An Investigation of Aryloxyphenoxypropionate Antagonism of Auxin-Type Herbicide Action on Proton-Efflux

Interactions between MCPA and diclofop-methyl, clopyralid and quizalofop-ethyl, and 2,4-D with fluazifop-acid and -butyl were examined in detail in a proton-efflux assay using etiolated coleoptile tissue from Avena sativa L. The aryloxyphenoxypropionic acid/esters alone were inactive but significantly inhibited synthetic auxin-induced proton-efflux in mixtures at physiologically relevant concentrations. Computer-assisted dose-response analysis revealed that these graminicides reduced both the amplitude of synthetic auxin-induced response and the relative receptor affinity toward the auxin-type herbicides, without significantly altering the "switch-like" nature of the auxin control mechanism. These findings suggest that the aryloxyphenoxypropionic acid/esters interfere directly with the processes controlling proton-efflux in the Avena coleoptile and act as competitive inhibitors of synthetic auxin-receptor binding, which in light of their lack of auxin activity suggests an anti-auxin role. Moreover, enantiomeric studies imply that the auxin receptor shows some stereospecificity for the herbicidally active R(+)-enantiomer. These observations are discussed in relation to recent findings on auxin binding in sensitive tissues and it is suggested that the basis of antagonism may reside at the receptor level.