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 b red for higher yields at supraoptimal temperatures under ample water s upply have shown that incremental increases in stomatal conductance ha ve 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 sele cted 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 whe at and Pima cotton. A gas exchange study showed that the stomatal resp onse to temperature, but not to light or to water vapour pressure defi cit (VPD), separated low and high-yielding Pima lines in the same orde r as their stomatal conductance in field conditions. Selection for hig her yields in Pima cotton and bread wheat appear to have generated sel ection 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 crit ical developmental stages associated with flowering and fruiting durin g July for Pima cotton in Arizona, and February for bread wheat in nor th-west Mexico appear to favour higher yields, Stomatal conductance co uld be a valuable selection criterion for higher yields in irrigated c rops grown at supra-optimal temperatures.