Objectives The application of exogenous sodium nitroprusside (SNP) can effectively alleviate alkali stress-induced damage in rice. We observed the effect differences of exogenous SNP on the carbon and nitrogen metabolism of rice cultivars when subjecting to alkali stress at germination satge.

Methods Alkali sensitive rice cultivar Zhonghua 11 (ZH11) and alkali resistant cultivar Ninggeng 52 (NG52) was chosed as test materials in a pot experiment. Firstly, rice seeds were soaked into alkali stress solutions (containing 20 mmol/L CO₃²⁻ and HCO₃⁻, pH 10.50) containing SNP 10, 30, 50, 70, 100 μmol/L, respectively. SNP 30 μmol/L treatment was recorded the highest germination index, so was used as the SNP concentrations for the following experiment. In the following experiment, normal nutrient solution control ( CK), normal solution containing SNP 30 μmol/L (SNP), alkali stress solution (AS), and alkali stress solution containing NSP 30 μmol/L (AS+SNP). The rice seeds were watered with the treatment solution, and the carbon and nitrogen metabolism of rice seedlings were measured.

Results AS+SNP treatment were recorded significantly lower sucrose content than AS treatment in both cultivar ZH11 and NG52. The sucrose contents of NG52 were significantly lower than that of ZH11, however, the expression of genes involved in sucrose synthesis (OsNIN1) and glucose synthesis (OsSPS1 and OsSPS11) in NG52 were significantly higher than that in ZH11. Compared with AS, the AS+SNP treatment markedly increased the levels of malic acid, citric acid, nitric oxide (NO), and proline content, as well as the activities of nitrate reductase (NR) and S-nitroso glutathione reductase (GSNOR) and expression of OsNR2 and OsGSNOR1, decreased the levels of NO₃⁻ and glutathione (oxidized glutathione, GSSG) in both cultivars. Furthermore, the contents of malic acid, citric acid, NO, proline, GSSG, together with GSNOR activity and the expression levels of OsNR1.2 and OsGSNOR1 were significantly higher in NG52 compared with ZH11. Positive correlations (P<0.05) were existed between genes involved in glucose and sucrose synthesis (OsNIN1) with malic acid, citric acid, proline, and NO. The principal component analysis indicated that glucose, fructose, NO, GSNO, and GSSG were key factors affecting the coordination of carbon and nitrogen metabolism.

Conclusions Treating rice seeds with 30 μmol/L SNP solution is effective in increasing sucrose breakdown, organic acid accumulations, NR and GSNOR activities, organic nitrogen levels, and stimulating the transformation of carbon metabolism to nitrogen metabolism, thereby maintaining the normal carbon and nitrogen metabolism of rice under alkali stress during germination.