Higii yields in advanced lines of Pima cotton are associated with higher stomatal conductance, reduced leaf area and lower leaf temperature

Advanced lines of Pima cotton {Gossypium barbadense L.) bred for higher yield potential and heat resistance have higher stomatal conductance and smaller leaf areas than those of obsolete !ines. In controlled experiments, five commercial lines of Pima cotton having increasing lint yield and heat resistance showed a gradient of increasing stomatal conductance and decreasing leaf size. In field experiments. heat-.sensitive, low yield Pima lines showed a lower stomatal conductance than high yielding, advanced lines. This indicates that selection for high yie!d potential and heat resistance has imposed a selection pressure for higher stomatal conductance and smaller leaf areas. The higher slomatal conductance and smaller leaf area in the advanced lines resulted in a lower ieaf temperature in both controlled environments and in the field. The largest leaf temperature differences belween obsolete and advanced lines were observed in the afternoon. These differences coincided with the largest differences in stomatal conductance and the highest air temperatures. Measurements of stomata! conductance and leaf temperature in field-grown progeny from a cross between the advanced line. Pima S-6, and the obsolete line, Pima 32, showed that genetical!y detennined differences in stomatal conductance resulted in corresponding differences in leaf temperature. None ofthe altered physiological traits were selected for in the breeding program, indicating that selection for the desired agronomic traits imposed selection pressures on the altered physiological traits. The increases in .stomata! conductance and decreases in leaf area could represent an integrated response to selection pressures on en!ianced evaporative cooling, ensuing from selection for heat resistance.