Nitrogen Regulates Rice Seedling Growth, Antioxidants, and Metabolites Under Salt Stress

Nitrogen plays a positive role in rice growth, whereas salt negatively affects it. However, rice varieties differ in nitrogen responses and whether nitrogen management can regulate the rice salt tolerance of all varieties remains largely unknown. We then investigated the effects of nitrogen levels on the responses of different rice varieties to salt stress through a comprehensive analysis including morphological, antioxidant physiological, and metabolomics-related indicators. A hydroponic experiment was conducted with three rice cultivars (Hanyou73, Meixiangzhan2, and Xiangyaxiangzhan) grown under two nitrogen treatments (HN: full nitrogen Kamura B nutrient solution and LN: 75% nitrogen reduction Kamura B nutrient solution) and two salt treatments (W: 0 mmol L^-1 NaCl and S: 100 mmol L^-1 NaCl). This resulted in four treatments: HNW, HNS, LNW, and LNS. Variety, salt, and variety x salt significantly affected the total fresh and dry weights of rice seedlings. Interestingly, compared with the HNS treatment, the LNS treatment increased the total dry weight of Hanyou73 by 32.88%, decreased the total dry weight of Xiangyaxiangzhan by 17.28%, whereas no significant change was detected in Meixiangzhan2. In addition, nitrogen and salt affected the photosynthetic pigment content, soluble protein content, hydrogen peroxide content, and antioxidant response. Moreover, metabolic analysis revealed that the metabolism of tryptophan, D-amino acid, and α-linolenic acid varied with nitrogen level across different salt treatments. This study demonstrated that nitrogen availability differentially regulates salt tolerance in rice varieties. For specific varieties, low nitrogen levels are more effective in reducing reactive oxygen species (ROS) and minimizing cellular damage in rice seedlings.