Morphological and Physiological Traits Associated with Wheat Yield Increases in Mediterranean Environments

MORPHOLOGICAL AND PHYSIOLOGICAL TRAITS ASSOCIATED WITH WHEAT

YIELDINCREASES INMEDITERRANEAN ENVIRONMENTS Stephen P. Loss and K. H. M. Siddique Division of Plant Industries Department of Agriculture, Western Australia South Perth, Western Australia 61 5 1, Australia

I. Introduction 11. Constraints in Mediterranean Environments A. Rainfall B. Solar Radiation C. Temperature D. Growing Season 111. Biomass Production and Partitioning A. Phenology B. Growth and Morphology W. Water Use A. Water-Use Pattern and Early Vigor B. Xylem Diameter C . Glaucousness D. Abscissic Acid Accumulation E. Osmoregulation F. Carbon Isotope Discrimination V. Radiation Use A. Interception B. Radiation-Use Efficiency VI. High-Temperature Stress VII. Use for Breeders VIII. Concluding Comments References

I. INTRODUCTION For several thousands of years, humans have been selecting wheats that are adapted to specific environments and cropping practices (Bell, 1987). With im229 Advances m Agronmny, Volume 52 Copyright 0 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.

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provements in crop management and plant selection, wheat spread from the mediterranean climate of west Asia to most other parts of the globe and it is now one of the most widely adapted plants in the world. Modern wheats perform best in the temperate regions of Europe and North America, where yields are higher and less variable than in the region where wheat originated and in other similar parts of the world that experience a mediterranean-type climate-west Asia, north Africa, South Africa, southern Australia, and in southwest North and South America (Fig. 1). It is somewhat ironic that plant scientists are currently trying to improve the adaptation of modern wheats to the environment where they originated. Nevertheless, about 10- 15% of the world’s wheat is produced in mediterranean-type environments. An important factor contributing to the widespread adoption of wheat was the recognition of the significance of the environment to adaptation. For example, after European settlement of Australia in 1788, the first two wheat crops failed miserably partly because the European wheats did not cope with the long, hot, and dry Australian summer (Macindoe, 1975). Australian settlers then began introducing from South Africa, India, and the mediterranean region earlymaturing wheats, which were better adapted to warm climates. With the development of plant breeding techniques during the twentieth century, breeders all over the world began to modify crops to increase yields, using

Figure 1 The distribution of mediterranean environments in the world

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mainly an empirical approach, that is, by trial and error. The progress of breeders in mediterranean environments has been slower than in other regions (Slafer et al., 1993), probably because of the limitations that the mediterranean environment places on plant growth, in particular, water stress. Yield increases associated with the genetic improvement of wheat have been demonstrated by comparisons of old and modern cultivars grown under the same conditions, and the mean yield increase attributed to wheat breeding in Western Australia is 6 kg ha-’ year-’ (Perry and D’Antuono, 1989), about one-eighth of that measured in Europe (Austin e t a ! . , 1980) and North America (Dalrymple, 1980). Breeders have successfully combined many desirable traits in cultivars, primarily by selecting for yield; however, except for two traits, time to anthesis and plant height, breeders have not been convinced of the value of selecting for other morphological and physiological traits recognized by physiologists as important in determining grain yield (Whan et a l . , 1993). Many studies have identified traits that have contributed to increased wheat yields in the past (Austin et a l . , 1980; Cox et a l . , 1988; Perry and D’Antuono, 1989; Kirby et a l . , 1989, Siddique et al., 1989a,b; Loss et al., 1989; Slafer et al., 1990; Siddique et a l . , 1990a,b; Slafer and Andrade, 1993), and the authors of these studies proposed that further improvements in many of these traits may lead to future yield increases. In addition, on the basis of physiological research, plant scientists have identified new unexploited traits that may increase wheat yields, for example, narrow xylem vessels (Richards and Passioura, 1981) and osmoregulation (Morgan, 1983). In the past, most physiologists and breeders operated independently, but recently a new level of cooperation has arisen. Future yield improvements may be hastened by a better understanding of factors that control growth, development, and yield of cereals (Shorter et a l . , 1991), and physiologists are helping breeders develop the most appropriate plants for particular environments. Richards ( 1 982) termed this breeding approach “analytical,” rather than empirical. This article reviews the wheat physiology/breeding work relative to the constraints of dryland cropping in mediterranean environments and explores opportunities for additional yield improvement associated with morphological and physiological traits. Other reviews have dealt with related aspects (Simmons, 1987; Ludlow and Muchow, 1990; Bidinger and Witcombe, 1989); however, these discussed a number of crops in a number of environments. By addressing wheat in mediterranean environments, we specifically review the progress of breeders and physiologists working in these regions and develop more concrete conclusions. Relatively few physiological studies have been conducted in mediterranean environments, therefore we occasionally draw on data from other environments. Improvements in disease and pest resistance, and increased tolerances to salinity, waterlogging, acidity, and mineral toxicities, are important contributions

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made by plant breeding; however, these are mainly localized stresses. Maintaining and improving grain quality is also an important breeding objective. Such types of breeding subprograms may also involve considerable physiological understanding, but they are not considered in this article. We begin by discussing the environmental constraints to crop growth in the parts of the world that experience mediterranean-type climates.

11. CONSTRAINTS IN MEDITERRANEAN ENVIRONMENTS Mediterranean climates are basically characterized by long, hot, dry summers and short, mild, wet winters. Cereals are mainly grown under dryland conditions in these areas, and although dryland implies unirrigated, water-limiting situations, the growth of cereals is not always limited by lack of water in mediterranean environments. Cereals are planted soon after the first autumn rains and they undergo vegetative growth in winter. They switch to reproductive growth as temperatures and photoperiods increase in spring, and they mature in early summer.

A. RAINFALL The constraints to cereal growth vary in mediterranean environments, but inadequate rainfall is usually the most limiting factor (Nix, 1975; Fischer, 1979). Cornish (1950) reported that 70-80% of the variation of yield in South Australia was due to variation in annual rainfall, and similar relationships exist in North Africa, west Asia and Western Australia (Srivastava, 1987; Blum and Pnuel, 1990; Karimi and Siddique, 1991a). According to Aschmann (1973), mediterranean environments receive between 275 and 900 mm annual rainfall, with the majority (>65%) in winter. Figure 2 illustrates the winter-dominated rainfall pattern at six sites that experience mediterranean climates, three in the Northern Hemisphere and three in the Southern Hemisphere. In general, winter rainfall exceeds crop demand because of mild temperatures, low evaporation, slow growth rates, and the high reliability of these rains. The coefficient of variation of the midwinter rainfall is about 6% at Merredin, Western Australia, whereas summer rainfall has a coefficient of variation of about 15%. Hence, intermittent drought during winter is rare, and, on the contrary, waterlogging can be a problem on some soil types in wet years. During spring, rainfall becomes less frequent, temperatures and vapor pressure deficits (VPDs) increase, and soil moisture is usually exhausted by the time

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-E E

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Figure 2 Mean monthly rainfall and temperature at six locations with mediterranean climates; (a) Merredin, Australia (31"20'S 118"17'E); (b) Cape Town, South Africa (3396's 19"29'E); (c) Rancagna, Chile (34"IO'S 7Oo45'W); (d) Aleppo, Syria (36"Il'N 37'13'E); (e) Rabat, Morocco (34"OO'N 6"50'E); and (f) Davis, California (38"32'N 121'45'W). Sites in the Southern Hemisphere are for January-December and those in the Northern Hemisphere are for July-June. Numbers in parentheses are the number of years of records used to calculate the means. Data from Wernstedt (1972).

the crop reaches maturity. This is often referred to as terminal drought and its timing varies according to the last spring rains, temperatures, soil type, and crop growth.

B. SOLARRADIATION Solar radiation has a large influence on temperature and evaporation regimes, and hence crop growth. In most mediterranean environments, midday solar radiation is about 6- 10 MJ m-* day-' in midwinter (Fig. 3), and it is unlikely that radiation limits crop growth, especially because temperatures and crop leaf areas are low at this time. In spring, however, when the leaf area index (LAI) is about 3, the lower canopy of the crop becomes shaded by the upper leaves and ear, and

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Month

Figure 3 Mean monthly climatic data for Aleppo, Syria (-) and Merredin, Australia (---); (a) solar radiation, (b) maximum and minimum temperatures, and (c) pan evaporation. Months are January-December for Merredin and July-June for Aleppo.

solar radiation may limit photosynthesis of the lower leaves. In midsummer, the elevation of the sun is high, there is a low incidence of cloud cover, and mean midday solar radiation is about 25-30 MJ m-* day-'.

C.TEMPERATURE As well as lack of rainfall, cereal growth can also be constrained by both high and low temperatures in mediterranean environments. Temperatures follow trends in solar radiation (Fig. 3). Summer maximum temperatures range between 25 and 40°C along western coasts and between 30 and 45°C inland and in the more easterly regions. Even if water was available in summer, as temperatures increase cereal development and respiration increase, while assimilation rates reach a plateaux, and thus growth at high temperatures (>30"C) is suboptimal. In most mediterranean environments, at least 1 month has an average temperature below 15°C (Fig. 2) and less than 3% of the year experiences minimum temperatures below 0°C (Aschmann, 1973). Mean monthly minimum tempera-

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tures in midwinter range from about 0 to 7°C and in some regions, especially inland areas, temperatures fall below 0°C during individual nights. Vegetative growth rates of cereals are restricted by low temperatures in midwinter and minimum temperatures are generally not low enough to cause long-term freezing damage to cereals during their vegetative stages of development. In northern Syria, severe frosts occasionally reduce the leaf area of cereal crops; however, the effect of frosts on grain yield during the vegetative stages is usually small because of compensation in later growth (Harris et al., 1989). Frost damage to the wheat stem and ear during early spring is an important constraint to wheat yields in mediterranean environments (Harris et al., 1989; Loss, 1989). Wheat becomes more susceptible to freezing damage as it enters its %productive stages of development, and the period from ear emergence to 2 weeks after anthesis is the most susceptible stage. Minimum temperatures may fall below - 2°C in early spring, killing the ear and/or restricting the movement of assimilates in the stem. Frosts can cause devastating yield losses to wheat crops that reach anthesis early in individual years. At these late stages of the life cycle, there is little opportunity for recovery from frost damage, although there is some compensation in the growth of unaffected grains and if spring conditions remain mild and moist, plants may be able to produce late tillers.

