A comparison of the rates and paths of metabolism of 2, 4-dichlorophenoxyacetic acid in resistant and susceptible plants Public Deposited

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  • Studies on the absorption of C¹⁴ carboxyl labeled 2, 4-dichlorophenoxyacetic acid by a susceptible plant (bean) and and resistant plants (corn and bluegrass) indicated that the chemical was readily absorbed by all species of plants from a foliar application. Approximately 22% and 42% of the applied chemical was absorbed by bean and corn plants respectively in three days following application of a 250 ppm solution of herbicide. There were no significant differences in rates of absorption of 2, 4-D in extended exposure. Approximately 60% and 65% of 2, 4-D solution were absorbed by either bean or corn plants in seven and 11 days respectively. In metabolic studies, corn plants were found to conjugate the C¹⁴ carboxyl labeled 2, 4-D much more rapidly than the susceptible bean plants. Analysis of the plant extract by paper chromatography revealed that corn had metabolized all of the absorbed chemicals in three days following application, while the beans had metabolized approximately 60% of the absorbed 2, 4-D. At the end of 11 days, the bean plants had metabolized approximately 88% of the herbicide to a conjugate. Enzymatic hydrolysis followed by paper and thin-layer chromatography of the plant extracts indicated that the metabolites of 2, 4-D were present in the plants as glucose adducts. Two metabolites beside the parent compound 2, 4-D itself were observed in both types of plants following hydrolysis by either acid, base, or enzyme. The metabolites were identified as 2, 5- and 2, 3-dichloro-4-hydroxyphenoxyacetic acid by means of R[subscript f] values in thin-layer chromatography and retention times in gas chromatography. Hence, the pathways of 2, 4-D metabolism in bean, corn, and bluegrass plants would seem to be through hydroxylation of the ring of phenoxy herbicide accompanied by a chlorine shift. Determination of the areas under the chromatographic peaks of these two metabolites indicated that they were present in a ratio of approximately 3:1, 6:1, and 10:1 respectively in 3, 7, and 11 days following treatment. The evolution of C¹⁴0₂ by plants revealed that this was a minor pathway of detoxification and there were no significant differences in rates of decarboxylation between susceptible bean and resistant corn plants. Furthermore, no significant amounts of unextracted radioactivity were found to be accumulated in plants. The foregoing evidence indicates the routes of metabolism of plants are through decarboxylation, conjugation, and hydroxylation accompanied by a chlorine shift. The ability of plants to conjugate 2, 4-D and its metabolites as glucose adducts becomes the major point of difference between the susceptible and resistant plants.
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