The interaction between bicarbonate and the herbicide ioxynil in the thylakoid membrane and the effects of amino acid modification on bicarbonate action

Bicarbonate depletion of chloroplast thylakoids reduces the affinity of the herbicide, ioxynil, to its binding site in Photosystem (PS) II. This herbicide is found to be a relatively more efficient inhibitor of the Hill reaction when HCO^-₃ is added to CO₂-depleted thylakoids in subsaturating rather than in saturating concentrations. The reason for this dependence of the inhibitor efficiency on the HCO^-₃ concentration is that the inactive HCO^-₃-deficient PS II reaction chains bind less ioxynil than the active PS II electron-transport chains that have bound HCO^-₃, and, thus, after addition of a certain amount of ioxynil the concentration of the free herbicide increases when the HCO^-₃ concentration decreases. Therefore, the inhibition of electron transport by ioxynil increases at decreasing HCO^-₃ levels. Measurements on the effects of modification of lysine and arginine residues on the rate of electron transport are also presented: the rate of modification is faster in the presence than in the absence of HCO^-₃. Therefore, we suggest that surface-exposed lysine or arginine residues are not involved in binding of HCO^-₃ (or CO₂ or CO^2-₃) to its binding protein, but that HCO^-₃ influences the conformation of its binding environment such that the affinity for certain herbicides and the accessibility for amino acid modifiers are changed.