D. GROWING SMON In mediterranean environments, the period of crop growth is usually restricted by lack of rainfall, water deficits, and high temperatures at the start and end of the season. Potential evaporation (Epan) exceeds rainfall for a large proportion of the year. The timing of the first autumn rains can vary considerably, and sowing times may vary from year to year over a period of 8- 10 weeks. Developments in machinery and weed control have enabled farmers to sow soon after the first autumn rains and maximize autumn water use (Perry et al., 1989; Anderson and Smith, 1990; Kerr et al., 1991); however, because of the limitations of rainfall and evaporation, there is little scope for improving yield by extending further the period for crop growth. With the adoption of early sowing, there is also the risk of an extended period of dry weather after the initial autumn rain, and under such conditions, early sown crops can be subjected to water stress soon after emergence (Ken and Abrecht, 1992). Winter can change abruptly into spring and the termination of the growing season varies considerably depending on rainfall, temperatures, and soil type. Soil type is an important factor affecting the moisture status of a crop and soils can vary considerably within a small area. For example, there is often a variety of soils in Western Australia within a transect of 100-200 m, ranging from deep infertile leached coarse-textured soils on the higher parts of the landscape to

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poorly drained clay loams in the valley floors, including transitional and duplex soils. Most crops in Western Australia depend largely on current rainfall because of the poor water-holding capacity of many soils, particularly in low-rainfall regions. Crops grown on fine-textured soils, such as those common in northern Syria and South Australia, tend to rely more heavily on stored soil moisture. Rainfall largely determines the length of the growing season in mediterranean environments and the pattern and efficiency of water use has a large effect on wheat yields. Ludlow (1989) groups strategies of plant adaptation to waterstressed environments into three categories. Reduced life cycle of the plant to match the average growing season is termed escape. To maximize long-term yields in mediterranean environments, it would be ideal to have a wheat cultivar that not only tolerates years when the terminal drought is early, but one that also takes advantage of years when spring rains and mild temperatures extend the growing season. Maximizing water uptake and minimizing water loss are termed avoidance, whereas mechanisms that enable a plant to cope with reduced water content are termed drought tolerance. These strategies are useful in mediterranean environments and wheat plants are capable of all three. We will now examine the morphological and physiological traits that have or are likely to increase wheat growth and yield through reduced water, radiation, and temperature stresses. We also discuss how these traits have been measured and possible selection techniques for these traits in breeding programs.

111. BIOMASS PRODUCTION AND PARTITIONING Donald and Hamblin (1 976) define grain yield (GY) as the product of the biomass produced and the harvest index (HI; the proportion of the aboveground biomass that is partitioned to the harvested grain).

GY

=

Biomass x HI

Biomass can be increased by agronomic manipulation (early sowing and increased seed rate and fertilizer) or by genetic means. However, unless the increased biomass is matched to the life cycle of the crop, there is a risk of exhausting water sources before maturity is reached. In this article, we only consider the genetic paths to increased biomass. In the past, genetic increases in wheat yields around the world have largely been associated with changes in HI, whereas increases in biomass production have been small or negligible (Deckerd et al., 1985; Cox et al., 1988; Perry and D’Antuono, 1989; Austin et al., 1989; Siddique er al., 1989a; Safer and Andrade, 1993). Results from Mexico and Canada are exceptions (Evans, 1987;

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Hucl and Baker, 1987). Up to the 1970s, biomass production had not increased for wheats bred by The International Maize and Wheat Improvement Centre (CIMMYT), but more recent yield increases in cultivars bred under irrigation have been associated with increased biomass under water-limited situations (Evans, 1987). Also, Hucl and Baker (1987) found a positive correlation between GY and biomass production when comparing old and modem Canadian spring wheats. Figure 4 illustrates changes in GY, biomass, HI, and maturity with the year of release of Western Australian wheats. This figure is based on the data of Perry and D’Antuono (1989), but also includes three new cultivars not included in their study. The inclusion of these latest cultivars indicates that the rate of increase in GY between 1965 and 1990 is greater than in the preceding 100 years, and the rate of biomass increase associated with breeding is small (Fig. 4b). Most of the GY increase can be attributed to increased HI (Fig. 4c), and the duration from sowing to anthesis has decreased with selection for yield (Fig. 4d). This and other studies (CIMMYT, 1991; Slafer et al., 1993) also demonstrate that modern cultivars outperform older cultivars even in dry environments with low yield potentials. Several authors (Donald and Hamblin, 1976; Richards, 1987; Turner and Nicholas, 1987) suggest future wheat yields can be increased by increasing biomass production. Certainly, there is the potential for increased biomass production in mediterranean environments in some circumstances; barley is capable of producing more biomass and grain than wheat using the same amount of water (Siddique et al., 1989b; Lopez-Castaneda, 1992; Gregory et a!., 1992; Simpson and Siddique, 1993). However, as with increased biomass caused by agronomic manipulation, genetically increased biomass is not always translated into increased GY. Where lack of water is the major limitation to growth, it appears that it will be difficult to increase the biomass production of wheat significantly, especially when soil water is completely exhausted at maturity. Under these circumstances, higher biomass will be translated into higher yield when rainfall is used more efficiently for photosynthesis and it may be difficult to improve these fundamental physiological processes with conventional breeding methods. We will discuss in more detail the potential for increasing water-use and radiation-use efficiencies later and deal with assimilate partitioning first. The genetics of HI is probably more easily modified than biomass production, but given the nature of moisture stress during grain filling, large increases in HI are unlikely in mediterranean environments (Siddique et al., 1989b; Hadjishristodoulou, 1991). Aspects of the genetic, physiological, and environmental regulation of partitioning of assimilates were reviewed by Snyder and Carlson (1984), Gifford er al. (1984), and Wardlaw (1990). Germination, ear initiation, terminal

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Figure 4 Mean cultivar grain yield, biomass, harvest index and maturity score of wheat cultivars released between 1860 and 1990 in Western Australia when grown under the same conditions. Maturity score is the time from sowing to anthesis relative to Gamenya = 100, which takes about 110 days. Data from Perry and D'Antuono (1989) for cultivars up until 1979 (28 experiments each) and from Siddique et al. (1989a.b). Regan et al. (1992), and Loss et al. (1989) for the three latest cultivars (five experiments each).

spikelet, and anthesis act as physiological switches for the allocation of assimilates to different organs of the plant, hence phenology (i.e., the duration of each of these development phases) and assimilate partitioning are closely related.

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A. PHENOLOGY Change in phenology is the single most important factor that accounts for increased wheat yields in Australia (Perry and D' Antuono, 1989; Richards, 1991). Early Australian pioneers and wheat breeders recognized that matching the crop life cycle to the length of the growing season is one of the most important factors influencing crop growth and yield. As we will discuss in more detail, changes in phenology have had secondary effects on assimilate partitioning, pattern of water use, and other traits. Several reviews (Kirby and Appleyard, 1987; Simmons, 1987; Hay and Kirby, 1991) have detailed the current understanding of wheat development, and here we outline only the main features applicable to mediterranean environments. The switches from one stage of development to the next are determined primarily by genes sensitive to photoperiod, and both high and low temperatures. In general, there has been a trend to select for less sensitivity to photoperiod and vernalization, especially in mediterranean environments, thereby advancing development and reducing the time to reach anthesis (Hake and Weir, 1970; Austin et a l . , 1980; Davidson et al., 1985; Perry et al., 1987; Cox et a l . , 1988; Van Oosterom and Acevedo, 1992). Of the regions that experience mediterranean climates, wheat phenology has been described in the most detail in Western Australia. In several comprehensive studies, Kirby and Perry (1987), Kirby et al. (1989), Siddique e t a l . (1989a,b), and Loss e t a l . (1989) observed the development pattern of old and modern wheats bred in Western Australia, and illustrated how the life cycle of wheat has changed with selection for yield. Hence, we give frequent examples from these studies.

1. Vegetative Development The rates of leaf initiation and emergence in wheat are relatively constant when plotted against thermal time (i.e. accumulated temperature, as defined by Weir et a l . , 1984), although photoperiod and cultivar can have a small effect on these rates. The rate of primordia initiation is about one every 50"Cd (degree days, above a 0°C base), and the rate of leaf emergence is about one every 100"Cd (Kirby and Perry 1987). Tillers are initiated in the axils of the leaves and, if conditions are suitable, the first tiller appears after 2.5-3 leaves have emerged. Subsequent tillers appear at intervals equal to about one phyllochron (the interval in thermal time between the appearance of one leaf and the next). Ear initiation signals the end of vegetative development and the start of reproductive development. In general, modern Australian cultivars have faster rates of vegetative development than old cultivars, including faster rates of leaf appearance, shorter durations of vegetative growth, fewer leaves, and, hence, fewer tillers (Kirby et

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Kulin 1986 Garnenya 1960 Purple Strau 1860 Thermal time from sowing (OCd)

Figure 5 Duration between sowing, double ridge (DR), terminal spikelet (TS), anthesis (A), and physiological maturity (PM) for Western Australian wheats (Kulin. Gamenya, and Purple Straw) grown at Perth under irrigation. Year of release is indicated. Data from Kirby ef al. (1989) and Loss er al. (1989).

al., 1989; Siddique et al., 1989a,b). For example, the phyllochron intervals varied between 97 and 126"Cd for Western Australian wheats, and a modern barley cultivar had a phyllochron interval of 84"Cd. The longest duration of vegetative development was shown by the cultivar Purple Straw, released in the 1860s, which reached double ridge 958"Cd after sowing (Fig. 5). In contrast, the cultivar Kulin, released in 1986, reached double ridge 424"Cd after sowing, and, consequently, Kulin produced only 8 leaves on the main stem while Purple Straw produced 14. Roots have been less well researched than shoots because of the difficulties in root collection and measurement, hence our understanding of root growth and development is less complete than that for the shoot, particularly in mediterranean environments. This is not to say that roots are less important. In fact, root growth is an important component of the adaptation of wheat to dryland environments. The relationships between root and shoot development have been described by Klepper et al. (1984) and were recently reviewed by Klepper (1992). Root growth, with reference to mediterranean environments, is described later in this article.

