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.