Photosystem II (PSII) uses solar energy to oxidize water and delivers electrons to fix CO₂. Although the atomic resolution structure and basic photophysical and photochemical functions of PSII are well understood, many important questions remain. The activity of PSII in vitro and in vivo is routinely monitored by recording chlorophyll-a fluorescence induction kinetics (ChlF). According to the ‘mainstream’ model, the rise from the minimum level (F_o) to the maximum (F_m) of ChlF of dark-adapted PSII reflects the closure of all functionally active reaction centers, and the F_v/F_m ratio is equated with the maximum photochemical quantum yield of PSII (F_v=F_m–F_o). However, this model has never been free of controversies. A range of recent experimental data confirmed that the first single-turnover saturating flash (STSF), generating closed state (PSII_C), produces F₁<F_m; and uncovered rate-limiting steps – Δτ_1/2 half-waiting times – in the multi-STSF induced F₁-to-F_m increments, originating from the gradual formation of light-adapted charge-separated state (PSII_L) with significantly increased stability of charges compared to PSII_C elicited by a single STSF. All data show that the interpretation of ChlF must be laid on new grounds. Here we discuss the underlying physical mechanisms and the significance of structural/functional dynamics of PSII – as reflected by ChlF and variations of the novel parameter Δτ_1/2.

Photosystem II (PSII) catalyses the photoinduced oxygen evolution and, by producing reducing equivalents drives, in concert with PSI, the conversion of carbon dioxide to sugars. Our knowledge about the architecture of the reaction centre (RC) complex and the mechanisms of charge separation and stabilisation is well advanced. However, our understanding of the processes associated with the functioning of RC is incomplete: the photochemical activity of PSII is routinely monitored by chlorophyll-a fluorescence induction but the presently available data are not free of controversy. In this work, we examined the nature of gradual fluorescence rise of PSII elicited by trains of single-turnover saturating flashes (STSFs) in the presence of a PSII inhibitor, permitting only one stable charge separation. We show that a substantial part of the fluorescence rise originates from light-induced processes that occur after the stabilisation of charge separation, induced by the first STSF; the temperature-dependent relaxation characteristics suggest the involvement of conformational changes in the additional rise. In experiments using double flashes with variable waiting times (∆τ) between them, we found that no rise could be induced with zero or short ∆τ, the value of which depended on the temperature - revealing a previously unknown rate-limiting step in PSII.

Recent chlorophyll‐a fluorescence yield measurements, using single‐turnover saturating flashes (STSFs), have revealed the involvement of a rate‐limiting step in the reactions following the charge separation induced by the first flash. As also shown here, in diuron‐inhibited PSII core complexes isolated from Thermosynechococcus vulcanus the fluorescence maximum could only be reached by a train of STSFs. In order to elucidate the origin of the fluorescence yield increments in STSF series, we performed transient absorption measurements at 819 nm, reflecting the photooxidation and re‐reduction kinetics of the primary electron donor P680. Upon single flash excitation of the dark‐adapted sample, the decay kinetics could be described with lifetimes of 17 ns (∼50%) and 167 ns (∼30%), and a longer‐lived component (∼20%). This kinetics are attributed to re‐reduction of P680^•+ by the donor side of PSII. In contrast, upon second‐flash (with Δt between 5 μs and 100 ms) or repetitive excitation, the 819 nm absorption changes decayed with lifetimes of about 2 ns (∼60%) and 10 ns (∼30%), attributed to recombination of the primary radical pair P680^•+Pheo^•–, and a small longer‐lived component (∼10%). These data confirm that only the first STSF is capable of generating stable charge separation – leading to the reduction of Q_A; and thus, the fluorescence yield increments elicited by the consecutive flashes must have a different physical origin. Our double‐flash experiments indicate that the rate‐limiting steps, detected by chlorophyll‐a fluorescence, are not correlated with the turnover of P680.

The purpose of this review is to outline our understanding of the nature, mechanism and physiological significance of light-induced reversible reorganizations in closed Type II reaction centre (RC) complexes. In the so-called ‘closed' state, purple bacterial RC (bRC) and photosystem II (PSII) RC complexes are incapable of generating additional stable charge separation. Yet, upon continued excitation they display well-discernible changes in their photophysical and photochemical parameters. Substantial stabilization of their charge-separated states has been thoroughly documented—uncovering light-induced reorganizations in closed RCs and revealing their physiological importance in gradually optimizing the operation of the photosynthetic machinery during the dark-to-light transition. A range of subtle light-induced conformational changes has indeed been detected experimentally in different laboratories using different bRC and PSII-containing preparations. In general, the presently available data strongly suggest similar structural dynamics of closed bRC and PSII RC complexes, and similar physical mechanisms, in which dielectric relaxation processes and structural memory effects of proteins are proposed to play important roles.