Published in Crop Sci 39:1611-1621 (1999)
© 1999 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Crop Science 39:1611-1621 (1999)
© 1999 Crop Science Society of America
SYMPOSIUM-1998 ASA MEETING -BALTIMORE
Physiological and Genetic Changes of Irrigated Wheat in the Post–Green Revolution Period and Approaches for Meeting Projected Global Demand
M.P. Reynoldsa,
S. Rajarama and
K.D. Sayrea
a International Maize and Wheat Improvement Centre (CIMMYT), Mexico. Mailing address: Apartado 370, P.O. 60326, Houston, TX 77205 USA
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ABSTRACT
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Global demand for wheat (Triticum aestivum L.) is growing faster than gains in genetic yield potential are being realized, currently a little under 1% per year in most regions. Improvement in yield of semidwarf wheat has generally been associated with increased harvest index (HI) and grain per square meter. For CIMMYT (International Maize and Wheat Improvement Center) varieties released between 1962 and 1988, yield increase was also associated with higher flag-leaf photosynthetic rate and related traits, but not higher biomass. Nevertheless, significantly higher biomass has been reported in more recent CIMMYT lines. Improved HI is associated with higher N use efficiency (yield per unit of available N) and improved yield of semidwarf lines is expressed at high and low levels of N input. Where interplant competition for light and soil factors are manipulated, yield improvement is associated with adaptation to high plant density. Studies have confirmed that the juvenile spike growth phase is critical in determining both grain number and kernel weight (sink) potential. Improving assimilate availability during this stage, perhaps by lengthening its relative duration, may be one way to improve yield potential. Traits that could potentially be exploited for improving assimilate (source) capacity include early vigor, stay-green, leaf-angle, and remobilization of stem reserves. Use of alien chromatin is a successful approach for introducing yield-enhancing genes into elite genetic backgrounds. Examples include the 1B/1R chromosome translocation from rye (Secale cereale L.), and more recently the LR19 segment from tall wheatgrass [Agropyron elongatum (Host) P. Beauv.] Improving the efficiency of early-generation selection may be another strategy for raising yield potential by increasing the probability of identifying physiologically superior lines by techniques such as infrared thermometry and spectral reflectance.
Abbreviations: CTD, canopy temperature depression • IR, infrared • HI, harvest index • IFPRI, International Food Policy Research Institute • NDVI, normalized difference vegetation index • NUE, N use efficiency • QTL, quantitative trait loci • RUE, radiation use efficiency • SIPI, structural independent pigment index • SR, simple ratio
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INTRODUCTION
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WHEAT PROVIDES
more than one-quarter of the total world cereal output, and constitutes the main source of calories for more than 1.5 thousand million people. The crop is grown on 
220 million ha worldwide, about half of which is in developing countries (CIMMYT, 1996). Approximately 45% of the total area sown to wheat in developing countries (excluding China) is irrigated, and accounts for more than 55% of the total wheat production (Byerlee and Moya, 1993). In India for example, at least 80% of the wheat area is irrigated, and the figure for China is around 75%.
While demand for wheat is growing faster than gains in genetic yield potential are being realized, investment into conventional breeding by national programs and development agencies is being reduced. (Genetic yield potential is defined as yield of adapted lines in a favorable environment in the absence of agronomic constraints.) Nonetheless, genetic gains in yield potential are still being made, and the physiological basis for this progress is presented here. While a full understanding of the physiological and genetic basis of yield is still incomplete, progress has been made in developing selection technologies that may improve the efficiency of empirical breeding. Their potential application in breeding to improve the probability of identifying higher-yielding genotypes is discussed.
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Trends in Yield Potential 1966 to Present, and Future Demand
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Though comprehensive studies quantifying genetic gains in yield potential under irrigated conditions have been conducted in relatively few locations, these studies suggest steady rates of gains of a little under 1% per year since the mid 1960s, for example in India (Kulshrestha and Jain, 1982) and Mexico (Waddington et al., 1986, 1987; Sayre et al., 1997). Similar results have been shown in studies employing irrigation in high-yielding environments such as Italy (Canevara et al., 1994) and Argentina (Calderini et al., 1995). Some studies suggest that on-farm wheat yields may have reached a plateau in recent years (Sayre, 1996; Calderini and Slafer, 1998). However, the reasons for this can be quite complex, involving agronomic and economic factors, and should not be confused with the issue of obtaining further gains in genetic yield potential from plant breeding. Indeed, even the most recent lines selected by the CIMMYT Wheat Program indicate a steady improvement in yield potential (Fig. 1) .
