Development of tolerance to the diphenyl ether herbicide acifluorfen-methyl in excised cucumber (Cucumis sativus L.) cotyledons

Etiolated cucumber (Cucumis sativus L.) cotyledons and cotyledons greened for 24 hr in low light (75 μE m⁻² sec⁻¹; measured as PAR, i.e., photosynthetically active radiation between 400 and 700 nm) were susceptible to 1 μM acifluorfen-methyl (AFM), methyl-5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoate, following an 8-hr exposure to high light (600 μE m⁻² sec⁻¹; PAR). Cotyledons greened in low light for 48 hr prior to treatment with 1 μM AFM in high light were tolerant. Injury was detected by monitoring efflux of 3-O-methyl-[¹⁴C]glucose from treated cotyledons excised from 6- or 7-day-old dark-grown seedlings. Although development of tolerance was light dependent, tolerance was not related specifically to seedling age. Tolerance to AFM injury was overcome partially by rubbing the adaxial surface of the leaf (i.e., the side of the cotyledon exposed to herbicide) with a wetted finger and was eliminated completely following abrasion with carborundum. Cuticle abrasion resulted in an increase in [¹⁴C]AFM uptake. Scanning electron microscopy revealed that abrasion resulted in disruption of the leaf surface. These induced aberrations in surface structure facilitated enhanced absorption of AFM. Nonpolar hydrocarbon constituents of cuticular and epicuticular waxes of 24- and 48-hr-greened cotyledons were examined using gas chromatography. There were no differences in these cuticular components. Transmission electron micrographs indicated there were also no differences in cuticle thickness. The light-dependent development of tolerance to AFM activity was due in part to a decrease in herbicide absorption. The mechanism(s) responsible for inhibition of herbicide uptake and tolerance are unknown.