Physiological and antioxidant responses of cotton varieties to pyrithiobac-sodium at different growth stages

This study systematically evaluated the physiological responses and tolerance mechanisms of four cotton varieties (CT2, C720, C48, C61) under pyrithiobac-sodium stress. Results demonstrated significant variability in herbicide sensitivity across growth stages, with the three-leaf stage identified as the most vulnerable period, characterized by pronounced reductions in plant fresh weight (maximum 43.01%) and chlorophyll content (e.g., 50.78% decrease in C48). In contrast, the five-leaf stage exhibited enhanced tolerance. Considerable intervarietal differences in tolerance were observed, with C61 displaying the highest tolerance level, featuring significantly elevated GST activity (299.57 U/mgprot) and a rapidly responsive antioxidant system (SOD, POD). Conversely, C720 showed delayed recovery of antioxidant capacity. Physiological analysis revealed that SOD and POD activities initially increased during early stress exposure (7 days) but were subsequently inhibited with prolonged treatment, leading to MDA accumulation and membrane lipid peroxidation damage. High herbicide concentrations induced chloroplast structural impairments, including thylakoid disorganization and reduced grana stacking, most severely in C48 and C720. Correlation analysis established a significant positive relationship between GST activity and MDA content. Principal Component Analysis confirmed antioxidant enzymes (SOD, POD) and GST as key indicators of herbicide tolerance. This research provides a scientific basis for the safe application of pyrithiobac-sodium in cotton production, recommending avoidance during the sensitive three-leaf stage and highlighting C61's resistance-related genes (e.g., GST and SOD isoforms) as potential molecular targets for resistance breeding.