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Fig. 1 Grain yield of a historic set of wheat cultivars developed in northwestern Mexico (1962–1996): For cultivars from 1962 through 1988 (open circles) yields are means of six growing seasons (from Sayre et al., 1997), while the latest two dates (closed circles) are based on data from 1997 to 1998 (Table 2) after adjustment based on the performance of common entries (i.e., Seri 82 and Bacanora 88). (Equation of line [1962–1988]
)
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Table 2 Phenotypic correlations between morphological traits and yield and year of release for eight cultivars released between 1962 and 1988, Obregon, Mexico, 1992 through 1995 (adapted from Sayre et al., 1997)
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While trends may seem reassuring, according to studies by the International Food Policy Research Institute (IFPRI), demand for wheat is expected to grow by 1.3% per year worldwide and by 1.8% per year in developing countries during the next 20 yr (Rosengrant et al., 1995; M. Rosengrant, 1998, personal communication). Therefore, current trends in genetic gains in yield are too low to keep pace with future demand. The prospect of meeting demands by increasing wheat productivity through increased land use, or narrowing the gap between potential yields and farmers' yields in irrigated environments is apparently not very likely. The cultivated area of wheat in developing countries is expected to rise by only 0.14% per year through 2020 (Rosengrant et al., 1995). Economic analyses on yield gaps showed that even 5 yr ago, wheat yields in the Punjab of India, for example, were already 70 to 80% of maximum yields obtained on experiment stations in the region (Byerlee, 1992). In a more recent analysis, Pingaly and Heisey (1999) state that in intensively cultivated, irrigated wheat areas such as the Yaqui Valley of Mexico or parts of Asia, the economically exploitable gap between the technology frontier and farmer performance is very small (<20%). They conclude that meeting the long-term requirement for cereals will require a shift in genetic yield frontiers. Given the environmental implications associated with increasing land use, and the cost and complexity of further reducing the relatively small yield gaps in high production areas, improving genetic yield potential of wheat would seem to be the most expedient solution for meeting future demands.
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Physiological Basis of Genetic Improvement in Yield
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Biomass and Other Morphological Traits
Just as the impact of the Green Revolution can be attributed mostly to improved partitioning of the products of photosynthesis to grain yield, progress in yield of irrigated wheat since the development of semidwarf lines is also most strongly associated with improved HI (Kulshrestha and Jain, 1982; Waddington et al., 1986, 1987; Calderini et al., 1995; Sayre et al., 1997). Morphological traits associated with increased yield potential in CIMMYT wheat varieties released between 1962 and 1988 include grain number and HI (Table 1)
and similar results have been shown in other studies (Calderini et al., 1998). The fact that increased HI still appears to account for improvements in genetic yield potential, even in the aftermath of the Green Revolution, could be worrying, especially in the light of the theoretical limit to HI, estimated at 60% (Austin et al., 1980a). Even if HI could be raised to 60% from its current maximum value (50%), it implies that yields could only be increased by a further 20% using HI as a selection criterion, unless total crop biomass is also raised. Furthermore, improved partitioning by greater reduction in plant height is unlikely since research suggests that optimal plant heights have already been achieved (Miralles and Slafer, 1995). However, some studies have shown increased biomass to be associated with yield increases (Waddington et al., 1986, 1987), and in other studies, correlation analysis across longer time spans may hide recent trends. A closer look at recent data from Mexico shows that in seven varieties chosen to represent breeding progress since 1962, the most recent release (Bacanora 88) has a biomass 9% higher than the average of those previously released. More recently developed high-yielding lines (Singh et al., 1998) also support the trend for higher biomass (Table 2)
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Photosynthesis and Related Traits
By definition, improved yield cannot be attributed to better overall radiation use efficiency (RUE) in cases where total biomass has not been improved. (RUE in a crop context represents the ratio of the total energy present in the crop's biomass to that of the solar energy incident on the crop across its growth cycle; see Loomis and Amthor, 1999, in this issue.) In a detailed study by Calderini, with Argentinean wheat cultivars released between 1964 and 1990, it was apparent that RUE had not been genetically improved during this period, when estimated either pre- or postanthesis. Nonetheless, research aimed at understanding the underlying basis for yield potential improvement (Fischer et al., 1998) has shown a consistent correlation between increases in yield potential achieved for CIMMYT semidwarf varieties since they were first introduced in the early 1960s and flag-leaf photosynthetic rate, stomatal conductance, canopy temperature depression (CTD), and 13C isotope discrimination of grain (Table 3) . A yield increase of 29% (1962–1988) corresponded with increases of 23% in photosynthetic rate, 63% in stomatal conductance, and 0.6°C in CTD (all measured during grain filling). The fact that photosynthetic rate during grain filling is correlated with yield, while final biomass is not, is perhaps explained by the lower preanthesis vegetative growth rates observed for the more modern cultivars in this study. Other flag leaf traits that were measured during grain filling included leaf area, specific leaf weight, N content, and greenness index (using a SPAD meter; Minolta SPAD-502, Tokyo, Japan), none of which were correlated with yield (Table 3). A recent review also indicated a general lack of association between yield progress and leaf area index or light extinction coefficient (Calderini et al., 1998).
