Whereas the structure and function of Photosystem II (PSII) and its reactioncenter (RC) is well known [1, 2], some important questions remain to beelucidated, e.g.: (i) does PSII RC possess structural dynamics associated withits function, and (ii) does the variable chlorophyll fluorescence reflect solelythe reduction of the first quinone acceptor (QA) [3]?Here we report on chlorophyll fluorescence transients elicited by single-turnoversaturating flashes (STSFs) in DCMU-treated samples between 170 and300 K. We found that a STSF raised the fluorescence yield from Fo to F1 andfurther flashes were required to reach the maximum level Fm. The fluorescenceincrement (FmF1)/(Fm?Fo) increased with decreasing temperatures. F1 relaxationdue to charge recombination involving QA? was observed only above 230 K;in contrast, the increment relaxed at all temperatures. In experiments usingdouble flashes with waiting time tw between them, in a train of STSFs, a strikingfinding was that the rise of the fluorescence yield from F1 to Fm depended on twrevealing a previously unknown, temperature-dependent rate limitation in PSII.We conclude that the reduction of QA is a necessary but not sufficientcondition to generate Fm and light-induced events, probably conformationalchanges [4], detected by fast-scan FTIR, occur in the F1-to-Fm rate-limited transition.We hypothesize that dielectric relaxation in the RC matrix exposed tothe strong local electric field due to charge separation explains the observedphenomena [5], in which STSF-induced ultrafast transient electric field effectsand/or dissipation-induced thermal transients may also play a role.