Light response curves of photosystem (PS) II electron transport from oxygen evolving complex to plastoquinone (PQ) were measured in sunflower (Helianthus annuus L.), cotton (Gossypium hirsutum L.) and tobacco (Nicotiana tabacum L.) leaves by recording O(2) evolution and fluorescence in 5-200 ms light pulses of 500-13500 micromol absorbed quanta m(-2) s(-1). The leaves were pre-adapted at 60-2000 micromol quanta m(-2) s(-1) for 60 min to obtain different nonphotochemical excitation quenching, which was predominantly of reversible q(I) type (relaxation time 30 min). PQ was completely oxidized by turning the actinic light off and illuminating with far-red light for 2 s before the pulse was applied in the dark, 4 s after the actinic light was turned off. Electron transport rate calculated from fluorescence transients considering PS II donor side resistance (V. Oja, A. Laisk, submitted) was maximal at the beginning of pulses (J(Fi)) and decreased immediately. The dependences of J(Fi) on pulse absorbed flux density were rectangular hyperbolas with K(m) about 7500 micromol m(-2) s(-1). Both the extrapolated plateau J(Fm) and initial slope (intrinsic quantum yield of PS II, Y(m)) decreased proportionally when q(I) increased from minimum to maximum (J(Fm) from 2860 to 1450 micromol 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 model of PS II electron transport for 2 micromol PS II m(-2), increased from 607 to 1315 microseconds with the activation of q(I) while the donor side time constant changed from 289 to 329 microseconds. These results show that changes in the electron transfer processes on the acceptor side of PS II occur in parallel with nonphotochemical (predominantly reversible q(I) type) excitation quenching.