(100-11) Genotypic variation in growth, photosynthetic responses, and acclimation potential of thylakoid processes in cotton exposed to high temperature extremes.
Body of Abstract: A number of physiological factors could potentially contribute to genotypic variation in heat tolerance in upland cotton. Leaf area responses, net assimilation rates, dark respiration, day and nighttime stomatal conductance, and acclimation of thylakoid processes could all influence whole-plant growth responses to temperature extremes. Consequently, the objective of this study was to assess the effects of growth temperature and genotype on plant growth, single-leaf physiology, and thermotolerance of thylakoid components for cotton exposed to optimal and supra-optimal temperature conditions. A diverse collection of cotton cultivars were selected based on previously documented differences in thylakoid-specific thermotolerance acclimation, and they were exposed to optimal (30/20 C) and two supra-optimal growth temperatures (35/25 C and 40/30 C). There was a significant interaction between genotype (G) and growth temperature (T) for all growth measures (leaf area, mainstem node production, and plant dry weight), indicating significant genotypic variation in tolerance to heat stress. Similarly, net assimilation rate, electron transport rate, nighttime respiration, nighttime stomatal conductance, and thermotolerance of photosystem II all exhibited significant G x T interactions. Genotypic averages at the highest growth temperature regime were converted into values that were relative to the optimum. Greater relative growth under high temperature was strongly and positively correlated with leaf area and strongly and negatively correlated with nighttime respiration and stomatal conductance. In contrast, photosynthetic parameters such as net assimilation rate and electron transport rate were not significantly correlated with any growth parameter. We conclude that genotypes with greater heat tolerance have higher relative leaf area and lower values for nocturnal respiration and stomatal conductance under high temperature conditions.