Effects of Mepiquat Chloride Application Timing and Concentration on Soybean Plant Architecture and Stem Mechanical Properties

Soybean plant architecture and stem mechanical properties are important determinants of lodging-resistance potential. To clarify the regulatory effects of mepiquat chloride (MC) applied at different growth stages and concentrations on soybean stem traits, two cultivars, Heinong 44 (HN44) and Heinong 65 (HN65), were used. Water-sprayed plants served as the control (MCK), and MC was applied foliarly at 50, 100, 200, or 400 mg/L at the V3 and R1 stages. Plant height, stem diameter, stem mechanical traits, and cell-wall components were measured. The effects of MC were strongly dependent on application timing, concentration, and cultivar. Following V3 application, 400 mg/L MC reduced the plant height of HN44 by 21.85% at R1; 200 mg/L MC increased basal stem diameter by 15.01% at R3 and fifth-internode diameter by 17.94% at R4. In HN65, 400 mg/L MC increased bending deflection by 27.01% at R1, whereas 100 mg/L MC increased cellulose content by 36.50%. In HN44, 100 mg/L MC increased lignin content by 77.39% at R2. Following R1 application, 400 mg/L MC increased bending deflection of HN44 by 29.40% at R2; 50 mg/L MC increased basal stem diameter by 20.85% at R6; and 400 mg/L MC increased stem breaking force by 36.23% at R4. In HN65, 200 mg/L MC increased hemicellulose content by 42.62% at R6. Correlation analysis showed a significant positive correlation between hemicellulose and lignin after both V3 and R1 applications, while some cell wall components were negatively correlated with stem bending deflection or breaking force. Overall, MC applied at different growth stages and concentrations regulated soybean plant architecture, stem mechanical properties, and cell wall composition; however, no single concentration was optimal across all cultivars and traits. V3 application produced a more pronounced early effect on plant architecture.