Field-Grown Cotton Exhibits Seasonal Variation in Photosynthetic Heat Tolerance without Exposure to Heat Stress or Water Deficit Conditions

Thermotolerance acclimation of photosystem II to heat and drought is well-documented, but studies demonstrating developmental impacts on heat tolerance in field-grown plants are limited. Consequently, climatic variables, estimated canopy temperature, predawn leaf water potential (Ψ_PD), and the temperature responses of maximum quantum yield of photosystem II (F_v/F_m), variable fluorescence (F_v/F₀), quantum yield of electron transport (φ_Eο), and efficiency of PSI electron acceptor reduction (REο/ABS) were characterized for Gossypium hirsutum at three sample times during the growing season (June 21, July 2, and July 18, 2013) under well-watered conditions. The temperature decreasing a given photosynthetic parameter 15% from the optimum is referred to as T₁₅ and served as a standardized measure of heat tolerance. Ambient and estimated canopy temperatures were well within the optimal range for cotton throughout the sample period, and leaves were verified well-watered using Ψ_PD measurements. However, T₁₅ varied with sample date (highest on July 2 for all parameters), being 2°C (F_v/F₀) to 5.5°C (φ_Eο) higher on July 2 relative to June 21, despite optimal temperature conditions and predawn leaf water potential on all sample dates. These findings suggest that even under optimum temperature conditions and water availability, heat tolerance could be influenced by plant developmental stage.