Light-induced lumenal acidification controls the efficiency of light harvesting by inducing thermal dissipation of excess absorbed light energy in photosystem II. We isolated an Arabidopsis mutant, pgr1 (proton gradient regulation), entirely lacking thermal dissipation, which was observed as little non-photochemical quenching of chlorophyll fluorescence. Map-based cloning showed that pgr1 had a point mutation in petC encoding the Rieske subunit of the cytochrome b(6)f complex. Although the electron transport rate was not affected at low light intensity, it was significantly restricted at high light intensity in pgr1, indicating that the lumenal acidification was not sufficient to induce thermal dissipation. This view was supported by (i) slow de-epoxidation of violaXanthin, which is closely related to lumenal acidification, and (ii) reduced 9-aminoacridine fluorescence quenching. Although lumenal acidification was insufficient to induce thermal dissipation, growth rate was not affected under low light growth conditions in pgr1. These results suggest that thermal dissipation is precisely regulated by lumenal pH to maintain maximum photosynthetic activity. We showed that pgr1 was sensitive to changes in light conditions, demonstrating that maximum activity of the cytochrome b(6)f complex is indispensable for short-term acclimation.
