The positive effects of exogenous sodium nitroprusside on the plant growth, photosystem II efficiency and Calvin cycle of tomato seedlings under salt stress

•Exogenous SNP triggers the up-regulation of endogenous NO level in salt-stressed tomato leaf.•SNP counteracted saline-induced inhibition of photosynthesis.•SNP overcomes stomatal limitations and make the leaf have higher Gs and Ci.•SNP can protect PSII from damage caused by excess energy and improve PSII efficiency.•SNP enhances the activity and genes transcript levels of Calvin cycle key enzymes. Although some studies have shown that exogenous sodium nitroprusside (SNP, an NO donor) participates in the tolerance of plants to stress, the protective role of the level of SNP-mediated endogenous nitrous oxide (NO) on the salt-induced inhibition of photosynthesis in salt-stressed tomato seedlings merits understanding. Thus, in this study, with the exception of tomato seedlings treated with 0 or 100 mmol· L−1 NaCl alone or together with 0.1 mmol· L−1 SNP, 100 μmol· L−1 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO, an NO scavenger) and 100 μmol· L−1 N-nitro-L-arginine methyl ester (L-NAME, an inhibitor of NO synthase) was applied jointly to saline-stressed tomato seedlings by a foliage spray of 0.1 mmol· L−1 SNP, respectively. The application of SNP counteracted the saline-induced growth inhibition and the reduction in net photosynthetic rate of tomato seedlings by enhancing their stomatal conductance, intercellular CO2 concentration, apparent photosynthetic quantum efficiency, PSII maximum photochemical efficiency, actual photochemical efficiency of PSII, non-photochemical quenching coefficient, and a decrease in the dark respiration rate, which is coupled with an increase in the endogenous levels of NO. In addition, treatment with SNP increases the maximum carboxylation rate of ribulose-1,5-bisphosphate carboxylase (Rubisco), the maximum regeneration rate of ribulose-1,5-bisphosphate (RuBP) and the upregulated total and initial activity of Rubisco; the activity in Rubisco activase and fructose-1,6-bisphosphate phosphatase; and the transcriptions of genes that encode Rubisco activase, the Rubisco large subunit, and the Rubisco small subunit, and sedoheptulose-1,7-bisphosphatase, 3-phosphogiyceric acid kinase, transketolase and ribulose-5-phosphate kinase, which are the key enzymes involved in the Calvin-Benson cycle. However, the positive effect of SNP was impaired by the application of PTIO and L-NAME to eliminate the generation of endogenous NO, respectively. Our findings conclusively revealed that SNP triggers the upregulation of endogenous