GENETIC-VARIATION IN CARBON-ISOTOPE DISCRIMINATION AND ITS RELATION TO STOMATAL CONDUCTANCE IN PIMA COTTON (GOSSYPIUM BARBADENSE)

Stable carbon isotope discrimination (Delta) was evaluated in primitiv e and cultivated Gossypium barbadense L. Significant differences among cultivated Pima lines were positively associated with the degree of s election for lint yield and heat resistance. A population mean study o f a cross between B368, a primitive, uncultivated G. barbadense, and P ima S-6, an advanced line, showed that Delta is probably under genetic control, and could be a suitable selection trait in breeding programs . Eleven uncultivated accessions of primitive G. barbadense of varying origins grown in one environment showed a broad range of Delta values (18.8-20.5 parts per thousand), pointing to substantial genetic varia tion of Delta in the G, barbadense germplasm. Delta was strongly corre lated with stomatal conductance (g(s)) in the commercial lines, a segr egating F-2 population of the B368 x Pima S-6 cross, and the collectio n of uncultivated G. barbadense. This relationship indicates that vari ation in gs is the main source of variation for Delta in both uncultiv ated and commercial G. barbadense. The positive correlation between De lta, g(s) and yield in the commercial Pima lines provides further evid ence for selection pressures on higher g(s) ensuing from selection for higher yield and heat resistance. Selection for higher Delta could in crease yield in crops grown in hot, high irradiance, and well-irrigate d environments.