PHOTOCONVERSION KINETICS OF CHLOROPLAST PHOTOSYSTEM-I AND PHOTOSYSTEM-II - EFFECT OF MG-2+

The effect of divalent cations on the primary photoconversion kinetics of pea chloroplast photosystems (PS) I and II was investigated by absorbance difference spectrophotometry in the UV (.DELTA.A320) and red (.DELTA.A700) regions and by fluorescence at room temperature. Three main chlorophyll (Chl) a fluorescence emission components were identified. Addition of 5 mM MgCl2 to unstacked chloroplasts caused a 5- to 7-fold increase in .**GRAPHIC**. the variable fluorescence yield controlled by the .alpha.-centers. The fluorescence yield .**GRAPHIC**. controlled by the .beta.-centers and the nonvariable fluorescence yield F0 were only slightly changed by the treatment. The absolute number of .alpha.- and .beta.-centers remained unchanged and independent of divalent cations. The rate constants K.alpha., K.beta. and KP-700 determined from the photoconversion kinetics of Q.alpha., Q.beta. and P-700 were also unchanged by divalent cations, suggesting a constancy of the respective absorption cross-sections. Apparently, the Mg2+ effect on Chl a fluorescence is not due simply to unstacking. In the absence of divalent cations from the chloroplast suspending medium, the variable fluorescence yield is not complementary to the rate of PS II photochemistry. A spillover of excitation from PS II to PS I in the absence of Mg2+ cannot account for the 7-fold lowering of the variable fluorescence yield .**GRAPHIC**. at room temperature. A model of excitation transfer and fluorescence emission in the pigment bed of PS II.alpha. and PS II.beta. is discussed.