Physiological evidence for a proton pump and sodium exclusion mechanisms at the plasma membrane of the marine angiosperm Zostera marina L.

The basic electrical plasma membrane characteristics of leaf cells from the seagrass Zostera marina L, have been investigated with respect to its prim ary transport system and its Na+/K+ selectivity. In natural seawater Z, mar ina exhibits a membrane potential of -156 +/- 10 mV, The phytotoxin fusicoc cin stimulates Hi extrusion and hyperpolarizes the plasma membrane. Ouabain , an inhibitor of the mammalian Na+/K+-ATPase did not depolarize the plasma membrane of Z. marina. Both flushing the leaves with CO, and 'light off' a cidified the cytoplasm and hyperpolarized the cells. It is suggested that a H+-ATPase rather than a Na+-ATPase is the primary pump in Z, marina. In th e presence of cyanide plus salicylhydroxamic acid the membrane potential ch anged to -64 +/- 11 mV, This so-called diffusion potential was sensitive to external [K+] from 0.05 to 0.5 mM in the presence of 0.5 M Na+ and reveale d a relative permeability PK+/PNa+ of 303, We suggest that this high ratio is the basic adaptation which permits Z, marina to grow in high [Na+] condi tions and to exhibit a rather negative resting potential. Since amiloride, an inhibitor of the nH(+)/Na+ antiporter, hyperpolarized the plasma membran e, it is suggested that this transporter could be present in the plasma mem brane of Z, marina acting as an overflow valve for Na+ which leaks into the cell.