Contributions of the free oxidized and Q_B-bound plastoquinone molecules to the thermal phase of chlorophyll-a fluorescence

Variable chlorophyll a (Chl a) fluorescence is composed of a photochemical and a thermal phases of similar amplitudes. The photochemical phase can be induced by a saturating single turnover flash (STF) and reflects the reduction of the Photosystem II (PS II) Q_A primary electron acceptor. The thermal phase requires multiple turnover flash (MTF) and is somehow related to the reduction of the plastoquinone (PQ) molecules. This article aimed to determine the relative contributions of the Q_B-bound and the free oxidized PQ molecules to the thermal phase of Chl a fluorescence. We thus measured the interactive effects of exogenous PQ (PQex), of an inhibitor (DCMU) acting at the Q_B site of PS II and of an artificial quencher, 2-methyl-1,4-naphtoquinone, on Chl a fluorescence levels induced by STF (F_F) and MTF (F_M) in spinach thylakoids. We observed that: (1) the incorporation of PQex in thylakoids stimulated photosynthetic electron transport but barely affected F_F and F_M in the absence of DCMU; (2) DCMU significantly increased the amplitude of F_F but slightly quenched F_M; (3) 2-methyl-1,4-naphtoquinone quenched F_M to a larger-extent than F_F; (4) DCMU increased the quenching effects of PQex on F_F and F_M and also, of methyl-1,4-naphtoquinone on F_F. These results indicate that: (1) the Q_B-bound and the free PQ molecules contribute to about 56% and 25%, respectively, to the thermal phase Chl a fluorescence in dark-adapted thylakoids; and (2) the thermal phase of Chl a fluorescence is more susceptible than the photochemical phase to the non-photochemical quenching effect of oxidized quinones.