Chlorophyll-a fluorescence induction on new grounds: quantum efficiency versus the light-adapted state of photosystem II.

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 (Fo) to the maximum (Fm) of ChlF of dark-adapted PSII reflects the closure of all functionally active reaction centers, and the Fv/Fm ratio is equated with the maximum photochemical quantum yield of PSII (Fv=Fm-Fo). 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 (PSIIC), produces F1<Fm; and uncovered rate-limiting steps - Δτ1/2 half-waiting times - in the multi-STSF induced F1-to-Fm increments, originating from the gradual formation of light-adapted charge-separated state (PSIIL) with significantly increased stability of charges compared to PSIIC 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 Δτ1/2.