Abstract: | Using glass capillary microelectrodes for the measurement ofpotential differences (PD) and antimony microelectrodes forthe measurement of pH, we investigated the light-induced changesof PD between the central vacuole and the external medium, ofpH in the vacuole (pHv), as well as of pH in the external medium(pHo) of the green marine alga Valonia ventricosa. PD in thedark was about +30 to +40 mV (vacuole positive), pHv 6.3, andthe resistance of the protoplast (cell wall-plasmalemma-tonoplast)17.8 kOhm cm2. Illumination caused an increase of the positivePD (after a few oscillations) up to +80 to +100 mV, acidificationof the vacuolar sap, alkalinization of the external medium,and a decrease in the resistance of the protoplast to 7.6 kOhmcm2. The kinetics of the changes of PD, pHv, and pHo were similarto each other. It is concluded that a light-stimulated activeH+ flow occurs from the external medium into the central vacuoleof Valonia ventricosa as a result of the onset of photosyntheticactivity. |