- 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.