Using Non-invasive Micro-test Technology (NMT), the Na⁺, K⁺ and H⁺ flux profiles in the root meristem regions were investigated in Nitraria sibirica Pall. seedlings under different NaCl concentrations. NaCl stress increased the K⁺ and Na⁺ contents in the roots of N. sibirica seedlings. NaCl stress significantly increased the steady Na⁺ efflux from the N. sibirica seedling roots. Steady K⁺ effluxes were measured in the control roots (without NaCl) and in the roots treated with 200 mM NaCl, and no significant differences were observed between the two treatments. The steady K⁺ efflux from roots treated with 400 mM NaCl decreased gradually. NaCl treatment significantly increased the H⁺ influx. Pharmacological experiments showed that amiloride and sodium vanadate significantly inhibited the Na⁺ efflux and H⁺ influx, suggesting that the Na⁺ efflux was mediated by a Na⁺/H⁺ antiporter using energy provided by plasma membrane H⁺-ATPase. The NaCl-induced root K⁺ efflux was inhibited by the K⁺ channel inhibitor tetraethylammonium chloride (TEA), and was significantly increased by the H⁺-ATPase inhibitor sodium vanadate. The NaCl-induced K⁺ efflux was mediated by depolarization-activated outward-rectifying K⁺ channels and nonselective cation channels (NSCCs). Under salt stress, N. sibirica seedlings showed increased Na⁺ efflux due to increased plasma membrane H⁺-ATPase and Na⁺/H⁺ antiporter activity. High H⁺ pump activity not only restricts the Na⁺ influx through NSCCs, but also limits K⁺ leakage through outward-rectifying K⁺ channels and NSCCs, leading to maintenance of the K⁺/Na⁺ balance and higher salt tolerance.

Nitraria sibirica Pall., a typical halophyte of great ecological value, is widely distributed in desert, saline, and coastal saline-alkali environments. Consequently, researching the salt tolerance mechanism of N. sibirica Pall. has great significance to the cultivation and utilization of salt-tolerant plants. In this research, RNA-seq, digital gene expression (DGE), and high flux element analysis technologies were used to investigate the molecular and physiological mechanisms related to salt tolerance of N. sibirica Pall. Integrative analysis and de novo transcriptome assembly generated 137,421 unigenes. In total, 58,340 and 34,033 unigenes were annotated with gene ontology (GO) terms and mapped in Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, respectively. Three differentially expressed genes (DEGs) libraries were subsequently constructed from the leaves of N. sibirica Pall. seedlings under different treatments: control (CK), light short-term salt stress (CL2), and heavy long-term salt stress (CL6). Eight hundred and twenty-six, and 224 differentially expressed genes were identified in CL2 and CL6 compared to CK, respectively. Finally, ionomic analysis of N. sibirica Pall. seedlings treated with 0, 100, 200 or 300 mM concentrations of NaCl for one day showed that the uptake and distribution of Ca, Cu, Fe, Mg and K in different organs of N. sibirica Pall. were significantly affected by salt stress. Our findings have identified potential genes involved in salt tolerance and in the reference transcriptome and have revealed the salt tolerance mechanism in N. sibirica Pall. These findings will provide further insight into the molecular and physiological mechanisms related to salt stress in N. sibirica Pall. and in other halophytes.