The control of the reduction kinetics in the dark of photooxidized chlorophyll alpha11+ by the inner thylakoid proton concentration

Renger, G.; Gläser, M.; Buchwald, H.E.

Biochimica et Biophysica Acta 461(3): 392-402


ISSN/ISBN: 0006-3002
PMID: 71160
DOI: 10.1016/0005-2728(77)90228-6
Accession: 068519342

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The 690 nm absorption change reflecting the turnover of the reaction center chlorophyll of System II, chlorophyll aII, was investigated as a function of pH of the chloroplast suspension. In the range 6.0 .ltoreq. pH .ltoreq. 8.0 the amplitude of the measured chlorophyll aII absorption change, .DELTA.A0 (chlorophyll-aII), markedly declines with increasing pH, whereas the average O2 yield per flash remains nearly unaffected. The dependency on actinic flash intensity of the amplitudes .DELTA.A0 (chlorophyll-aII) at pH 6.0 closely resembles that for the amplitudes of the chlorophyll aI absorption change. At pH 7.5 a higher flash intensity is required for half saturation of .DELTA.A0 (chlorophyll-aII), whereas the corresponding curve for .DELTA.A0 (chlorophyll-aI) is practically invariant to pH. Addition of ionophores at concentrations sufficiently high for a complete collapse of proton gradients across the thylakoid membrane shifts the pH-dependence of .DELTA.A0 (chlorophyll-aII) by more than 2 units towards the acidic range. On the basis of a hypothesis presented very recently, it is assumed that the reduction in the dark of photooxidized chlorophyll aII+ is regulated by the proton concentration in the inner thylakoid space. At lower pH the reaction proceeds predominantly via the well known 35 .mu.s kinetics, whereas at higher pH a very rapid kinetics (.ltoreq. 1 .mu.s), too fast to be resolvable by our measuring device, additionally contributes to a significant degree to the overall reduction of chlorophyll aII+. On the basis of Lavorels model the close interrelationship between .DELTA.A0 (chlorophyll-aII) and delayed fluorescence intensity in the .mu.s range is interpretable mainly by kinetic rather than by energetical effects.