Intensifying heat stress impacts cotton flowering and boll development efficiency.

Cotton-growing regions face heat stress during flowering and boll development, adversely affecting reproductive fitness, yield, and quality. Identifying and incorporating superior physiological and reproductive traits into elite genetic background may improve yield potential under stress. This study quantified the effects of heat stress (36/24 °C, HS) on 25 traits of 16 upland cotton cultivars during the reproductive stage, compared to control conditions (32/24 °C, CNT). Under HS, all cultivars exhibited transpirational cooling, with stomatal conductance increasing by 1-fold and transpiration by 1.6-fold compared to CNT. Under HS, leaf temperature was 3.5 °C higher than CNT plants, and PSII quantum efficiency dropped by 20% compared to CNT. HS also reduced the ratio of reproductive to vegetative dry mass (57%), indicating poor resource partitioning towards reproductive organs. The specific leaf area was increased by 34%, whereas the leaf chlorophyll index dropped by 28% under HS compared with CNT. When exposed to HS, pollen germination was reduced by 71% across cultivars compared to the CNT. While some cultivars maintained similar boll and seed numbers between CNT and HS, a corresponding tolerance was not observed in seed cotton yield. On average, seed cotton yield and lint yield decreased by 19% and 26% under HS, respectively. In contrast, seed yield remained stable, suggesting a disproportionate reduction in intra-boll components. Seed oil content and fiber quality, except for fiber uniformity, displayed susceptibility to HS. Multidimensional responses of cotton cultivars to HS highlight the need to map genetic loci governing heat tolerance. This knowledge is essential for pinpointing effective breeding targets.