Studies on the mechanism of the selective action of chlortoluron in cereals and cereal weeds

Abstract

The studies reported in this thesis have been primarily concerned with the mechanism of the selective action of the substituted phenylurea herbicide chlortoluron (N 1-(3-chloro-4-methylphenyl)-N,N-dimethylurea) in cereals and monocotyledonous cereal weeds. The introduction reviews relevant aspects of photosynthetic electron transport, the use of the substituted phenylureas as herbicides and the mode of action of the substituted phenylureas with reference to structure/activity relationships. In addition, their selective properties are discussed particularly in relation to their metabolic fate in biological systems.Studies on the Hill activity of chloroplasts isolated from resistant and sensitive plants and incubated in vitro with chlortoluron or the related phenylurea isoproturon (NT-(3-chloro-4-isopropylphenyl)-N,N -dimethylurea) demonstrated a similar inhibition of DCPIP photoreduction in each case. Examination of the Hill activity of chloroplasts isolated from plantstreated with either chlortoluron or isoproturon, however, demonstrated a greater inhibition of DCPIP photoreduction in sensitive compared to resistant plants. Studies on the recovery of photosynthetic activity both in whole plants and in leaf segments following inhibition by prior treatment with chlortoluron or isoproturon demonstrated a greater ability to restore oxygen evolution in the case of resistant plants, suggesting a more rapid detoxification of the herbicides in these varieties. Metabolism studies made using (carbonyl-14C) chlortoluron indicated degradation by both N-demethylation and ring-methyl oxidation pathways, followed by conjugation. The ring-methyl oxidation pathway predominated in the resistant cereal varieties to a greater extent than in the sensitive cultivars. In contrast, the N-demethylation pathway constituted the major metabolic route in the case of the cereal weeds. Studies onthe inhibition of the Hill reaction by the metabolites of chlortoluron indicated that the initial product of N-demethylation retained significant phytotoxicity, whereas all products of ring-methyl oxidation were essentially non-phytotoxic. Further metabolism studies made using 14(carbonyl-C)diuron (N1-(3,4-dichlorophenyl)-N,N-dimethylurea) and 14(carbonyl-C) N1-(3-methyl-4-chlorophenyl)-N,N-dimethylurea emphasised the importance of both the presence and the positioning of the ring-methyl group in the selective action of the substituted phenylureas.Studies on the polar conjugated metabolites of chlortoluron revealed that the glycone moiety was B-D-glucose in each case. In vitro experiments indicated that chlortoluron metabolism was achieved by a microsomal-linked system requiring molecular oxygen and NADPH as cofactors. Experiments on the absorption of chlortoluron from both soil and nutrient media indicated a greater uptake of the herbicide by the cereal weeds compared to the cereal varieties examined. A sub-cellular organelle localisation study of absorbed chlortoluron and its metabolites demonstrated the association of significantly larger amounts of phytotoxic material with the chloroplasts of sensitive compared to resistant plants. In addition, post-mitochondrial fractions prepared from resistant plants contained correspondingly larger quantities of non-phytotoxic metabolites.The various experimental results are discussed in terms of their significance in accounting for the observed differences in the response of cereals and cereal weeds to chlortoluron.