Abstract
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 CO 2-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 CO 2 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.
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