Membrane Response to Diclofop Acid Is pH Dependent and Is Regulated by the Protonated Form of the Herbicide in Roots of Pea and Resistant and Susceptible Rigid Ryegrass

Abstract

Electrophysiological studies in roots of pea (Pisum sativum 1.) and rigid ryegrass (Lolium rigidum Caud.) seedlings were conducted to elucidate the mechanism involved in the membrane response to the herbicide diclofop. In pea, a dicotyledonous plant insensitive to diclofop, membrane depolarization at varying pH values and herbicide concentrations increased at higher concentrations of the protonated form of diclofop acid (pK, 3.57). In unbuffered nutrient solution (pH 5.71, diclofop acid (50 PM) depolarized the membrane potential (E,) in roots of both resistant and susceptible biotypes of rigid ryegrass, whereas recovery of E, occurred only in the resistant biotype following removal of the herbicide. This differential response was correlated with an increase (450%) in the rate of acidification of the externa1 solution by the susceptible biotype, and the E , differences between biotypes were eliminated in solutions buffered at pH 5.0 or 6.0. In addition, p-chloromercuribenzene-sulfonic acid did not prevent the depolarization of E , by 50 PM diclofop acid. It is concluded that the differential membrane response to diclofop acid in herbicide-resistant and -susceptible biotypes of rigid ryegrass is due to pH differences at the cell wall/plasmalemma interface. Although the membrane response is probably not involved in the primary inhibitory effect of diclofop on plant growth, it could reduce the concentration of the permeant protonated form of the herbicide and possibly could contribute to increased tolerance to diclofop and other weak acid herbicides.

Effect of herbicide diclofop‐methyl on proton extrusion from Lolium multiflorum seedlings differing in resistance and in fungal endophyte (Neotyphodium sp.) infection

“…The susceptible populations tested in this work, growing in the mineral solution (control treatment), were able to acidify the external medium more than the resistant ones. This finding agrees with that found by Ha¨usler et al (1991) and Di Tomaso (1993) in which L. rigidum DM susceptible roots have a greater capability of liberating H 1 than the resistant ones in the absence of DM. This has introduced the idea that resistant Lolium populations through a reduced capability of acidification may inhibit transport of weak acids (diclofop) across the plasma membrane due to a relative higher apoplastic pH (Di Tomaso 1994).…”

Enantioselective Degradation and Ecotoxicity of the Chiral Herbicide Diclofop in Three Freshwater Alga Cultures

“…Most studies on the environmental behavior of diclofop-methyl and diclofop considered their enantiomers as a single compound (15,(18)(19)(20). The chemical structures of the two chiral herbicides are shown in Figure 1.…”

Differential enantioselectivity of quizalofop ethyl and its acidic metabolite: Direct enantiomeric separation and assessment of multiple toxicological endpoints

“…In most cases, contaminant-modified membrane dysfunction such as varied membrane permeability is believed to be a first step, triggering membrane depolarization and inducing other toxicological actions including growth inhibition [27,28]. Initial changes of membrane permeability and subsequent depolarization, for example, occur both with diclofop methyl (DM) and diclofop acid (DA), analogues of QE and QA, respectively, and appear to be correlated with the phytotoxicity [15,40,41].…”