Effects of Drought Stress During Branching and Pod Formation Stages on Morpho-Physiology and Yield in Soybean

Background: Recurrent drought events in the Huaibei Plain of China pose a major constraint to soybean production. The peak drought period in July and September coincides with two critical stages of soybean, branching (B) and pod formation (PF), yet current research still focuses predominantly on the flowering-podding (FP) stage, leaving the physiological and yield response mechanisms to drought stress during these stages poorly understood. This study was designed to investigate the differential responses of soybean to varying intensities of drought stress during critical growth stages. Methods: A pot experiment was conducted under drought conditions in the Huaibei Plain in 2024, with three limits of soil water content set at both the B and PF stages: light drought (LD, 55%), moderate drought (MD, 45%) and well-watered control (CK, 75%). Morphological, photosynthetic, yield and pod-related traits of soybean under varying drought levels were examined and Pearson correlation analysis was applied to assess their relationships with yield. Result: The results indicated that, overall, drought stress during the B stage resulted in severe reductions in plant height (PH), root length (RL) and seed moisture content (MC) compared to the PF stage. In contrast, photosynthetic parameters (SPAD value, stomatal conductance (Gs), net photosynthetic rate (Pn), transpiration rate (Tr) and intercellular CO2 concentration (Ci), yield, pod-related traits (total seed number (TSN), total pod number (TPN), filled pod number (FPN), empty pod number (EPN) and pod wall weight (PWW), as well as seed oil content (OC), exhibited an opposite trend. Furthermore, the magnitude of change in all aforementioned traits (except for EPN) relative to the CK followed the pattern MD greater than LD at the same growth stage. Correlation analysis showed that soybean yield was significantly and positively associated with TSN, TPN, FPN and PWW (p less than 0.05), while exhibiting negative correlations with EPN, Ci and MC.