EPOXIDATION OF ZEAXANTHIN AND ANTHERAXANTHIN REVERSES NONPHOTOCHEMICAL QUENCHING OF PHOTOSYSTEM-II CHLOROPHYLL-A FLUORESCENCE IN THE PRESENCE OF TRANS-THYLAKOID DELTA-PH

The xanthophyll cycle apparently aids the photoprotection of photosystem IT by regulating the nonradiative dissipation of excess absorbed light energy as heat. However, it is a controversial question whether the resulting nonphotochemical quenching is soley dependent on xanthophyll cycle activity or not. The xanthophyll cycle consists of two enzymic reactions, namely deepoxidation of the diepoxide violaxanthin to the epoxide-free zeaxanthin and the much slower, reverse process of epoxidation. While deepoxidation requires a transthylakoid pH gradient (Delta pH), epoxidation can proceed irrespective of a Delta pH. Herein, we compared the extent and kinetics of deepoxidation and epoxidation to the changes in fluorescence in the presence of a light-induced thylakoid Delta pH. We show that epoxidation reverses fluorescence quenching without affecting thylakoid Delta pH. These results suggest that epoxidase activity reverses quenching by removing deepoxidized xanthophyll cycle pigments from quenching complexes and converting them to a nonquenching form. The transmembrane organization of the xanthophyll cycle influences the localization and the availability of deepoxidized xanthophylls is to support nonphotochemical quenching capacity. The results confirm the view that rapidly reversible nonphotochemical quenching is dependent on deepoxidized xanthophyll.