Inhibition of the reoxidation of the secondary electron acceptor of photosystem II by bicarbonate depletion.

In bicarbonate-depleted chloroplasts, the chlorophyll a fluorescence decayed with a halftime of about 150 ms after the third flash, and appreciably faster after the first and second flash of a series of flashes given after a dark period. After the fourth to twentieth flashes, the decay was also slow. After addition of bicarbonate, the decay was fast after all the flashes of the sequence. This indicates that the bicarbonate depletion inhibits the reoxidation of the secondary acceptor R2- by the plastoquinone pool; R is the secondary electron acceptor of pigment system II, as it accepts electrons from the reduced form of the primary electron acceptor (Q-). This conclusion is consistent with the measurements of the DCMU (3-(3,4-dichlorophenyl)-),)-dimethylurea)- induced chlorophyll a fluorescence after a series of flashes in the presence and the absence of bicarbonate, if it is assumed that DCMU not only causes reduction of Q if added in the state QR-, but also if added in the state QT2-.