Light-induced slow changes in chlorophyll a fluorescence in isolated chloroplasts: effects of magnesium and phenazine methosulfate.

We have investigated the possible relationships between the cation-induced and phenazine methosulfate (PMS)-induced fluorescence changes and their relation to light induced conformational changes of the thylakoid membrane. 1. In isolated chloroplasts, PMS markedly lowers the quantum yield of chlorophyll a fluorescence (fir) when added either in the presence or the absence of dichlorophenyldimethylurea (DCMU). In contrast, Mg 2+ causes an increase in ~f. However, these effects are absent in isolated chloroplasts fixed with glutaraldehyde that retain (to a large extent) the ability to pump protons, suggesting that structural alteration of the membrane--not the pH changes--is required for the observed changes in fir. The PMS triggered decrease in fff is not accompanied by any changes in the emission (spectral) characteristics of the two pigment systems, whereas room temperature emission spectra with Mg 2+ and Ca 2+ show that there is a relative increase of System II to System I fluorescence. 2. Washing isolated chloroplasts with 0.75 mM EDTA eliminates (to a large extent) the PMS-induced quenching and Mg2+-induced increase of fie, and these effects are not recovered by the further addition of dicyclohexyl carbodiimide. It is known that washing with EDTA removes the coupling factor, and thus, it seems that the coupling factor is (indirectly) involved in conformational change of thylakoid membranes leading to fluorescence yield changes. 3. In purified pigment System II particles, neither PMS nor Mg 2+ causes any change in fir. Our data, taken together with those of the others, suggest that a structural modification of the thylakoid membranes (not macroscopic volume changes of the chloroplasts) containing both Photosystems I and II is necessary for the PMS-induced quenching and Mg 2 +-induced increase of fie. These two effects can be explained with the assumption that the PMS effect is due to an increase in the rate of internal conversion (kh), whereas the Mg 2+ effect is due to a decrease in the rate of energy transfer (kt) between the two photosystems. 4. From the relative ratio of ~r with DCMU and DCMU plus Mg ~+, we have calculated k t (the rate constant of energy transfer between Photosystems II and I to be 4.210 s s -~, and fit (quantum-yield of this transfer) to be 0.12.