| Abstract: | Decay-associated fluorescence spectra of the green alga Scenedesmus obliquus have been measured by single-photon timing with picosecond resolution in various states of light adaptation. The data have been analyzed by applying a global data analysis procedure. The amplitudes of the decay-associated spectra allow a determination of the relative antenna sizes of the photosystems. We arrive at the following conclusions: (a) The fluorescence kinetics of algal cells with open PS II centers (F0 level) have to be described by a sum of three exponential components. These decay components are attributed to photosystem (PS) I (τ ≈ 85 ps, λmaxem ≈ 695-700 nm), open PS II α-centers (τ ≈ 300 ps, λmaxem = 685 nm), and open PS II β-centers (τ ≈ 600 ps, λmaxem = 685 nm). A fourth component of very low amplitude (τ ≈ 2.2-2.3 ns, λmaxem = 685 nm) derives from dead chlorophyll. (b) At the Fmax level of fluorescence there are also three decay components. They originate from PS I with properties identical to those at the F0 level, from closed PS II α-centers (τ ≈ 2.2 ns, λmaxem = 685 nm) and from closed PS β-centers (τ ≈ 1.2 ns, λmaxem = 685 nm). (c) The major effect of light-induced state transitions on the fluorescence kinetics involves a change in the relative antenna size of α- and β-units brought about by the reversible migration of light-harvesting complexes between α-centers and β-centers. (d) A transition to state II does not measurably increase the direct absorption cross-section (antenna size) of PS I. Our data can be rationalized in terms of a model of the antenna organization that relates the effects of state transitions and light-harvesting complex phosphorylation with the concepts of PS II α,β-heterogeneity. We discuss why our results are in disagreement with those of a recent lifetime study of Chlorella by M. Hodges and I. Moya (1986, Biochim. Biophys. Acta., 849:193-202). |
