Comparative uptake, translocation, and metabolism of root-applied isoxaben in ajuga (Ajuga reptans) and two ornamental Euonymus species

The uptake, translocation, and metabolism of root-applied [C-14]isoxaben were compared in sensitive ajuga (Ajuga reptans L., cv. Alba), dwarf burning bush [Euonymus alatus (Thunb) Sieb., cv. Compacta], and tolerant wintercreeper [Euonymus fortunei (Turcz.) Hand.-Mazz., cv. Colorata). Divisions of ajuga and rooted cuttings of the two Euonymus species were treated with radiolabeled isoxaben in nutrient solution for 1, 3, 7, or 14 days. Uptake of radioactivity increased with time in all three species. Dwarf burning bush absorbed more radioactivity at 1 and 3 days after treatment (DAT) than ajuga and wintercreeper. At 14 DAT ajuga had absorbed more radioactivity than the two Euonymus species, translocation of absorbed radioactivity from roots to shoots at 1 and 3 DAT was greatest in ajuga, intermediate in wintercreeper, and least in dwarf burning bush. Thin-layer chromatography analysis of root extracts showed a slower rate of isoxaben metabolism in wintercreeper and dwarf burning bush than in ajuga. Ajuga roots metabolized isoxaben to unknown polar metabolites at all exposure times. Polar metabolites of isoxaben were also formed in shoot extracts of the three plant species at all exposure times. Following a 3-h incubation, isoxaben at 1 mM did not affect the incorporation of radiolabeled glucose into acid-insoluble materials of root tips excised from dwarf burning bush or wintercreeper, but reduced slightly the incorporation of glucose into ajuga roots. The greater absorption of root-applied isoxaben by ajuga and dwarf burning bush may explain their sensitivity to this herbicide. The faster translocation of root-applied isoxaben, to ajuga shoots and leaves may also contribute to the sensitivity of ajuga. Metabolic detoxification does not appear to play a role in the observed differential response of ajuga and the two Euonymus species to isoxaben. (C)1998 Academic Press.