Nitrogen Application Regulates Antioxidant Capacity and Flavonoid Metabolism, Especially Quercetin, in Grape Seedlings under Salt Stress

Salt stress is a typical abiotic stress in plants that causes slow growth, stunting, and reduced yield and fruit quality. Fertilization is necessary to ensure proper crop growth. However, the effect of fertilization on salt tolerance in grapevine is unclear. In this study, we investigated the effect of nitrogen fertilizer (0.01 and 0.1 mol L−1 NH4NO3) application on the salt (200 mmol L−1 NaCl) tolerance of grapevine based on physiological indices, and transcriptomic and metabolomic analyses. The results revealed that 0.01 mol L−1 NH4NO3 supplementation significantly reduced the accumulation of superoxide anion (O2.-), enhanced the activities of superoxide dismutase (SOD) and peroxidase (POD), and improved the levels of ascorbic acid (AsA) and glutathione (GSH) in grape leaves compared to salt treatment alone. Specifically, joint transcriptome and metabolome analyses showed that the differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) were significantly enriched in the flavonoid biosynthesis pathway (ko00941) and the flavone and flavonol biosynthesis pathway (ko00944). In particular, the relative content of quercetin (C00389) was markedly regulated by salt and nitrogen. Further analysis revealed that exogenous foliar application of quercetin improved the SOD and POD activities, increased the AsA and GSH contents, and reduced the H2O2 and O2.- contents. Meanwhile, 10 hub DEGs, which had high Pearson correlations (R2 > 0.9) with quercetin, were repressed by nitrogen. In conclusion, all the results indicated that moderate nitrogen and quercetin application under salt stress enhanced the antioxidant system defense response, thus providing a new perspective for improving salt tolerance in grapes.