Absorption and translocation of atrazine and linuron by plants with implications concerning linuron selectivity

The absorption and translocation of atrazine and linuron by carrot, parsnip, lettuce, and turnip seedlings in culture solutions were measured. There were marked differences among species in the distribution of herbicide within the plant. A high proportion of the linuron absorbed by carrot and parsnip seedlings was retained in their root systems whereas in lettuce and turnip over 60 per cent of the herbicide absorbed was translocated to the shoot. With atrazine, differences were also apparent but were much less marked. The amounts of herbicide translocated were directly proportional to the amounts of water transpired by the plants but the effective concentrations in the transpiration stream were always less than those in the treatment solutions. Examination of the extracts from the different plant species by thin-layer chromatography showed that up to 45 per cent of the linuron translocated in parsnip and carrot was present as metabolites, but little metabolism had occurred in the shoots of lettuce and turnip or in the roots of any of the species. The data suggest that the tolerance of parsnip and carrot seedlings to linuron results from a combination of root fixation and metabolism in the shoot.