This thesis assessed the effect of temperature and cultivar on growth and physiological responses, the thermotolerance of photosynthetic processes in cotton seedlings, and identified the most important plant traits contributing to seedling vigor. The lowest temperature regime (20/15 ⁰C) consistently caused significant declines for all growth and physiological plant traits. 30/20 and 35/25 °C were optimal for shoot growth, and where no interactive effect existed, the large-seeded cotton cultivar had higher values for most growth traits. Leaf area was the most important driver of seedling vigor, and whole-canopy photosynthesis was a more precise predictor of total biomass than net assimilation per unit leaf area. The net assimilation rate was lowest at 20/15 ⁰C, and reductions were due to metabolic limitations. Inter-photosystem electron transport was the most heat-sensitive thylakoid response. Cotton plants grown under the sub-optimal temperature were least thermotolerant, and cultivars exhibited differences in heat acclimation of photosystem I.