A downy brome population in a winter wheat field at Córdoba, Spain, survived use rates of chlortoluron (2.5 to 3.5 kg ai ha⁻¹) over 2 consecutive yr, where wheat monoculture and multiple annual chlortoluron applications had been carried out. The resistant (CR) biotype showed a higher ED₅₀ value (7.4 kg ai ha⁻¹; the concentration required for 50% reduction of fresh weight) than the susceptible (S) control (2.2 kg ai ha⁻¹), with a 3.4-fold increase in chlortoluron tolerance. Chlortoluron resistance in the CR downy brome biotype was not caused by altered absorption, translocation, or modification of the herbicide target site but by enhanced detoxification. The inhibition of both the recovery of photosynthetic electron transport and chlortoluron metabolism in the CR biotype due to the presence of the Cyt P450 inhibitor 1-aminobenzotriazole (ABT) indicates that herbicide metabolism catalyzed by Cyt P450 monooxygenases is related to chlortoluron resistance in CR plants. Although both biotypes degraded chlortoluron by N-dealkylation and ring-methyl hydroxylation and seem to share the same ability to form polar conjugates, degradation in the resistant biotype is more efficacious as this biotype metabolizes the parent herbicide faster and to a greater extent than its susceptible counterpart. The ability of the susceptible biotype to ring-hydroxylate chlortoluron, albeit at much slower rate, probably explains its moderate tolerance to chlortoluron observed in the growth assays and its minor photosynthetic electron transport recovery observed in fluorescence measurements.

Nomenclature: Chlortoluron; downy brome, Bromus tectorum L. BROTE.