THE ROLE OF THE PLASMA-MEMBRANE PROTON PUMP IN SHORT-TERM PH REGULATION IN THE AQUATIC LIVERWORT RICCIA-FLUITANS L

The question is raised, to what extent is the plasma membrane proton pump involved in short-term pH regulation of plant cells? For this purpose the cytosolic pH (pH(c)) of Riccia fluitans rhizoid and thallus cells has been measured continuously using pH-sensitive microelectrodes (Felle and Bertl, 1986a). It is demonstrated that pH perturbations (light, weak acids, external pH) in both directions are completely or at least partly eliminated within minutes. The pH(c) recovery occurs regardless of the activation state of the proton pump. The proton pump reacts to changes in cytosolic pH as expected, namely with increased proton extrusion to decreased pH(c); however, changes in pump activity (fusicoccin, CCCP, cyanide) do not necessarily result in cytosolic pH shifts. These results suggest that several proton transport mechanisms (including the proton pump) co-operate in the restoration of a perturbed cytosolic pH. It is concluded, however, that the proton pump, although most important for the energization of the plasma membrane, does not regulate cytosolic pH.