PHOTOSYNTHETIC RATE, STOMATAL CONDUCTANCE AND LEAF-AREA IN 2 COTTON SPECIES (GOSSYPIUM BARBADENSE AND GOSSYPIUM-HIRSUTUM) AND THEIR RELATION WITH HEAT-RESISTANCE AND YIELD

Gossypium barbadense L. (Pima) and Gossypium hirsutum L. (upland) cott ons are the two major fibre producing species grown in the south-weste rn United States, where lint yields are adversely affected by high tem peratures. In these environments, heat-adapted upland cultivars show h igher yields and heat resistance than advanced Pima cultivars. Recent studies with an historical series of commercial Pima cultivars have sh own that increases in lint yield and heat resistance are tightly coupl ed to increases in stomatal conductance and photosynthetic rate, and t o decreases in leaf area. In the present study, Pima S-6 and Pima S-7 (advanced Pima cultivars) and Deltapine 90 (advanced upland cultivar) were compared under field and laboratory conditions to determine wheth er the physiological and morphological gradients found in the Pima his torical series extrapolate to upland cotton. In the field, Deltapine 9 0 showed 25-35% higher stomatal conductance, 35-50% higher photosynthe tic rate and 45% smaller leaf area than Pima S-6. The higher photosynt hetic rate and stomatal conductance of Deltapine 90 leaves were partia lly related to their sun-tracking ability. In gas exchange experiments that prevented sun-tracking, the two cultivars had comparable photosy nthetic rate as a function of incident radiation, while stomatal condu ctance was higher in upland cotton. In the 25-35 degrees C range, phot osynthetic rate as a function of temperature remained nearly constant in both cultivars, and was higher in upland cotton at all temperatures . Stomatal conductance showed a strong temperature-dependence, and con ductance value and the slope of the stomatal response to temperature w ere higher in Deltapine 90. In progeny from a cross between Deltapine 90 and Pima S-7, the segregation of stomatal conductance in F-1 and F- 2 populations showed a clear genetic component. These results indicate that the differences in photosynthetic rate, stomatal conductance and leaf area associated with increases in lint yield and heat resistance in the Pima historical series are also evident in a comparison betwee n advanced cultivars of upland and Pima cotton. Upland cotton could je used as a source of genetic variation for high stomatal conductance i n Pima breeding programs.