LIGHT-INDUCED H+ ACCUMULATION IN VACUOLE OF NITELLOPSIS-OBTUSA

The effects of light on the pH in the vacuole and the electric potential difference across the plasmalemma and the tonoplast of Nitellopsis obtusa were investigated by means of conventional and H+-specific glass or antimony microelectrodes. Illumination is found to bring about a decrease in the pH of the vacuolar sap by 0.1-0.5 units concomitant with a depolarization of the cell. The light-induced changes of the potential difference and the vacuolar pH depend in different ways on the pH of the external medium (pHo). At pHo 9.0 cells exhibit great light-induced potential changes (up to 100 mV), but only small pH changes of the vacuolar sap. At neutral or slightly acidic pHo values the amplitude of the light-induced pH changes in the vacuole increases up to 0.3-0.5 pH units, but the amplitudes of the potential changes at the plasmalemma are relatively small. At pHo 9.0 a transient acidification of the medium is observed upon illumination whereas at lower pH values light-induced alkalinization was only seen. Transfer of the cells from pHo 9.0 to pHo 7.5 results in a cell hyperpolarization by 60-80 mV and a decrease of the 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 in the plasmalemma and the tonoplast. The observed acidification in the vacuole appears to be determined by a light-induced increase of the concentration of H+ in the cytoplasm. The H+ conduction of the plasmalemma seems to increase on illumination. The pattern of the light-induced H+ fluxes across the tonoplast and the plasmalemma depends crucially on the extent of the light-induced changes in the H+ conductance and on the electrochemical gradient for H+ at the plasmalemma. © 1978 Oxford University Press.