The aim of this study was to test the effects of salt stress on nitrogen metabolism and ion balance in rice plants. The contents of inorganic ions, total amino acids, and NO<sub>3</sub><sup>–</sup> in the stressed seedlings were then measured. The expressions of some critical genes involved in nitrogen metabolism were also assayed to test their roles in the regulation of nitrogen metabolism during adaptation of rice to salt stress. The results showed that when seedlings were subjected to salt stress for 4 h, in roots, salt stress strongly stimulated the accumulations of Na<sup>+</sup> and Cl<sup>–</sup>, and reduced K<sup>+</sup> content; however, in leaves, only at 5 days these changes were observed. This confirmed that the response of root to salt stress was more sensitive than that of leaf. When seedlings were subjected to salt stress for 4 h, salt stress strongly stimulated the expression of OsGS1;1, OsNADH-GOGAT, OsAS, OsGS1;3, OsGDH1, OsGDH2, OsGDH3 in both leaves and roots of rice, after this time point their expression decreased. Namely, at 5 days most of genes involved in NH<sub>4</sub><sup>+</sup> assimilation were downregulated by salt stress, which might be the response to NO<sub>3</sub><sup>–</sup> change. Salt stress did not reduce NO<sub>3</sub><sup>–</sup> contents in both roots and leaves at 4 h, whereas at 5 days salt stress mightily decreased the NO<sub>3</sub><sup>–</sup> contents. The deficiencies of NO<sub>3</sub><sup>–</sup> in both roots and leaves can cause a large downregulation of OsNR1 and the subsequent reduction of NH<sub>4</sub><sup>+</sup> production. This event might immediately induce the downregulations of the genes involved in NH<sub>4</sub><sup>+</sup> assimilation.
Peer Review #3 of "The relationship between carbon and nitrogen metabolism in cucumber leaves acclimated to salt stress (v0.1)"
