Revisiting the QA model of chlorophyll-a fluorescence induction: new perspectives to monitor the photochemical activity and structural dynamics of photosystem II

The technique of chlorophyll-a fluorescence induction (ChlF) is widely used in plant biology. The 'mainstream', so-called Q_A model of ChlF, posits that the reaction centers (RCs) of Photosystem-II (PSII) exist in two states, quenched (F_o) or open (PSII_O), and unquenched (F_m) or closed (PSII_C), containing the primary quinone acceptor, Q_A, in oxidized and reduced state, respectively; and that the quantum yield of PSII photochemistry of a dark-adapted sample is Y(II) = F_v/F_m, where F_v=F_m-F_o. The widespread application of ChlF, with user-friendly instruments, and the use of the Q_A 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 PSII_C; 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 Q_A 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.