Bicarbonate effects on chlorophyll a fluorescence transients in the presence and the absence of diuron

We investigated the effect of HCO^-₃ addition to CO₂-depleted thylakoids by means of fluorescence techniques. (1) In the presence of diuron (3-(3,4-dichlorophenyl)-1,1-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 NH₂OH-treated samples are used. This effect of HCO^-₃ is, therefore, not on the O₂-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 BH₂ oxidation in the absence of HCO^-₃. (3) No change in the midpoint potential of Q is observed when HCO^-₃ is added to CO₂-depleted membranes. HCO^-₃ not only has a large (on/off) effect on the reoxidation of BH₂, 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.