A mathematical model of electron transport. Thermodynamic necessity for photosystem II regulation: 'light stomata’

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Abstract
A model of the electron transport chain (ETC) and carbon reduction cycle (PCR cycle) of photosynthesis is described. The structure of the model includes photosystems II and I (PSII and PSI), with their donor and acceptor sides, and plastoquinone and NADP as the corresponding secondary acceptors of electrons from PSII and PSI primary acceptors. Plastoquinone oxidation/P700 reduction is coupled to proton transport and build-up of the transmembrane ∆pH. The latter creates a shift in the free energy change, ∆G, of the ATP-synthetase towards ATP synthesis. Cyclic electron flow from ferredoxin to plastoquinone is allowed. The carbon reduction cycle is described as in Laisket al. (1989) but RuBP regeneration and sucrose synthesis are significantly simplified. Electron transport reactions are described by their standard redox potentials and maximum rates (Vm) of the corresponding enzymes, PCR cycle reactions are described by their Michaelis constant,Km, values in addition toVmand ∆G'0. Analysis of the model at different photon flux densities and CO2con­centrations reveals an absolute thermodynamic necessity for a control mechanism that dissipates the surplus of light energy reaching photochemically active PSII centres. At the same time, the excitation rates of the PSII and PSI centres must be quantitatively balanced in such a way that over-reduction or over-energization of the electron transport chain is avoided. It is assumed that part of the light harvesting complex in PSII is inactivated by a control mechanism that simultaneously involves ∆pH and plastoquinone reduction status. If it is also assumed that the fluorescence signal comes only from the reduced active PSII centres, the ‘Kautzky effect’ and some observations related toqE-non-photochemical quenching of fluorescence can be explained. The model provides a basis to suggest a possible mechanism of light and shade tolerance in plants.