Synergistic Regulation of Planting Density and Mepiquat Chloride on Yield and Plant Architecture of Short-Season Cotton in the Yangtze River Basin, China

Optimizing planting density and mepiquat chloride (MC) is essential for simplified, machine-harvestable cotton production in the Yangtze River Basin. A two-year field experiment was conducted to explore the synergistic regulatory mechanisms of MC and planting density on plant architecture, physiology, and yield in short-season direct-seeding cotton. A split-plot design was employed with varying gradients of MC dosage and planting density. The results indicate that density and MC function complementarily in shaping plant architecture: MC primarily controls vertical growth (“dwarfing”), while density elevates the initial fruiting node (“elevation”), with no antagonistic interaction between the two. Regarding canopy structure, increasing density is the primary driver for improving the leaf area index (LAI), while MC optimizes light distribution during the critical boll stage. In terms of yield formation, high density significantly enhances seed cotton yield by increasing the number of bolls per unit area, which effectively overcompensates for the reduction in bolls per plant. Notably, a dose-dependent synergistic effect was observed where high MC dosage maximized the yield potential of high-density populations. Furthermore, fiber quality remained stable across treatments, driven primarily by interannual climate factors rather than agronomic regulation. Consequently, an independent synergistic optimization strategy is recommended, combining high density to secure population yield with medium-to-high MC dosage to shape an ideal machine-harvestable architecture. This approach provides a theoretical basis and technical pathway for high-yield and efficient cotton cultivation in the region.