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Table 3 Phenotypic correlations between physiological traits and yield and grains per square meter for eight cultivars released between 1962 and 1988, Obregon, Mexico, 1993 through 1995 (adapted from Fischer et al., 1998)
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The physiological basis of the association of yield with photosynthetic rate and traits like stomatal aperture and CTD are unknown. Expression of higher photosynthetic rate in the absence of significant changes in biomass could be a pleiotropic effect of improved partitioning to yield driven by high demand for assimilates during grain filling. Canopy temperature depression is a direct function of evapotranspiration rate, which itself is determined largely by stomatal conductance. These traits could also be pleiotropic effects of genetic variability among lines for a number of physiological and metabolic processes including sink strength, photosynthetic rate, vascular capacity, and hormonal signals.
Improvement in Nutrient Use Efficiency
Using a historic series of CIMMYT lines similar to those described above, studies have been conducted in the high-yielding, irrigated environment of northwest Mexico to estimate genetic gains in N use efficiency (NUE), defined as grain yield per unit of N available to the plant (Ortiz-Monasterio et al., 1997). While NUE almost doubled with the introduction of height reduction (Rht) genes in the early 1960s, progress since the Green Revolution has continued at a lower rate in parallel with the more modest improvements in partitioning to yield (Fig. 2)
. Improvement in NUE has been associated with improvements in both total N uptake, as well as efficiency of utilization in terms of grain yield. This study also revealed the interesting and controversial fact that Green Revolution varieties demonstrated genetic gains in yield even under severely N-limited conditions, that is 2 to 2.5 t ha-1 yield levels. This trend has continued since 1966 with the varieties of the mid 1980s yielding more than 3 t ha-1 under the same conditions. Work in Argentina has demonstrated improvement in both N and P use efficiency in modern varieties (Calderini et al., 1995). In contrast to the previous study, data for varieties released in the last decade in Argentina indicated increasing rates of improvement in nutrient use efficiency.
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Fig. 2 Nitrogen use efficiency (NUE) and harvest index (HI) of a historic series of cultivars; average for three levels of N (75, 150, and 300 kg ha-1), northwestern Mexico, 1987 through 1989 (adapted from Table 3 in Ortiz-Monasterio et al., 1997). (LSDs [0.05] are 3.0 kg yield kg-1 N for NUE, and 2.5% for HI.)
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Adaptation to Density
The idea that higher yield potential could be achieved by designing a plant type that is well adapted to the commercial practice of sowing high density monocultures was introduced 30 yr ago by Donald (1968). Yield progress in CIMMYT lines seemed to be associated with the "communal" trait, when defined as the relative lack of yield response of higher-yielding lines to a reduction in interplant competition, in contrast to lower-yielding lines that responded considerably to removal of neighboring plants after flag-leaf emergence (Reynolds et al., 1994a). The study demonstrated that yield of older wheat varieties were no different from modern varieties when interplant competition was reduced, a result very similar to that found in a comparison of 40 maize (Zea mays L.) hybrids released between 1934 and 1992 (Duvick, 1992). One important implication of these results is that improvement in yield potential would appear to be more a function of improved adaptation to canopy microenvironment, rather than macroenvironmental factors such as climate. Several studies have shown that selection for yield potential in early generations can be enhanced by reducing interplant competition between genotypes in bread wheat (Lungu et al., 1987), and durum wheat (T. turgidum L.) (Mitchell et al., 1982). None of these studies has yet shown a physiological basis for the communal trait, but they do suggest that conventional breeding approaches may lose yield potential by selecting against it.
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Improving the Ideotype
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Relatively few systematic studies, such as those described above, have been conducted to examine the physiological basis of yield improvement in the post–Green Revolution era. Perhaps for this reason, at a recent workshop on raising wheat yield potential (Reynolds et al., 1996), it was suggested that further investment be made in understanding which plant traits could be optimized to further improve yield. Many traits have been suggested in the literature as having potential to raise yield, but very few have been examined in a systematic way for their potential to increase genetic gains when used as selection criteria. They have not generally been introgressed into high-yielding backgrounds, and little if any work has been conducted to assess potential complementarity between the many morphological and physiological traits which have potential to improve the crop ideotype.
