Light-dependent pH changes in leaves of C3 plants : II. Effect of CO2 and O 2 on the cytosolic and the vacuolar pH.

Illumination of leaves of C3 plants caused cytosolic alkalization and vacuolar acidification in the mesophyll cells. Both phenomena were particularly pronounced when CO2 was absent, were suppressed by CO2, and were related to the activation state of the photosynthetic apparatus. The cytosolic alkalization reaction has at least two major components. Trivalent cytosolic phosphoglycerate must be protonated before it can be transferred into the chloroplasts for reduction. Pumping of protons from the cytosol into the vacuole also contributes to cytosolic alkalization. The dependence of light scattering by chloroplast thylakoids on the energy fluence rate was closely related to that of vacuolar acidification under different conditions for chloroplast energization. This indicates (i) transport of energy from the chloroplasts to the cytosol in the light and (ii) use of this energy for the transport of protons into the vacuoles. The light-dependent vacuolar acidification is interpreted to be caused by the increase in the activity of a proton-translocating enzyme of the tonoplast. The decrease of vacuolar acidification during photosynthetic carbon reduction or photorespiration is indicative of decreased cytosolic energization. In low light, the light-dependent vacuolar acidification was stimulated in the absence of CO2 when photorespiration was inhibited. The data do not support the view that photorespiration is capable of increasing the cytosolic energy state in the light.