Stomatal conductance predicts yields in irrigated Pima cotton and bread wheat grown at high temperatures

Recent studies of historical series from Pima cotton and bread wheat bred for higher yields at supraoptimal temperatures under ample water supply have shown that incremental increases in stomatal conductance have accompanied lint and grain yield increases in successive commercial releases. Pima cotton studies showed that the differences in stomatal conductance are under genetic control. F-4 progeny of F-2 plants selected solely for high stomatal conductance had higher lint yields than progeny from low conductance plants. Carbon isotope discrimination is positively correlated with stomatal conductance and yields in both wheat and Pima cotton. A gas exchange study showed that the stomatal response to temperature, but not to light or to water vapour pressure deficit (VPD), separated low and high-yielding Pima lines in the same order as their stomatal conductance in field conditions. Selection for higher yields in Pima cotton and bread wheat appear to have generated selection pressures for higher stomatal conductances that are independent of operating pressures for higher photosynthetic rates. The adaptive advantage of higher stomatal conductance appears to be associated with leaf cooling, which provides an avoidance type of heat resistance at supra-optimal temperatures. Lower leaf and canopy temperatures at critical developmental stages associated with flowering and fruiting during July for Pima cotton in Arizona, and February for bread wheat in north-west Mexico appear to favour higher yields, Stomatal conductance could be a valuable selection criterion for higher yields in irrigated crops grown at supra-optimal temperatures.