Models of Chlorophyll a Fluorescence Transients

Abstract

In this chapter we describe modeling efforts of fluorescence rise (FLR) transients over the last 20 years. During this period the complexity of the models has increased significantly. Nowadays, the more complex models consist of a combination of the Kok model for the reactions on the donor side of photosystem II (PS II), the reversible radical pair model for the primary PS II photochemistry, the two-electron gate model for electron transport on the acceptor side of PS II, reactions related to reduction and oxidation of plastoquinone (PQ) and, in some cases, of cytochrome b6f, plastocyanin and photosystem I. In some models additional processes are considered like electric field effects and dark reactions of photosynthesis occurring in the stroma and cytosol. The chapter begins with an introduction of topics important for the construction of a model: relevant fluorescence theories, measuring techniques, the physiology behind the FLR, the role of the integrity of the sample, enzyme kinetics and rate constants. Subsequently several published models are discussed. A major problem for many FLR models is that the fluorescence rises much faster (often by a factor 10) than experimentally observed. Possible reasons for this mismatch are discussed in the context of different models. The large majority of models is based on the postulate that the redox state of QA is the major determinant of the variable fluorescence yield. In several models P680+ and quenching by the PQ pool are added, but this is still insufficient to correctly model the slowest rise phase. The question is raised whether additional assumptions are needed to correctly simulate the O—J—I—P transient. At the end of the chapter the fluorescence decrease following the initial rise is discussed. Only a few models include this part of the fluorescence transient. A flaw of these models is that they ignore the experimentally observed transient block at the acceptor side of photosystem I, limiting both electron flow and proton transport during the FLR. As a consequence, activation of photosynthesis occurs in models with considerably faster kinetics than observed experimentally.