The effect of bicarbonate on photosynthetic oxygen evolution in flashing light in chloroplast fragments

The ability of bicarbonate ion (HCO₃^-) to stimulate photosynthetic oxygen evolution in maize chloroplast fragments exposed to continuous light depends on light intensity. Stimulation by HCO₃^- is less at low intensities. In HCO₃^--depleted chloroplasts exposed to brief saturating light flashes, period 4 oscillations (in O₂ yield per flash) are damped within three cycles. Readdition of HCO₃^- to these preparations restores the oscillatory pattern to higher flash numbers, indicating that HCO₃^- reduces the probability of “misses” in the photosystem II reaction center. The rate of the dark relaxation reaction S_n^′ → S_n+1 (where S refers to the oxidation state of the oxygen-evolving mechanism and n = 0, 1, or 2), after a photoact in the photosystem II reaction center, is retarded in HCO₃^--depleted chloroplasts compared to the rate for this reaction in depleted chloroplasts to which HCO₃^- has been resupplied. However, the final oxygen-evolving reaction after the accumulation of four positive charges appears to be independent of HCO₃^-. Bicarbonate has no effect on the dark deactivation of the higher oxidation states (S₂ and S₃) of the positive charge-accumulating system. We propose two alternate ways in which the kinetic model of oxygen evolution developed by Kok et al. [(1970) Photochem. Photobiol. 11, 457-475] can be extended to include the action of HCO₃^-.