Source and Sink: Kernel Number
It is widely believed that yield gains are most likely to be achieved by simultaneously increasing both source (photosynthetic rate) and sink (partitioning to grain) strengths (Slafer et al., 1996; Richards, 1996; Kruck et al., 1997). While most experiments indicate that yield is primarily limited by growth factors prior to anthesis, source capacity may have become more limited in modern cultivars. For example, experiments on a historic series of spring wheats from Russia indicated that, while sink capacity has been improved in the post–Green Revolution period, improvement has also resulted in modern lines that are now more source limited than those in previous eras (Koshkin and Tararina, 1989). Using field-grown plots, spikelets from one side of the spike were completely removed at flowering. Potential yield (i.e., in the absence of source limitation during grain filling) was calculated from the doubled grain weight of semidegrained spikes. Data showed that, while the potential yield per spike had been improved (i.e., the doubled grain weight of semidegrained spikes was higher in more modern lines), the extent to which the potential was realized (i.e., potential vs. actual spike yield) had declined in modern lines.
The reproductive stages of development, from initiation of floral development to anthesis, are pivotal in determining yield potential, and especially the rapid spike-growth phase (Fischer, 1985) which has a duration of 25 d in irrigated spring wheat in northwest Mexico (Fischer, 1985) and Argentina (Abbate et al., 1997). During this period, final grain number is determined, a major factor determining subsequent partitioning of assimilates to yield, as well as heavily influencing the assimilation rate of the photosynthetic apparatus during grain filling. The duration of spike growth relative to other phenological stages shows genetic variation (Slafer and Rawson, 1994). This is associated with sensitivities to photoperiod, vernalization, and developmental rate independent of these stimuli (i.e., earliness per se).
Fischer (1975, 1985) established the critical nature of the rapid spike-growth phase in determining yield. Based on this, it has been suggested that the possibility exists of improving final grain number and yield potential by manipulating the genes associated with sensitivity to photoperiod (Ppd) and vernalization (Vrn), as well as earliness per se (Slafer et al., 1996). The hypothesis is based on the idea that by increasing the partitioning of assimilates to spike growth, and therefore spike biomass, potential floret survival will be increased and hence yield potential raised (Bingham, 1969). Experiments in which different radiation regimes were compared during this critical phase are consistent with the hypothesis (Fischer, 1985; Abbate et al., 1997). Numerous studies comparing old and modern lines have demonstrated increased partitioning of biomass to spike growth to be a pleiotropic effect of dwarfing genes (see Abbate et al., 1998). Most recently, the duration of the rapid spike growth has been successfully manipulated using photoperiod, revealing a strong relationship between its duration and the number of fertile florets per spike (Miralles and Richards, 1999). By maintaining plants at a relatively short photoperiod during this growth phase, the number of days from terminal spikelet to heading was increased from 50 to 70 d, with 13- and 9-h photoperiods, respectively, while the number of fertile florets per spike increased from 77 to 108.
However, other recent work conducted on a set of Argentinean cultivars released between 1984 and the present (Abbate et al., 1998) show no significant variation in the duration of the spike-growth phase, or crop growth rate in that same period, despite a range of yields from 4.5 to 7 t ha-1. While partitioning of biomass to the spikes showed significant genetic variation (from 28–34%), the trait was not associated with yield or grain number (Fig. 3) , suggesting that some factor other than partitioning of dry weight to spike growth is determining grain number and yield in these lines. Earlier work showed that grain number responded directly to N supply to the spike during its development (Abbate et al., 1995), and its seems unlikely that any single factor during this critical growth phase would ultimately determine grain number. Another approach to evaluating traits involved in determination of grain number would be to study these phenomena in populations of recombinant inbred lines made from contrasting parents. Such populations could also be used to identify molecular markers associated with the genes involved.
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Fig. 3 Relationship between partitioning of biomass to spikes at anthesis, and final grain number and yield of six modern Argentinean cultivars (Adapted from Tables 4 and 5 in Abbate et al., 1998). (LSDs [0.05] are 112 g m-2 for yield, and 2900 grains for grains per square meter.)
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Table 4 Average characteristics of best-yielding advanced lines (n = 4, averaged across three cycles), selected for increased spike fertility as well as agronomic traits, in comparison with standard cultivars (n = 2) (adapted from Dencic, 1994)
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From a practical point of view, breeders have tried to modify the sink capacity of wheat by modifying the spike morphology. A good example of this approach was reported by Dencic (1994) who crossed genotypes with branched tetrastichon (two spikelets per node of the rachis) with high-yielding lines that contained other desirable traits such as high yield, disease resistance, and quality. Materials were crossed using single, back, and top crossing, and desirable lines selected using a pedigree approach. After 10 yr of breeding and selection, 229 lines with desirable characteristics were yield tested, of which four lines had yield superior to the standard checks (Table 4)
. Average yield of the four lines was 13% (i.e., 1 t ha-1) higher than the standards (Jugoslavija and Skopljanka), and the following morphological traits were improved over the standards: spike length (16%), spikelets per spike (10%), grains per spikelet (9%), grains per square meter (18%). This progress in yield was achieved in spite of the fact that the tetrastichon donor lines had problems of empty florets or shriveled grain with very low kernel weight. At CIMMYT, a current research goal is to attempt the simultaneous improvement of source and sink. Lines with "big-spikes" (and therefore a high potential kernel number) are crossed with others having a number of attributes that may increase source capacity such as large semierect leaves, high chlorophyll concentration, and improved green area duration.
