Kinetic analysis of P-700 photoconversion: Effect of secondary electron donation and plastocyanin inhibition

Abstract

The kinetics of P-700 photoconversion under weak continuous actinic illumination were quantitatively analyzed to provide information on the relative absorption cross-section σ PSI of the light-harvesting pigments associated with photosystem I and on the number of electrons stored between the two photosystems in dark-adapted chloroplasts. The theory of chemical kinetics for a system of monomolecular consecutive first-order reactions is reviewed briefly to provide support for the experimental approach taken. A complete inhibition of plastocyanin by cyanide eliminated all secondary electron donation to P-700 + and allowed the registration of the exponential (monomolecular) P-700 photoconversion at room temperature. The rate constant K p-700 of the exponential kinetics was independent of the ionic (± Mg 2+) and osmotic (± sucrose) strength of the chloroplast suspension medium, and of the oxidation-reduction state of photosystem II. The extent of plastocyanin inhibition in partially inhibited samples was greater under low ionic and low osmotic conditions. In dark-adapted chloroplast samples that were not cyanide treated, the number of electrons stored between the two photosystems was 3.9 ± 0.2 and independent of divalent cations. It is concluded that plastocyanin inhibition by cyanide is favored under low ionic and low osmotic conditions. The Mg 2+ ion and redox state of photosystem II-independent photoconversion of P-700 does not support significant changes in the spillover of excitation from photosystem II to photosystem I in isolated chloroplasts.