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Abstract
Past cotton breeding programs have primarily focused on enhancing yield through increasing lint percent and selecting a limited set of desirable agronomic traits. However, without targeted efforts to select for other functional yield drivers or tolerance to abiotic stress, cotton crops are at a greater risk of experiencing yield losses caused by environmental extremes. To further improve cotton cultivars, breeding programs should incorporate diverse germplasm and prioritize the selection of traits that contribute to yield under varying environmental conditions. This research included two field experiments and one controlled-environment experiment. The objectives of the field experiments were to assess genotypic variation in thermotolerance of thylakoid component processes for diverse cotton genotypes and quantify differences in physiological (∑IPAR, RUE, and HI) and yield component contributors to yield in a diverse set of field-grown cotton genotypes. The first experiment highlighted genotypic variations in thermotolerance of photosystem II, intersystem electron transport, and photosystem I. Intersystem electron transport exhibited greater heat sensitivity than photosystem II or I. In the second experiment, lint yield, biomass production, light interception, and harvest index were affected by genotype, with harvest index being a better predictor of lint yield than other traits. Boll production and intra-boll yield components were also genotype-dependent. The objective of the controlled-environment experiment was to assess the effects of growth temperature and genotype on early plant growth, single-leaf physiology, and thermotolerance of thylakoid processes for cotton exposed to optimal and supra-optimal temperature conditions. Significant interactions between genotype and growth temperature were observed for all growth metrics, and some of the physiological processes, and for thermotolerance of photosystem II. Genotypes with higher growth-specific thermotolerance showed greater leaf area and lower nighttime respiration and stomatal conductance under high temperatures relative to the optimum. Net photosynthesis was not predictive of growth response to high temperature. Photosystem II acclimated more readily to high temperature in some genotypes than in others, but high temperature thresholds for PSII were not consistently predictive of growth under high temperature extremes. It is concluded that genotypic differences in thermotolerance will depend on acclimation of multiple processes.