Reshaping plant architecture for high yield: effects of combined mepiquat chloride and growth regulators on post-wheat soybeans in Northwest China

High planting density helps maintain soybean productivity in double-cropping systems but often causes excessive stem elongation, poor canopy light distribution, and severe lodging. A three-year field experiment was conducted from 2023 to 2025 to evaluate the agronomic, physiological, and biochemical responses of high-density post-wheat soybean to mepiquat chloride-based regulators. Four treatments were compared: water control, 375 g ha⁻¹ mepiquat chloride alone (MC) ; 375 g ha⁻¹ MC + 150 g ha⁻¹ prohexadione-calcium (MCD); and 375 g ha⁻¹ MC + 150 g ha⁻¹ prohexadione-calcium + 150 g ha⁻¹ thidiazuron (MCT). Among these treatments, MCT was identified as the most beneficial formulation for yield improvement, increasing seed yield by 13.8%–17.6% compared with the control across three growing seasons. The yield advantage of MCT was associated with higher seed number per plant, greater 100-seed weight, prolonged rapid biomass accumulation, and enhanced assimilate allocation to reproductive organs. Physiologically, MCT optimized canopy architecture by promoting more erect leaf angles and improving diffuse light penetration, thereby supporting reproductive biomass formation. In contrast, MC at 375 g ha⁻¹ was the most effective treatment for lodging control, reducing lodging rate by 80.7%–94.3%. This lodging reduction was closely related to shorter internodes, increased stem diameter, higher stem potassium concentration, and greater lignin and cellulose deposition. Seed quality analysis showed that regulator treatments increased protein content but reduced oil content, indicating a protein–oil trade-off. Overall, MCT can be recommended as the yield-optimal formulation for high-density post-wheat soybean, whereas MC alone provides the strongest lodging-control effect. These findings demonstrate that yield improvement and lodging resistance are regulated through coordinated canopy physiological adjustment, biomass allocation, and stem biochemical reinforcement.