Regulation of excitation energy distribution in Photosystem-II fragments by magnesium ions

Fluorescence characteristics of Photosystem-II subchloroplasts (TSF-II and TSF-IIa) fractionated by Triton X-100 treatment were studied in relation to cation-induced regulation of excitation-energy distribution within subchloroplast fragments. Absorption spectra and fluorescence-emission spectra at 77 K showed that TSF-II contains the light-harvesting chlorophyll-protein complex in addition to the reaction-center complex, which is present alone in TSF-IIa.Mg²⁺ increased the ratio of F_695nm to F_685nm in the fluorescence-emission spectrum of TSF-II particles at 77 K, but had no effect on TSF-IIa particles. Mg²⁺ also induced a quenching of chlorophyll fluorescence at room temperature in TSF-II, an effect that was insensitive to the presence of DCMU. The DCMU-insensitive fluorescence quenching was not observed in the TSF-IIa preparation. These results suggest an existence of cation-induced regulation of excitation-energy transfer in TSF-II preparations. Presence of antenna chlorophyll molecules alone does not seem to be sufficient for observing energytransfer regulation by cations in Photosystem-II preparations.