Effects of Diclofop and Diclofop-Methyl on Membrane Potentials in Roots of Intact Oat, Maize, and Pea Seedlings

Abstract

Growth and electrophysiological studies in roots of intact diclofop-methyl susceptible and resistant seedlings were conducted to test the hypothesis that the herbicide acts primarily as a proton ionophore. The ester formulafton of diclofop, at 0.2 micromolar, completely inhibited root growth in herbicide-susceptible oat (Avena Lucas et al. (16) and later revised by Wright and Shimabukuro (25), the ester form of diclofop crosses the plasmalemma and is rapidly hydrolyzed to the free acid form. This undissociated acid then releases a proton on the cytoplasmic side of the membrane. The deprotonated anionic species then moves back across the plasmalemma to the apoplasmic side in response to the root-cell Em (inside negative) and is again protonated. This cycle is repeated until the proton gradient is dissipated and could eventually lead to a reduction in cellular ATP levels by short circuiting the proton-translocating plasmalemma ATPase. The diclofop concentration causing half-maximal depolarization of the Em in excised and peeled oat coleoptiles was determined to be 10 to 20 jtM (25). Ratterman and Balke (20)

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

“…From these results, they concluded that the membrane response was a significant initial reaction in the eventual phytotoxic action of the herbicide. However, we recently provided evidence (DiTomaso et al, 1991) that casts some doubt on the relevance of the membrane response observed at relatively high concentrations of diclofop acid. We discovered that 100 FM diclofop acid was capable of depolarizing the E, of susceptible com and oat roots, as well as moderately tolerant barley roots and highly insensitive pea roots.…”

Effects of Acetyl-Coenzyme A Carboxylase Inhibitors on Root Cell Transmembrane Electric Potentials in Graminicide-Tolerant and -Susceptible Corn (Zea mays L.)

“…The demonstration of the nonselective effect of the graminicides (particularly the aryloxyphenoxypropionic acids) on the E,,, of susceptible and tolerant species (Figs. 1-3, and 5; DiTomaso et al, 1991) suggests that the membrane effect reported for these herbicides does not play an important role in their herbicidal activity under field conditions.…”

“…Based on this hypothesis, a membrane response should not occur in highly resistant dicotyledonous species at comparable concentrations. However, DiTomaso et al (1991) reported that 100 PM diclofop caused the same degree of depolarization of E, in diclofop-susceptible corn and diclofop-resistant pea. Furthermore, they showed that a 96-h exposure to 0.8 HM diclofop completely inhibited corn root growth but did not depolarize root cortical cell E, .…”

“…Evidence that the mechanism of action of the aryloxyphenoxypropionic acids in corn and oat tissue is not related to their effects on E, was provided by DiTomaso et al (1991).…”

Diclofop-methyl: A phenoxy propionate herbicide with multiple toxic effects in mouse embyro fibroblast (NIH/3T3) cell line

“…Aryloxy phenoxy propionic acid herbicides are one of the major pesticide groups. Diclofop-methyl is a selective, post emergence aryloxy phenoxy propionic acid herbicide primarily used to control wild oat (Avena fatua L.) and other annual graminaceous weeds in wheat, barley, and soybeans [3,17]. Exposure to some of these pesticides may lead to modifications in the genetic material thereby causing mutagenicity, carcinogenicity, teratogenicity and immunotoxicity [16].…”