Electron transport through photosystem II in leaves during light pulses: acceptor resistance increases with nonphotochemical excitation quenching

Light response curves of photosystem (PS) II electron transport from oxygen evolving complex to plastoquinone (PQ) were measured in sunflower (Heliant hus annuus L.), cotton (Gossypium hirsutum L.) and tobacco (Nicotiana tabac um L.) leaves by recording O-2 evolution and fluorescence in 5-200 ms light pulses of 500-13 500 mu mol absorbed quanta m(-2) s(-1). The leaves were p re-adapted at 60-2000 mu mol quanta m(-2) s(-1) for 60 min to obtain differ ent nonphotochemical excitation quenching, which was predominantly of rever sible q(I) type (relaxation time 30 min). PQ was completely oxidized by tur ning the actinic light off and illuminating with far-red light for 2 s befo re the pulse was applied in the dark, 4 s after the actinic light was turne d off. Electron transport rate calculated from fluorescence transients cons idering PS II donor side resistance (V. Oja, A. Laisk, submitted) was maxim al at the beginning of pulses (J(Fi)) and decreased immediately. The depend ences of J(Fi) On pulse absorbed flux density were rectangular hyperbolas w ith K-m about 7500 mu mol m(-2) s(-1). Both the extrapolated plateau J(Fm) and initial slope (intrinsic quantum yield of PS II, Y-m) decreased proport ionally when q(I) increased from minimum to maximum (J(Fm) from 2860 to 145 0 mu mol e(-) m(-2) s(-1) and Y-m from 0.41 to 0.23). The time constant for electron transfer away from the PS II acceptor side, calculated from a mod el of PS II electron transport for 2 mu mol PS II m(-2), increased from 607 to 1315 mus with the activation of q(I) while the donor side time constant changed from 289 to 329 mus. These results show that changes in the electr on transfer processes on the acceptor side of PS II occur in parallel with nonphotochemical (predominantly reversible q(I) type) excitation quenching. (C) 2000 Elsevier Science B.V. All rights reserved.