Exogenous SNP alleviates high-temperature stress in rice at flowering via coordinated antioxidant, nitrogen, and hormone regulation.

High-temperature stress during flowering poses a severe threat to global rice production. Although nitric oxide (NO) is known to mediate plant stress responses, its protective role during this critical developmental stage remains poorly understood. In this study, we investigated the protective effects of the NO donor sodium nitroprusside (SNP) on a heat-tolerant cultivar (N22) and a heat-sensitive cultivar (YR343) exposed to a dynamic high-temperature regime (comprising five gradual transitions over a 24-h cycle within 30-38 °C). The results showed that foliar application of SNP at 150 μmol L-1, the most effective concentration among those tested in this study, significantly alleviated high-temperature-induced yield losses in both cultivars, with a notable genotype-dependent effect. In the heat-sensitive YR343, yield recovered by 74.3%, compared with only 35.8% in the heat-tolerant N22. This differential responsiveness was associated with a coordinated multi-physiological network. SNP promoted transpirational cooling, lowering panicle and leaf temperatures by up to 1.87 °C, preserved photosynthetic performance, and activated antioxidant enzymes (SOD, POD, and CAT) to alleviate oxidative membrane damage. Concurrently, SNP sustained nitrogen assimilation (via NR, GS, GOGAT, and GDH) and osmolyte accumulation, while rebalancing endogenous hormone profiles by elevating IAA and CTK levels and suppressing ABA accumulation. Notably, these protective effects were more pronounced in the sensitive genotype, suggesting that exogenous NO compensates for inherent physiological weaknesses in heat-susceptible cultivars. Collectively, these findings support the use of SNP as an effective strategy for improving rice thermotolerance at flowering, particularly for stabilizing yield in heat-susceptible genotypes. Moreover, this work provides physiological insights consistent with a proposed NO-coordinated stress mitigation and presents a genotype-sensitive strategy for sustaining rice productivity under a warming climate.