Alkaline stress suppresses soybean waterlogging tolerance by exacerbating energy expenditure and ROS accumulation

To clarify the synergistic inhibition mechanisms of saline-alkali and waterlogging combined stress on soybean physiology, this study systematically analyzed phenotypic traits, photosynthetic characteristics, reactive oxygen species (ROS) metabolism, and energy metabolism under saline-alkali (A + NW), waterlogging (NA + W), and combined stress (A + W). Results demonstrated that saline-alkali stress significantly impaired waterlogging-induced morphological adaptations, with A + W reducing dry weight and adventitious root number compared to W alone. Synergistic photosynthetic damage was observed: net photosynthetic rate (Pn) under A + W decreased markedly versus controls, while chlorophyll b content increased, suggesting PSII light-harvesting complex reorganization to mitigate photoinhibition. Saline-alkali conditions disrupted ROS homeostasis in waterlogged plants by exacerbating ion toxicity (elevated root Na⁺) and membrane peroxidation (increased electrolyte leakage). Despite heightened leaf SOD activity, root CAT activity declined. Carbon metabolism dysregulation under A + W was evidenced by reduced C/N ratio, sucrose, and starch levels compared to W. Moreover, compound stress may disrupt the stability of energy metabolism through the crosstalk between ethylene and abscisic acid. Gene expression analysis revealed that saline-alkaline stress significantly upregulated the expression levels of GmADH and GmPDC1 in the root systems of waterlogged plants, confirming that the sustained activation of anaerobic respiration exacerbated the energy crisis. Correlation network analysis highlighted UGPase's negative association with biomass and Na⁺-MDA positive linkage, indicating synergistic ion toxicity and carbon-nitrogen imbalance. This study demonstrated that saline-alkali stress reduced soybean waterlogging tolerance by inhibiting morphological remodeling, enhancing oxidative damage, and disrupting energy homeostasis, providing a theoretical basis for regulating combined stress and offering insights for breeding and management strategies.