Distinguishing High Daytime From Nighttime Temperature Effects During Early Vegetative Growth in Cotton

High-temperature limits early season vegetative growth of cotton, and the physiological response of cotton (Gossypium hirsutum L.) to high daytime or nighttime temperature needs to be explored. The objectives of the current study were to determine (1) plant growth response, (2) physiological contributors to variation in biomass production and (3) mechanisms driving variation in net photosynthetic rate (A_N) in response to different combinations of high daytime and nighttime temperatures. Beginning at planting, cotton was exposed to four different growth temperature regimes: (1) optimum (30/20°C day/night), (2) high nighttime (30/30°C), (3) high daytime (40/20°C) and combined high daytime and nighttime (40/30°C) for 4 weeks. Relative to the 30/20°C treatment, plant growth was positively affected by high nighttime temperature and negatively affected by high daytime temperature and combined high day and night temperature. Increased leaf area mainly contributed to increased biomass production in high nighttime temperature; higher nighttime respiration (R_N) drove reductions in biomass in combined high daytime and nighttime temperature; and decreased leaf area and A_N and increased R_N drove reductions in biomass under high daytime temperature alone. A_N was not impacted by high nighttime temperature, while decreased under high daytime temperature and increased with combined high daytime and nighttime temperature. Adjustments in leaf traits contributed to increases in A_N in combined high daytime and nighttime temperature, and increased photorespiration and respiration contributed to reductions in A_N under high daytime temperature. Overall, early season vegetative growth of cotton exhibited differential responses to high daytime and nighttime temperatures.