Assessment of PSII Heat Tolerance for Six Commercially-Available Cotton Cultivars Across a Range of Production Environments.

Chlorophyll fluorescence provides a rapid measure of photosystem II efficiency. Leaf samples can be collected in the field and chlorophyll fluorescence assessed under a range of temperatures to define heat tolerance for a given variety or environment. Heat tolerance assessments at statewide variety testing sites should provide a robust indication of cultivar differences in heat tolerance plasticity to environment. In the current study we assess heat tolerance for six cultivars (CG3787, DP1555, PHY333, PHY499, ST6182, ST6448) across a borad range of environments. The temperature at which photosystem II maximum quantum yield (F_v/F_m) declined 15% from the optimum temperature served as a standardized measure of heat tolerance. Thermotolerance was exceptionally responsive to environment. Cultivar differences in heat tolerance were observed but only in two environments in 2015, indicating that the cultivar differences in heat tolerance may be governed by local conditions preceding each heat tolerance assessment. Previous work has indicated that heat tolerance may be influenced by plant water status and ambient temperature conditions. Thus, this work will also assess the impact of the aforementioned factors on PSII thermotolerance.