COLE CROP (BRASSICA-OLERACEA) TOLERANCE TO CLOMAZONE

A laboratory bioassay was conducted to determine the differential tolerance of cole crops to clomazone as measured by extractable total chlorophyll and carotenoids. Clomazone concentrations causing 50% inhibition (I50) in the biosynthesis of total chlorophyll in broccoli, cauliflower, and green and red cabbage cotyledons were 16, 11, 3, and 11-mu-M, respectively, while I50 values for carotenoid levels were 20, 10, 4, and 8-mu-M clomazone, respectively. Therefore, broccoli was the most tolerant to clomazone based upon extractable chlorophyll and carotenoid concentrations. Further laboratory studies were performed to investigate the basis for differential clomazone tolerance in 3-wk-old cole crop seedlings. No differences in total root uptake of C-14-clomazone were observed between these crops after 24 h. There were no differences in rate of metabolism of C-14-clomazone to methanol-soluble metabolites in roots of these crops. Percentage of polar metabolites in roots remained fairly constant over time. There were also no differences between crops in percentage of methanol-soluble C-14-clomazone metabolites formed in shoots between 24 and 96 h. In all crops, levels of C-14-clomazone decreased in a similar manner over time in methanolic extracts of roots and shoots while nonextractable C-14 levels increased, indicating a conversion of clomazone to insoluble, nonextractable forms. Differential uptake, translocation, and metabolism do not appear to account for clomazone selectivity differences between cole crop seedlings. Nomenclature: Clomazone, 2-[(2-chlorphenyl)methyl]-4,4-dimethyl-3-isoxazolidione; cole crops, Brassica oleracea L. broccoli 'Green Comet', cauliflower 'Snow Crown', green cabbage 'Bravo', and red cabbage 'Solid Red hybrid 781'.