Fine-Tuning Mepiquat Chloride Application in Soybean: Interactive Effects of Concentration and Growth Stage on Photosynthetic Apparatus, Carbohydrate Metabolism, and Grain Yield

Soybean is an important protein source, economic crop, and strategic agricultural product, which is related to nutrition and health, industrial development, and food security. Mepiquat chloride (MC) can influence yield, but its effectiveness depends on application timing and concentration. We hypothesized that optimal MC application enhances photosynthetic efficiency and carbohydrate metabolism, thereby increasing yield. A pot experiment was conducted using two cultivars (HN44, HN65) with four MC concentrations (50–400 mg/L) applied at seedling (V3), flowering (R1), or both stages (V3 + R1). Dry matter, photosynthesis, carbohydrates, and yield were evaluated. MC significantly altered dry matter partitioning; 200 mg/L at seedling stage increased total dry matter in HN65 at R6. MC enhanced net photosynthetic rate and PSII efficiency in a concentration-dependent manner, particularly improving photoprotective capacity in HN65. Leaf carbohydrate accumulation was also affected. Yield gains were stage- and cultivar-specific: seedling-stage 200 mg/L increased grain weight per plant in HN65, while flowering-stage 50 mg/L was most effective for HN44. Combined applications (200–400 mg/L) produced the highest yields for both cultivars. This is the first comparative evidence that HN44 and HN65 exhibit divergent yield responses to MC timing—HN44 benefits from flowering-stage, HN65 from seedling-stage application. Optimizing MC concentration and timing enhances yield by modulating photosynthesis and assimilate partitioning in a cultivar-dependent manner.