2. Ear Initiation Ear initiation begins with spikelet initiation, progresses during a period of leaf growth, and ends at terminal spikelet formation. Double ridge is easier to recognize than ear initiation, and for practical purposes these have been considered the same. The rate of spikelet initiation is faster than that of leaf initiation (Kirby and Perry, 1987). Modern wheats have faster rates of spikelet initiation than do old cultivars; one every 12"Cd for Kulin and 33"Cd for Purple Straw (Kirby et al., 1989; Siddique et al., 1989b). Old cultivars have some vernalization requirement

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and when grown in areas with effective vernalizing temperatures, they have shorter durations between ridge and terminal spikelet than do modern cultivars-238"Cd for Kulin and 167"Cd for Purple Straw (Kirby et al., 1989). However, in warm areas with similar photoperiods, the vernalization requirements of the old cultivars are met more slowly and their duration between double ridge and terminal spikelet is extended, whereas the duration in modern cultivars is decreased (Fig. 5). Bingham (1969) suggested extending the period of ear development to increase sink capacity of the grain, and, interestingly, this has occurred with selection for yield in Western Australia. The period between double ridge and anthesis was 184"Cd longer in Kulin than in Purple Straw when grown in cool parts of Western Australia (Siddique et al., 1989b). However, warm temperatures during winter can cause rapid rates of development in wheats with little or no vernalization requirements and this results in reduced numbers of spikelets and number of grains per spike (Warrington et al., 1977; Shpiler and Blum, 1986).

3. Floret Initiation and Stem Elongation Floret development starts just before terminal spikelet formation, and coincides with the beginning of stem internode elongation. During this critical stage of development, the potential number of grains and the yield potential of the crop are determined, while there is an overlap of leaf, stem, root, and ear growth. Under favourable conditions, the central spikelet of a developing ear can produce up to 10 floret primordia, but only two to four survive and set grain in mediterranean environments (Siddique et al., 1989a; Slafer and Andrade, 1993). The number of tillers reaches a maximum at terminal spikelet and declines until anthesis. While old cultivars produce many more tillers than do modern cultivars, tiller survival is much lower-35% in old and 5 1 % in modern cultivars (Siddique et al., 1989b). The duration between terminal spikelet and anthesis of modern cultivars was shorter than in old cultivars (Kirby et al., 1989)-825"Cd for Kulin and 927"Cd for Purple Straw (Fig. 5 ) .

4. Anthesis Anthesis signals the end of vegetative growth and the start of grain filling, and its timing can have a large effect on cereal yields in mediterranean environments. The timing of anthesis is particularly important for determinate plants, such as cereals, because they only have a single opportunity for producing grain, as opposed to indeterminate plants, which are able to produce flowers over a considerable period of time. The time of anthesis that produces the highest longterm yield is a compromise between sowing time and the risks of frost, low biomass production, disease, high temperatures, and drought during grain filling. Several studies have demonstrated that cereal yields increase when anthesis is

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advanced because of decreased high temperature and moisture stresses during grain filling (Fischer and Kohn, 1966; Woodruff and Tonks, 1983; Kerr et ul., 1991). In addition, metabolic energy is required for the storage and retranslocation of assimilates (Geiger and Fondy, 1980), and once the minimum required structures of the plant have been produced, that is, the stem and leaves; then it is more efficient to partition growth directly into ears and grain, rather than producing additional vegetative growth and retranslocating the assimilates to the grain at some later stage. Anthesis has been advanced by genetic means and by the adoption of sowing very soon after the first autumn rains, when temperatures are warmer than in winter, hence improving early growth. Unlike ear initiation or terminal spikelet development, anthesis is easily visible and there has been a conscious selection for early anthesis in many environments (Halse and Weir, 1970; Austin et al., 1980; Davidson et ul., 1985; Perry and D'Antuono, 1989; Cox et al., 1988; Van Oosterom and Acevedo, 1992). Consequently, the duration between sowing and anthesis has decreased considerably (Fig. 5). In the study of Loss er al. (1989), Purple Straw flowered after 2132"Cd, whereas Kulin flowered after 1416"Cd. Early anthesis can also have detrimental effects. Unfortunately, during the period after emergence, the ear is very susceptible to frost damage, and as discussed earlier, frosts can cause devastating yield losses. Rapid development may also reduce the amount of biomass produced at anthesis, the number of sites for grain filling, and, hence, potential yield (Fischer, 1979). As was shown for sunflowers in Spain (Fereres et al., 1986) and sorghum in Texas (Blum and Arkin, 1984), very early-maturing plants may have restricted rooting depth and water use. In some early-sown crops that flower early, the high temperatures during vegetative growth increase the crop's susceptibility to diseases, especially on the early-emerging flag leaf (Wilson, 1989). Breeding for rapid development, that is, less photoperiod sensitivity and less vernalization requirement, has caused more variation in date of anthesis. In individual years and at locations where the optimum period of anthesis is very narrow, small variations in temperature can change crop development such that the crop reaches anthesis beyond the optimum time. For example, in Western Australia, farmers should sow early when the first autumn rains commence early to make use of the available moisture and warm temperatures (Perry et af., 1989; Kerr et al., 1991); however, there is a need for midseason cultivars that reach anthesis during the optimum period from these early sowing times, particularly in years when winter temperatures are above average. This could be achieved by incorporating a small vernalization requirement into existing cultivars (Anderson and Smith, 1990). Cultivars with a small vernalization requirement are less affected by temperature variations than are cultivars that require no vernalization, because vernalization prevents rapid pre-ear initiation development should an early break of the season be followed by a warm winter (Loss et al., 1990; Van

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Oosterom and Acevedo, 1992). Ludlow and Muchow (1990) also suggested that a greater sensitivity to photoperiod will overcome the effect on the timing of anthesis of year-to-year variations in temperature. With a large photoperiod sensitivity, anthesis is mainly triggered by daylength at a particular time of the year, irrespective of temperature. In contrast, yields may be increased by very early anthesis in dry areas with a low risk of frost, especially in seasons when autumn rains are delayed and sowing is later than average. Very early anthesis, that is, less than 95 days from sowing (<1 100-130O0Cd), may be appropriate for areas of North Africa (Van Oosterom and Acevedo, 1992) and Western Australia.

5. Grain Development The development of wheat culminates with the formation of grain. Grain filling can be divided into three phases. After anthesis there is a short period of exponential growth, sometimes referred to as the lag phase, during which time the cells of the endosperm divide rapidly and the potential size of the grain is determined. During the second phase, starch is deposited in the endosperm and the rate of growth is constant when expressed as thermal time. The final phase begins when lipids are deposited in the phloem strands supplying the grain and the growth rate declines until maximum grain weight is achieved. The process of grain growth can be considered as two components-rate, which is reflected in the rate of biochemical reactions involved in the synthesis of starch and protein, and duration, which is a reflection of the developmental program of the grain (Jenner et al., 1991). Loss et al. (1989) and Austin et al. (1989) studied grain growth and development of old and modern wheats under irrigation in a mediterranean and a temperate climate, respectively. Cultivars with a short duration from sowing to anthesis, which was the case for most modem cultivars, also had a long duration of grain growth, i.e., about 800°Cd, or 200"Cd longer than the oldest cultivars (Fig. 5). This may explain why modern cultivars are more able to exploit seasons and environments where conditions in spring are mild and grain filling is not terminated by drought and high temperatures. Modern cultivars that reached anthesis quickly also had shorter lag phases than did old cultivars-6% of the duration of grain growth for Kulin and 18% for Purple Straw. We will discuss changes in the rate of grain growth later.

B. GROWTH AND MORPHOLOGY Breeders have changed the structure of cereals considerably, both indirectly through changes in phenology and directly through the introduction of dwarfing genes.

