Salt-Stress-Induced Ion Transport Contributes to K+/Na+ Homeostasis in Roots of Ping'ou Hybrid Hazelnut

Soil salinity is a worldwide problem that adversely affects plant growth and development. Soil salinization in Xinjiang of China is very serious. Ping'ou hybrid hazelnut, as an important ecological and economic tree species, as well as a salt-tolerant plant, has been grown in Xinjiang for over 20 years. Understanding the salt-tolerance mechanism of Ping'ou hybrid hazelnut is of great significance for the breeding of salt-tolerant varieties and the rational utilization of salinized land. In this study, 'Liaozhen 7 ', a fine variety of Ping'ou hybrid hazelnut, was selected as test material, and seedlings were treated with 0 (control), 50, 100 and 200 mM NaCl. Subsequently, the pattern of NaCl-induced fluxes of Na+, K+ and H+ in the root meristematic zone and their response to ion transport inhibitors were studied using non-invasive micro-test technology (NMT). Different concentrations of NaCl stress significantly increased the Na+ concentration in roots, while K+ concentration decreased first and then increased with the increase of NaCl concentration. Meanwhile, NaCl stress induced a significant decline in K+/Na+ ratio. Control and 200 mM NaCl-induced Na+ and K+ fluxes in roots exhibited an outward efflux, whereas an inward flux was observed for H+. Under 200 mM NaCl stress, the average rates of net Na+ and K+ efflux, as well as H+ influx in roots were significantly increased, which were 11.6, 6.7 and 2.3 times higher than that of control, respectively. Furthermore, pharmacological experiments showed that 200 mM NaCl-induced Na+ efflux; H+ influx was significantly suppressed by amiloride, an inhibitor of plasma membrane (PM) Na+/H+ antiporter, and sodium vanadate, an inhibitor of PM H+-ATPase. Net Na+ efflux and H+ influx induced by NaCl decreased by 89.9% and 135.0%, respectively. The NaCl-induced Na+ efflux was mediated by a Na+/H+ antiporter using energy provided by PM H+-ATPase. The NaClinduced K+ efflux was significantly restricted by tetraethylamine chloride, a K+ channel inhibitor, and promoted by sodium vanadate, which decreased by 111.2% and increased by 80.8%, respectively, indicating that K+ efflux was regulated by depolarization-activated outward-rectifying K+ channels and non-selective cation channels (NSCCs). In conclusion, NMT data revealed that NaCl stress up regulated the root Na+/H+ antiporter and H+ pump (an activity of PM Na+/H+ antiport system) of 'Liaozhen 7 ', which compelled the Na+/H+ exchange across the PM and restricted K+ loss via depolarization-activated K+ channels and NSCCs simultaneously, thereby maintaining the K+/Na+ homeostasis and higher salt tolerance.