Regulatory effects of mepiquat chloride on soybean yield formation and physiological responses to drought stress.

Mepiquat chloride (DPC) regulates crop architecture, but its effects on soybean yield formation and drought-related physiological responses remain insufficiently understood. We conducted field experiments to assess the effects of five DPC concentrations (0, 125, 200, 275, and 500 mg L-1) on plant architecture, yield components, and grain yield in two soybean cultivars with contrasting drought tolerance, Heinong 44 (HN44) and Heinong 65 (HN65). We then used a sand-culture experiment with PEG-6000-induced osmotic stress to examine DPC-associated changes in root and shoot growth and stress-related physiology. Under field conditions, DPC reduced plant height and internode elongation and increased stem diameter. It also altered pod-set composition and increased seed number per plant, resulting in higher grain yield. The greatest yield increase in HN44 occurred at 200 mg L-1 DPC (29.44%), whereas the largest numerical increase in HN65 occurred at 275 mg L-1 DPC (12.99%). However, yield did not differ significantly between the 200 and 275 mg L-1 treatments in HN65; therefore, 200 mg L-1 DPC was selected for the subsequent osmotic-stress experiment. Under PEG-induced osmotic stress, DPC restricted shoot and leaf-area expansion while promoting root growth and biomass accumulation. DPC-treated plants maintained higher SPAD values and PSII-related fluorescence parameters and showed lower non-photochemical quenching. In leaves and roots, DPC treatment was associated with lower hydrogen peroxide and malondialdehyde contents, higher antioxidant-enzyme activities, and greater osmotic-adjustment capacity. DPC treatment was also associated with enhanced phenylpropanoid metabolism, greater lignin accumulation, and tissue-specific changes in endogenous hormone profiles. These findings show that DPC can improve yield formation under the non-drought field conditions examined and modulate morphological and physiological responses under PEG-induced osmotic stress. They provide a basis for optimizing DPC application in soybean production and for further evaluating its role in drought adaptation.