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1. Leaves and Tillers One path to improved wheat yields is to increase HI by decreasing the proportion of biomass in leaves and tillers (Donald and Hamblin, 1976). Selection of wheats for Western Australia and southwestern Iran has favored cultivars that produce fewer main stem leaves and fewer tillers than the old cultivars (Siddique ef al., 1989b; Ehdaie and Waines, 1989). Modern cultivars produce only primary tillers associated with the first two or three leaves. These tillers have three or more of their own leaves, their own nodal roots, and they are largely independent of the subtending leaf for assimilate supply, hence their survival rate is high, about 50%. Older wheats produce many more tillers and subtillers than do modern cultivars, of which 35% survive to produce grain (Siddique et al., 1989b). Given the inefficiencies in retranslocation (Geiger and Fondy, 1980), tiller death represents a net loss of assimilate by the plant and increases the water used before anthesis. Siddique el al. (1989b) suggested that additional increases in yield in mediterranean environments may result from further decreases in tiller number and increases in tiller survival, particularly in low-rainfall areas where biomass production is low. They acknowledged that this may also reduce the ability of the plant to take advantage of years when conditions are favorable and tiller survival is inherently high. The single mainstem wheat, or uniculm, was first proposed by Donald ( 1 968) as an important characteristic for dry environments. Islam and Sedgley (1981) found that wheats that were surgically restricted to two shoots per plant used more water after anthesis and produced greater GY s than did unrestrictedtillering wheats. Contrasting results were observed by Marshall and Boyd (1985), who found that two Israeli biculms, despite having larger ears, yielded 25% less biomass and 30% less grain than conventional cultivars from Western Australia. In addition to the differences in rainfall and soil types between the two studies, the comparison within the Marshall and Boyd (1985) study was probably complicated by the poor adaptation of the Israeli cultivars outside of Israel. More recently, Whan et al. (1989) and Yunusa and Sedgley (1992) reported no advantage of limited-tillering wheat breeding lines in water use or GY under dryland conditions in Western Australia. Reduced tillering was also of no advantage in the semiarid wheat-growing regions of western Canada (Hucl and Baker, 1991) or for barley in South Australia (McDonald, 1990). In these cases, the lack of yield increases with reduced-tillering wheats is probably related to their larger leaves and to the absence of changes in leaf area index or biomass when compared to conventional wheats (Richards, 1988). In addition, reducedtillering wheats tend to have other inefficient assimilate partitioning, that is, large specific leaf weights, high stem and ear densities, and a high proportion of the ear as chaff. Turner and Nicolas (1987) proposed that rapid, vigorous seedling growth should be advantageous on coarse-textured soils in water-limiting environments

MEDITERRA”

WHEAT YIELD INCREASES

2 45

because of more efficient use of water. Seedling growth has important consequences for the pattern of water use and is discussed later in this article.

2. Roots Root growth is strongly influenced by moisture and nutrient availability, soil type, and cultivation (Hamblin et al., 1990). During seedling and tillering growth, more assimilate is usually partitioned to roots than to shoots, but after anthesis root growth is reduced, and when grown under favorable conditions, roots frequently account for as little as 10%of the total crop biomass at maturity (Lupton et al., 1974). Under drought conditions, however, roots may comprise as much as 60% of the total crop biomass at maturity (Gregory et al., 1984; Hamblin et al., 1990; Siddique et al., 1990b). Hamblin and Tennant (1987) argue that rooting depth or rate of root elongation is a better selection criteria for maximizing water uptake than either root length, weight, or density in the mediterranean environment of Western Australia. Cereal root densities in the top 30 cm of soil are very high, compared with grain legumes (Gregory, 1988), and this has been interpreted as evidence that less roots in the surface soil may increase rooting depth and cereal yields (Richards, 1991); however, differences in root morphology and physiology are also important. Hamblin and Tennant (1987) also measured greater root lengths for cereals when compared to grain legumes, but water uptake was better correlated with rooting depth than with total root length. Water uptake per unit root length was greater in the grain legumes than in the cereals, probably due to lower axial resistances in the xylem vessels of the grain legumes. Smucker (1984) suggests that cereals adopt a “conservative rooting strategy,” typified by an extensive root morphology that uses water slowly. In contrast, legumes tend to have an “opportunistic rooting strategy,” which is characterized by a less extensive root system that uses water rapidly. Cereals have evolved from a predominantly nodal to a predominantly seminal root system, particularly when grown at high densities (MacKey, 1986). Seminals develop earlier and deeper, and are finer and more efficient at water uptake per unit dry weight than are nodal roots (Passioura, 1976). Seminal roots also have a higher resistance to water flow and tend to conserve soil water more than nodal roots. Producing more roots, particularly deeper roots to obtain more water, appears to be a logical drought avoidance mechanism; however, roots are a major sink for assimilates, requiring twice as much assimilate to produce the same amount of biomass as shoots (Passioura, 1983). In cases where a small amount of water is stored in the subsoil, the cost of producing deep roots to obtain this water may be less than the extra assimilate that can be produced from the additional transpiration and photosynthesis. In m e d i t e ~ n environments, e~ soil moisture is almost always exhausted at maturity (Cooper et al., 1987; Siddique et al., 1990b; Gregory et al., 1992),

246

S. P. LOSS AND K. H. M. SIDDIQUE

and cultivars that produce less roots, particularly in the top soil, may be at an advantage. In fact, this is what has occurred with selection for yield in Western Australia (Siddique et al., 1990a,b). Modern wheats produce less root dry matter and lower root: shoot ratios than do old wheats, which probably relates to their earlier ear sink development (double ridge), fewer tillers, and fewer adventitious roots associated with the tillers. Patterns of root and shoot growth for old and modern wheats are illustrated in Fig. 6. Modem cultivars have root densities of 10 cm ~ m in-the~ top 10 cm of soil, about half the density of the old wheats. Passioura (1983) estimated that root densities of 0.5 cm ~ m are- adequate ~ for removing all the water stored in soils, although higher densities may be required for nutrient uptake. In the past, roots for nutrient uptake were less important because nutrient uptake could be improved by increasing fertilizer application, but, recently, more economical use of fertilizer and reduced groundwater pollution from fertilizer leaching are considered desirable. MacKey (1973) claimed that because root growth tends to mirror shoot growth, semidwarf cereals may have reduced root growth when compared to tall cereals; however, the studies of Siddique et al. (1990b) and Holbrook and Welsh (1980) demonstrate this is not the case in dry environments. The growth of shoots by modern semidwarf and old tall cultivars is similar, and modern cultivars produce deep roots earlier than do old cultivars (Siddique et al., 1990b). By anthesis, the roots of old and modern wheats reach the same depth. MacKey (1986) suggested that root growth in modern wheats is reduced soon after anthesis because they

0

100

200 300 Root dry matter (g.m-2)

400

Figure 6 Patterns of root and shoot growth for Western Australian wheats grown at Merredin. Terminal spikelet (TS)and anthesis (A) are indicated with arrows. Reproduced with permission from Siddique eta!. (19Wa).

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247

have fewer lower leaves that provide the roots with assimilates, compared to old wheats. Although advocated by Bums (1980), Passioura (1983), and Richards (1991), it is not known if yields can be increased by further reductions in the growth of roots in the surface soil. Using a simulation model, Miglietta et al. (1987) predicted that for a silty clay-loam, increasing rooting depth below 80 cm increased water uptake early in the season but left little stored in the soil for use later in the season, hence yields were reduced. Techniques for measuring rooting depth are unsuitable for screening large numbers of breeding lines, and we are aware of only one example where rooting depth has been successfully incorporated into a wheat breeding program. In the semiarid region of western Canada, Hurd et al. ( I 972) selected parents for deep and prolific root systems in sloping plastic boxes and produced a deep-rooted and high-yielding line that was later released as a commercial cultivar. Using similar techniques, others have observed a wide variation in wheat root growth (Derera el al., 1969; MacKey, 1983; Sharma and Lafever, 1992). Other techniques for examining roots, such as hydroponics, herbicide placement at various soil depths, and minirhizotrons, have also been examined (Gregory, 1989), but these are also unsuitable for selection in routine breeding programs.

3. Grain Growth Temperatures above 30°C are common during grain filling in mediterranean environments, causing an increase in the rate of grain growth. However, this increased rate does not compensate for the reduction in duration of grain filling (Sofield et al., 1977; Wardlaw et al., 1980, 1989) and, consequently, grain sizes are smaller in mediterranean than in temperate environments. In fact, selection for yield in Australia has favored cultivars with many more grains but smaller grain weights (Perry and D’Antuono, 1989). Cultivars that are able to fill their grain quickly may reach physiological maturity before moisture stress limits grain growth (Bruckner and Frohberg, 1987), and in areas of high frost risk, cultivars with delayed anthesis and fast grain growth rates reduce the risk of frost damage without increasing the risk of moisture stress during grain filling (Loss et al., 1989). The proportion of GY that is derived from assimilates produced after anthesis varies from 70 to 95% depending on the degree of moisture stress (Rawson and Evans, 1971; Austin et al., 1977; Bidinger et al., 1977; Pheloung and Siddique, 1991; Kobata et al., 1992). Although modem cultivars have fewer leaves, the rate of grain growth is not reduced because most of the assimilate used for grain growth is produced by the upper canopy. These assimilates are derived mainly from the spike, the flag leaf, and its sheath (Austin and Jones, 1975; Rawson et al., 1983), and it has long been recognized that awns can also substantially

248

S. P. LOSS AND K. H. M. SIDDIQUE

increase spike photosynthesis and yields under dry conditions (Atkins and Norris, 1955; Bremner and Rawson, 1972; Evans et al., 1972; Olugbemi et al., 1976). Under postanthesis moisture stress, remobilized assimilates that were produced before anthesis make a considerable contribution to GY. These assimilates are nonstructural soluble carbohydrates that are stored in the stem before anthesis and during the lag phase of grain growth. They are rapidly depleted during grain filling, and other structural substances are also remobilized as the vegetative parts of the plant senesce. Modern semidwarf cultivars are more efficient at remobilizing dry matter assimilated before and after anthesis compared to old tall cultivars, and under irrigated conditions the old tall cultivars retained some of the stored assimilate in the stem at maturity (Pheloung and Siddique, 1991). Hence, modern cultivars are better able to take advantage of favorable conditions after anthesis, and further increases in the efficiency of remobilization could increase HI and GY. The application of chemical desiccants on leaves and stems has been used to simulate postanthesis water stress and to screen genotypes for their ability to retranslocate assimilates to the grain (Blum et af., 1983; Nicolas and Turner, 1993; Regan et al., 1993). Selection for large grains in breeding populations treated with the chemical dessicant effectively increased GY under postanthesis moisture stress with no change in the number of days to ear emergence or plant height, whereas selection without the chemical desiccant treatment did not improve grain filling under postanthesis stress (Blum et al., 1991). This method is best conducted in the absence of leaf diseases and under irrigated conditions or in wetter environments (Nicolas and Turner, 1993; Whan et al., 1993). Under very dry conditions, GY is hardly affected by the desiccant and there is little discrimination between genotypes. Chemical desiccants are also more convenient to apply to genotypes of similar maturity because the treatment can be applied to all genotypes in a single application, otherwise the desiccant has to be applied individually to each genotype after they reach anthesis (Regan et al., 1993). Loss et al. (1989) and Austin et al. (1989) examined the growth of individual grains of old and modern wheats under well-watered conditions. Although there were significant differences in the rates of grain growth between cultivars, the rates were not related to grain size; several old cultivars had slow grain growth rates and large grain sizes. Large grains have been associated with a low number of grains, and hence these genotypes are of no yield advantage (Siddique et al., 1989b; Slafer et al., 1993). However, Carlton (personal communication, 1993) had some success in combining high growth rates with high grain numbers in breeding populations in Western Australia. The effects of grain growth rate, duration, and number must be considered simultaneously to increase GY. The

MEDITERRANEAN WHEAT YIELD INCREASES

2 49

broadsense heritability of grain growth rate ranged from 60 to 92%, depending on the genetic background, and grain growth rate was stable over generations (F2-FJ. In contrast to the frequent sampling used in other studies, Carlton (personal communication, 1993) took only two samples during the linear phase of growth to provide an estimate of grain growth rate. This technique was able to differentiate between breeding lines without being too laborious, and considering the high heritability of grain growth rate, it may be useful for the selection of parents.

