Redox transients of P680 associated with the incremental chlorophyll‐ a fluorescence yield rises elicited by a series of saturating flashes in diuron‐treated photosystem II core complex of Thermosynechococcus vulcanus

Recent chlorophyll-a fluorescence yield measurements, using single-turnover saturating flashes(STSFs), have revealed the involvement of a rate-limitingstep in the reactions following the charge separation induced by the first flash(Magyar et al. 2018). 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 flashexcitation 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 QA ; and thus, the fluorescence yield increments elicited by the consecutive flashesmust have a different physical origin. Our double- flashexperiments indicate that the rate-limiting steps, detectedby chlorophyll-a fluorescence, are not correlated with the turnover of P680. This article is protected by copyright. All rights reserved.