In Vivo and In Vitro Characterization of the Foliar Entry of Glyphosate in Hemp Dogbane (Apocynum cannabinum)

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Abstract
Detached leaves of hemp dogbane (Apocynum cannabinum L.) were treated with 14C-glyphosate [N-(phosphonomethyl)glycine] with and without adjuvants in the treatment solution. In the absence of adjuvants, 14C absorption was not significant after the first 30 min harvest. Adjuvants increased the amount of glyphosate initially absorbed but did not extend the period of absorption. The pattern of glyphosate absorption in isolated leaf cells was similar to the pattern found in detached leaves. Significant absorption was obtained at the first 15-min sampling with no additional absorption occurring during the remainder of the 2-h incubation period. The quantity of 14C absorbed by cells was proportional to the external concentration, but the pattern of cellular absorption was unaffected by the glyphosate concentration. After 2 h of incubation, an average of 0.1% of the glyphosate had been absorbed by the leaf cells. In similar studies using 14C-leucine, 1.0% of the 14C was absorbed after 2 h with absorption still increasing in a linear fashion. A uniform distribution of 14C was obtained in detached leaves that had absorbed 14C-glyphosate through the stem. In vitro and in vivo binding studies indicated this absorbed 14C was not tightly bound to cellular components and was probably free to be absorbed by and translocated out of the cells. Since the absorption pattern in detached leaves was similar to the pattern obtained in isolated cells, cellular absorption appears to represent the major barrier in the foliar absorption of glyphosate by hemp dogbane.