Excitonic connectivity between photosystem II units: what is it, and how to measure it?

In photosynthetic organisms, light energy is absorbed by a complex network of chromophores embedded in light-harvesting antenna complexes. In photosystem II (PSII), the excitation energy from the antenna is transferred very efficiently to an active reaction center (RC) (i.e., with oxidized primary quinone acceptor Q _A), where the photochemistry begins, leading to O₂ evolution, and reduction of plastoquinones. A very small part of the excitation energy is dissipated as fluorescence and heat. Measurements on chlorophyll (Chl) fluorescence and oxygen have shown that a nonlinear (hyperbolic) relationship exists between the fluorescence yield (Φ _F ) (or the oxygen emission yield, $ \Phi _{{{\text{O}}_{2} }} $ ) and the fraction of closed PSII RCs (i.e., with reduced Q _A). This nonlinearity is assumed to be related to the transfer of the excitation energy from a closed PSII RC to an open (active) PSII RC, a process called PSII excitonic connectivity by Joliot and Joliot (CR Acad Sci Paris 258: 4622–4625, 1964). Different theoretical approaches of the PSII excitonic connectivity, and experimental methods used to measure it, are discussed in this review. In addition, we present alternative explanations of the observed sigmoidicity of the fluorescence induction and oxygen evolution curves.