Drought response modelling of leaf photosynthetic parameters in two Gossypium species

Cotton is well adapted to dry areas, but progressive water deficits can lead to declines in net photosynthesis (A), ultimately reducing yield. However, the exact mechanism responsible for this decline in net photosynthesis (stomatal or non-stomatal) is not fully understood under field conditions, partially due to limitations in the ability to collect critical data. To our knowledge, no other study has compared the drought responses of Pima and upland cotton using both CO₂ response and chlorophyll fluorescence under field conditions. To this end, a field study was conducted to quantify the impact of progressive mild drought, as measured by midday stomatal conductance to water vapour (g_s), on cotton leaf metabolism in Pima and upland cotton. Survey gas exchange and rapid photosynthetic CO₂ response (RACiR) were conducted during flowering on the same leaf. The study observed decline in A as g_s declined for both species. Correlation analysis indicated typical relationships with A and parameters associated with stomatal limitations such as decreased CO₂ inside the leaf and at the site of carboxylation; however, it was found that while Pima exhibited a strong relationship between maximum electron transport rate (J_max) and electron transport rate (ETR), upland cotton did not. Furthermore, when ETR is broken down into proportions contributing to net photosynthesis and photorespiration (ETR_A, ETR_P, respectively), we found that a greater proportion of ETR is being shuttled to the photorespiratory pathway in upland, relative to Pima as g_s decreases. Our results fill critical knowledge gaps that can be useful for modellers and breeders when preparing for future climate change scenarios.