Importance of the oxidised plastoquinone molecules on the thermal phase of chlorophyll-a fluorescence

Abstract

Variable Chla fluorescence is composed of two distinct phases (photochemical and thermal) of nearly equal amplitude. The photochemical phase is related to the reduction of the photosystem II (PSII) QA electron acceptor and can be induced by a saturating single turnover flash (STF). The thermal phase requires multiple turnover flash (MTF) and is thought to reflect the reduction of the plastoquinone (PQ) pool. The objective of this report was to determine the relative contributions of the free oxidised PQ molecules and those bound to the QB site to the thermal phase of Chla fluorescence. To this end, we measured the variable Chla fluorescence yields induced by STF and MTF (corresponding to Fvf and Fvm respectively) emitted from spinach thylakoids resuspended in presence of different inhibitors acting at the QB site, of exogenous PQ (PQex) or in presence of the artificial quencher 2-methyl¿1,4-naphtoquinone. The results showed that 1) the inhibitors DCMU, bromoxynil and o-phenanthroline similarly increased Fvf but quenched Fvm, 2) PQex markedly quenched Fvf and Fvm in presence of DCMU but not in its absence, 3) Fvm was quenched to a much larger extent than Fvf by 2-methyl¿1,4-naphtoquinone. These results indicate that I) the thermal phase of Chla fluorescence is more susceptible than the photochemical phase to the non-photochemical quenching of oxidised quinones and II) both QB and the PQ pool contribute to the thermal phase, so that the oxidised forms of QA QB and of the PQ pool are responsible for about 50%, 25% and 25% respectively of the total amplitude of variable Chla fluorescence.