Analysis of fluorescence induction in thylakoids with the method of moments reveals two different active Photosystem II centres

The oxidation/reduction kinetics of the plastoquinone pool controls the appearance of the I-peak in the O-J-I-P chlorophyll fluorescence rise: effects of various electron acceptors

Quantitative analysis of the fluorescence induction (FI) rise was used in this study to elucidate the complex effects of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) on thylakoids. Reduced TMPD molecules, responsible for the ADRY agent effect, caused an increase in the amplitude of the O-J rise. Also, only oxidized TMPD molecules were shown to have the ability to bind the Q(B) pocket of photosystem II (PSII). On the other hand, the I-P rise was slowed in proportion with the oxidized TMPD concentration, inducing the clear appearance of the I-peak. While this property was previously thought to be unique to TMPD, this study shows that some artificial electron acceptors of PSII, silicomolybdate, 2,5-dichloro-p-benzoquinone, and phenyl-p-benzoquinone, have a similar effect. These results demonstrated a major role of the oxido-reduction kinetics of the PQ-pool in the resolution of J-I and I-P phases in the FI of isolated thylakoids.

Quantitative analysis of the experimental O-J-I-P chlorophyll fluorescence induction kinetics. Apparent activation energy and origin of each kinetic step

Fluorescence induction has been studied for a long time, but there are still questions concerning what the O-J-I-P kinetic steps represent. Most studies agree that the O-J rise is related to photosystem II primary acceptor (Q(A)) reduction, but several contradictory theories exist for the J-I and I-P rises. One problem with fluorescence induction analysis is that most work done to date has used only qualitative or semiquantitative data analysis by visually comparing traces to observe the effects of different chemicals or treatments. Although this method is useful to observe major changes, a quantitative method must be used to detect more subtle, yet important, differences in the fluorescence induction trace. To achieve this, we used a relatively simple mathematical approach to extract the amplitudes and half-times of the three major fluorescence induction phases obtained from traces measured in thylakoid membranes kept at various temperatures. Apparent activation energies (E(A)) were also obtained for each kinetic step. Our results show that each phase has a different E(A), with E(A O-J) <E(A J-I) < E(A I-P), and thus a different origin. The effects of two well-known chemicals, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, which blocks electron transfer to the photosystem II secondary electron acceptor (Q(B)), and decylplastoquinone, which acts similarly to endogenous reducible plastoquinones, on the quantitative parameters are discussed in terms of the origin of each kinetic phase.

On the rates of cyclic electron transport around Photosystem II in the presence of donor side limitation

Photosystem II cyclic electron transport was investigated at low pH in spinach thylakoids and PS II preparations from the cyanobacteriumPhormidium laminosum. Variable fluorescence (Fv) quenching at a very low light intensity was examined as an indicator of cyclic electron flow. A progressive quenching of Fv was observed as the pH was lowered; however, this was shown to be mainly due to an inhibition of oxygen evolution. Cyclic electron flow in the uninhibited centres was estimated to occur at a rate comparable to or smaller than 1 μ mole O2 mg Chl(-1) h(-1) in the pH range 5.0 to 7.8.The quantum yeeld of oxygen production is known to decrease at low pH and has been taken to indicate cyclic electron flow (Crofts and Horton (1991) Biochim Biophys Acta 1058: 187-193). However, a direct all-or-none inhibition of oxygen production at low pH has also been reported (Meyer et al. (1989) Biochim Biophys Acta 974: 36-43). We have analysed the effects of light intensity on the rates of oxygen evolution in order to calculate ΦU, the quantum yield of open and uninhibited centres. ΦU was found to be constant over a broad pH range, and by using ferricyanide and phenyl-p-benzoquinone as electron acceptors the maximum possible rate of cyclic electron transport was equivalent to no more than 1 μmole O2 mg Chl(-1) h(-1). The rate was no greater when the acceptor was adjusted to provide the most favourable conditions for cyclic flow.