Objectives The mechanism by which exogenous sodium nitroprusside (SNP) improves reactive oxygen species (ROS) metabolism during the germination stage of rice seeds was investigated.

Methods A seed soaking experiment was conducted using an alkali-sensitive rice cultivar, ‘Zhonghua 11’ (ZH11), and an alkali-tolerant cultivarn ‘Ningjing 52’ (NJ52). Four treatments were applied: distilled water (CK), SNP 30 μmol/L solution (CK+SNP), 20 mmol/L NaHCO₃:Na₂CO₃ solution (AS), and 20 mmol/L AS + 30 μmol/L SNP (AS+SNP). Seeds were treated for 7 days, after which germination characteristics were evaluated, and the contents of phytohormones, activities of antioxidant enzymes, and expression levels of genes related to ROS metabolism were analyzed in the seedlings.

Results Compared with the control, alkali stress significantly inhibited seed germination and seedling growth in both cultivars, with more pronounced inhibition in shoot and root lengths in ZH11. However, the AS+SNP treatment significantly increased germination rate and root lengths in both cultivars relative to AS treatment, and the ameliorative effect was more pronounced in NJ52 than in ZH11. AS treatment significantly reduced gibberellin (GA) content and increased abscisic acid (ABA) content in both cultivars, up regulated the expression levels of GA synthesis genes (OsGA3ox2, OsGA20ox1) and decreased that of ABA metabolism genes (OsNCED2, OsABA8ox2) significantly, compared to the control. AS+SNP treatment observed significantly increased GA content and decreased ABA content by up-regulating GA synthesis genes and down-regulating ABA metabolism genes, relative to that of AS treatment. These hormonal changes helped to break dormancy and promote seed germination. AS treatment significantly induced excessive accumulation of ROS (H₂O₂ and \rmO_\small 2^\overline \,\cdot\, ), activated the antioxidant system as evidenced by increased activities of peroxidase (POD), catalase (CAT), glutathione reductase (GR), dehydroascorbate reductase (DHAR), and glutathione peroxidase (GPX), along with up-regulated expression of related genes. The content of oxidized glutathione (GSSG). AS+SNP treatment significantly reduced ROS levels, synergistically enhanced the antioxidant defense system, further increased the activity of antioxidant enzymes and the expression of their corresponding genes, and significantly improved the efficiency of the ascorbic acid-glutathione (AsA-GSH) cycle. Notably, NJ52 exhibited a stronger antioxidant capacity than ZH11.

Conclusions Exogenous SNP effectively maintains cellular redox homeostasis by synergistically regulating ROS metabolism, enhancing the antioxidant defense system, and modulating hormonal balance, thereby promoting seed germination under alkali stress. The alkali-tolerant cultivar exhibited a more pronounced response to SNP regulation.