4. Harvest Index and Ear Growth Harvest index is important for agronomists and breeders concerned with crop production, and because water stress usually occurs after anthesis in mediterranean environments, wheat yields are often characterized by a low HI when compared to temperate environments (Austin et al., 1980). In mediterranean environments, the HI has risen from about 23% for old wheats to about 38% for modern cultivars and is well correlated with yield increases (Perry and D’Antuono, 1989; Siddique et al., 1989a,b; Ehdaie and Waines, 1989; Slafer et al., 1990). The increase in HI of modern compared to old cultivars can be attributed to earlier anthesis and reduced investment in the stem and, to a lesser extent, the roots. The worldwide introduction of the dwarfing genes from the Japanese cultivar ‘Norin 10’ has changed assimilate partitioning and increased HI. This has been clearly demonstrated with wheat isolines that contain differing numbers of the dwarfing genes (Brooking and Kirby, 1981; Fischer and Stockman, 1986; Siddique et al., 1989a; Youssefian et al., 1992). The time of maximum stem, root, and ear growth coincide, and it has been proposed that ear growth is limited by competition for assimilates at this stage (Siddique et al., 1989a; Slafer et al., 1990; Slafer and Andrade, 1993). This is also supported by results of shading during this time, which reduces ear growth and floret survival (Thorne and Wood 1987; Savin and Slafer, 1991). As a result of the shortage of assimilates or some hormonally mediated process, about 50% of the florets in old, tall wheats die before ear emergence, whereas in modern semidwarfs the stem weight is reduced and considerably more assimilates are available for investment in the ear. Consequently, semidwarfs set more grains per ear and per unit area, and increased yields are strongly related to increased grain number (Perry and D’Antuono, 1989; Ehdaie and Waines, 1989; Slafer et al., 1990; Slafer and Andrade, 1993). Although many studies indicate that HI and yield are well correlated, selection for the HI in simulated breeding populations was less effective at increasing GY than was selecting for yield (Whan et al., 1982).

S. P. LOSS AND K. H. M. SIDDIQUE

250 0.5

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.

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Figure 7 Relationships between ear: stein ratio and harvest index for a range of old and modern Western Australian wheats grown at Wongan Hills, Perth, and Merredin. Reproduced with permission from Siddique er ul. (1989a).

The ear:stem ratio, that is, the size of the ear relative to the stem, is an index of the competitive strength of the ear and it is highly correlated with the HI, grains per ear, and GY (Siddique et al., 1989a; Slafer et af., 1990; Slafer and Andrade, 1993). The relationships between ear:stem ratio and HI vary between sites (Fig. 7); however, the ranking of cultivars remains the same. The ear:stem ratio gives a better indication of yield potential than does HI because it is determined early in the life cycle and, unlike HI, it is not affected by environmental stresses after anthesis. When ear dry weight is about 1 mg, stem dry weight varies from 50 mg in modern cultivars to 250 mg in old cultivars (Siddique et a l . , 1989a). The effects of selecting for higher ear: stem ratio have been investigated in breeding populations and ear: stem ratio offers promise as a predictor of HI and yield potential (Siddique and Whan, 1994). Broad sense heritabilities for ear: stem ratio ranged from 50 to 90% and this trait was strongly correlated between generations and sites. Ear: stem ratio is a highly stable characteristic and has the potential for effective selection in early generations. Selections for high ear: stem ratio in the F2 generation resulted in greater HI and GY values in some lines than in their parents (Siddique and Whan, 1994). Unlike HI, ear:stem ratio is independent of GY because it does not contain GY as a component of the ratio. It is unlikely that ear: stem ratio will be adopted as a routine measurement in breeding programs, because it is laborious and time consuming. However, this trait could be used to identify superior parental genotypes and early generation

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selections from special crosses, because it is an important physiological attribute that reflects the ability to partition assimilates (Siddique and Whan, 1994).

IV.WATER USE We have already discussed how rainfall is the most limiting factor in dryland mediterranean cropping, but we only mentioned in passing how plant morphology and physiology can affect water use. Passioura (1977) proposed that in water-limited environments, biomass production is a function of the water used by the crop (WU) and the efficiency with which it is converted into biomass (WUE); Biomass

=

WU x WUE

Hence, GY = WU

X

WUE

X

HI

This model has become a framework for examining ways to improve crop yields, especially in water-limited environments (Acevedo, 1987; Turner et al., 1989; Ludlow and Muchow, 1990; Richards, 1991). WU is usually considered as soil evaporation (E) plus transpiration (T), and water runoff and drainage are often negligible in dryland areas and hence are ignored. A frequent feature of crops grown in mediterranean environments is that despite differences in biomass production, all the soil water is depleted by maturity (Siddique et al., 1990b; Cooper et al., 1987; Gregory et al., 1992). In mediterranean environments, E usually accounts for about 40% of the WU, most of which is lost early in the season when the crop biomass and ground cover are small (Cooper et al., 1983; French and Schultz, 1984; Siddique et al., 1990b). WUE is the amount of dry matter that is produced per unit of WU. It sometimes refers to the efficiency of WU for grain production; however, we shall restrict its use to biomass, unless specified otherwise. Any mechanism that reduces E and increases T will usually increase WUE because T is closely tied with photosynthesis and biomass production. When water deficits cause stomata1 closure, T and CO, assimilation rates are decreased. Increases in WUE will only increase growth and yield when WU and HI are maintained. For example, in a time of sowing experiment, Connor et al. (1992) observed that maximum WUE did not correspond to maximum growth or yield because late-sown crops, which had the highest WUE, also used much less water than early-sown crops. Increasing the proportion of WU that is used through T can be achieved agronomically, for example, by early sowing, mulching the soil, narrow row spacing,

2s2

S. P. LOSS AND K. H. M. SIDDIQUE

or encouraging early growth with fertilizer application (Cooper et al., 1987; Anderson, 1992). Here we only consider the genetic mechanisms of increasing T and WUE.

A. WATER-USE PATTERN AND EARLY VIGOR In mediterranean environments, the pattern of water use through the season is almost as important as the size of the water supply in determining GY (Passioura, 1983). The efficiency of transpiration and WUE are high when the VPD between the leaf and the air is low, and hence in mediterranean environments, plants use water most efficiently in winter when humidity is high and temperatures are low. Consequently, yields may be increased by increasing the proportion of water transpired during winter and this can be achieved genetically, through early vigor and/or appropriate phenology. French and Schultz ( 1984) suggested increasing preanthesis WU to improve yields of wheat in South Australia, although this is not the case elsewhere in Australia. Nix and Fitzpatrick (1969), Passioura (1977), and Richards and Townley-Smith (1987) advocated more postanthesis WU, whereas Siddique et a / . (1990a) showed a strong negative relationship between preanthesis WU and GY in Western Australia. This result was largely related to the early maturity of the modern wheats, so that even though their leaf conductances and transpiration rates were higher than those of the old wheats in winter (20-80 days after sowing), the durations from sowing to anthesis were shorter for the modern wheats and so they used less water before anthesis (Fig. 8). These WU patterns reflect the patterns of crop growth rates observed by Karimi and Siddique ( 1991b). The WUE for biomass production did not improve with selection for yield (about 45 kg ha-‘ mm-I), but as a result of earlier anthesis and improved HI, the WUE for grain production increased by 46% (Siddique et al., 1990a). Passioura (1 983) suggested an optimum ratio of pre- and postanthesis WU of 2: 1 in eastern Australia. French and Schultz (1984) measured 2.5: 1 in the mediterranean environment of South Australia, and in a drier part of the Western Australia wheatbelt, higher ratios (3-5: 1) have been measured (Rickert et al., 1987; Siddique et al., 1990a). It is apparent that the ratio of water used before and after anthesis in each environment relates to the severity of moisture stress in the postanthesis period. Siddique et al. (1990a) showed that modern wheat cultivars rapidly decreased their leaf conductances when the soil moisture content began to fall in spring, whereas the old wheats had a gradual decrease in leaf conductances. They described modern wheats as “opportunistic” in that they develop rapidly, reach anthesis early, and use water rapidly when it is most available, but they markedly reduce their water uptake when soil moisture becomes limiting. In contrast, the

MEDITERRANEAN WHEAT YIELD INCREASES *.OI

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253

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Garneya

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-

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0.5

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0

40

80

Days After Sowing

120

160

Figure 8 Transpiration patterns of old and modern Western Australian wheats grown at Merredin. Average annual rainfall is illustrated and arrows indicate anthesis data. Data from Siddique er al. (1990a).

