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 μM, respectively, while I50values for carotenoid levels were 20, 10, 4, and 8 μM 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 of14C-clomazone were observed between these crops after 24 h. There were no differences in rate of metabolism of14C-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-soluble14C-clomazone metabolites formed in shoots between 24 and 96 h. In all crops, levels of14C-clomazone decreased in a similar manner over time in methanolic extracts of roots and shoots while nonextractable14C 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.