UC Irvine

We present here a comparative study on the decay of chlorophyll (Chl) a fluorescence yield in thylakoid membranes and photosystem II (PSII)-enriched samples, measured with multifrequency cross-correlation phase fluorometry. These measurements confirm the general conclusions of Van Mieghem et al. (Biochim. Biophys. Acta 1100, 198-206, 1992), obtained with a flash method, on the effects of reduction of the primary quinone acceptor (QA) on Chl a fluorescence yield of PSII. Different states of the reaction centers of PSII were produced by: (1) pretreatment with sodium dithionite and methyl viologen followed by laser illumination: the doubly reduced QA (QAH2) centers; (2) with laser illumination or pretreatment with diuron: QA- centers; and (3) the addition of micromolar concentration of dichlorobenzoquinone (DCBQ): oxidized QA centers. The data were analyzed with Lorentzian distribution as well as with multiexponential fluorescence decay functions. The analysis with Lorentzian 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 (pea thylakoid membranes) and from 0.24 ns to 1.31 ns (core PSII). However, when QAH2 was formed, the lifetime distribution peaks shifted back to shorter lifetimes (0.57-0.77 ns) both in thylakoids and PSII membranes. Multiexponential analysis showed three lifetime components: fast (40-400 ps), middle (300-1500 ps) and slow (5-25 ns). When QA- was formed in PSII centers, the amplitude of the fast component decreased, but both the amplitude and the lifetime of the middle component increased severalfold.(ABSTRACT TRUNCATED AT 250 WORDS)