“conservative” old cultivars are less effective at avoiding drought. They reach anthesis later, use water slowly when soil moisture is high, and slowly reduce their water uptake when soil moisture becomes limiting. On coarse-textured soils, rainfall is in excess of crop use during early winter and a significant proportion is lost through E and deep drainage. By selecting plants for rapid early growth and leaf area development, and a prostrate growth habit that covers the soil surface, drainage and E can be reduced significantly. In addition, plant growth is exponential in nature, so that a small increase in growth during the vegetative stages of development will often cause a considerable increase in transpiration and biomass production during the later stages, and if harvest index can be maintained, yields will also be increased (Richards, 1987). Turner and Nicolas (1987) observed a strong positive relationship between GY and aboveground biomass at the five- to six-leaf stage, and Siddique et al. (1989b) showed that compared to wheat, barley had a fast emergence rate after sowing and a more rapid development of leaf area that was related to its high biomass production and GY. Van Oosterom and Acevedo (1992) also found a relationship between ground cover and vigor and barley GY in Syria under terminal drought conditions. However, in other experiments, the relationship between early vigor and GY was equivocal (Acevedo, 1987; Siddique ef d., 1990a,b; Damisch and Wiberg, 1991). In Western Australia, Regan el al. (1992) found no correlation between the early biomass production of introduced geno-

2 54

S. P. LOSS AND K. H. M. SIDDIQUE

types and their GY; however, these genotypes were not adapted to local conditions and thus were inappropriate to assess this relationship. Whan et al. (1993) examined the biomass production and GY of the progeny of these genotypes when crossed with a number of standard local cultivars. High biomass was associated with high GY at dry sites but no relationship was evident at wet cool sites. Broad sense heritabilities for biomass production varied from 60 to 80%, depending on the cross. Clearly, the choice of parents and the target environment will play an important role in the success of selecting for improved early biomass production and GY. Increased rooting depth associated with early vigor may improve water extraction in coarse-textured soils where water is stored at depths of greater than 1 m. Studies with temperate grasses suggest that improved pasture yield and high growth rates in winter are associated with earlier commencement of reproductive development (Kemp, 1988; Kemp et al., 1989); however, recent studies with wheat found that rapid early dry matter production was not related to stage of development (Regan et al., 1992; Rawson, 1991). A better understanding of the effects of early vigor on WU and GY will enable physiologists and breeders to pinpoint the environments where early vigor is likely to be of benefit. Determining aboveground biomass by destructive sampling is a timeconsuming and labor-intensive task, and visual ratings of early vigor are unable to categorize breeding lines accurately. Spectral reflectance measurement in the visible, near-infrared, and mid-infrared regions is a new and promising technique for estimating early vigor in breeding programs (Smith et al., 1992; Elliott and Regan, 1993). This technique has the advantages of being nondestructive, rapid, and accurate, but further work is required to investigate the effects of environmental factors and genotypic differences in morphological characters such as tiller number and growth habit on the reflectance measurements.

B. XYLEMDIAMETER Passioura (1972) proposed that where crops rely heavily on water stored in the soil and the soil water is almost exhausted by anthesis, slowing the rate of water extraction during vegetative growth should increase the amount of water available after anthesis, and hence improve HI and yield. Richards and Passioura (1981) calculated that decreasing the metaxylem diameter in the upper part of the seminal wheat roots to less than 60 p m would increase the hydraulic resistance and slow the rate of water uptake from the soil. They developed a selection technique wherein roots of seedlings grown in tubes were washed, cut, and examined under a microscope. They crossed a landrace wheat from Turkey that had narrow xylem vessels with local cultivars and selected progeny with narrow xylems, which out-yielded the standard cultivars by up to 11% under stored

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moisture conditions in subtropical eastern Australia (Richards and Passioura, 1989). Biomass production was slightly increased and HI was greater in the narrow xylem wheats than in the standard cultivars, suggesting increased postanthesis WU and a longer period of grain filling. Narrow xylem progeny and standard cultivars produced similar GYs at the wet sites because nodal roots of the narrow xylem genotypes proliferated in the top soil, so that water uptake was not impaired. Little moisture is stored in the coarse-textured soils of Western Australia, where crops tend to rely heavily on current rainfall; hence this “conservative” drought-avoidance strategy is probably of little use in this environment. It may be of value for other mediterranean environments with fine-textured soils, especially in years with early terminal drought, but it has yet to be tested in these areas.

C. GLAUCOUSNESS Studies with isogenic lines have shown that under postanthesis water stress, epicuticular wax or glaucousness increases radiation reflectance, reduces leaf temperature, increases T, and hence increases leaf survival (Johnson et al., 1983; Richards et al., 1986). Glaucous wheats out-yielded nonglaucous wheats by 7% under drought conditions and suffered no yield penalty under irrigated conditions. Photosynthesis was reduced in wheats with glaucousness but not to the extent of transpiration, hence WUE was increased and HI was unaffected. Glaucousness develops mainly on the leaf sheaths and on the abaxial surfaces of leaves, and it reaches a maximum at flag leaf emergence (Richards et al., 1986). Water stress enhances the development of glaucousness, but the amount under stressed and unstressed conditions is positively correlated, hence it can be selected for under either condition (Nizam Uddin and Marshall, 1988). Glaucousness is simply inherited, being controlled by a single gene, with minor genes controlling intensity (Richards, 1983). Leaf pubescence or hairs in sorghum also have effects on WUE and yield under water stress similar to the effects of glaucousness (Baldocchi et al., 1983), but this has not been examined in wheat.

D. ABSCISSIC ACIDACCUMULATION Abscissic acid (ABA) plays an important role in the regulation of plant water relations, and its synthesis and accumulation are a natural drought avoidance mechanism in most plants (Turner, 1986). Within a few minutes of a reduction in the turgor pressure of leaf and root cells, ABA rapidly accumulates in the leaves, where it causes stomata1 closure, reduced transpiration, and decreased photosynthesis. In the growing points, ABA inhibits cell expansion and division.

256

S. P. LOSS AND K. H. M. SIDDIQUE

Other effects of ABA include advancing the rate of plant development and increased assimilate partitioning to the roots. When the water stress is relieved, ABA concentrations decrease while transpiration and growth recommence. Henson and Quarrie (1981) developed a detached-leaf test to screen for ABA accumulation under moisture stress and found a threefold variation in the rate of ABA accumulation within wheats. High-ABA lines were selected in the F2-F4 generations and tested at F, under irrigated and pre- and postanthesis drought in the United Kingdom. This trait was simply inherited (Quarrie, 1981). High-ABA wheat lines were smaller, flowered earlier, had fewer spikelets per ear, and used less water (Innes and Quarrie, 1987). High-ABA lines out-yielded low-ABA lines by about 5% in the test environment and had an improved WUE for grain production, probably because of lower leaf conductances that were sufficient to reduce water loss but not photosynthesis. In contrast, Read et al. (1991) tested these lines in Oklahoma and found that the lines selected as “low ABA” from the detached-leaf test had slightly higher ABA contents than did the high-ABA lines. The low-ABA lines also had lower stornatal conductance, much greater biomass, and higher WUE. Genotypes that are specifically selected for variation in ABA accumulation are being studied to predict more reliably the role of ABA in regulating WUE in the field. Although rapid ABA accumulation may improve yields under pre- and postanthesis drought, short periods of ABA accumulation in the ear near anthesis cause pollen sterility and drastic reductions in grain set (Morgan and King, 1974; Morgan, 1980; Saini and Aspinall, 1982). Consequently, high-ABA lines have a risk of major failure in most environments, unless greater pollen tolerance to ABA can be found and utilized.

E. OSMOREGULATION Osmoregulation or osmotic adjustment is a decrease in cell osmotic potential due to the accumulation of solutes rather than to a decrease in cellular volume (Turner and Jones, 1980). Under water stress, plants that accumulate solutes within cells maintain turgor pressure, stornatal opening, transpiration, photosynthesis, and growth, and in this way, plants tolerate mild dehydration. The maintenance of turgor pressure can prevent large increases in ABA near anthesis, thereby maintaining seed set. Although osmoregulation does not affect WUE, it enables roots in a dry surface soil to survive until the next rainfall event, or alternatively, it can maintain root growth and increase WU if water is stored at depth (Morgan and Condon, 1986). The rate of leaf senescence may also be reduced, hence retranslocation of assimilates is enhanced and HI is improved. Blum (1988) concluded that osmoregulation is one of the most important and effective components of drought resistance. In field experiments in eastern Australia, segregating lines selected for high

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osmoregulation in the glasshouse showed greater turgor maintenance and yielded 7-50% more than lines with low osmoregulation (Morgan, 1983; Morgan et al., 1986). This advantage was evident in a range of moisture regimes, indicating that this trait is environmentally robust and has no detrimental effects in highrainfall years. The capacity for high osmoregulation appears to be controlled by one or two genes and is simply inherited. Because cell expansion involves the accumulation of solutes to maintain turgor pressure as the cell volume increases, plants can be selected for high osmoregu~ationby measuring coleoptile expansion under water stress in petri dishes (Morgan, 1988). This technique is rapid and inexpensive, and hence is suitable for routine screening of breeding populations. Wheats with a high capacity for osmoregulation have not been tested in mediterranean environments and more work is required in this area.

