Topic 26.1

Stomatal Conductance and Yields of Irrigated Crops

In the last few decades, the breeding of agricultural crops for higher yields has been very successful. Breeders usually select for high-yielding genotypes empirically, without particular attention to specific plant traits that might be conducive to higher yields. However, comparing old, low-yielding lines of any crop with advanced, high-yielding lines shows clearly that many morphological, physiological, and biochemical traits have been altered by the intense selection pressures for higher yields. These changes indicate that selection for high-yielding genotypes has generated indirect selection pressures on the altered traits. If we exclude effects of genes that regulate the expression of two or more unrelated traits (pleiotropic genes), the study of high-yielding lines might reveal specific traits and genes associated with the higher yield. With this information, breeders could explicitly select for yield-enhancing traits to improve yields further.

Changes in plant traits associated with yield increases can be studied conveniently in a historical series of a crop. Historical series include successive commercial releases within a breeding program, with each member of the series representing an incremental increase in yield. In studies aimed at characterizing relationships between specific traits and higher yields, all the members of the series are grown together under the same conditions, and the traits of interest, such as plant architecture or photosynthetic rates, are measured in parallel with yields. Recent studies of this type focusing on historical series of Pima cotton (Gossypium barbadense) and bread wheat (Triticum aestivum) have shown a remarkable positive correlation between yield increases and increases in stomatal conductance (Web Figure 26.1.A) (Lu et al. 1998).

Web Figure 26.1.A Stomatal conductance has increased in parallel with agronomic yields in irrigated Pima cotton (Gossypium barbadense) selected for higher yields at a high temperature. The figure shows the relationship between lint yield and stomatal conductance in a historical series of Pima cotton grown in Arizona. The abbreviations P32 and PS-1 through PS-7 designate successive commercial releases between 1949 and 1996. (From Lu et al. 1998.)

Pima cotton is a high-quality fiber cotton grown in hot environments under intensive irrigation. The historical series used in the Pima studies included eight members, released between 1949 and 1996, and encompassing a nearly threefold increase in lint yield. The yield increase attained in each commercial release was accompanied by a corresponding increase in stomatal conductance. As Web Figure 26.1.A shows, stomatal conductance increased by about 30 mmol m^–2 s^–1 for each 100 kg ha^–1 increase in yield.

Genetic crosses between low- and high-yielding Pima strains have shown that stomatal conductance in Pima cotton has a clear-cut genetic component (Web Figure 26.1.B) (Percy et al. 1996). The studies with stomata of Pima cotton are the first to show a clear-cut genetic regulation of stomatal conductance proper, of the stomatal response to temperature, and of proton pumping rates of isolated guard cells (Lu et al. 1998).

Web Figure 26.1.B Frequency distribution of stomatal conductance in parental populations of the high-yielding Pima cotton line (P73), the old Pima line (P32), and their F₁ and F₂ progeny. (A) Nonoverlapping distribution of stomatal conductance in the two parental populations. (B) Distribution of stomatal conductance in an F₁ population from a cross between the two parents. (C) Distribution of stomatal conductance in an F₂ population derived from F₁ parents. (After Percy et al. 1996.)

In the absence of explicit selection for higher stomatal conductance in the Pima breeding program, what could be the nature of the indirect selection pressures that caused the increases in conductance paralleling the increases in yield? Higher stomatal conductance increases CO₂ diffusion into the leaf and favors higher photosynthetic rates (see textbook Chapter 9). Higher photosynthetic rates could in turn favor a higher biomass and higher crop yields. Advanced Pima lines show a higher photosynthetic capacity than older, low-yielding lines, but photosynthetic rates measured in the same leaves used to measure stomatal conductance were not positively correlated with yields (Radin et al. 1994). Thus, higher stomatal conductance appears to favor higher yields by a mechanism not directly related to photosynthesis.

Evapotranspiration at the leaf surface lowers leaf temperature (see textbook Chapter 9), and higher stomatal conductance enhances this leaf cooling. Optimal daytime temperatures for growth, photosynthesis, and reproduction in Pima cotton are below 30°C, while afternoon air temperature in the hot Pima-growing areas often exceeds 40°C. Evaporative cooling of the leaves thus reduces the gap between optimal growth temperatures and air temperature, and provides heat resistance. Studies with the Pima historical series showed that, because of leaf cooling, leaf temperatures were several degrees lower than air temperature, and leaf and canopy temperature were lower in the advanced, high-yielding lines than in low-yielding lines.

The same relationship among yields, stomatal conductance, and leaf temperature was found in a historical series of bread wheat grown in the warm Yaqui Valley of northwestern Mexico (Web Figure 26.1.C) (Lu et al. 1998). These studies indicate that selection for higher yields in irrigated crops grown at high temperatures imposes indirect selection pressures for high stomatal conductance that lowers leaf temperature and appears to reduce deleterious effects of heat stress on critical flowering and fruiting stages, thus resulting in higher crop yields.

Web Figure 26.1.C The relationship between grain yield and stomatal conductance in a historical series of semidwarf bread wheat grown in Ciudad Obregón, Mexico. The abbreviations H1 through H8 designate successive commercial lines released by the International Maize and Wheat Improvement Center between 1962 and 1988. (From Lu et al. 1998.)

If selection for higher yields generates indirect selection pressures for higher stomatal conductance, can breeders explicitly select for higher conductance and thus obtain lines with higher yields? Studies with Pima cotton have shown that selection of high-conductance F₂ progeny from a cross between high- and low-conductance parents produces high-conductance F₄ lines with higher lint yields than low-conductance lines have (Radin et al. 1994; Ulloa et al. 2000). These experiments indicate that high stomatal conductance could be used as a selection trait for high yields in irrigated crops grown at high temperature (Barbour et al. 2000).

The observed increases in stomatal conductance in crops grown at high temperature also have valuable implications for models developed to predict global climate changes, which are expected to occur because of increases in atmospheric CO₂ (see textbook Chapter 9). Under atmospheric CO₂ concentrations that are twice as high as the present ones, evapotranspiration would decrease over the continents and air temperatures would increase significantly over the tropical land masses, amplifying the changes resulting from atmospheric radiative effects (Sellers et al. 1997). However, these models have yet to take into consideration the effect of higher temperatures on the stomatal control of evapotranspiration. Incorporation of the stomatal response to temperature in climate change models is likely to improve the accuracy of the predictions generated by the models. These studies illustrate both the complex interactions between physical and biological factors in the biosphere, and the advantage of a thorough understanding of the physiological properties of plants for efforts aimed at increasing agronomic yields and at improving models that predict global climate changes.