| Summary: | The studies reported in the present thesis have been concerned with the biochemistry of the varietal susceptibility of winter wheat (Triticum aestivum L.) cultivars to the substituted phenylurea herbicide, metoxuron (N'-(3-chloro-4-methoxyphenyl)-N ,N-dimethylurea). The introduction includes an historical account of the substituted phenylureas and reviews of the relationships between structure and phytotoxicity and mechanism of action. In addition selective properties of phenylureas are discussed together with their metabolic fate not only in plants but also in microorganisms and mammalian systems. Present concepts of the photosynthetic process are also outlined with special emphasis on those aspects directly pertinent to the mode of action of the substituted phenylureas. Studies of Hill activity of isolated chloroplasts incubated with metoxuron showed a similar inhibition of DCPIP and potassium ferricyanide photoreductions in both resistant (Capelle Desprez, Cama) and susceptible (Maris Nimrod, Maris Huntsman) plants. Examination of the Hill activity of chloroplasts isolated from herbicide-treated plants showed a greater inhibition of DCPIP and ferricyanide photoreductions in susceptible compared with resistant varieties. Whereas a severe inhibition of Hill activity in susceptible varieties was followed by scorching and eventual death of the plant, tolerant cultivars recovered from an initial slight visible injury in parallel with a return of Hill activity to normal levels. A reduction in the proportion of ethanol soluble material of leaves was also observed 24 h following treatment of wheat plants with metoxuron, the susceptible varieties showing the greater decrease. The distinction between resistant and susceptible varieties was more apparent when plants that had contacted the herbicide for longer periods were investigated. These results were also reflected in a greater inhibition of CC fixation by metoxuron-treated leaves of susceptible plants compared to similarly treated resistant strains. Studies of absorption and translocation of (methoxy-C)-metoxuron in resistant and susceptible wheat varieties, indicated that a differential rate of herbicide uptake may represent a factor which contributes to the observed difference in varietal response. A subcellular organelle localisation study of absorbed (methoxy-C)-metoxuron indicated that on the basis of protein content the greatest amounts of radioactivity were associated with the chloroplast fraction. In addition, chloroplasts from susceptible varieties contained a quantity of metoxuron several fold greater than those of resistant plants. Metabolism studies made using (methoxy-C)-metoxuron indicated that a major degradative pathway in wheat involves a two-step N-dealkylation followed by hydrolysis of the ureido group to give the corresponding aniline derivative. The various metabolites were separated by thin-layer chromatography and their identity was revealed by a comparison of their Rf values with those of known synthesised standards. Time course studies indicated that metoxuron metabolites represented a greater proportion of the radioactivity of the total methanol extract from resistant plants than from susceptible types. The metabolic route outlined was shown to result in an effective detoxication of metoxuron in in vitro assays of Hill reaction activity where the N-monodes methylated derivative was found to be some 80% as inhibitory as metoxuron and the N-bisdesmethyl and aniline derivatives were ineffective as photosynthetic inhibitors. Experiments made using leaf discs contained in Warburg flasks in14 14the presence of a CO2 trap demonstrated a loss of CO from (methoxy-C)-metoxuron indicating that cleavage of the methoxy substituent of the aromatic ring represents a significant additional step in the metabolism of the herbicide. Cleavage of the methoxy group occurred at an appreciably greater rate in leaf discs of resistant compared to susceptible plants. Preliminary experiments would indicate that metabolism of metoxuron by wheat leaves is achieved by the microsomal fraction of the cells which contains an N-demethylase activity requiring molecular oxygen and reduced pyridine nucleotide as co-factors. This activity was most apparent in the microsomal fraction prepared from leaves of resistant varieties. The various experimental results are discussed in terms of their significance in accounting for the differences in response to metoxuron of the wheat varieties studied.
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