The control of the reduction kinetics in the dark of photooxidized chlorophyll aII+ by the inner thylakoid proton concentration

The 690 nm absorption change reflecting the turnover of the reaction center chlorophyll of System II, chlorophyll a II , (see Doring, G., Renger, G., Vater, J. and Witt, H. T. (1969) Z. Naturforsch. 31c, 712–721 and Glaser, M., Wolff, Ch., Buchwald, H. E. and Witt, H. T. (1974) FEBS. Lett. 42, 81–85) has been investigated as a function of pH of the chloroplast suspension. It was found: 1. (1) In the range 6.0 ⩽ pH ⩽ 8.0 the amplitude of the measured chlorophyll a II absorption change, ΔA 0 (chlorophyll- a II ), markedly declines with increasing pH, whereas the average oxygen yield per flash remains nearly unaffected. 2. (2) The dependency on actinic flash intensity of the amplitudes ΔA 0 (chlorophyll- a II ) at pH 6.0 closely resembles that for the amplitudes of the chlorophyll a I absorption change. On the contrary, at pH 7.5 a higher flash intensity is required for half saturation of ΔA 0 (chlorophyll- a II ), whereas the corresponding curve for ΔA 0 (chlorophyll- a I ) is practically invariant to pH. 3. (3) Addition of ionophores at concentrations sufficiently high for a complete collaps of proton gradients across the thylakoid membrane shifts the pH-dependence of ΔA 0 (chlorophyll- a II ) by more than 2 units towards the acidic range. On the basis of a hypothesis presented very recently (Glaser, M., Wolff, Ch. and Renger, G. (1976) Z. Naturforsch. 31c, 712–721) it is assumed, that the reduction in the dark of photooxidized chlorophyll a II + is regulated by the proton concentration in the inner thylakoid space. At lower pH the reaction proceeds predominantly via the well known 35 μs kinetics, whereas at higher pH a very rapid kinetics (⩽ 1 μs ), too fast to be resolvable by our measuring device, additionally contributes to a significant degree to the overall reduction of chlorophyll a II + . Furthermore, on the basis of Lavorels model (Lavorel, J. (1975) in Bioenergetics of Photosysthesis (Govindjee, ed.), pp. 223–317, Academic Press, New York) the close interrelationship between ΔA 0 (chlorophyll- a II ) and delayed fluorescence intensity in the μs range is shown to be interpretable mainly by kinetical rather than by energetical effects.