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a Dept. of Plant Agriculture, Crop Science Division, University of Guelph, Guelph, ON, N1G 2W1, Canada
ttollena{at}plant.uoguelph.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONResource Capture and UseThe Role of Stress...The Potential for Future...REFERENCES |
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1.5% yr-1 during the last five decades. Comparison of short-season hybrids representing yield improvement from the late 1950s to the late 1980s showed that genetic yield improvement was 2.5% per year and that most of the genetic yield improvement could be attributed to increased stress tolerance. Differences in stress tolerance between older and more recent hybrids have been shown for high plant population density, weed interference, low night temperatures during the grain-filling period, low soil moisture, low soil N, and a number of herbicides. Yield improvement is the result of more efficient capture and use of resources, and the improved efficiency in resource capture and use of newer hybrids is frequently only evident under stress. Improved resource capture has resulted from increased interception of seasonal incident radiation and greater uptake of nutrients and water. The improved resource capture is associated with increased leaf longevity, a more active root system, and a higher ratio of assimilate supply by the leaf canopy (source) and assimilate demand by the grain (sink) during the grain-filling period. Improvements of resource use under optimum conditions have been small, as leaf photosynthesis, leaf-angle distribution of the canopy, grain chemical composition, and the proportion of dry matter allocated to the grain at maturity (i.e., harvest index) have remained virtually constant. Genetic improvement of maize has been accompanied by a decrease in plant-to-plant variability. Results of our studies indicate that increased stress tolerance is associated with lower plant-to-plant variability and that increased plant-to-plant variability results in lower stress tolerance.
Abbreviations: CV, coefficient of variability • PPFD, photosynthetic photon flux density
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONResource Capture and UseThe Role of Stress...The Potential for Future...REFERENCES |
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per hectare in Ontario has increased at a rate of approximately 1.5% yr-1 during the five-decade period since the 1940s, whereas little or no improvement in grain yield was evident during the five decades before the introduction of hybrids in the 1940s (Fig. 1) . Yield improvements of a similar magnitude have been recorded for maize hybrids in the USA and Europe (Tollenaar et al., 1994). Yield improvement generally can be attributed to genetic improvement, changes in cultural management, climate change, and the interactions among these factors. The genetic improvement can be estimated from side-by-side comparisons of hybrids that are representative of the period under study. Estimates of the contribution of genetic improvement to the overall yield improvement in maize range from 40 to 100% (e.g., Derieux et al., 1987; Tollenaar, 1989; Duvick, 1992). However, the relative contributions of genetic gain and of gains due to agronomic and environmental influences are difficult to separate, as genotype x environment interaction is a prominent feature of yield improvement in maize.
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The observation that recent single-cross hybrids are more stress tolerant than open-pollinated varieties and older hybrids contradicts the notion that genotypic and phenotypic variability are positively associated with yield stability. Commercial maize genotypes in the U.S. Corn Belt evolved from open-pollinated varieties before the 1930s to double-cross hybrids from the 1930s to the 1960s and single-cross hybrids since the late 1960s. The evolution of open-pollinated varieties of the 1930s to single-cross hybrids of the 1990s was associated with an increase in rate of yield improvement and an increase in area of adaptation (Troyer, 1996). This evolution was also associated with a decrease in genetic variability within and among commercial maize genotypes: hundreds of open-pollinated varieties were grown up to the 1930s in the U.S. Corn Belt, but the parentage of virtually all relatively recent, commercial U.S. hybrids involve only six inbred lines or their close relatives (Goodman, 1990). The notion that variability within a community of plants, including mixtures of cultivars, increases yield stability (Wolfe, 1985) suggests that yield stability of an open-pollinated variety should be greater than that of a single-cross maize hybrid. However, the opposite effect is indicated by the increase in area of adaptation and the increase in stress tolerance in the comparison of older and newer commercial maize varieties and hybrids. We have found that increases in yield and stress tolerance of more recent maize hybrids grown in Ontario are associated with enhanced crop stand uniformity (Wu, 1998).
