Thermal phase and excitonic connectivity in fluorescence induction

Chl fluorescence induction (FI) was recorded in sunflower leaves pre-adapted to darkness or low preferentially PSI light, or inhibited by DCMU. For analysis the FI curves were plotted against the cumulative number of excitations quenched by PSII, nq, calculated as the cumulative complementary area above the FI curve. In the +DCMU leaves nq was <1 per PSII, suggesting pre-reduction of QA during the dark pre-exposure. A strongly sigmoidal FI curve was constructed by complementing (shifting) the recorded FI curves to nq = 1 excitation per PSII. The full FI curve in +DCMU leaves was well fitted by a model assuming PSII antennae are excitonically connected in domains of four PSII. This result, obtained by gradually reducing QA in PSII with pre-blocked QB (by DCMU or PQH2), differs from that obtained by gradually blocking the QB site (by increasing DCMU or PQH2 level) in leaves during (quasi)steady-state e− transport (Oja and Laisk, Photosynth Res 114, 15–28, 2012). Explanations are discussed. Donor side quenching was characterized by comparison of the total nq in one and the same dark-adapted leaf, which apparently increased with increasing PFD during FI. An explanation for the donor side quenching is proposed, based on electron transfer from excited P680* to oxidized tyrosine Z (TyrZox). At high PFDs the donor side quenching at the J inflection of FI is due mainly to photochemical quenching by TyrZox. This quenching remains active for subsequent photons while TyrZ remains oxidized, following charge transfer to QA. During further induction this quenching disappears as soon as PQ and QA become reduced, charge separation becomes impossible and TyrZ is reduced by the water oxidizing complex.