Antagonistic effects of mono‐ and divalent cations on polarization of chlorophyll fluorescence in thylakoids and changes in excitation energy transfer

Monoand divalent cations affect several primary photoprocesses in thylakoids: initial energy distribution between the two pigment systems [1 -7 ] , excitation energy redistribution or 'spillover' from pigment system (PS) II to PSI [2,4,6-15], rate constant of thermal dissipative transitions [6,7,16,17], and activation of reaction center II [18-20] . Several of these effects may occur concurrently [2,6,7,17] with the largest effect on the excitation energy redistribution process [2,7]. It is generally accepted that excitation energy transfer in the photosynthetic system is by F6rster's inductive resonance mechanism [21,22] in which the pair-wise transfer rate is dependent upon the distance (oc r -6, where r is the distance), the orientation factor, K2(K 2 <_ 4) between the donor and acceptor molecules, and the overlap of the donor fluorescence with the acceptor absorption spectrum [23,24]. Since the degree of polarization of fluorescence is an indicator of the extent of excitation energy migration and/or of the orientation of the molecules [22,25], we have measured, at room temperature, the effects of cations on chlorophyll a (Chl a) fluorescence polarization: (i) At wavelengths selected to monitor preferentially PSII or PSI emission; 0i) At 760 nm (to avoid artifacts due to scattering of