We will analyze the physiological basis of genetic improvement in maize in general and specifically for short-season maize hybrids grown in Ontario. Yield improvement is the result of more efficient capture and use of resources, and factors involved in the improved efficiency will be reviewed. Second, the effect of stress tolerance and stand uniformity will be discussed because improved efficiency in resource capture and utilization of newer hybrids is frequently only evident under stress. Finally, we will speculate about the potential for genetic improvement of maize in the future.
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Resource Capture and Use |
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TOPABSTRACTINTRODUCTIONResource Capture and UseThe Role of Stress...The Potential for Future...REFERENCES |
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Canopy Closure
Whereas enhanced seedling vigor advances time of canopy closure and increases seasonal PPFD interception, our results with short-season maize hybrids show that total aboveground dry matter before the 12-leaf stage of a newer hybrid was less than that of an older hybrid in growth-cabinet studies (Tollenaar et al., 1991; McCullough et al., 1994), and aboveground dry matter of newer hybrids was lower than that of older hybrids in a field study that involved six hybrids (Tollenaar, 1996, unpublished data). Results of the growth-cabinet studies indicated that lower early vigor was associated with a greater root/shoot ratio. Root/shoot ratio can influence rate of dry matter accumulation during early development, because leaf area expansion is directly related to amount of dry matter partitioned to the leaves. Leaf photosynthetic rate did not differ between the two hybrids (Tollenaar et al., 1991; McCullough et al., 1994) and, consequently, plant dry matter accumulation and rate of leaf area expansion during early development were inversely related to the root/shoot ratio.
Senescence
Delayed leaf senescence or "stay green" is associated with yield improvement of maize hybrids in North America (e.g., Crosbie, 1982; Tollenaar, 1991; Duvick, 1997). Most of the differences in dry matter accumulation between older and newer hybrids can be attributed to the grain-filling period (Tollenaar, 1991; Tollenaar and Aguilera, 1992). The "stay-green" characteristic of the newer hybrid compared with an older hybrid was associated with a larger source/sink ratio during the grain-filling period (Rajcan and Tollenaar, 1999a).
Nutrient and Water Uptake
Nutrient uptake is related to root mass and energy supply. Limited data on root mass during later phases of maize development have been reported in the literature. A comparison of the older hybrid Pride 5 with the newer hybrid Pioneer 3902 grown in a hydroponic system (Tollenaar and Migus, 1984) in the field in 1992 and 1993 showed that the root/shoot ratio was 20% greater in the newer than in the older hybrid during the grain-filling period (Nissanka, 1995). Higher source/sink ratio in the newer hybrid (Rajcan and Tollenaar, 1999a) may imply that assimilate supply to the roots is greater in the newer hybrid. Rajcan and Tollenaar (1999b) reported that the proportion of grain N derived from postsilking N uptake was 60% for the newer hybrid and 40% for the older hybrid. Continuous N uptake during the grain-filling period has been associated with the ability to maintain root growth after silking (Mackay and Barber, 1986), which may be a function of assimilate supply. Water uptake also may be related to root mass. Nissanka et al. (1997) showed that decline in plant photosynthesis during a drying cycle occurred on average 1 d earlier in the older hybrid compared with the newer hybrid, and the recovery of canopy photosynthesis after rewatering was greater in the newer than in the older hybrid.
Resource Use
There are four avenues for increasing resource utilization: an increase in gross leaf photosynthetic rate, an increase in gross canopy photosynthetic rate, a reduction in plant respiration, and an increase in harvest index (i.e., grain yield as a proportion of total aboveground dry matter at maturity).
Gross Leaf Photosynthetic Rate
An increase in gross leaf photosynthesis may be the result of either an increase of photosynthesis at saturating PPFD (i.e., Pmax), an increase of photosynthesis at low PPFD (i.e., quantum yield), or both. Differences in leaf net photosynthesis between older and newer hybrids were not significant under optimal conditions (Crosbie, 1982; Dwyer and Tollenaar, 1989).
