A. Laisk et V. Oja, Electron transport through photosystem II in leaves during light pulses: acceptor resistance increases with nonphotochemical excitation quenching, BBA-BIOENER, 1460(2-3), 2000, pp. 255-267
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.
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