Regulation of the Hill reaction by cations and its abolishment by uncouplers

Concurrent measurements of P-700 turnover and the reduction of K₃Fe(CN)₆ revealed an identical relative quantum yield for both reactions in isolated pea chloroplasts as well as chloroplast particles from wild type Scenedesmus. On the other hand, chloroplast particles of wild type Scenedesmus showed the same relative quantum yield for the Hill reaction as those of the P-700-free mutant No. 8, indicating that P-700 is not required for ferricyanide reduction.Several metal ions, such as Mg²⁺, Ca²⁺, Na⁺ and K⁺ stimulated the reduction of K₃Fe(CN)₆. In short wavelength light, the stimulation was a function of light intensity, varying in magnitude from an approximate doubling of the yield in low intensities to only a slight increase at light saturation. P-700 was totally unaffected by the cations.The effect of the metal salts was abolished in the presence of uncouplers of photophosphorylation.The data reconcile several divergent results concerning the effect of divalent cations on the reduction of ferricyanide which have been reported in the recent literature. On the whole the experiments suggest that the Hill acceptor can be reduced at two sites. The stimulation of the Hill reaction by metal ions is proposed to be due to an activation of Photosystem II and a more efficient utilization of quanta at the expense of radiationless de-excitation.