Improving water and plant growth regulator (PGR) management practices in cotton: Cultivar physiological responses and PGR rates

Cotton (Gossypium hirsutum L.) is grown in an increasingly unpredictable climate and often under rainfed conditions. Without irrigation to alleviate common drought conditions, cultivar selection becomes a primary tool in mitigating the negative effects of water-deficit stress. Cultivars can differ in water use under water-limited environments, which can affect crop performance. Moreover, cotton management must involve balancing vegetative and reproductive growth through the use of plant growth regulators (PGR) due to the plant’s perennial growth pattern. The efficacy of PGRs are dependent on many variables including environmental conditions, irrigation, and genetics (cultivar). In 2020 and 2021, a series of growth chamber, greenhouse, and field studies were conducted to identify drought tolerance traits as well as physiological responses to variable rates of PGR among a variety of cotton cultivars. In the greenhouse, cultivar transpiration rates (TR) were measured in response to progressive soil drying. In growth chambers, cultivar TR and shoot hydraulic conductance (K_shoot) were measured in response to increasing vapor pressure deficit (VPD) and temperature. To establish a connection between the performance of the cotton cultivars under both progressive soil drying and high VPD environments, an extreme water-deficit stress field evaluation was performed in 2020. In a second field study in 2021, cultivar’s stomatal conductance (g_s), plant height, wilting, relative water content (RWC), specific leaf area (SLA), and yield were measured in response to varying irrigation and PGR amounts. In the greenhouse, genotypic variation among cultivars contributed to differences in TR decline in response to soil. In the growth chamber, only one cultivar, PHY 400 W3FE, had expressed limited TR in response to increasing VPD (TR_lim). Furthermore, cultivars differed significantly in their K_shoot response to high VPD at 38° [degree] C and correlations were measured between K_shoot and a variety of drought tolerance parameters. In the field studies, cultivars demonstrated their significant variation in physiological response to an extreme water-deficit stress as well as PGR application rates under varying irrigation amounts. The results of these studies provide a better understanding of the impact of cultivar selection as well as PGR rate response under varying water regimes in field-scale cotton production.