Gross Canopy Photosynthetic Rate
A more uniform distribution of incident solar radiation across the crop canopy can result in an increase in gross canopy photosynthetic rate (Tollenaar and Dwyer, 1998). An increase in leaf angle results in a more uniform distribution of solar radiation across the canopy, and an increase in leaf angle has been reported for Corn Belt hybrids between 1930 and 1990 (Crosbie, 1982; Duvick, 1997). Effect of leaf angle on canopy photosynthesis can be estimated from the change in PPFD distribution across the leaf canopy and the photosynthesis-PPFD response curve. Our estimates indicate a yield improvement in the order of 20% when leaf angle increases from 30 to 60° (Tollenaar and Dwyer, 1998). In contrast with maize hybrids in the U.S. Corn Belt, canopy architecture of short-season hybrids in Central Ontario had not changed by the late 1980s (Tollenaar and Aguilera, 1992), although leaf angle seems to have increased in hybrids introduced during the 1990s (Tollenaar, 1996, unpublished data).
Respiration
The two major components of respiration are growth respiration, which is a function of the composition of the plant dry matter, and maintenance respiration, which is a function of the energy required to maintain plant function and structure (Penning de Vries et al., 1974; Penning de Vries, 1975). Little has been reported on changes in plant composition associated with yield improvement in maize. Vyn and Tollenaar (1998) reported that changes in grain composition associated with yield improvement in Ontario were minor and Duvick (1997) reported that grain protein concentration of U.S Corn Belt hybrids declined by 1.8% yr-1 from the 1930s to the 1990s. A decline in protein concentration will result in a reduction of growth respiration per unit grain, but no data have been reported on total plant respiration and its components. Earl and Tollenaar (1998) reported a strong negative correlation between mature-leaf respiration during the growing season and total seasonal dry matter accumulation among three older and three newer Ontario maize hybrids.
Harvest Index
The different responses of grain yield to plant population density among maize hybrids representing various eras of breeding have led some to conclude that harvest index has been positively associated with genetic improvement in maize (Russell, 1985). Harvest index declines when plant population density is increased beyond the optimum plant population density for grain yield in maize, and the optimum plant population density for grain yield is lower for older hybrids than for newer hybrids, which may explain the association between harvest index and era of release for hybrids grown at high plant population density (Tollenaar et al., 1994). Harvest index did not differ among hybrids when maize hybrids representing three decades of yield improvement in Ontario were adjusted for optimum plant population density (Tollenaar, 1989).
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The Role of Stress Tolerance and Stand Uniformity in Resource Use |
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TOPABSTRACTINTRODUCTIONResource Capture and UseThe Role of Stress...The Potential for Future...REFERENCES |
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Results of our studies with short-season hybrids representing grain-yield improvement in Central Ontario have shown that newer hybrids are more tolerant to stress than older hybrids. The differential response to stress between older and newer hybrids has been shown for low night temperature during the grain-filling period (Dwyer and Tollenaar, 1989), low soil moisture in the field (Dwyer et al., 1992), and under controlled-environment conditions (Nissanka et al., 1997), low soil N (McCullough et al., 1994), and the herbicide bromoxynil (3,5-dibromo-4-hydroxybenzonitrile, a Photosystem II inhibitor) (Tollenaar and Mihajlovic, 1991). We also have shown that tolerance of compound stresses such as plant population density (Tollenaar, 1992) and weed interference (Tollenaar et al., 1997) is greater in newer than in older hybrids.
It has been suggested that selection under high plant population density has been a key component in grain yield improvement in corn (Troyer and Rosenbrook, 1983) and, consequently, changes in stress tolerance may have been the result of indirect rather than direct responses to selection. One could speculate that such indirect responses resulted from either or both (i) progressively higher plant population densities in yield testing programs and (ii) wide area testing , which has enabled the evaluation of hybrids under a wide range of environmental conditions.
