Mechanisms of the plastoquinone pool oxidation in the dark after illumination

Abstract

Abstract Oxidation of the PQ-pool in leaves, intact chloroplasts and isolated thylakoids after its reduction by flash of light intensity 3000 µmol photons m− 2 s− 1 for 1.5 s was studied as a time-dependence in the dark of area above curve of fluorescence, Afl. The fluorescence was excited with the same flash applied after dark intervals from 0.1 s to 10 min. The samples were either dark-adapted or pre-illuminated with a continuous light. The results showed a two-phase kinetic of Afl across all samples, consistent with prior findings in thylakoids without added electron acceptors and in leaves. A comprehensive analysis of the kinetics in isolated thylakoids conducted under both aerobic and anaerobic conditions, with or without Fd and NADP+, and in the presence of catalase, enabled the proposition of mechanisms responsible for the observed two phases. Our findings suggest that the “fast” phase is likely a combination of residual electron flow from the PQ-pool to downstream acceptors and the oxidation of PQH2 by the superoxide radical, formed during illumination and preserved in thylakoid membrane. The "slow" phase involves the oxidation of the PQ-pool through slow autocatalytic reactions with molecular oxygen, and this oxidation is enhanced in the presence of hydrogen peroxide. The role of the above processes in intact chloroplasts and leaves is discussed.