Light-induced cytoplasmic pH changes and their interrelation to the activity of the electrogenic proton pump in Riccia fluitans

Abstract

In green thallus cells of the aquatic liverwort Riccia fluitans light-induced pH changes have been measured, using a turgor-resistant pH-sensitive microelectrode. (1) Light-off/-on causes oscillations of the cytoplasmic pH (pH c), as well as of the membrane potential difference across the plasmalemma (ψ). Beside the well-known ψ m changes, the first detectable pH c change following light-off is a transient acidification of about 0.3 pH units, whereas light-on causes a transient alkalinization of roughly 0.4 pH units. (2) 1 μM DCMU eliminates these transients. (3) In the presence of 0.2 mM procaine, which alkalizes the cytoplasm to over pH 8, the light-induced ψ m transients are enhanced, but are almost absent, if pH c is acidified to 6.9 by 1 mM acetate. It is suggested that the transient light-induced changes in pH c are caused by light-dependent proton translocation across the thylakoid membranes, and it is concluded that the subsequent changes in ψ m are essentially the result of altered activities of the electrogenic proton pump in the plasmalemma, due to the observed fluctuations of its substrate, the proton.