Alkali grass resists salt stress through high [K+] and an endodermis barrier to Na+

In order to understand the salt-tolerance mechanism of alkali grass (Puccinellia tenuiflora) compared with wheat (Triticum aestivum L.), [K+] and [Na+] in roots and shoots in response to salt treatments were examined with ion element analysis and X-ray microanalysis. Both the rapid K+ and Na+ influx in response to different NaCl and KCl treatments, and the accumulation of K+ and Na+ as the plants acclimated to long-term stress were studied in culture- solution experiments. A higher K+ uptake under normal and saline conditions was evident in alkali grass compared with that in wheat, and electrophysiological analyses indicated that the different uptake probably resulted from the higher K+/Na+ selectivity of the plasma membrane. When external [K+] was high, K+ uptake and transport from roots to shoots were inhibited by exogenous Cs+, while TEA (tetraethylammonium) only inhibited K+ transport from the root to the shoot. K+ uptake was not influenced by Cs+ when plants were K+ starved. It was shown by X-ray microanalysis that high [K+] and low [Na+] existed in the endodermal cells of alkali grass roots, suggesting this to be the tissue where Cs+ inhibition occurs. These results suggest that the K+/Na+ selectivity of potassium channels and the existence of an apoplastic barrier, the Casparian bands of the endodermis, lead to the lateral gradient of K+ and Na+ across root tissue, resulting not only in high levels of [K+] in the shoot but also a large [Na+] gradient between the root and the shoot.