Photoelectrochemical control of the balance between cyclic- and linear electron transport in photosystem I. Algorithm for P700 induction kinetics |
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Authors: | Wim J Vredenberg Alexander A Bulychev |
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Institution: | a Department of Plant Physiology, Wageningen University and Research, Wageningen, The Netherlands b Department of Biophysics, Faculty of Biology, Moscow State University Moscow, Russia |
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Abstract: | Redox transients of chlorophyll P700, monitored as absorbance changes ΔA810, were measured during and after exclusive PSI excitation with far-red (FR) light in pea (Pisum sativum, cv. Premium) leaves under various pre-excitation conditions. Prolonged adaptation in the dark terminated by a short PSII + PSI− exciting light pulse guarantees pre-conditions in which the initial photochemical events in PSI RCs are carried out by cyclic electron transfer (CET). Pre-excitation with one or more 10 s FR pulses creates conditions for induction of linear electron transport (LET). These converse conditions give rise to totally different, but reproducible responses of P700− oxidation. System analyses of these responses were made based on quantitative solutions of the rate equations dictated by the associated reaction scheme for each of the relevant conditions. These provide the mathematical elements of the P700 induction algorithm (PIA) with which the distinguishable components of the P700+ response can be resolved and interpreted. It enables amongst others the interpretation and understanding of the characteristic kinetic profile of the P700+ response in intact leaves upon 10 s illumination with far-red light under the promotive condition for CET. The system analysis provides evidence that this unique kinetic pattern with a non-responsive delay followed by a steep S-shaped signal increase is caused by a photoelectrochemically controlled suppression of the electron transport from Fd to the PQ-reducing Qr site of the cytb6f complex in the cyclic pathway. The photoelectrochemical control is exerted by the PSI-powered proton pump associated with CET. It shows strong similarities with the photoelectrochemical control of LET at the acceptor side of PSII which is reflected by release of photoelectrochemical quenching of chlorophyll a fluorescence. |
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Keywords: | α fraction with a maximum number of buffering groups (N) in vicinity of Fd-PQ redox domain at acceptor side of PSI (with N &minus 1 groups in complemental fraction) Acc complex of primary and secondary acceptors of PSI CET cyclic electron transport around PSI FCCP carbonyl cyanide p-trifluorometoxyphenylhydrazone Fd ferredoxin FIA fluorescence induction algorithm FNR ferredoxin NADPH oxidoreductase γ CET sub-fraction populated with an oxidized PQ-pool ks rate constant of electron donation to PC by the PSI electron donor kL excitation rate of photosystem in light pulse k&minus PE rate constant of passive trans-thylakoid proton flow kqbf rate constants of the proton efflux out of the Fd-PQ redox domain towards the Qr site of the cytb6f complex kqr rate constant of the reduction of PQ (at the Qr site) by (reduced) Fd kfnr rate constant of the reduction of FNR by (reduced) Fd LET linear electron transport through PSI P700 primary chlorophyll electron donor of PSI PC plastocyanin ODE ordinary linear differential equation PIA P700 induction algorithm PQ plastoquinone PSII photosystem II θ fraction of PSI RCs in which only cyclic electron transport occurs (RCs operating in CET mode) Qr plastoquinone reducing side of cytb6f complex (stromal side) Q0 plastoquinol oxidizing side of cytb6f complex (lumenal side) qPE degree of photoelectrochemical quenching of chlorophyll fluorescence RC reaction center of PS |
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