F. CARBON ISOTOPE DISCRIMINATION The CO, in the atmosphere contains about 1% of the naturally occurring carbon isotope, 13C, but during photosynthes~splants discriminate against I3C in favor of the lighter isotope, I2C. The amount of discrimination is largely determined by the ratio between intercellular and atmospheric partial pressures of C 0 2 and hence indicates the efficiency of transpiration and photosynthesis. Carbon isotope discrimination (A) is a measure of the ratio of I3C and I2C in piant material compared with the same ratio in the atmosphere and depends on the balance between stomatal conductance, photosynthetic capacity, and transpiration efficiency ~ F ~ q and u hRichards, ~ 1984). The relationship between A and GY varies according to the environment. Increased salinity, decreased water availability, soil compaction, and increased VPD all cause lower A in plant material because of their effects on stomatal conductance or photosynthetic capacity (Condon et al., 1992). Also variations in E, WU patterns, root growth, and boundary layer conductances can affect the relationship between A and growth (Richards et al., 1993; Turner 1993). In a diverse range of wheat lines grown in eastern Australia, biological yields and GYs were positively correlated with A measured on the peduncle when grown in a wet environment but negatively correlated in a dry environment (Richards, 1991). No relationship was evident in a dry environment in Western Australia (Turner et al., 1989). In nothern Syria, A measured on grain samples was ~ s i t i v e l ycorrelated with barley GYs at dry sites and showed no relationship at wet sites, presumably because of the lower stomatal conductances under stress and the large contribution of preanthesis assimilate to GY at the dry sites (ICARDA, 1987; Craufurd et al., 1991). Differences in the relationships between A and yield may also relate to differences in the plant samples taken for measuring A in these studies. The A measure may reflect the improved WUE of early-vigor crops that pro-

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duce a rapid groundcover early in the season when the VPD is low. In California field experiments, A was negatively correlated with WUE, positively correlated with biomass, and there was also a strong negative relationship between A and days to ear emergence (Ehdaie et al., 1991). At a dry site in northern Syria, where early anthesis was a large advantage, A was also negatively correlated with days to ear emergence (Craufurd et al., 1991). Read et al. (1991) found that A was positively related to GY under drought conditions in Oklahoma, and the relationship had a greater slope in wheats with high ABA accumulation. Richards et ul. (1993) conclude that low-A genotypes should yield more in longduration environments when soil E is a small component of evapotranspiration or in environments where there is little variation in VPD during periods of rapid growth. Hence, the use of A to select for high GY in mediterranean environments is probably limited. This trait can be measured quickly and accurately with a mass spectrometer and shows considerable genetic variation in wheat (Condon et al., 1987). Condon and Richards (1992) measured a high heritability of A (68-97%) and a small genotype by environment interaction. The heritability of A was highest and the genotype by environment interaction was lowest when measured on plant samples taken during seedling and tiller development, the stages least likely to encounter moisture stress. The coefficient of variation of A is about 2%, which is about one-fifth of that for GY (Richards and Condon, 1993).

V. RADIATION USE Biscoe and Gallagher (1977) proposed that biomass production can be defined by the amount of radiation intercepted (RI) and the radiation-use efficiency (RUE), that is, the efficiency of the conversion of this radiation to dry matter. Biomass = RI x RUE Hence, GY

=

RI x RUE x HI

The use of radiation and water is linked together in photosynthesis, but, in general, radiation is much less limiting than water in mediterranean environments. In fact, excessive radiation can have detrimental effects during postanthesis moisture stress. Therefore, radiation use may be less easily manipulated to improve biomass production compared to WU. The amount of RI is related to leaf appearance, size, orientation, tillering capacity, and senescence, and RI can influence leaf temperature, T, and WUE.

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A. INTERCEPTION Canopy structure has a large effect on RI. During the seedling stages of development, photosynthesis is limited by the ability of the plant to intercept radiation. As the crop develops, the green area index (GAI) increases until the crop intercepts all the radiation and photosynthesis reaches a maximum. Crops with prostrate habits generally give rapid groundcover and have a high RI value during the early stages of plant development, crops with erect habits have greater radiation penetration and illumination of the lower canopy after anthesis. The duration of the leaf area has been found to be highly correlated with GY in temperate environments (Fischer and Kohn, 1966; Bingham, 1977; Borojevic et al., 1980). Siddique et al. (1989b) and Yunusa et al. (1993) showed that old wheat cultivars had more leaves with a prostrate orientation, a greater GAI, and a greater fraction of groundcover than modem wheats. These results suggest that old cultivars had a greater RI, but paradoxically the efficiency of radiation interception, as measured by the extinction coefficient, was greater for modern cultivars probably because of their large leaf size and erectophile leaf habit, which caused less shading once stem elongation commenced. Yunusa et al. (1993) demonstrated that the increased RI of modern wheats was partly due to their large-awned ears, which constituted the majority of the GAI at the top of the canopy. Awns are better placed than leaves to intercept radiation and to dissipate unused radiation because they are narrow structures at the top of the canopy. Awn weight and length and flag leaf area were positively correlated with GY in Jordanian wheats (Al-Shalaldeh and Duwayri, 1986), presumably due to effects on RI. Under moisture stress, wheats with erect leaves yielded better than prostrate lines because they intercepted less radiation and had more favorable water relations (Innes and Blackwell, 1983). Leaf movements, such as paraheliotropism, rolling, and wilting under water stress, reduce RI and T (Begg, 1980). Clarke (1986) and Ludlow and Muchow (1990) consider these traits as essentially survival mechanisms and proposed that they are likely to have little effect on yield in terminal drought situations. However, leaf rolling, a familiar response in cereals, can reduce effective leaf area and transpiration by up to 50% and the amount during early growth was significantly correlated with biological and GY in northern Syria (ICARDA, 1987). In later stages of growth, leaf rolling is indicative of susceptibility to moisture stress and loss of turgor, and delayed leaf rolling is an important selection criteria for drought avoidance in rice (O’Toole and Cruz, 1979).

B. RADIATION-USE EFFICIENCY Comparisons of wild diploid and domesticated hexaploid wheats indicate that the photosynthetic rate per unit leaf area has fallen considerably in the course of plant domestication and selection (Khan and Tsunoda, 1970; Austin et al., 1982)

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and that photosynthetic rate is negatively correlated with leaf area (Evans and Dunstone, 1970; Rawson er al., 1983; Morgan and LeCain, 1991). Within modern field crops, yield and photosynthetic rate are poorly correlated (Evans, 1975) and selection for wheats with high photosynthetic capacity often results in reduced leaf area or thicker leaves and no increase in RUE and yield (Rawson et al., 1983). The higher photosynthetic rate of the diploid wheats appears to be related to genes affecting chlorophyll protein complexes, and Austin er al. (1988) are attempting to incorporate these genes into bread wheats, while breaking the link with leaf area. Walker and Sivak (1986) and Evans (1987) saw a remote chance of using genetic engineering to improve the activity of the rate-limiting photosynthetic enzyme, ribulose- 1,5-bisphosphate carboxylase-oxygenase (rubisco). There have been no reports in the literature of success in this area. Unlike comparisons of diploid and hexaploid wheats, Blum (1990) showed that selection for yield among Israeli wheats had increased RUE that was associated with increased photosynthesis and WUE. Yields were positively correlated with the GAI after anthesis as a result of early anthesis. The cultivars that reached anthesis earlier had a longer duration of photosynthetic area during grain filling and a higher yield than the later cultivars. Similarly, Siddique et al. (1989b) demonstrated that the modem Australian cultivars had a longer green area duration in the postanthesis period than the old cultivars. Recent studies (N. Watanabe, personal communication) using a subset of Australian wheat cultivars (Siddique et al. 1989b) showed that selection for yield has decreased the amount of chlorophyll associated with the core complex in photosystem I1 relative to the total amount of chlorophyll. However, the rate of CO, assimilation, total chlorophyll content, and N content per unit leaf area has increased from old (Purple Straw) to modern cultivars (Kulin). In contrast, Gent and Kiyomoto (1985) did not measure any difference in the net CO, assimilation rates per unit leaf area of New York wheats, although they only compared two winter cultivars. In the study of Siddique et al. (1989b), modern cultivars converted the intercepted photosynthetically active radiation to aboveground biomass more efficiently compared to older wheats. The RUE increased from 1.08 g MJ-I for the old cultivars to 1.31 g MJ-’ for the modern cultivars and this appeared to be related to the reduced investment in root biomass by the modern cultivars. Similar RUE values were measured by Gregory et al. (1992). Increased RUE may be related to early vigor because photosynthesis is more efficient at low VPD and RUE decreases with the increasing VPD during the growing season (Stockle and Kiniry, 1990). High levels of solar radiation can damage proteins in the chloroplasts and impair photosynthesis when the electron transport systems are inhibited by feedback mechanisms caused by a low demand for photosynthate. Wheat is most susceptible to this type of damage, known as photoinhibition, during early summer when growth rates are limited by water and heat stress and radiation levels

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are high. Light green leaves have a higher capacity for photosynthesis at near light saturation point and are less susceptible to photoinhibition than darker leaves. The development of a light green color was positively correlated with GY under drought in durum wheats and barleys in northern Syria (ICARDA, 1987, 1988). Landrace wheats and adapted lines were darker during their vegetative growth stages when radiation levels were low and became paler as radiation increased, whereas lines from South Australia and Europe maintained an intermediate color throughout the season. Respiration is a large component of assimilate use in plants, and studies with several species have established a link between respiration and biomass production (Heichel, 1971; Wilson, 1975; Jones and Nelson, 1979; Gifford et ul., 1984). Winzeler et al. (1989) measured higher rates of respiration and lower rates of growth in wheat than in rye or triticale. The mechanism for more efficient respiration is unknown and there have been no reports in the literature of wheat genotypes with reduced respiration. In the future, respiration enzymes may be modified using genetic engineering to increase the amount of assimilate available for growth, and hence increase RUE and WUE. Clearly, a better understanding of the respiratory pathway and its effect on crop production is required before this could be attempted. Most methods for measuring photosynthesis or RUE are currently laborious and time consuming, and unsuitable for use in breeding programs. However, strong relationships between leaf N content and photosynthetic rate have been demonstrated in maize, rice, soybean (Sinclair and Horie, 1989), and wheat (N. Watanabe, personal communication), and selecting for high N content may increase the efficiency of photosynthesis.