Source and Sink: Kernel Size
Genetic progress in yield potential is strongly associated with increases in grain number while weight per grain has generally declined (Slafer et al., 1994). Nonetheless, some studies have shown increased kernel weight has contributed to improved yield potential in irrigated wheat (Calderini et al., 1995). Understanding the physiological and genetic basis of potential kernel size remains an obvious challenge for yield improvement. An important question is whether grain weight potential can be increased independently of increases in grain number. Simplistically, it can be argued that this inverse relationship is a necessary tradeoff when more grains are competing for limited assimilates during grain filling. However, studies that have examined the relationship between kernel size and number at different spike positions using lines from different eras conclude that the size of kernels at low potential weight spikelet positions are independent of kernel number or year of release (Slafer et al., 1994, 1996). It is suggested that grain weight is colimited by both source and sinks, such that grain weight potential would be most likely determined during spike growth, resulting in different potential sizes at different spike positions. Realization of potential would be determined by assimilate availability during grain filling. Very recent research with synthetic hexaploid wheat, which tends to have larger kernel weights than conventional cultivars, has demonstrated significant increases in grain weight when assimilate supply was increased by partial degraining treatments during rapid spike growth. The effect was greatest at grain positions generally showing lower grain weight. No effect on grain size was apparent when degraining occurred a week after anthesis. The data confirmed the physiological potential for increasing kernel weight at distal spikelet positions by as much as 16%, strongly endorsing the objective of breeders to raise yields through increasing grain weight potential. On the basis of these data, it is proposed that extending the duration of the rapid spike-growth phase may increase yield potential not only by increasing potential grain number, but also by increasing grain weight potential through extending the window of opportunity for individual kernel development (Calderini and Reynolds, 2000).
Leaf Angle
The erectophile leaf canopy has been proposed as a trait that could increase crop yield potential by improving light use efficiency in high radiation environments. A number of studies support the hypothesis. It has been associated with a 4% yield advantage in wheat isolines in the United Kingdom (Innes and Blackwell, 1983). The physiological basis of the trait was studied in near isogenic CIMMYT lines and showed that more erect leaf posture was associated with higher grain number and higher stomatal conductance, based on C isotope discrimination measurements of mature grain (Araus et al., 1993). In barley (Hordeum distichum L.), two varieties contrasting in leaf angle were compared for photosynthetic rate at different depths of the canopy. The erect leaf variety showed a more even distribution of photosynthetic rate throughout the canopy, as well as higher rates of stem photosynthesis (Angus et al., 1972). Based on this hypothesis, a large number of accessions from germplasm collections were screened for erect leaves at CIMMYT in the early 1970s. The trait was introgressed into the wheat germplasm base, and it is present in some of CIMMYT's highest-yielding durum and bread wheat lines (Fischer, 1996).
Stem Reserves and Green Leaf Area Duration
There are a number of additional physiological traits that have implications for yield potential and are related to increasing assimilate availability (i.e., source). One is the ability to reach full ground cover as early as possible after emergence to maximize the interception of radiation (Richards, 1996). Another is remobilization of soluble carbohydrates (stem reserves) during grain filling (Stoy, 1965). A third is the ability to maintain green leaf area duration ("stay-green") throughout grain filling (Jeneer and Rathjen, 1975). Direct evidence for the contribution of these traits to high yield potential is lacking. Stem reserves apparently make a greater contribution to performance in relative low-yielding lines where contrasting lines have been examined (Stoy, 1965; Austin et al., 1980b). It has also been suggested that use of stem reserves and stay-green may be mutually exclusive, since loss of chlorophyll and stem reserve mobilization seem to be consequences of plant senescence (Blum, 1998). A greater understanding of the genetics of these traits is called for to establish the potential for breaking such linkage. As yield potential is raised by improving reproductive sinks, additional assimilation capacity will become more important. In theory, extra assimilates gained by increasing early ground cover could contribute to increased stem reserves and be tapped at later reproductive stages to enhance potential kernel number and size. A longer stay-green period would improve the likelihood of realizing that potential.
