IMPORTANCE OF PLASMALEMMA ATPASE IN THE RETENTION AND EXCLUSION OF INORGANIC-IONS

The experiments were focused on the question whether the plasmalemma ATPase activity (proton pump) has an influence on the efflux of major inorganic ion species. Efflux from roots of intact Trifolium pratense, Hordeum vulgare, Glycine max. and Zea mays was examined into a solution containing 100 muM CaCl2 and 500 muM NH4+ as sulfate in the control solution and 100 muM CaCl2 and 500 muM NH4+ as vanadate in the test solution. Vanadate being an inhibitor of the plasmalemma ATPase depressed significantly the H+ secretion of roots into the outer solution but had no major impact on the efflux of cation species. In the presence of vanadate significantly higher amounts of sulfate. phosphate. and nitrate were released into the outer solution by roots of soya and maize as compared with the control treatment (no vanadate). In the absence of vanadate, virtually no nitrate was released by all species examined whereas in the vanadate treatment significant amounts of NO3- were released. Vanadate inhibited the uptake of Cl- in barley and maize and increased the uptake of Ca2+ in soya. It is concluded that the plasmalemma ATPase activity plays a major rule in the ''ionic stat'' of cells in providing protons to the apoplast for the reabsorption of sulfate, phosphate, and particularly nitrate which have leaked out of the cytosol.