System Analysis and Photoelectrochemical Control of Chlorophyll Fluorescence in Terms of Trapping Models of Photosystem II: A Challenging View

This chapter gives an outline, quantitative description and a critical evaluation of the essentials of ‘classic’ single hit two-state trapping models of Photosystem II (PS II) and of a recently proposed extension of it. This double hit three-state trapping model (TSTM) suggests that closing of the photosynthetic reaction center of PS II requires at least two successive trapping events. It emphasizes that each of the primary and secondary acceptors of PS II, pheophytin (Pheo, Phe, Ph) and Q_A act as an electron trap and that the electron trapping efficiency of centers with reduced Q_A is low but not necessarily zero. The extended model enables quantitative analyses of the chlorophyll (Chi) fluorescence induction curve in terms of excitation rate and rate constants of electron transfers at the donor and acceptor sides of PS II. Predicted critical differences in interpretation and explanations of, for instance, sigmoidicity of fluorescence time curve, PS II antenna size heterogeneity and some other parameters challenge interpretations, accepted in other chapters in this book. Further research is needed to judge if the current thinking needs to be overhauled in view of the ideas presented here. Examples are given which show that the model offers a supreme diagnostic tool for localizing and quantifying the effect of stress conditions on photosynthetic performance.

Evidence for photoelectrochemical control of Chi fluorescence of PS II is presented. This is based on elec-trophysiological and fluorescence data that show that changes in electric fields in the vicinity of the reaction center site, generated by photosynthetic charge separation and secondary charge displacements, are associated with changes in Chi fluorescence of the antennas.