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 photosyst em IT by regulating the nonradiative dissipation of excess absorbed li ght energy as heat. However, it is a controversial question whether th e resulting nonphotochemical quenching is soley dependent on xanthophy ll cycle activity or not. The xanthophyll cycle consists of two enzymi c reactions, namely deepoxidation of the diepoxide violaxanthin to the epoxide-free zeaxanthin and the much slower, reverse process of epoxi dation. While deepoxidation requires a transthylakoid pH gradient (Del ta 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 thylako id Delta pH. We show that epoxidation reverses fluorescence quenching without affecting thylakoid Delta pH. These results suggest that epoxi dase activity reverses quenching by removing deepoxidized xanthophyll cycle pigments from quenching complexes and converting them to a nonqu enching form. The transmembrane organization of the xanthophyll cycle influences the localization and the availability of deepoxidized xanth ophylls is to support nonphotochemical quenching capacity. The results confirm the view that rapidly reversible nonphotochemical quenching i s dependent on deepoxidized xanthophyll.