Integrated physiological markers of drought tolerance and yield stability in cotton under deficit irrigation

The selection of drought-resistant cotton genotypes with high productivity and improved water-use efficiency is an increasingly pressing challenge in arid and semi-arid cotton-growing regions, where climate variability is intensifying water scarcity. This study evaluated two widely cultivated cotton varieties in Texas, NG 4190 B3XF and ST 4990 B3XF, under full and moderate deficit irrigation to identify physiological markers associated with contrasting yield responses under water limitation. A comprehensive set of physiological traits was assessed, including plant water status, leaf gas exchange, carbon isotope composition ({delta}13C), total nitrogen and C/N ratio, chlorophyll fluorescence, light and CO{square} response curves, and canopy temperature. While no yield differences were observed under full irrigation, moderate water deficit resulted in stable seed and fiber yield in NG 4190, but significant yield reduction in ST 4990. Yield differences were not explained by instantaneous gas exchange or direct biochemical limitations of photosynthesis, but rather by integrated physiological behavior over time. Key discriminating traits included midday relative water content (RWC), photosynthetic light-response parameters ( and Pm), stomatal optimization parameter (g{square}), and {delta}13C. NG 4190 exhibited higher RWC, more negative {delta}13C, and higher g{square} values, indicating a less conservative stomatal regulation strategy that supported sustained carbon assimilation under water stress. These findings provide insight into the physiological mechanisms underlying cotton performance under drought and support the use of integrated physiological markers for the selection of resilient genotypes in water-limited environments.