STUDIES IN PLANT METABOLISM. III. ABSORPTION, TRANSLOCATION AND METABOLISM OF RADIOACTIVE 2,4-D IN CORN

The use of 2,4-dichlorophenoxyacetic acid (2,4-D) as a selective herbicide has become quite extensive. In general, dicotyledonous plants are more sensitive to 2,4-D treatment than are most plants of the monocotyledonous types (7), although very little is known regarding the factors responsible for the difference in sensitivity. Recently, by using an isotopically labeled herbicide, Wood et al. (10) found that bean plants absorbed and translocated 2-iodo131-3-nitrobenzoic acid (INBA) more readily than barley plants. Later, Mitchell et al. (8) extended their work to oat and corn plants. They suggested that the growth-inhibiting effect of INBA in the bean plant and its failure to produce significant inhibition in barley, oat and corn plants might be due to the differences in the manner in which INBA reacts with the plant constituents in each case. Gallup and Gustafson (3) using 2,4-dichloro-5iodo181 -phenoxyacetic acid were also able to demonstrate that the absorption of this growth regulator by the monocotyledonous plants is slower than by the dicotyledonous ones. The translocation to the apical portion of the plant is very rapid in broadleaved plants, but much less rapid in the grasses. Holley, Boyle and Hand (4) using carboxyl-C14labeled 2,4-D demonstrated the presence of free 2,4-D in bean plant homogenate. Later Holley (5) using the same labeled preparation was able to show the presence of an ether-insoluble, water-soluble compound. Weintraub et al. (9), by using either carboxylor methylene-C14-labeled 2,4-D, demonstrated that the C14 from the applied 2,4-D in bean plants is incorporated into other substances within a few days. The radioactive carbon becomes distributed among a variety of plant constituents, including acids, sugar, dextrins, starch, pectin, protein and cell wall substances. In a publication from this laboratory (2) it was found that with bean plants the radioactive 2,4-D was readily absorbed and the radioactivity was found in various parts of the bean plant. In a continuation of this study (6) two radioactive compounds, in addition to unchanged 2,4-D, were found in an 80% alcohol extract of the stems of bean plants treated with either a-methyleneor carboxyl-C14-labeled 2,4-D. It seemed worthwhile to extend this work to some 2,4-D resistant plants. The results here report the absorption, translocation and metabolism of radioactive 2,4-D by corn a d wheat plants.