Roots
Studies with roots under irrigated conditions are very limited. Work with Rht isolines in Argentina indicated that the shorter lines had a higher investment in root length and dry weight at anthesis than the tall ones, in the top 30 cm of soil (Miralles et al., 1997). Studies of root length density (0–20 cm) on a historic series of CIMMYT wheats revealed no statistically significant differences (G. Manske, 1997, unpublished). The proportion of total respiration resulting from root activity has apparently declined in more modern Russian spring wheat (Koshkin and Tararina, 1989). While progress in measuring and understanding root anatomy and its relationship to yield is likely to be slow, the discovery of root signals should raise a note of caution for breeders wishing to increase yield in irrigated wheat. Biochemical signals, elicited by reduced soil water potential, have been shown to cause reduced stomatal conductance well in advance of leaf water deficit (Davies and Zhang, 1991). The trait probably evolved to increase the likelihood of completing a genotype's life cycle under unpredictably dry conditions. However, researchers need to test the hypothesis that root signalling may limit productivity in irrigated crops by reducing stomatal conductance (and therefore the potential for CO2 assimilation) as roots detect progressively drying soils prior to scheduled irrigations.
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Genetic Basis of Yield Improvement
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The wheat improvement program at CIMMYT has expanded the genetic base of modern wheat lines by conventional breeding as well as cytogenetic approaches (Table 5)
. The highly successful Veery lines produced in the early 1980s (Rajaram et al., 1990) resulted from a cross to a winter wheat parent containing the 1B/R translocation from rye. As well as being widely adapted, the Veery lines show outstanding yield potential and other physiological characteristics. `Seri 82', for example, has been shown to have superior leaf photosynthetic rate, stomatal conductance, and leaf greenness relative to a set of hallmark varieties developed both before and after its release (Fischer et al., 1998). Part of the success may also be related to increased stress tolerance (Villareal et al., 1997a). The use of winter x spring wheat germplasm at CIMMYT, which began in the early 1970s, has had the largest impact on increasing the genetic diversity of the spring wheat gene pool. (The exploitation of the same germplasm for winter wheat improvement was undertaken by Oregon State University.) The reasoning at the time was that since both gene pools were so distinct in their adaptability, they must have "coevolved" in terms of agronomic characters. This implied that they could therefore be combined readily to produce superior genotypes for yield and disease resistance.
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Table 5 Examples of the successful application of wide crossing to introduce alien genes into the hexaploid wheat genome
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Wide crosses, between durum wheat and wild (D genome) diploids, have been employed to create synthetic hexaploid wheat with the purpose of introducing disease resistance into hexaploid wheat (Villareal et al., 1995; CIMMYT, 1996). Many synthetic lines also have good yield characteristics (Villareal et al., 1997b). Synthetic wheat lines are used now in 15% of all crosses made by CIMMYT's wheat improvement program. Recently, Singh et al. (1998) reported that chromosome translocation containing the LR19 gene from A. elongatum is associated with a significant increase in yield and biomass when introduced into an already high-yielding background (Seri 82).
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Promising New Approaches to Accelerate Yield Gains
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To date, wheat breeding programs worldwide have achieved significant genetic gains in yield potential without the aid of new selection technologies (Rajaram and van Ginkel, 1996). Nonetheless, there is consensus among breeders as well as physiologists that while the contribution from physiology has been modest, its contribution to breeding is expected to be larger in the next 20 yr (Jackson et al., 1996). There are perhaps two main reasons for this. One is based on need. Over the next quarter of a century demand for wheat is expected to grow by 2% per year in developing countries (Rosengrant et al., 1995). This means that current trends in the improvement of genetic yield potential are too low to keep pace with future demand. The second is probably based on justifiable optimism. Several studies suggest that some new selection technologies have real potential to complement conventional wheat breeding programs in the areas of biotechnology (Tanskley and Nelson, 1996) and physiology (Richards et al., 1996; Fischer et al., 1998; Reynolds et al., 1998). At a recent consultation on raising yield potential, successful breeders suggested a number of strategies for increasing genetic gains in yield potential. While the pivotal role of recombining elite germplasm was recognized (Rasmusson, 1996), it was also agreed that significant jumps in yield potential will almost certainly require introgression of new genes from diverse sources (Kronstad, 1996). This will permit evaluation of new yield-determining genes in different backgrounds. Development of improved early-generation selection criteria was also among the recommendations highlighted by the group.
