The aim of this study was to characterise the response of CO2 assimilation (A) of cotton (Gossypium hirsutum L.) to short- and long-term exposures to night chilling. We hypothesised that short-term exposures to night chilling would induce reductions in gs and, therefore, A during the following days, while growth of cotton plants for several weeks in cool night conditions would cause elevated leaf carbohydrate content, leading to the down-regulation of the capacity for A. Transferring warm-grown seedlings of wild type cotton, transgenic cotton with elevated sucrose-phosphate synthase activity (SPS+) that might produce and export more sucrose from the leaf, and a segregating null to cool nights (9°C minimum) for 1 or 2 d caused a small reduction in A (12%) and gs (21–50%) measured at 28°C. Internal CO2 did not change, suggesting some biochemical restriction of A along with a gs restriction. After 30 d, new leaves that developed in cool nights exhibited acclimation of A and partial acclimation of gs. Despite the elevated leaf carbohydrate content when plants were grown to maturity with night chilling, no reduction in A, gs, carboxylation capacity, electron transport capacity, or triose-phosphate utilisation capacity occurred. Instead, growth in cool nights tended to retard the diminishing of photosynthetic parameters and gs for aging stem and subtending leaves. However, elevated SPS activity did not affect any photosynthetic parameters. Therefore, when cotton that is well fertilised with nitrogen is grown with continuous night chilling, photosynthesis should not be negatively affected. However, an occasional exposure to cool nights could result in a small reduction in A and gs for leaves that have developed in warm night conditions.
