[Photo-induced H+ transport in Nitellopsis obtusa cells]

Light-induced changes of pH in the vacuole as well as changes of the electric potential difference across the plasmalemma and the tonoplast of Nitellopsis obtusa were measured simultaneously by means of conventional and H+-specific glass or antimony microelectrodes. Illumination is found to produce a decrease in pH of the vacuolar sap by 0.1-0.5 units concomitant with cell depolarization. Cells suspended in a medium with pH 9.0 exhibit great (up to 100 mV) light-induced potential changes but only small pH changes of the vacuolar sap. When pH of the external medium (pH0 in shifted from 9.0 to more acid values the amplitude of photoinduced changes of pH in the vacuole rises up to 0,3-0.5 pH units and the amplitude of the potential changes at the plasmalemma gets smaller. At pH0 = 9.0 a transient acidification of the medium is observed upon illumination whereas at lower pH light-induced alkalinization was only seen. Transition of the cells from pH0 9.0 to pH0 7,5 results in cell hyperpolarization by 60-80 mv and a decrease of vacuolar pH by 0.4-0.5 units under light conditions but has no significant effect on the potential and the vacuolar pH in the darkness. It is proposed that mechanisms of active H+ extrusion from the cytoplasm are located both at the plasmalemma and tonoplast. Light-induced depolarization seems to be determined by the increase of H+-conductance of the plasmalemma and by a correspondent decrease in the electrogenic components of the membrane potential. The ratio of light-induced H+-fluxes across the tonoplast and the plasmalemma depends crucially on the level of H+-conductance of the plasmalemma.