity

Increases in agricultural productivity are closely related to irrigation. Irrigation practices, however, lead to increased soil salinity. Irrigation water may contain from 0.1 to 4 kg/m³ of salt, which - considering 1.0 to 1.5 m of irrigation water applied annually - contributes from 1 to 60 tonnes of salt per hectare (Shannon, 1997).

Soil salinity affects crop plants in three major ways: (i) osmotic stress, decreasing water availability; (ii) ionic stress; and (iii) changes in the cellular ionic balance (Kirst, 1989). Physiologically, many processes are affected, but the most notable are reduced cell growth and decreased leaf area, biomass and yield. Wheat has a moderate tolerance to salinity (Shannon, 1997).

Salinity effects on crop growth

Soil salinity above 4.5 dS/m electrical conductivity of the saturation extract decreases the percentage of plants established per unit area. At 8.8 dS/m the wheat plants emerged decrease to 50 percent (Francois et al., 1986).

Soil salinity accelerates apex development and increases the phyllochron, reducing the number of leaves in the main shoot (Maas and Grieve, 1986). It also decreases the number of spikelets in the main spike (Frank et al., 1987) and anticipates crop maturity.

Soil salinity affects the normal development and viability of tillers; it also decreases the number of primary and secondary tillers. A salinity level of 7.5 dS/m eliminates the secondary tillers and reduces the number of primary tillers. Root growth is generally less affected and may even be stimulated by low to medium salinity levels (Ayers et al., 1952).

All phenological phases are accelerated under salinity stress in wheat (Grieve et al., 1994). Spike development is accelerated, decreasing the time to terminal spikelet and the crop cycle.

The yield component most affected under salinity stress is the reduction of the number of culms that bear ears (Maas et al., 1994). The toxicity to the plant caused by salinity stress is particularly evident after anthesis. It is characterized by early senescence and low kernel weight (Wyn Jones and Gorham, 1991), as well as the abortion of distal spikelets (Grieve et al., 1992).

Wheat yield is decreased by 50 percent at soil saturation extracts of 13 dS/m (Ayers and Wescot, 1976). Durum wheat has a higher sensitivity to salinity than bread wheat. The threshold at which grain yield starts to decrease with increasing soil salinity is 5.9 dS/m for durum wheat and 8.6 dS/m for bread wheat. Furthermore, durum wheat decreases its yield at a higher rate with increasing salinity than bread wheat (Maas and Grieve, 1986; Acevedo et al., 1998b). Durum wheat also has a lower genetic variability in biomass and yield when grown under salinity stress than bread wheat (Acevedo et al., 1998b).

The higher salinity tolerance observed in bread wheat appears to be related to a lower sodium (Na⁺) concentration, as well as to a lower sodium to potassium ratio (Na⁺/K⁺) in the leaves (Table 3.10). The discrimination against Na⁺ in the foliar tissue (Shah et al., 1987; Gorham et al., 1987) and in favour of K⁺ (Schachtman et al., 1991) is due to genes present in the D genome of hexaploid wheat, not present in the tetraploid. The donor of the D genome to bread wheat is the diploid Aegilops tauschii (syn. Triticum tauschii).

TABLE 3.10
Salinity concentration in flag leaf of various Triticum species

^aBread wheat genotype reputed to be resistant to soil salinity.
^bNumbers followed by a different letter at P = 0.05.
Source: Acevedo at al., 1998b.

Yield under stress depends on yield potential, stress resistance per se and the phenology of the genotype (Bidinger et al., 1987a, 1987b; Acevedo, 1991a; Acevedo et al., 1998b). Some authors point out that selection for high yield potential could be the best strategy to increase wheat yields in salty soils due to the spatial heterogeneity in salt distribution in the soil, which would allow expression of the yield potential in some plants growing in areas with lower salinity (Richards et al., 1987). Acevedo et al. (1998b), however, found that many of the spring wheat genotypes reputed to have high yield under salinity stress also have salinity resistance per se.

The salinity resistance per se found by Acevedo et al. (1998b) was negatively correlated to Na⁺ concentration in the flag leaf. Ashraf and O’Leary (1996) and Chhipa and Lal (1995) also found a close negative correlation between wheat grain yield under salinity stress and Na⁺/K⁺ ratio in the grains. Salama et al. (1994) also observed a lower Na⁺ concentration in foliar tissue of tolerant as compared to sensitive wheat genotypes.

It appears that there are at least two mechanisms of salt tolerance in wheat: (i) low Na⁺ accumulation regulated at the root level; and (ii) ionic compartmentalization (Schachtman and Munns, 1992). It may be that the first mechanism could be located at the level of the plasma membrane, inducing a lower affinity for Na⁺ uptake, while the second could be located in the tonoplast with a higher affinity for Na⁺ uptake and isolation of Na⁺ in the vacuole. Other mechanisms, such as osmotic adjustment, have been widely cited as responsible for salinity tolerance in wheat and other crops (Richards, 1954).

CONCLUDING REMARKS

Much is known about the physiology of wheat that can be of direct use to agronomists and breeders. The generation of potential grain yield in wheat is quite well understood in addition to the avenues for improving yield potential. Yield under stress is generally less understood, but available physiological knowledge should allow better and more rapid progress in the future. Important aspects of wheat physiology, such as lodging resistance, the use of growth regulators for wheat growth, weed competition, soil mechanical impedance and nutrient toxicities/deficiencies, were not discussed here since a priority was given to yield and yield-forming processes with the idea that the application of these concepts would have a higher impact on wheat production around the world.

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