title: Thermal phase and excitonic connectivity in fluorescence induction.
abstract: 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, n q, calculated as the cumulative complementary area above the FI curve. In the +DCMU leaves n q was <1 per PSII, suggesting pre-reduction of Q A during the dark pre-exposure. A strongly sigmoidal FI curve was constructed by complementing (shifting) the recorded FI curves to n q = 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 Q A in PSII with pre-blocked Q B (by DCMU or PQH2), differs from that obtained by gradually blocking the Q B 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 n q 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 (TyrZ(ox)). At high PFDs the donor side quenching at the J inflection of FI is due mainly to photochemical quenching by TyrZ(ox). This quenching remains active for subsequent photons while TyrZ remains oxidized, following charge transfer to Q A. During further induction this quenching disappears as soon as PQ and Q A become reduced, charge separation becomes impossible and TyrZ is reduced by the water oxidizing complex.
authors.affiliation: Institute of Molecular and Cell Biology, University of Tartu, Tartu, Estonia, alaisk@ut.ee.
authors.forename: Agu
authors.initials: A
authors.lastname: Laisk
authors.forename: Vello
authors.initials: V
authors.lastname: Oja
mesh_terms: D000222:Adaptation, Physiological / Q000187:drug effects; D003624:Darkness; D004237:Diuron / Q000494:pharmacology; D004579:Electron Transport / Q000187:drug effects; D004583:Electrons; D005453:Fluorescence; D006368:Helianthus / Q000187:drug effects / Q000378:metabolism*; D017785:Photons; D045332:Photosystem II Protein Complex / Q000378:metabolism; D018515:Plant Leaves / Q000187:drug effects / Q000378:metabolism; D013696:Temperature
chemical_list: D045332:Photosystem II Protein Complex; D004237:Diuron