VI.HIGH-TEMPERATURESTRESS Under dryland conditions, high-temperature, radiation, and moisture stresses often occur simultaneously, and heat stress alone rarely plays a role in reducing wheat yields. In mediterranean environments, high-temperature stress is an important constraint after anthesis. As temperatures rise, photosynthesis reaches a maximum at about 20°C (Al-Khatib and Paulsen, 1984) while respiration continues to increase, hence the assimilates available for growth are reduced (Gusta and Chen, 1987). However, if radiation and moisture sources are increased, the rate of photosynthesis increases with increasing temperature and biomass production is largely unaffected by high temperature (Rawson, 1988). Short periods of high temperature near anthesis can dramatically reduce wheat yields. In a study by Saini and Aspinall (1982), well-watered wheat plants were exposed to 30°C for 3 days before anthesis, and floret fertility was reduced by

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80%. As already mentioned, although high temperatures increase the rate of grain filling, the duration of grain filling is reduced considerably and grain weights are decreased (Sofield et al., 1977; Wardlaw et al., 1980, 1989). Wheat cultivars from the Middle East and tropical areas are more tolerant of high temperatures than are cultivars from North America and Europe, whereas Australian and Indian cultivars are intermediate (Shpiler and Blum, 1986; Wardlaw et al., 1989). Wheat and a number of other plants respond to heat stress by producing a large number of low-molecular-weight proteins, but the significance of these heat-shock proteins is poorly understood (Lindquist, 1986) and the physiological and biochemical basis for heat tolerance needs to be investigated further. Chlorophyll fluorescence may be a useful technique for screening wheats of similar maturity for high-temperature tolerance (Moffat et al., 1990).

VII. USE FOR BREEDERS We have discussed many morphological and physiological attributes that have or are likely to contribute to increased wheat yields, and yet relatively few breeding programs are currently selecting for specific physiological traits. Several authors advocate the analytical approach to plant breeding (Richards, 1982; Rasmusson and Gengenbach, 1983; Blum, 1983, Acevedo and Ceccarelli, 1989; Whan et al., 1993), although many have had variable success. There are several reasons for the lack of adoption of the analytical approach by breeders. First, empirical breeding programs have been very successful at producing consistent increases in yield, especially in high-yielding environments (Turner and Begg, 1981; Slafer et al., 1993). Breeders have not been convinced that a physiological approach will give better results, and they believe that improvements in field experimentation and computerization will ensure continued success of the empirical approach. As evident from this article, many traits highlighted by plant physiologists as useful for breeders have not been unequivocally proved to increase yield. Given the complex physiological process that determine yield and the large genotype by environmental interaction of yield, it is often easier to show that a trait improves a short-term plant function or characteristic under stress (e.g., root growth, T, or HI), rather than improving yield itself. Passioura (198 1) suggests that often physiologists begin their studies at too low a level of organization (e.g., molecular or cellular) and in controlled environments that are of little relevance to the breeder who is interested in the GYs of crops in the field. In addition, organ number and size are often inversely related (Grafius, 1978) so that selection for a morphological trait may result in compensation in some other trait and expected yield increases are often not achieved (e.g., reduced tillering lines may have larger leaves and WU is unaffected).

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Frequently, the yield increases associated with a particular trait are small, and breeders have not been convinced that selecting for the trait is more efficient than selecting for yield. For example, Whan et al. (1982) showed that selecting for harvest index was less effective at improving GY than direct selection for GY. Passioura (1981) argued that there is a low probability that a single plant trait will have a sufficient effect to cause a statistically significant increase in yield. Ceccarelli et al. (1991) argued that selection for a single trait is often unsuccessful, particularly in unpredictable environments where the frequency, timing, and severity of stresses are unknown. In these situations, different combinations of many traits may produce the same GY. Also, genes for the desired traits are sometimes found in exotic stock that are poorly adapted to the target environment and crossing with this stock incorporates other deleterious genes that reduce growth and yield in early crosses. The use of isolines has been advocated by several authors for proving increased yields with the inclusion of a trait because they overcome any genetic effects that other traits may have on yield (Pugsley, 1983; Rasmusson and Gengenbach, 1983; Richards, 1991; Shorter et al., 1991). Although isolines generate information on the effect of a trait in a particular genetic background, they are sometimes of limited use to breeders who want to incorporate the trait into many different backgrounds (Acevedo and Ceccarelli, 1989). In addition, it is costly and time consuming to develop isolines and they can only be produced for traits controlled by one or two genes. Many traits are measured with complex, time-consuming techniques that are unsuitable for screening large numbers of progeny in breeding programs. Consequently, most traits are evaluated in a small number of genotypes without testing in breeding populations. Some physiological techniques have been modified, and although not as accurate, they may provide a useful method for plant screening (e.g., osmoregulation in coleoptiles) (Morgan, 1988). Other more complicated and time-consuming techniques are only useful for screening a small number of genotypes for use as parents. Some traits can be combined in a single measurement. For example, infrared thermal sensing of canopy temperatures can be used to screen for deep roots, maintenance of higher leaf water potentials, increased stomata1 conductance, and general drought avoidance (Blum et al., 1982, 1989; Pinter et al., 1990), provided variations in measurements due to time of day, weather, and groundcover can be avoided (Turner, 1986). The high degree of environmental influence on the expression of some traits also poses problems for selecting for the trait within breeding programs. And yet, the environmental effects on GY are usually greater than those on morphological and physiological traits, hence the low heritability of GY. Often the use of irrigation, time of sowing, rain-out shelters, or artificial water stress treatments, such as chemical desiccants, is required to examine the trait under a range of moisture stresses. Many breeders are unsure of the conditions under which they should select for stress adaptation. A review of widely adapted wheat geno-

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types by Ceccarelli ( 1989) concluded that in environments where the average GYs are greater than 2-3 t ha-’, selection for traits is successful under optimum conditions. But where yields are less than 2 t ha-’, direct selection in the target environment is the most efficient strategy. In these harsh environments, yield stability is often a high priority. It would be a mistake to ignore the contribution that modeling can make to this area of breeding/physiology. Our ability to assess accurately the interaction of the numerous processes over a crop life cycle is limited and the development of models can remove much of the “hunch taking” in selecting relevant physiological traits for breeding (Moorby, 1987; Shorter et al., 1991). New genotypes are tested in the field for several seasons before release, and using historical weather and other data, models can also be used to predict how genotypes perform in other seasons and sites and in a changing climate. Many simulation models have been used successfully for wheat (Stapper, 1984; Weir et al., 1984;Ritchie et al., 1985; Hammer et al., 1987). For example, Stapper and Harris (1989) used the SIMTAG simulation model to illustrate how long-term, early-maturing wheats have higher yields in short-seasonenvironmentsthan do late-maturing wheats, and

I

’‘O (a) Breda

I I

0.0I 0

I

200

I

400

I

I

600

800

Grain yield (gm-2)

The cumulative frequency distribution of predicted yields of two maturity types, early (4, at (a) Breda (275 mm annual rainfall) and (b) Jindiress (479 mm annual rainfall), Syria. Reproduced with permission from Stapper and Harris (1989). Figure 9

(---) and late

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vice versa, in long-season environments (Fig. 9). Models can also predict much more complicated crop growth processes and interactions.

VIII. CONCLUDING COMMENTS Wheat is one of the most domesticated plants in the world and yet we are still trying to improve its performance in the mediterranean environment where it originated. The physiology of wheat is probably better understood than any other plant and the contribution that physiology has to make to wheat breeding is probably greater than for other crops. Water stress is a major limitation to wheat growth and yield in mediterranean and other environments, and physiologists have a role to play in improving WUE, RUE, and yield in these environments. Empirical breeding approaches have been very successful and we do not suggest that breeders abandon this methodology. However, over the last 15 years, wheat breeders in mediterranean environments have relied heavily on germ plasm for CIMMYT and they are concerned that the germ plasm may become too narrow and that yield may be approaching a plateau. It is important that some attempt is made to develop new parental lines with the potential for increasing yields significantly. As advocated by Whan et al. (1993), we believe this is best achieved by using a physiological approach to identify parental genotypes with superior traits. Rasmusson (1987) and Acevedo and Ceccarelli (1989) wisely suggest that breeding programs make a modest investment in the physiological breeding approach of about 15-25% of the total breeding resources. Given the equivocal evidence associated with some traits and the difficulty of selection, we believe these resources are probably most efficiently used in a parental identification subprogram. Because of the relatively small number of genotypes involved in parental screening, some of the more laborious physiological techniques can be used. Crosses should be made between superior parents and locally adapted cultivars and the progenies tested for yield and other standard measurements (e.g., grain quality) in the routine evaluation procedure. Some of the crossbred material rejected by the evaluation procedure may be useful parental germ plasm. In order to reap the large potential benefits that molecular biology has to offer plant breeding, breeders will require greater cooperation from physiologists because the genetic engineering technologies are of little use to breeders unless they understand the cellular and molecular processes that control stress tolerance and yield. Molecular biology can also assist the selection of traits that are expensive or slow to measure, that are sensitive to environmental factors, or that can only be measured in the mature plant. DNA markers that are visually recognized

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in the seedling can be linked to the genes for the trait, thereby increasing the efficiency of selection dramatically. We predict that, in the future, the most successful breeding programs will involve breeders, physiologists, agronomists, modelers and molecular biologists.

ACKNOWLEDGMENTS The authors thank the National Committee on Crop Improvement and Protection of the Grains Research and Development Corporation for financial assistance. We thank Mohan Saxena for the detailed climatic data for Aleppo and we are also grateful to Michael Perry, Neil Turner, and Richard Richards for useful comments on the manuscript.

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