Ionic Relations of Garden Orache,A triplex hortensisL.: Growth and Ion Distribution at Moderate Salinity and the Function of Bladder Hairs

The growth of garden orache, A triplex hortensis was studied under conditions of mild NaCl or Na2SO4 salinity. Growth, dry matter production and leaf size were substantially stimulated at 10 mM and 50 mM Na+ salts. Increased growth, however, appeared to be due to a K+-sparing effect of Na+ rather than to salinity per se. The distribution of K+ and Na+ in the plant revealed a remarkable preference for K+ in the roots and the hypocotyl. In the shoot the K/Na ratio decreased strongly with leaf age. However, the inverse changes in K+ and Na+ content with leaf age were dependent on the presence of bladder hairs, which removed almost all of the Na+ from the young leaf lamina. Measurements of net fluxes of K+ and Na+ into roots and shoots of growing Atriplex plants showed a higher K/Na selectivity of the net ion flux to the root compared to the shoot. With increasing salinity the selectivity ratio SK, Na* of net ion fluxes to the roots and to the shoots was increased. The data suggest that recirculation of K+ from leaves to roots is an important link in establishing the K/Na selectivity in A. hortensis plants. The importance of K+ recirculation and phloem transport for salt tolerance is discussed.