The Effect of Bicarbonate on Photosynthetic Oxygen Evolution in Flashing Light in Chloroplast Fragments

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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 Sn′ → Sn+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-.

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