BICARBONATE EFFECTS ON CHLOROPHYLL a FLUORESCENCE TRANSIENTS IN TH E PRESENCE AND T H E ABSENCE OF DIURON

We investigated the effect of HCO 3 addition to C02-depleted thylakoids by means of fluorescence techniques. (1) In the presence of diuron (3-(3,4-dichlorophenyl)-l,l-dimethylurea), the net reduction of the primary quinone-type electron acceptor (Q) of Photosystem (PS) II is about 2-times faster in the absence of HCO 3 than in its presence, whether normal, heat-treated or NH 2OH-treated samples are used. This effect of HCO 3 is, therefore, not on the 02-evolving apparatus. It is, however, interpreted to be due to an influence of HCO 3 on the kinetics of the reduction of Q, perhaps combined with an effect on the back reaction of Q with P-680 +, the oxidized form of the PS II reaction center chlorophyll a. (2) Fluorescence experiments in the absence of diuron indicate that the absence of HCO 3 results in a complete block at the quinone level; the area over the fluorescence induction curve in the absence of HCO 3 was found to be 2.2-times higher in the absence than in the presence of diuron, pointing to a complete block of BH 2 oxidation in the absence of HCO 3 . (3) No change in the midpoint potential of Q is observed when HCO 3 is added to CO 2-depleted membranes. HCO 3 not only has a large (on/of f ) effect on the reoxidation of BH z, but also a smaller effect between P-680 and Q. We propose that HCO 3 binding to its specific site in the thylakoid membrane results in a conformational change, allowing normal electron transport between the two photosystems.