Effects of short-term salt stress on photosynthetic characteristics, physiological metabolism, and ion homeostasis in soybean seedlings

Salt stress is a major abiotic stress limiting soybean production, with the seedling stage being the most salt-sensitive period. Elucidating the growth, photosynthetic, and physiological response mechanisms of soybean seedlings to salt stress holds significant theoretical value for breeding salt-tolerant cultivars. In this study, we systematically investigated the effects of varying NaCl stress intensities (0–160 mmol⋅kg⁻¹) on plant growth, photosynthetic gas exchange, chlorophyll fluorescence, antioxidant physiology, and ion homeostasis using a salt-tolerant cultivar (Zhonglongdou 136, ZLD136) and a salt-sensitive cultivar (Henong 85, HN85). The results demonstrated that increasing salt concentrations significantly inhibited plant growth and photosynthetic carbon assimilation. However, ZLD136 exhibited superior phenotypic stability and photoprotective mechanisms, evidenced by a significant increase in non-photochemical quenching (NPQt). Salt stress triggered a substantial rise in malondialdehyde (MDA) content and relative electrical conductivity (REC); notably, ZLD136 effectively mitigated membrane lipid peroxidation by maintaining higher antioxidant enzyme (superoxide dismutase SOD, peroxidase POD, and catalase CAT) activities and accumulating more osmolytes, including proline (Pro) and soluble sugars (SS). Furthermore, the salt-tolerant cultivar more efficiently coordinated the mitochondrial respiratory electron transport chain (cytochrome c oxidase COX and alternative oxidase AOX pathways) to dissipate excess reducing power and maintain cellular energy homeostasis. ZLD136 also possessed a stronger capacity for ion regulation, significantly outperforming HN85 in maintaining higher leaf K⁺/Na⁺ and Ca²⁺/Na⁺ ratios. Salt tolerance in soybean seedlings involves a comprehensive synergistic network, and the salt tolerance advantage of ZLD136 primarily stems from its superior ability to maintain ion balance, highly efficient photoprotective mechanisms, and a robust antioxidant defense system. Comprehensive analysis revealed that with intensifying salt stress, the physiological responses of soybean gradually transitioned from relying on the antioxidant enzyme system to the synthesis of osmolytes. Moreover, maintaining ion homeostasis (particularly a high leaf K⁺/Na⁺ ratio) is the core prerequisite for alleviating physiological and metabolic disorders, safeguarding photosynthetic functions, and ensuring short-term plant growth.