Oxygen evolution and chlorophyll fluorescence from multiple turnover light pulses: charge recombination in photosystem II in sunflower leaves

Oxygen evolution and Chl fluorescence induction were measured during multiple turnover light pulses (MTP) of 630-nm wavelength, intensities from 250 to 8,000 μmol quanta m−2 s−1 and duration from 0.3 to 200 ms in sunflower leaves at 22 °C. The ambient O2 concentration was 10–30 ppm and MTP were applied after pre-illumination under far-red light (FRL), which oxidized plastoquinone (PQ) and randomized S-states because of the partial excitation of PSII. Electron (e−) flow was calculated as 4·O2 evolution. Illumination with MTP of increasing length resulted in increasing O2 evolution per pulse, which was differentiated against pulse length to find the time course of O2 evolution rate with sub-millisecond resolution. Comparison of the quantum yields, YIIO = e−/hν from O2 evolution and YIIF = (Fm − F)/Fm from Chl fluorescence, detected significant losses not accompanied by fluorescence emission. These quantum losses are discussed to be caused by charge recombination between QA− and oxidized TyrZ at a rate of about 1,000 s−1, either directly or via the donor side equilibrium complex QA → PD1+ ↔ TyrZox, or because of cycling facilitated by Cyt b559. Predicted from the suggested mechanism, charge recombination is enhanced by damage to the water-oxidizing complex and by restricted PSII acceptor side oxidation. The rate of PSII charge recombination/cycling is fast enough for being important in photoprotection.