The technique of chlorophyll-a fluorescence induction (ChlF) is widely used in plant biology. The 'mainstream', so-called QA model of ChlF, posits that the reaction centers (RCs) of Photosystem-II (PSII) exist in two states, quenched (Fo) or open (PSIIO), and unquenched (Fm) or closed (PSIIC), containing the primary quinone acceptor, QA, in oxidized and reduced state, respectively; and that the quantum yield of PSII photochemistry of a dark-adapted sample is Y(II) = Fv/Fm, where Fv=Fm-Fo. The widespread application of ChlF, with user-friendly instruments, and the use of the QA model, have substantially contributed to our understanding of the operation of the photosynthetic machineries under different environmental conditions. However, recent experimental data - multiple light-induced fluorescence increments in PSIIC; the complex, pH and temperature dependent kinetic and spectral features of key ChlF parameters; twith enhanced stabilization of the charges - cannot be reconciled with the QA model. These features are explained by subtle conformational transitions driven by stationary and transient electric fields and associated dielectric relaxation processes. This interpretation, while invites further studies, places the hitherto unknown structural and functional plasticity of the RC matrix in the context of its physiological significance.

Department of Physics Faculty of Science University of Ostrava Ostrava Czech Republic

Institute of Plant Biology HUN REN Biological Research Centre Szeged Hungary

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