Absorption, Translocation, and Metabolism of 2,4-D and Glyphosate in Hemp Dogbane (Apocynum cannabinum)

Low recoveries of total applied 14 C in translocation studies and erratic control of hemp dogbane ( Apocynum cannabinum L.) in the field showed a need for a balance-sheet study of absorption, translocation, and metabolism of 14 C-2,4-D [(2,4-dichlorophenoxy) acetic acid] and 14 C-glyphosate [ N -(phosphonomethyl)glycine]. Total recovery of 14 C-herbicides applied to hemp dogbane in the laboratory was 97% for 2,4-D and 105% for glyphosate. Of the 14 C recovered after 12 days in the hemp dogbane, 34 to 55% was parent-2,4-D after 2,4-D treatment, and 93 to 96% was parent glyphosate after glyphosate treatment. Only negligible amounts of 14 C were lost via volatilization or evolution as 14 CO 2 . A broadcast treatment with unlabeled herbicide did not significantly affect subsequent absorption, translocation, or metabolism of either herbicide. Total herbicide absorbed and translocated out of the treated area of the leaf generally increased during the subsequent 12 days for 2,4-D but only 3 days for glyphosate. A greater percentage of the total applied 2,4-D (31 vs. 14%) and glyphosate (14 vs. 8%) was translocated from upper rather than lower leaves of hemp dogbane, respectively. Higher temperatures (30 vs. 25 C) resulted in greater translocation of glyphosate (39 vs. 18%) but not 2,4-D (35 vs. 39%). Higher light intensities resulted in greater accumulations of 2,4-D into roots and of glyphosate into untreated areas of the treated leaf. Autoradiographs showed that both herbicides moved through hemp dogbane in a typical symplastic pattern and accumulated in roots and new leaves.