Stomatal Effects on Leaf Metabolism in Two Cotton Species with Alternative Stress Response Strategies.

Cotton is well adapted to dry areas, but progressive water deficits can lead to decline in net photosynthesis (A_N), 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 this end, a field study was conducted to quantify the impact of progressive mild drought, as measured by midday stomatal conductance to water vapor (g_sw), 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_N as g_sw declined for both species. Correlation analysis indicated typical relationships with A_N and parameters associated with stomatal limitations (Ci, Cc, g_sw, E); however it was found that while pima exhibited a strong relationship between J_max and 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_sw decreases.