REGULAR PAPER Thermal phase and excitonic connectivity in fluorescence induction Agu Laisk • Vello Oja Received: 8 March 2013 / Accepted: 19 August 2013 Ó Springer Science+Business Media Dordrecht 2013 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 comple- mentary 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 excitoni- cally connected in domains of four PSII. This result, obtained by gradually reducing Q A in PSII with pre-blocked Q B (by DCMU or PQH 2 ), differs from that obtained by gradually blocking the Q B site (by increasing DCMU or PQH 2 level) in leaves during (quasi)steady-state e - transport (Oja and Laisk, Photosynth Res 114, 15–28, 2012). Explanations are dis- cussed. Donor side quenching was characterized by compar- ison 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. Keywords Leaf  Fluorescence induction  Photosystem II  Electron transport Abbreviations a II Excitation partitioning to PSII CA Complementary area Cyt Cytochrome DCMU (3-(3,4-Dichlorophenyl)-1,1-dimethylurea ETR Electron transport rate FI Fluorescence induction FRL Far-red light F 0 , F in , F m Fluorescence yields, minimum, initial, and final (maximum) F v Fluorescence, variable LED Light-emitting diode n e , n q Number of PSII excitations, total and quenched P680 PSII donor pigment complex PC Plastocyanin P D1 PSII donor pigment PFD, PAD Photon flux density, incident and absorbed PQ, PQH 2 Plastoquinone, oxidized and reduced PSII, PSI Photosystems, II and I Q A , Q B Primary and secondary quinone acceptors in PSII STF, SSTF Single turnover flash, saturating TyrZ (ox) Tyrosine Z (oxidized) WOC Water oxidizing complex Foreword My acquaintance with photosynthesis began with the book ‘‘Photosynthesis’’ by Rabinowitch and Govindjee (1969). A. Laisk (&)  V. Oja Institute of Molecular and Cell Biology, University of Tartu, Tartu, Estonia e-mail: alaisk@ut.ee 123 Photosynth Res DOI 10.1007/s11120-013-9915-1