Wide Crossing and Potential Application of Molecular Approaches
The potential value of genes from relatives of cultivated wheat is undoubted, but to date the disadvantages of wide crossing have frequently outweighed the benefits, especially in raising yield potential, since the genetic basis of yield is so poorly understood. However, recent work in rice (Oryza sativa L.) has demonstrated molecular markers which were associated with yield-enhancing genes from the wild relative, O. rufipogon, in generations that had been back-crossed to the high-yielding parent (Xiao et al., 1996). If specific markers for yield-enhancing quantitative trait loci (QTLs) from wheat relatives can be identified in wide-cross progeny, it should be possible to speed up subsequent introduction of beneficial alleles by screening for them in early back-cross generations. By crossing an elite line with a wild relative of interest, and developing the segregating population until the second or third back-cross without making any selection, a random breaking up of the wild recipient genome can be achieved. Comparing phenotypic performance and allelic composition for the different genotypes, favorable alleles contributed by the wild relative can be identified. Those QTLs are less likely to be associated with linkage drag from unfavorable alleles, or to be dependent on epistatic interactions among alleles from the wild parent, since the genome will be skewed in favor of that of the recurrent parent (Tanskley and Nelson, 1996). This technique has the benefit that favorable QTLs will be identified at the same time they are being introgressed. (This of course assumes that beneficial genes are present in wild diploid species that will show favorable epistatic interaction with improved tetraploid genomes.) Knowledge of yield QTLs could also significantly improve the efficiency of breeding by permitting the identification of genetically complementary parents.
Using Physiological Tools to Complement Empirical Selection
While morphological traits associated with yield, such as grain number and HI, can be used in visual selection of breeding lines, neither trait is reliably expressed in small plots, or at low density in early generations. However, there is now good evidence that certain physiological traits have potential for improving selection efficiency. For example, under warm, irrigated conditions, CTD measured on yield trials in Mexico was significantly associated with yield variation in situ, as well as with the same lines grown at a number of international testing sites (Reynolds et al., 1994b).
An integrated CTD value can be measured almost instantaneously using an infrared (IR) thermometer on scores of plants in a small breeding plot, thus reducing error normally associated with traits measured on individual plants. Leaf temperatures are depressed below air temperature when water evaporates from their surface. The trait is affected directly by stomatal conductance, and therefore indirectly by many physiological processes, including vascular transport of water as well as C fixation and other metabolic activity. As such, CTD is a good indicator of a genotype's fitness in a given environment. Canopy temperature depression measured during grain filling also seems to be influenced by the ability of a genotype to partition assimilates to yield. This is indicated by the fact that CTD frequently shows a better association with yield and grain number than it does with total aboveground biomass (unpublished data). Investigations into methodology (Amani et al., 1996) have shown that CTD was best associated with performance when measured at higher vapor pressure deficits (i.e., on warm, sunny afternoons). Irrigation status was not a confounding factor within the normal frequencies of water application.
Measurements made at CIMMYT's main wheat breeding station (Obregon, northwestern Mexico) under temperate, irrigated conditions have demonstrated potential genetic gains in response to selection for CTD. The trait measured on F5:8 recombinant inbred lines from the cross Seri82 x 7Cerros explained >40% of the variation in yield (Fig. 4)
. Other work has demonstrated the effectiveness of using the trait in selection nurseries to predict performance of advanced lines in target environments (Reynolds et al., 1998). When CTD was compared with other potential selection traits measured in the selection environment, including grain number, biomass, phenological data, and yield, none of the other traits showed a greater association with performance in the target environment than CTD.
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Fig. 4 Relationship between canopy temperature depression (CTD) measured during grain filling and yield of random derived F5:8 sister lines from the spring wheat cross Seri 82 x 7Cerros-66, Obregon, northwestern Mexico, 1996 through 1997
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In addition to yield, breeding objectives must take into account multiple factors, such as disease tolerance and phenology. Therefore, when incorporating CTD into a selection protocol, it would be logical to select for relatively genetically simple traits such as agronomic type and disease resistance in the earliest generations (e.g., F2–F3). Selection for CTD could be employed in subsequent generations, when more loci are homozygous, as well as perhaps in preliminary yield trials. The possibility of combining selection for both CTD (on bulks) and stomatal conductance (on individual plants) is another interesting possibility. In fact, work which evaluated a number of indirect early generation (F2) selection criteria for yield (Table 6)
demonstrated the value of stomatal (i.e., leaf) conductance as a predictor of yield more than 20 yr ago (Wall, 1977). In more recent work at CIMMYT (Gutierrez et al., 2000), stomatal conductance was measured on individual plants in F2:5 bulks and showed significant phenotypic and genetic correlation with yield of F5:7 lines (Fig. 5)
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Measuring Canopy Temperature Depression with Aerial Infrared Imagery
In terms of selection technologies on the horizon, one may be the use of aerial IR imagery to significantly increase the efficiency of conventional selection for yield. Work conducted recently in northwestern Mexico showed that aerial IR images had sufficient resolution to detect CTD differences on relatively small yield plots (1.6 m wide). Data were collected using an IR radiation sensor mounted on a light aircraft that was flown at a height of 800 m above the plots. Information from the image was subsequently digitized to provide individual plot canopy temperatures to within an accuracy of 0.1°C. Data of plot temperatures showed significant correlation with final grain yield for random derived recombinant inbred lines as well as advanced breeding lines, and a set of elite varieties (Table 7)
. Data from IR imagery were compared with a spot reading taken a few days earlier with a handheld IR thermometer under clear and sunny conditions. Considering that conditions were suboptimal at the time of IR imagery measurement (intermittent cloud cover introduced significant error into the measurements), the correlation with yield compared quite favorably with that of data from handheld IR thermometers (Table 7). For both methodologies, correlation with yield was higher with the random derived lines than with advanced or elite lines that had already been screened for performance. (This is to be expected since nurseries would be skewed in favor of physiologically superior lines after selecting for yield.) The results validated the potential of aerial IR imagery as a means of screening hundreds, or potentially thousands, of breeding plots in a few hours for CTD, and hence for their genetic yield potential.
