RECOVERY OF TRANSMEMBRANE POTENTIALS IN PLANTS RESISTANT TO ARYLOXYPHENOXYPROPANOATE HERBICIDES - A PHENOMENON AWAITING EXPLANATION

Aryloxyphenoxypropanoate (APP) herbicides, such as diclofop, depolariz e membranes in parenchyma cells of coleoptiles and root tips, and isol ated tonoplast or plasma membrane vesicles from a variety of plant spe cies. Some APP-resistant biotypes of rigid ryegrass and wild oat repol arize membranes after removal of herbicide from a bathing medium. The repolarization ability does not require presence of either APP-insensi tive acetyl coenzyme A carboxylase or an increased capacity for herbic ide detoxification. The kinetics of depolarization and repolarization depend upon the herbicide, the herbicide concentration, the biotype, a nd 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 muM diclofop acid. Both the nonherbicidal S(-) and the herbicidal R(+) enantiomers of diclofop acid depolarized membranes in susceptible and resistant r yegrass. Susceptible biotypes regenerated transmembrane potentials fol lowing removal of the S(-) but not the R(+) enantiomer, whereas resist ant 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 de polarize and repolarize. It is postulated that the intracellular conce ntration of diclofop acid in susceptible and resistant plants is not t he same due to differences in the partitioning of diclofop acid betwee n the extracellular spaces and the cytoplasm. The mechanism producing the postulated difference is unknown, but observations on the proton e xtrusion capacity of both ryegrass and wild oats, the responses of rye grass to [K+] and PCMBS, and the single-gene inheritance pattern of re sistance in wild oats indicate that changes in the diclofop sensitivit y of a plasma membrane protein involved in the generation of proton or ion gradients may be involved.