Context-Dependent Responses of Cotton to Supplemental Early-Season Mepiquat Chloride Under Contrasting Field Conditions

Abstract Early vegetative growth regulation in cotton has received renewed interest as a strategy for moderating canopy development and potentially improving plant resilience to later-season stress. Mepiquat chloride is typically applied from first square onward, but the consequences of an additional, earlier application under non-stress field conditions are not well defined. We tested supplemental early-season mepiquat chloride (two rates at the two-leaf and 6–8 leaf stages) in two commercial fields in western Türkiye that differed in growth potential because of cultivar and management. Each site used a randomized complete block design with four replications; plant height, leaf traits, growth indices, yield components, and yield were measured across development. Supplemental early-season mepiquat chloride significantly reduced seed cotton yield at both sites, with a larger reduction in the higher-vigor field context (approximately 20%; p < 0.01) than in the lower-vigor field context (approximately 9%; p < 0.01), while associated trait responses differed between sites. In the higher growth potential site, canopy-scale suppression (reduced leaf area, specific leaf area, and plant height) coincided with limited reproductive compensation. In the lower growth potential site, leaf structural adjustment and greater allocation to reproductive sinks (higher boll number, boll weight, and reproductive biomass ratio) partially buffered yield loss. Relative growth rate declined in selected intervals, whereas evidence for increased net assimilation rate was limited to the first growth interval at Site 1, suggesting that any physiological compensation was transient and context-dependent. Overall, responses to supplemental early-season mepiquat chloride differed between the two commercial production contexts, indicating that under well-managed, non-stress conditions, its use should be considered site- and management-specific rather than routine.