Fluorescence Induction From Photosystem II: Analytical Equations for the Yields of Photochemistry and Fluorescence Derived From Analysis of a Model Including Exciton-Radical Pair Equilibrium and Restricted Energy Transfer Between Photosynthetic Units

Abstract

The theoretical relationships between the fluorescence and photochemical yields of photosystem II (PSII) and the fraction of open reaction centres are examined in a model based on the following assumptions: (a) a homogeneous, infinite PSII domain; (b) exciton-radical pair equilibrium; and (c) different rates of exciton transfer between 'core' and 'peripheral' antenna beds. Simple analytical relations are examined for the yields and their time-courses in induction experiments. Variation of the inter-unit transfer rate allows continuous transition from the case of 'separated units' to the pure 'lake' model. Widely used relations for estimating the fraction of closed reaction centres from the complementary area of the fluorescence, or the photochemical yield from fluorescence levels are derived. An experimental induction curve is analysed, considering its composition of 'α' and 'β' centres. The sigmoidicity of the induction kinetics is characterised by a single parameter J (corresponding to Joliots' 'p'), that is shown to depend on both the connectivity of the photosynthetic units and on reaction centre parameters. On the other hand, the relation between J and the extreme fluorescence levels (or the deviation from the linear Stern-Volmer dependence of 1 /Φ_f, on the fraction of open traps) is only controlled by antenna connectivity. Experimental data are more consistent with a model of 'connected units' for PSII_α than with the pure 'lake' model.