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Divalent cations are known to decrease the "spillover" of excitation energy from photosystem II (PS II) to PS I. At low light intensities, this does lead to an increased quantum yield of photo­reactions in PS II. Contrary to some suggestions that PS II is activated at high light intensities, or in strong flashes of light by the addition of Mg 2+ , we suggest here that Mg 2+  ions may not activate reaction center II, P 680. After brief light flashes, we do not find any significant enhancement in (a) O 2  yield/flash, (b) proton yield/flash, (c) the amplitude of charge separation in PS II, as monitored by 515 nm absorbance change, and (d) the amplitude of the reduced primary acceptor, as monitored by 320 nm absorbance change. Data on absorbance change at 820 nm suggest that the absence of Mg 2+  does not lead to inactivation of electron donation to P 680 + .

zeitschrift für naturforschung c

Cation Effects on System II Reactions in Thylakoids: Measurements on Oxygen Evolution, the Electrochromic Change at 515 Nanometers, the Primary Acceptor and the Primary Donor