Bicarbonte stimulation, of oxygen evolution, ferricyanide reduction and photoinactivation using isolated chloroplasts

Dependence of Hill reaction (ferricyanide reduction) by isolated (broken) chloroplasts on bicarbonate ion increases with time of illumination (upto 4 min) in HCC>3~ free reaction mixture. The stimulation caused by HCOs" is independent of light intensity down to very low intensities indicating an involvement of this ion in early photochemical events of photosystem II. Oxygen evolution was found to be more dependent than ferricyanide reduction on HCOs". The existence of an endogenous non-oxygen evolving electron donor in chloroplasts is thus suggested. HCOa" is also shown to greatly increase the rate of photoinactivation during Hill reaction.