Photodamage and excitation energy quenching in PSII: A time-resolved fluorescence study in Arabidopsis

Photosynthesis is driven by light absorbed in photosystem (PS) I and II. Paradoxically, light can also inactivate photosynthesis, mainly by damage to PSII. The light-dependent decrease in functional PSII, referred to as photoinhibition, is initially accompanied by an increase of excitation quenching, energy dissipation characterized by a decline in the lifetime and yield of chlorophyll fluorescence. In plants, research has not yet been performed on the effect of photoinhibition on the fluorescence lifetime of PSII in conditions where the PSII reaction centers are closed or remain open (capable of performing photochemistry). In this work, we studied the effect of photoinhibition on the fluorescence lifetime of PSII in Arabidopsis thaliana using time-resolved fluorescence measurements with a streak-camera setup in both closing (F_m) and non-closing (F_o) conditions. Measurements under F_m conditions in the chlorina mutant, lacking peripheral antenna, demonstrate formation of a photoinhibitory quencher in the PSII core complex. In F_o_, the average fluorescence lifetime of PSII increases upon induction of photoinhibition. This could be due to the degradation of quenched PSII core reaction center protein by FtsH proteases, which leads to unquenched and dysfunctional PSII. We tested this hypothesis by comparing WT plants with the FtsH2 lacking mutant. Based on the similar behavior, we conclude that degradation by FtsH proteases is not the main cause of the increase. Instead this increase is caused by the larger antenna size of still functional PSII. These findings provide new insights into the impact of photoinhibition on the PSII fluorescence lifetime in A. thaliana.