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Vredenberg WJ, Bulychev A.
Photoelectric effects on chlorophyll fluorescence of
photosystem II in vivo. Kinetics in the absence and presence of
valinomycin.
Bioelectrochemistry. 2003 Aug;60(1-2):87-95
Laboratory of Plant Physiology, Wageningen University and
Research, Arboretumlaan 4, NL6703 BD Wageningen, The
Netherlands.
wim.vredenberg@wur.nl
Fluorescence induction curves (F(t)) in low intensity 1s light
pulses have been measured in leaf discs in the presence and
absence of valinomycin. Addition of valinomycin causes: (i) no
effect on the initial fluorescence level Fo and the initial
(O-J) phase of F(t) in the 0.01-1 ms time range. (ii) An
approximately 10% decrease in the maximal fluorescence Fm in the
light reached at the P level in the O-J-I-P induction curve.
(iii) Nearly twofold increase in the rate and extent of the F(t)
rise in the J-I phase in the 1-50 ms time range. (iv) A 60-70%
decrease in the rise (I-P phase) in the 50-1000 ms time range
with no appreciable effect, if at all, on the rate. System
analysis of F(t) in terms of rate constants of electron transfer
at donor and acceptor sides have been done using the Three State
Trapping Model (TSTM). This reveals that valinomycin causes: (i)
no, or very little effect on rate constants of e-transfer
reactions powered by PSII. (ii) A manifold lower rate constant
of radical pair recombination (k(-1)) in the light as compared
to that in the control. The low rate constant of radical pair
recombination in the reaction center (RC) in the presence of
valinomycin is reflected by a substantial increase in the
nonzero trapping efficiency in RCs in which the primary quinone
acceptor (Q(A)) is reduced (semi-open centers). This causes an
increase in their rate of closure and in the overall trapping
efficiency. Data suggest evidence that membrane chaotropic
agents like valinomycin abolish the stimulation of the rate
constant of radical pair recombination by light. This light
stimulation that becomes apparent as an increase in Fo has been
documented before [Biophys. J. 79 (2000) 26]. It has been
ascribed to effects of (changes in) local electric fields in the
vicinity of the RC. The decrease of the I-P phase is attributed
to a decrease in the photoelectric trans-thylakoid potential in
the presence of valinomycin. Such effects have been hypothesized
and illustrated.
PMID: 12893314
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