Impacts of growth temperature, water deficit and heatwaves on carbon assimilation and growth of cotton plants (Gossypium hirsutum L.)

Increased variability in growing season climates continues to threaten the growth and yields of many crops. The impacts of individual climate stress conditions on crops has been documented frequently, yet how crops respond to multiple abiotic stress components is less well understood. Here, we report on the main and interactive effects of growth temperature, water deficit and a heatwave on leaf physiology and biomass production of cotton plants (Gossypium hirsutum L.). Plants were raised under two day/night growth temperature regimes (28/18 °C and 32/22 °C) and their corresponding nocturnal warming (+4 °C) scenarios (i.e. 28/22 °C and 32/26 °C). Following the emergence of the first square (flower bud), plants were subjected to two water treatments (well-watered and water deficit) until the beginning of the flowering stage, and then half of the plants in all temperature treatments were exposed to a 5-day heatwave treatment (40/26 °C). We found that elevated growth temperature increased growth rate (as defined by plant height) and leaf-level carbon gain, but decreased total aboveground biomass. Water deficit stress decreased leaf level carbon gain and biomass, but these impacts were generally less pronounced. Nocturnal warming moderately decreased leaf carbon gain for plants grown under the cool temperature regime (i.e. 28/18 °C), but not the warm temperature regime (i.e. 32/22 °C), and its impacts on biomass were also thermal regime specific. In contrast, leaf carbon gain was promoted by the heatwave under the cool daytime temperature treatment, but not the warm daytime temperature treatment. However, total aboveground biomass was less affected by the heatwave due to high resilience of gas exchange, although there was decreased fruit biomass. Overall, both short- and long-term increases in daytime temperature decreased cotton fruit biomass, while nocturnal warming had limited capacity to buffer that impact. Moderate soil water deficit will not strongly reduce carbon gain and growth. This study adds to the knowledge regarding the response of cotton plants to climate change and underscores the complexity of plant response to multiple environmental factors.