Does Thermotolerance Plasticity Influence Yield Stability?

In cotton, Photosystem II responds to conditions of high temperature and drought by increasing thermotolerance such that the photosynthetic efficiency of PSII (the F_v/F_m ratio measured using chlorophyll a fluorescence) for field grown cotton does not decline appreciably until leaf temperatures are artificially raised to temperatures well-above those experienced in the field. Both innate heat tolerance (maintaining high levels of PSII thermotolerance without prior exposure to abiotic stress conditions) and acclimation (increasing PSII heat tolerance) have been proposed as possible mechanisms for improving plant performance under heat and drought; however, the relationship between thermotolerance plasticity and yield stability has not been elucidated for field grown cotton. To address this and to evaluate the sensitivity of other fluorescence-based parameters to high temperature, 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) will be evaluated from the fluorescence transient for leaves of two cotton cultivars, at three sample times during the 2013 and 2014 growing seasons, under three irrigation regimes. The temperature at which each parameter declines 15% from the optimum (T₁₅) will serve as the high temperature threshold for all parameters. Linear regression of T₁₅ for each cultivar on an environmental index will provide a measure of thermotolerance plasticity for each cultivar; a similar approach will be used to assess yield stability. Preliminary data from the 2013 growing season indicated differences in thermotolerance plasticity between the two cultivars chosen. Consequently, the aforementioned study should address whether PSII heat tolerance plasticity is associated with yield stability, and whether chlorophyll a fluorescence parameters derived from the same fluorescence trace will exhibit notable differences in heat sensitivity.