Recovery of Transmembrane Potentials in Plants Resistant to Aryloxyphenoxypropanoate Herbicides: A Phenomenon Awaiting Explanation

Aryloxyphenoxypropanoate (APP) herbicides, such as diclofop, depolarize membranes in parenchyma cells of coleoptiles and root tips, and isolated tonoplast or plasma membrane vesicles from a variety of plant species. Some APP-resistant biotypes of rigid ryegrass and wild oat repolarize membranes after removal of herbicide from a bathing medium. The repolarization ability does not require presence of either APP-insensitive acetyl coenzyme A carboxylase or an increased capacity for herbicide detoxification. The kinetics of depolarization and repolarization depend upon the herbicide, the herbicide concentration, the biotype, and the pH of the bathing solution. For rigid ryegrass, depolarization in the presence of diclofop acid is more rapid than in the presence of diclofop-methyl, and 50% depolarization required about 4 μM diclofop acid. Both the nonherbicidal S(–) and the herbicidal R(+) enantiomers of diclofop acid depolarized membranes in susceptible and resistant ryegrass. Susceptible biotypes regenerated transmembrane potentials following removal of the S(–) but not the R(+) enantiomer, whereas resistant biotypes repolarized following exposure to either enantiomer or a mixture of the two. The herbicide 2,4-D affected, in a complex manner, the ability of both susceptible and resistant ryegrass biotypes to depolarize and repolarize. It is postulated that the intracellular concentration of diclofop acid in susceptible and resistant plants is not the same due to differences in the partitioning of diclofop acid between the extracellular spaces and the cytoplasm. The mechanism producing the postulated difference is unknown, but observations on the proton extrusion capacity of both ryegrass and wild oats, the responses of ryegrass to [K+] and PCMBS, and the single-gene inheritance pattern of resistance in wild oats indicate that changes in the diclofop sensitivity of a plasma membrane protein involved in the generation of proton or ion gradients may be involved.