Effects of Diclofop and Diclofop-Methyl on the Membrane Potentials of Wheat and Oat Coleoptiles

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Abstract
Electrophysiological measurements were made on the mesophyll cells of wheat (triticum aestivum L. cv aldron) and oat (Avena sativa L. cv Garry) coleoptiles treated either with the herbicide diclofop-methyl (methyl 2-(4-2'',4''-dichlorophenoxy)phenoxy)propanoate), or it''s primary metabolite dichlofop, (2-(4-(2'',4''-dichlorphenoxy)phenoxy)-propanoic acid). Application of a 100 micromolar solution of diclofopmethyl to wheat coleoptiles had little or no effect on the membrane potential (EM), however in oat, EM slowl depolarized to the diffusion potential (ED). At pH 5.7, 100 micromolar diclofop rapidly abolished the electrogenic component of the membrane potential in both oat and wheat coleoptile with half-times of 5 to 10 minutes and 15 to 20 minutes, respectively. The concentrations giving half-maximal depolarizations in wheat were 20 to 30 micromolar compared to 10 to 20 micromolar in oat. The depolarizing response was not due to a general increase in membrane permeability as judged from the EM''s response to changes in K+, Na+, Cl-, and SO42-, before and after treatment with diclofop and from its response to KCN treatment. In both plants, diclofop increased the membrane permeability to protons, making the Em strongly dependent upon the external pH in the range of pH 5.5 to pH 8.5. The effects of diclofop can best be explained by its action as a specific proton ionophore that shuttles protons across the plasmalemma. The rapidity of the cell''s response to both diclofop-methyl (15-20 minutes) and diclofop (2-5 minutes) makes the ionophoric activity a likely candidate for the earliest herbicidal event exhibited by these compounds.