Light-dependent pH changes in leaves of C3 plants

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Etiolated leaves and the inhibitors of photosynthesis 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and DL-glyceraldehyde were used to study the relationship between thylakoid energization, photosynthesis, the light-dependent alkalization of the cytosol of mesophyll cells and the acidification of mesophyll vacuoles. No light-dependent pH changes were observed in etiolated leaves. As chloroplasts developed in the light and became photosynthetically competent, mesophyll vacuoles became more acidic when the leaves were illuminated in CO2-free air. Acidification was suppressed and even replaced by a small light-dependent alkaline shift during photoassimilation of CO2. In green leaves, DCMU and DL-glyceraldehyde inhibited the cytosolic and vacuolar pH responses to illumination. Inhibition was also observed under anaerobiosis. In the absence of DCMU and glyceraldehyde, the extent of the light-dependent vacuolar acidification corresponded closely to the extent of thylakoid energization by light. Because, in contrast to DCMU, glyceraldehyde did not inhibit thylakoid energization while inhibiting the extrachloroplast pH responses, it is concluded that the signal transfer from the chloroplasts to the cytosol which results in increased vacuolar acidification in the light involves metabolites of the Calvin cycle. The observations do not support the view that the cytosolic energy state is increased in the light by the mitochondrial oxidation of the NADH generated during the oxidation of photorespiratory glycine in the mitochondria.