Stand Uniformity
Although stand uniformity of field crops has long been recognized as an important aspect of high-yielding cultivars and breeders have been pursuing it since the beginning of modern agriculture, it also has frequently been claimed that cultivar mixtures can give higher and more stable yields than pure lines in cereal crops (Wolfe, 1985). However, Marshall and Brown (1973) emphasized that blends of highly adapted cultivars will seldom be more stable than the best or better component. Reports of mixtures of barley (Hordeum vulgare L.) with yields higher than the highest yielding component were very few (Jedel et al., 1998). In most previous studies, yields of mixtures have been neutral, in other words about the same as, or slightly higher than, that of the weighted means of their components (Wolfe, 1985; Jokinen, 1991). Hoekstra et al. (1985) compared all two-component mixtures of seven maize hybrids with their pure stands under different densities. In both years, no mixture at a standard agronomic plant population density yielded significantly more than either of the two highest yielding hybrids in pure stand. In a 7-yr experiment in Nepal comparing barley cultivar mixtures with pure lines, it was found that even during epidemic years of yellow rust, the disease reduction in the mixture was not enough to significantly increase the yield of the mixture above that of the mean yield of the components (Pradhanang and Sthapit, 1995).
Size differences within plant communities can be caused by several factors, some of which affect plant growth rates, while some affect size in other ways. Size variation in plant communities may develop because of (i) age differences; (ii) genetic differences; (iii) environmental heterogeneity; (iv) maternal (seed size) effects; (v) differential effects of herbivores, parasites, or pathogens; or (vi) competition. In most cases, size variation will be the result of interactions among these factors.
Bonan (1988) showed in a model that competing plants have a higher size inequality than noncompeting plants. He interpreted these results as indicating that the greater degree of size inequality among competing plants than among noncompeting plants reflects, in part, variation in relative growth rates induced by neighborhood competition. Bonan (1991) further proposed that increased plant-to-plant variability at higher plant densities is a direct manifestation of neighborhood competition.
Stand Uniformity, Yield, and Stress Tolerance
Resource use efficiency of a crop is inversely related to plant-to-plant variability because the response of photosynthesis and grain yield plant-1 to input of resources (i.e., PPFD, N, water) is curvilinear (i.e., the law of diminishing returns). For instance, Edmeades and Daynard (1979) reported a curvilinear relationship between assimilate flux plant-1 at anthesis and grain yield plant-1, with a positive threshold for assimilate flux at which grain yield was zero. In nonuniform stands of field crops, bigger or taller plants have a competitive advantage over the smaller or shorter ones. There is much evidence that interactions between plants of the same genotype at commercial production densities are always undercompensatory; gains from competition fail to counterbalance losses (Pendleton and Seif, 1962; Glenn and Daynard, 1974; Hoekstra et al., 1985; Fasoula and Fasoula, 1997). Pendleton and Seif (1962) studied the effects of competition between a tall maize hybrid `US 13' and its near-isogenic dwarf version. All combinations of tall and dwarf plants produced lower yields than tall and dwarf plants grown alone. A row of tall maize plants bordered by dwarf plants yielded 6% more than when bordered by tall plants. A row of dwarf plants bordered by tall plants yielded 30% less than when bordered by dwarf plants. A study by Glenn and Daynard (1974) on the effects of plant-to-plant variation on maize grain yield demonstrated that plant-to-plant variation per se lowered grain yield. They suggested that cultural procedures designed to encourage uniform plant establishment, like uniform seed bed and constant planting depth, should maximize maize grain yield. Ford and Hicks (1992) observed a 5% yield reduction in maize when half of the stand was delayed in sowing by 7 d, and a 12.8% yield reduction when half of the plant stand was delayed by 14 d of late sowing. The reduction in yield increased both with an increase in the proportion of the stand that was delayed and with an increase in plant population density.
