ABSTRACT
Yield of Pima cotton (Gossypium barbadense L.) has tripled over the last forty years with the development of new cultivars and strains. We report elsewhere in these proceedings that in the greenhouse, lines bred for heat tolerance (yield in a hot environment) maintained higher stomatal conductance (g(s)) than did obsolete cultivars. Genetic differences in g(s) were also strongly expressed in irrigated field-grown Pima cotton at Maricopa, Arizona. Rankings of numerous lines for g(s) were similar to rankings for heat tolerance. Plant temperatures were followed using both infrared thermometry and thermocouples affixed to tissues. Canopy temperature was inversely related to g(s) because of differences in evaporative cooling by transpiration. The difference between improved and unimproved lines was as much as 3 C. Genetic differences in leaf temperature were maintained for most of the day and the night. In the bracts subtending the flower buds (squares), diurnal patterns of genetic temperature differences were similar to those in the leaves, but in the flower buds themselves, genetic differences in temperature were minimal. If improved tolerance results from lowered temperature, the primary effects must occur outside the developing reproductive structures. The findings are consistent with the hypothesis that evaporative cooling of the canopy is an important component of heat tolerance in Pima cotton.
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