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

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Abstract
The ability of bicarbonate ion (HCO 3 - ) to stimulate photosynthetic oxygen evolution in maize chloroplast fragments exposed to continuous light depends on light intensity. Stimulation by HCO 3 - is less at low intensities. In HCO 3 - -depleted chloroplasts exposed to brief saturating light flashes, period 4 oscillations (in O 2 yield per flash) are damped within three cycles. Readdition of HCO 3 - to these preparations restores the oscillatory pattern to higher flash numbers, indicating that HCO 3 - 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 HCO 3 - -depleted chloroplasts compared to the rate for this reaction in depleted chloroplasts to which HCO 3 - has been resupplied. However, the final oxygen-evolving reaction after the accumulation of four positive charges appears to be independent of HCO 3 - . Bicarbonate has no effect on the dark deactivation of the higher oxidation states (S 2 and S 3 ) 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 HCO 3 - .