THE LIGHT DEPENDENT QUENCHING OF CHLOROPLAST FLUORESCENCE BY COFACTORS OF CYCLIC ELECTRON FLOW IN PHOTOSYSTEM I

Abstract—When 3–(3',4'-dichlorophenyl)-1,1-dimethylurea poisoned, intact thylakoids of isolated chloroplasts are illuminated in salt free suspension media, N -methylphenazinium cations (MP⁺) are reversibly taken up. Simultaneously, the chlorophyll fluorescence is reversibly lowered. When inorganic salts in the reaction medium provide membrane permeant charge balancing ions, the extent of the MP⁺ association with the thylakoids is strongly increased, but the fluorescence lowering is hardly affected. lonophoretically active agents inhibit specifically the salt dependent increment of the MP⁺ interaction with the thylakoids, but have only insignificant effects on the fluorescence lowering provided the experimental conditions do not allow the formation of a proton gradient across the thylakoid membrane. On the basis of these results, and of data obtained from comparative studies with other cofactors of cyclic electron transport in PS I, it is suggested that the 'energy dependent' fluorescence lowering is linked to a binding of cationic cofactors to nucleophilic sites in or at the thylakoid membrane. Such sites appear to become exposed in the wake of a light dependent transport of the cofactor, or of protons, into the thylakoid.