Variation in thermotolerance of photosystem II energy trapping, intersystem electron transport, and photosystem I electron acceptor reduction for diverse cotton genotypes

Cotton breeding programs have focused on agronomically-desirable traits. Without targeted selection for tolerance to high temperature extremes, cotton will likely be more vulnerable to environment-induced yield loss. Recently-developed methods that couple chlorophyll fluorescence induction measurements with temperature response experiments could be used to identify genotypic variation in photosynthetic thermotolerance of specific photosynthetic processes for field-grown plants. It was hypothesized that diverse cotton genotypes would differ significantly in photosynthetic thermotolerance, specific thylakoid processes would exhibit differential sensi-tivities to high temperature, and that the most heat tolerant process would exhibit substantial genotypic variation in thermotolerance plasticity. A two-year field experiment was conducted at Tifton and Athens, Georgia, USA. Experiments included 10 genotypes in 2020 and 11 in 2021. Photosynthetic thermotolerance for field-collected leaf samples was assessed by determining the high temperature threshold resulting in a 15% decline in photo-synthetic efficiency (T15) for energy trapping by photosystem II (0Po), intersystem electron transport (0Eo), and photosystem I end electron acceptor reduction (0Ro). Significant genotypic variation in photosynthetic ther-motolerance was observed, but the response was dependent on location and photosynthetic parameter assessed. 0Eo was substantially more heat sensitive than 0Po or 0Ro. Significant genotypic variation in thermotolerance plasticity of 0Eo was also observed. Identifying the weakest link in photosynthetic tolerance to high temperature will facilitate future selection efforts by focusing on the most heat-susceptible processes. Given the genotypic differences in environmental plasticity observed here, future research should evaluate genotypic variation in acclimation potential in controlled environments.

Automated summary

Cotton breeding programs need to select for tolerance to high temperature extremes to prevent environment-induced yield loss. Methods that combine chlorophyll fluorescence induction measurements with temperature response experiments can be used to identify genotypic variation in photosynthetic thermotolerance. Cotton genotypes differ significantly in photosynthetic thermotolerance, and different photosynthetic processes have varying sensitivities to high temperature, with intersystem electron transport being the most heat sensitive.