Ph.D. Student University of Georgia Tifton, Georgia
Body of Abstract: Upland cotton is most widely grown cotton species but Pima cotton is grown under the extreme high temperatures of the desert southwest. Heat stress negatively affects the net photosynthetic rate (AN) but the relative heat sensitivities of photosynthetic component processes and carbon loss mechanisms remain relatively unexplored for these two cotton species. Therefore, the objective of this study was to compare the heat stress tolerance of photosynthesis and its component processes in Upland and Pima cotton. Upland and Pima cotton were grown under two temperature regimes (optimum: 30/20 ˚C day/night temperature and heat stress: 40/30 ˚C day/night temperature). There were no significant temperature X genotype interactions for all measured parameters except specific leaf N content, specific leaf weight, leaf area and total mainstem nodes. AN was not impacted by the growth temperature, however, it was significantly higher in Upland cotton compared to Pima cotton. Among the potential diffusional limitations evaluated, both stomatal and mesophyll limitations were not contributing to decline in AN under heat stress. Among the metabolic component processes evaluated, electron transport rate and maximum rate of RuBP regeneration increased while maximum rate of Rubisco carboxylation was not impacted by the heat stress. Among the carbon loss processes, both photorespiration and dark respiration increased under heat stress. Based on the results of this study, it can be concluded that carbon loss processes were the major contributor to a decline in AN under heat stress in both cotton species. At whole plant level, decline in leaf area was the major contributor to decline in plant growth and development. Between two cotton species, Upland cotton performed better than Pima cotton at both temperature regimes.