PHOTOSYNTHETIC RESPONSE OF TWO COMMERCIAL COTTON CULTIVARS TO IMPOSED DROUGHT USING PREDAWN LEAF WATER POTENTIAL AS AN IRRIGATION TRIGGER

Cotton producers in southern Georgia typically adopt a rainfall budget or more commonly a checkbook approach to irrigation scheduling, where water lost to evapotranspiration is replaced by balancing rainfall with supplemental irrigation. With increased interest in resource conservation, many irrigation-scheduling methods based on estimates of crop water status have been proposed. Previous research on cotton indicates that direct measurements of plant water status before sunrise are strong indicators of midday leaf metabolic trends (Chastain et al., 2014; Snider et al., 2014). In these studies, low predawn leaf water potentials in dryland, relative to fully irrigated cotton were observed. When these differences were detected, dryland treatments typically had lower photosynthetic rates. Carbon loss mechanisms, such as respiration and photorespiration were also shown to increase. All cultivars responded similarly to drought stress. In the current study, we were interested in assessing the feasibility of using predawn leaf water potential as an irrigation trigger to induce a range of water-deficit stress. In addition, it was our objective to determine if differences in physiological response to drought stress existed between two different, commercially available cotton cultivars.