PICOSECOND TIME-RESOLVED STUDY ON THE NATURE OF HIGH-ENERGY-STATE QUENCHING IN ISOLATED PEA THYLAKOIDS - DIFFERENT LOCALIZATION OF ZEAXANTHIN DEPENDENT AND INDEPENDENT QUENCHING MECHANISMS

The influence of the transthylakoid proton gradient on the kinetics of picosecond fluorescence decay was examined using isolated pea thylako ids having high or low zeaxanthin contents. Fluorescence lifetime meas urements were performed with open (Fo) and closed (Fm) PS II reaction centers. Zeaxanthin formation in membrane energized isolated thylakoid s led to a marked decrease of the average fluorescence lifetime at bot h Fm and Fo. In contrast, when zeaxanthin synthesis was blocked by the inhibitor DTT, the fluorescence lifetime decrease was less pronounced in the Fm state and totally missing in the Fo state. Samples containi ng the uncoupler ammonium chloride did not exhibit any zeaxanthin infl uence on the fluorescence decay kinetics. By detailed kinetic analysis of the fluorescence data based on the exciton/radical pair equilibriu m model it was possible to separately locate and quantify the effects of zeaxanthin, on the one hand, and the proton gradient, on the other hand, in terms of rate constants of individual primary processes withi n PS II. It is shown that the enhanced non-photochemical fluorescence quenching (NPQ) in the presence of zeaxanthin mainly originates in the antenna, while without zeaxanthin smaller changes in the Fm state are owing to changes in processes located at the reaction center. Possibl e mechanisms of zeaxanthin dependent and independent nonphotochemical fluorescence quenching in open and closed PS II are discussed.