Improvement in Nitrogen-Use Efficiency Increases Salt Stress Tolerance in Rice Seedlings and Grain Yield in Salinized...

Salt stress has become a major limiting factor of rice (Oryza sativa L.) yield worldwide. Appropriate nitrogen application contributes to improvement in the salt tolerance of rice. Here, we show that improvement in nitrogen-use efficiency increases salt stress tolerance in rice. Rice varieties with different nitrogen-use efficiencies were subjected to salt stress; they were stimulated with 50, 100, and 150 mmol/L of NaCl solution at the seedling stage and subjected to salinities of 0.2, 0.4%, and 0.6% at the reproductive growth stage. Compared with nitrogen-inefficient rice varieties, the nitrogen-efficient rice varieties showed significant increases in the expression levels of nitrogen-use-efficiency-related genes (TOND1 and OsNPF6.1), nitrogen content (5.1-12.1%), and nitrogen-use enzyme activities (11.7-36.4%) when under salt stress conditions. The nitrogen-efficient rice varieties showed a better adaptation to salt stress, as shown by the decrease in leaf-withering rate (4.7-10.3%), the higher chlorophyll (3.8-9.7%) and water contents (1.1-9.2%), and the better root status (7.3-9.1%) found in the rice seedlings under salt stress conditions. Analysis of physiological indexes revealed that the nitrogen-efficient rice varieties accumulated higher osmotic adjustment substances (9.7-79.9%), lower ROS (23.1-190.8%) and Na⁺ (15.9-97.5%) contents, higher expression levels of salt stress-related genes in rice seedlings under salt stress conditions. Furthermore, the nitrogen-efficient rice varieties showed higher yield under salt stress, as shown by a lower salt-induced decrease in 1000-grain weight (2.1-6.2%), harvest index (1.4-4.9%), and grain yield (2.8-4.1%) at the reproductive growth stage in salinized soil. Conversely, the nitrogen-efficient rice varieties showed better growth and physiological metabolism statuses under severe salt stress conditions. Our results suggest that nitrogen-efficient rice varieties could improve nitrogen-use and transport efficiency; accordingly, their use can improve the gene expression network, alleviating salt damage and improving grain yield under severe salt stress conditions.