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Table 7 Comparison of CTD data from aerial infrared (IR) imagery with handheld IR thermometers, Obregon, 1996 through 1997
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Spectral Reflectance
Another promising technology is spectral reflectance, which can be used to estimate a range of physiological characteristics including plant water status, leaf area index, chlorophyll content, and absorbed PAR (Araus, 1996). The technique is based on the principle that certain crop characteristics are associated with the absorption of very specific wavelengths of electromagnetic radiation (e.g., water absorbs energy at 970 nm). Solar radiation reflected by the crop is measured, and calibrated against light reflected from a white surface. Different coefficients can be calculated from specific bands of the crop's absorption spectrum, giving a semiquantitative estimate (or index) of a number of crop characteristics. In preliminary experiments, the indexes NDVI (normalized difference vegetation index), WI (water index), SR (simple ratio), and SIPI (structural independent pigment index) all showed significant correlation with yield, biomass, and leaf area index. The measurements were made during grain filling on 25 advanced lines selected for diverse morphology, with yields ranging from 5 to 9 t ha-1 in an irrigated spring wheat environment in northwestern Mexico (Reynolds et al., 1999). Performance was best correlated with NDVI and was a little higher
for biomass (Fig. 6)
than for yield
. For further explanation of these and other spectral reflectance indexes, the reader is referred to Araus et al. (1996, 1999). Spectral reflectance devices record the intensity of reflected radiation from 400 to 1200 nm, producing a unique SR signature for a genotype. Given the wealth of information present in a genotype's SR signature, the possibility exists of using parallel processing to search out characteristic reflectance patterns associated with performance, to complement some of the existing indexes mentioned above.
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Fig. 6 Relationship between spectral reflectance index NDVI (normalized difference vegetation index) measured during grain filling and biomass of irrigated spring wheat advanced lines, Obregon, northwestern Mexico, 1996 through 1997
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Conclusions
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It is reassuring that genetic gains in wheat yield are holding steady, and that a genetic plateau has not yet emerged. To assist with the challenge of accelerating rates of genetic gains, there is renewed hope that alien genes, such as those associated with the chromosome segment containing Lr19, can contribute significant gains in yield. In addition, there is evidence that conventional approaches of breeding for improved morphology, such as the branched rachis node, can still lead to substantial yield gains. Breeders can consider new tools that may improve the efficiency of selection, such as the measurement of stomatal conductance or CTD and the use of molecular markers in advanced back-cross generations with wild relatives.
With respect to understanding the physiological basis of yield improvement perhaps the most interesting developments are: (i) the direct manipulation of spike growth duration which resulted in substantially increased spike fertility and (ii) the observation that yield is not necessarily associated with improved partitioning of assimilates to spike growth challenging the hypothesis that a simple increase in biomass of the juvenile spike would necessarily result in yield gains. If we are to understand the physiology of the wheat crop, perhaps we also need to consider the evolution of the wheat species. Wheat evolved and was subsequently selected under relatively low-yielding conditions. Physiological traits conferring survival were strongly favored for most of the crop's evolution. Blum (1996) suggests that subtle expression of "conservative" traits may still hold back yield potential in modern wheat. As a result, any degree of competition for assimilates from alternate sinks, for example root and tiller growth, osmotic adjustment, or carbohydrate reserves in stems, may reduce partitioning of assimilates to grain yield. Obviously, these types of traits are not advantageous when a single genotype is grown at high density with ample water and nutrients. Perhaps another example of a conservative trait is root signalling, which can cause reduced stomatal conductance in response to soil water deficits that are not actually limiting potential evapotranspiration (Davies and Zhang, 1991). If in response to an environmental cue, the water relations of a plant can be regulated, it is conceivable that subtle stresses at critical growth stages may also lead to conservative responses in reproductive growth. Nitrogen availability is another critical factor determining growth potential, and spike fertility apparently can respond directly to N supply to the spike (Abbate et al., 1995). Whether or not significant genetic diversity exists for sensitivity to subtle stresses needs investigation, as does the potential role of these conservative traits in determining yield potential in modern wheat varieties.
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TOP
ABSTRACT
INTRODUCTION
