Abstract

— We prescnt here a comparative study on the decay of chlorophyll a fluorescence yield in thylakoid membranes and photosystem 11 ‐enriched samples, measured with multifrequency cross‐correlation phase fluorometry. These measurements confirm the general conclusions of Van Mieghem ef al., obtained with a flash method, on the effects of reduction of the primary quinone acceptor on ChI a fluorescence yield of PSI. Different states of the reaction centers of PSII were produced by: pretreatment with sodium dithionite and mcthyl viologen followed by laser illumination: the doubly reduced QA centers: with laser illumination or pretreatment with diuron: QA‐ centers; and the addition of micromolar concentration of dichlorobenzoquinonc : oxidized QA centers. The data were analyzed with Lorentzian distribution as well as with multiexponential fluorescence decay functions. The analysis with Lorcntzian distribution function showed that upon formation of QA. the major lifetime distribution peak shifted to longer lifetimes: from 0.25 ns to 1.66 ns and from 0.24 ns to 1.31 ns. However, when QAHZ was formed, the lifetime distribution peaks shifted back to shorter lifetimes both in thylakoids and PSIl membranes. Multiexponential analysis showed three lifetime components: fast, middle and slow. When QA was formed in PS11 centers, the amplitude of the fast Component decreased, but both thc amplitude and the lifetime of the middle component increased severalfold. However, when QA was doubly reduccd, the amplitude of the fast component increased and the amplitude of the middle component decreased: in addition, the lifetime of the slow component increased. All of the above results are consistent with the conclusions that PS11 charge separation is decreased when QA is formed and increased when doubly reduced QA is formed. Copyright © 1993, Wiley Blackwell. All rights reserved.