Results of our studies indicate that stand uniformity and stress tolerance are highly associated (Wu, 1998). In the first experiment, the relationship between stand uniformity and stress tolerance was investigated in an older and a more recent hybrid grown at two plant densities, 3.5 and 11 plants m-2. At each density seeds were sown either all on the same day to produce uniform stands (control), or on alternative sowing dates of three to produce nonuniform stands (treatment). At physiological maturity a significant (P < 0.05) density x treatment interaction was observed for grain yield, total aboveground dry matter, and coefficient of variability (CV) of the individual plant yield and dry matter. The CV is the most widely used parameter to quantify variability among individual plants of a crop stand (e.g., Edmeades and Daynard, 1979; Barnes, 1982; Fasoula and Fasoula, 1997). At the lower density, stands of uniform sowing and nonuniform sowing did not differ in leaf area index, total aboveground dry matter, and grain yield, nor did they differ in the plant-to-plant variability for these traits. At the higher plant population density, higher plant-to-plant variability in stands of both hybrids was observed. Nonuniform stands yielded less than uniform stands in both hybrids at the higher plant population density. The difference in grain yield between the newer and the older hybrid was 30% in the uniform stand and 46% in the nonuniform stand (Fig. 2) . Overall, stand uniformity declined with an increase of interplant competition, and its impact on grain yield was greater in the older than in the newer hybrid. In a second experiment, the association between genetic improvement and stand uniformity among six maize hybrids released from 1959 to 1995 was explored. Hybrids were grown at two plant population densities (7 and 11 plants m-2) and two soil N levels. Results depicted in Fig. 3 show that CV was not associated with yield when CV of the crop stand was <30%, which indicates that changes in standard deviation were proportional to changes in yield (i.e., CV = standard deviation/mean). In contrast, a small decline in yield was associated with a steep increase in CV when CV of the crop stand was >30%. In both studies density-tolerant hybrids exhibited less plant-to-plant variability than nontolerant ones at plant densities close to or higher than their optimum densities.
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The Potential for Future Genetic Improvement |
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TOPABSTRACTINTRODUCTIONResource Capture and UseThe Role of Stress...The Potential for Future...REFERENCES |
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In contrast, some of the factors that have shown either no or little improvement in the past may contribute to future yield improvement. For instance, early plant vigor, possibly combined with cold tolerance, could increase PPFD interception during early phases of development. Also, maize researchers in China (Zhao and Li, 1998) have selected for high Pmax and reported that Pmax during the growing season was 15% greater in selected lines than in the control, which was associated with an increase in grain yield. In addition, harvest index could increase if the increase in grain yield would be proportionally greater than the increase in total dry matter (i.e., an increase in total aboveground dry matter does not have to be associated with an increase in leaf area per plant, as PPFD interception of maize grown at commercial plant densities is currently close to 95%). However, the contribution to future yield improvement of these three factors is likely to be relatively small.
Increased stress tolerance, combined with increased stand uniformity under stress conditions, will probably continue to provide the highest potential for yield improvement in maize in the next decades. A large proportion of the genetic improvement of maize in the past is attributable to increased stress tolerance, and improved stress tolerance can continue to contribute to genetic gain in the future. Maximum yields obtained under field conditions represent an indication of genetic yield potential of current maize hybrids (all yields cited herein are expressed at 0% grain moisture). A farm-scale yield of 19.6 Mg ha-1 has been recorded by a maize producer in Illinois (Warsaw, 1985) and a yield of 15.5 Mg ha-1 has been reported for large field research plots in Ontario (Stevenson, 1985). Average yields of maize in the USA and Ontario during the period those yields were recorded were 6.0 Mg ha-1. Alternatively, yield potential is indicated by estimates of grain yield obtained under close to no-stress conditions. Mean yield during a 4-yr period was 11.5 Mg ha-1 of a short-season hybrid grown in a growth room in which daily accumulated PPFD was only 50% of average daily accumulated PPFD during summer months in Ontario (Tollenaar and Migus, 1984). The radiation use efficiency in this study was 7.6 g of dry matter per megajoule of incident photosynthetic active radiation, which is approximately two times greater than reported values for field-grown maize (Tollenaar and Aguilera, 1992). Overall, these results show that the potential for continued genetic yield improvement through improved stress tolerance is substantial.
Received for publication December 28, 1998.
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TOPABSTRACTINTRODUCTIONResource Capture and UseThe Role of Stress...The Potential for Future...REFERENCES |
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