The coordinated regulation of Na+ and K+ in Hordeum brevisubulatum responding to time of salt stress

Hordeum brevisubulatum, called as wild barley, is a useful monocotyledonous halophyte for soil improvement in northern China. Although previously studied, its main salt tolerance mechanism remained controversial. The current work showed that shoot Na+ concentration was increased rapidly with stress time and significantly higher than in wheat during 0-168 h of 100 mM NaCl treatment. Similar results were also found under 25 and 50 mM NaCl treatments. Even K+ was increased from 0.01 to 50 mM in the cultural solution, no significant effect was found on tissue Na+ concentrations. Interestingly, shoot growth was improved, and stronger root activity was maintained in H. brevisubulatum compared with wheat after 7 days treatment of 100 mM NaCl. To investigate the long-term stress impact on tissue Na+, 100 mM NaCl was prolonged to 60 days. The maximum values of Na+ concentrations were observed at 7th in shoot and 14th day in roots, respectively, and then decreased gradually. Micro-electrode ion flux estimation was used and it was found that increasing Na+ efflux while maintaining K+ influx were the major strategies to reduce the Na+ concentration during long-term salt stress. Moreover, leaf Na+ secretions showed little contribution to the tissue Na+ decrease. Thereby, the physiological mechanism for H. brevisubulatum to survive from long-term salt stress was proposed that rapid Na+ accumulation occurred in the shoot to respond the initial salt shock, then Na+ efflux was triggered and K+ influx was activated to maintain a stable K+/Na+ ratio in tissues.