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

Bouchra Yaakoubd, Roxane Andersen, Yves Desjardins, Guy Samson

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18 Citations (Scopus)


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) QA 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 QB-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 QB site of PS II and of an artificial quencher, 2-methyl-1,4-naphtoquinone, on Chl a fluorescence levels induced by STF (FF) and MTF (FM) in spinach thylakoids. We observed that: (1) the incorporation of PQex in thylakoids stimulated photosynthetic electron transport but barely affected FF and FM in the absence of DCMU; (2) DCMU significantly increased the amplitude of FF but slightly quenched FM; (3) 2-methyl-1,4-naphtoquinone quenched FM to a larger-extent than FF; (4) DCMU increased the quenching effects of PQex on FF and FM and also, of methyl-1,4-naphtoquinone on FF. These results indicate that: (1) the QB-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.

Original languageEnglish
Pages (from-to)251-257
Number of pages7
JournalPhotosynthesis Research
Issue number3
Publication statusPublished - 2002


  • Chlorophyll a fluorescence
  • Naphtoquinone
  • Non-photochemical quenching
  • Photosystem II
  • Plastoquinone pool
  • Q site


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