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 0-168h of 100mM NaCl treatment. Similar results were also found under 25 and 50mM NaCl treatments. Even K was increased from 0.01 to 50mM 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 7days treatment of 100mM NaCl. To investigate the long-term stress impact on tissue Na, 100mM 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.