Discussion
In partial support of our first hypothesis, warmer daytime temperatures did lead to increased leaf level carbon gain, but not biomass production. Water deficit stress slightly reduced carbon gain and biomass production independent of temperature. Our second hypothesis was largely rejected given that both photosynthesis and respiration, and in turn leaf level carbon balance, were not strongly altered by nocturnal warming. Additionally, the impact of nocturnal warming on biomass production was only found for plants under elevated daytime temperature, with the direction of effect depending on water regime. Contrary to the third hypothesis that warm-grown plants would be less affected by the heatwave, leaves of plants in warmer temperature regimes showed decreased carbon gain during short-term heat stress despite the occurrence of thermal acclimation in photosynthesis. On the other hand, leaf carbon gain was promoted by the heatwave in cool-grown plants. Although a legacy effect of heatwave on carbon gain was observed on the second day of recovery, differences in gas exchange variables triggered by heat stress were generally small and disappeared in some cases on the seventh day of recovery, thereby supporting our fourth hypothesis. Consequently, the heatwave generated minor effects on total biomass production, but cotton final reproductive biomass was reduced. Taken together, these results demonstrate the complexity regarding the potential impacts of climate change on cotton plants, and highlight the important role of growth temperature in regulating cotton carbon assimilation and growth.