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

Abstract

The basic electrical plasma membrane characteristics of leaf cells from the seagrass Zostera marina L. have been investigated with respect to its primary transport system and its Na+/K+ selectivity. In natural seawater Z. marina exhibits a membrane potential of 156 ± 10 mV. The phytotoxin fusicoccin stimulates H+ 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 CO2 and 'light off' acidified 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 the presence of cyanide plus salicylhydroxamic acid the membrane potential changed 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 revealed a relative permeability P(K)+P(Na)+ of 303. We suggest that this high ratio is the basic adaptation which permits Z. marina to grow in high [Na+] conditions and to exhibit a rather negative resting potential. Since amiloride, an inhibitor of the nH+/Na+ antiporter, hyperpolarized the plasma membrane, it is suggested that this transporter could be present in the plasma membrane of Z. marina acting as an overflow valve for Na+ which leaks into the cell.