Untitled

Abstract

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.

Photosynthetic sea slugs induce protective changes to the light reactions of the chloroplasts they steal from algae

“…timida. We base this on the notion that full reduction of the PQ pool is a prerequisite for reaching maximum fluorescence when fluorescence rise kinetics are measured using multiple turnover saturating light pulses, such as the ones used in the current study (Kramer et al, 1995;Yaakoubd et al, 2002;Suggett et al, 2003;Osmond et al, 2017). Maintaining an oxidized PQ pool could be advantageous for chloroplast longevity, as it would help mitigate electron pressure and ROS formation in PSII.…”

“…The ~ 300 ms delay in reaching the maximum fluorescence during the OJIP measurements in E. timida in Figure 3A indicates that the PQ pool is more oxidized in the slugs than in Acetabularia even in aerobic conditions, and full reduction of the PQ pool simply takes longer in E. timida . We base this on the notion that full reduction of the PQ pool is a prerequisite for reaching maximum fluorescence when fluorescence rise kinetics are measured using multiple turnover saturating light pulses, such as the ones used in the current study ( Kramer et al, 1995 ; Yaakoubd et al, 2002 ; Suggett et al, 2003 ; Osmond et al, 2017 ). Maintaining an oxidized PQ pool could be advantageous for chloroplast longevity, as it would help mitigate electron pressure and ROS formation in PSII.…”