The Mechanisms of Improved Grain Yield and Nitrogen Use Efficiency in Salt-Tolerant Rice Varieties Under Salt Stress

Nitrogen (N) uptake and utilisation are crucial for improving rice yields, but the mechanisms of N absorption and transportation under salt stress conditions are still unclear. To address this gap, therefore, a field experiment was conducted from 2022 to 2023 to evaluate the characteristics of N metabolism, nitrogen use efficiency, and yield in rice under varying saline conditions. The experiment was comprised of three irrigation treatments i.e., freshwater irrigation (S0) and seawater and freshwater mixtures i.e., 5 dS m⁻¹ (S1), and 11 dS m⁻¹ (S2) to simulate salt stress in salt tolerant (ST), i.e., ‘Chaoyou 1000’ and ‘Longliangyou 506’, and in salt sensitive (SS), i.e., ‘Ruanhuayou1179’ and ‘Ruanhuayoujinsi’ rice varieties. Compared with S0, the 2-year average decreases in total nitrogen accumulation, nitrogen grain production efficiency and nitrogen harvest index were 27.1%, 17.8% and 10.5% under S1 treatment and 46.8%, 53.7% and 41.2% under S2 treatment for ST, respectively, all of which were lower than those for SS. Additionally, the gene expression of N-metabolising enzymes such as OsNR1, OsGS1.1 and OsGOGAOT and N transporter genes such as OsNRT2.1 and OsNRT2.3 were higher in ST than SS during the heading stage under salt stress, potentially enhancing N transport and utilisation. Moreover, a significant and positive correlation was found between total N accumulation at maturity and yield under salt stress. Overall, the findings not only provide significant theoretical support for the breeding and improvement of salt-tolerant rice varieties but also offer scientific guidance for optimising nitrogen management strategies in saline conditions.