Dynamics of salt secretion by Sporobolus spicatus (Vahl) Kunth from sites of differing salinity

Secretion of salts by bicellular salt glands and the water relations of the grass Sporobolus spicatus were investigated at four sites along the coast of the Red Sea in Egypt that differed in the extremity of salinity and drought. Salt eliminated by the leaves was similar in its composition at all sites. Na+ and Cl- were the dominant ions in the soil, and together comprised about 93 % of the dry weight of secreted salt. The molar ratio of K+:Na+ in the plant leaves was more than ten-fold that in the interstitial soil solution and thirteen-times that in the secreted salts, reflecting the high selectivity of the secretion mechanism for Na+. The concentration of Na+ in the solution transported to the leaves between 0900 and 1500 h was less than 0.1 % of that in the soil solution. Accumulation of salts by the plant shoots, which increased with increasing soil salinity and drought, was maximal during the day when the extent of secretion greatly reduced. The ionic osmotic potential (psi (i)) of leaves was lowest at midday and this was mainly due to the decreasing water content and increasing accumulation of NaCl which contributed about 68 % of total leaf ionic osmotic potential. Increasing secretion during the night resulted in removal of salts that were accumulated during the day. In this way the mineral content of the plant was regulated and compartments in the leaves that contained salt during the day were emptied during the night to become available for a new supply the following day. (C) 2001 Annals of Botany Company.