Absorption and Translocation of Glyphosate in Aspen (Populus tremuloides Michx.) as Influenced by Droplet Size, Droplet Number, and Herbicide Concentration1

When herbicide concentration was constant, ab- sorption of 14C-glyphosate increased with increasing droplet size (326 to 977 gim). Amount of 14C-glyphosate translocated away from the treated area, expressed as percent of ab- sorbed, increased as droplet size decreased. Herbicide con- centration of the droplet was more important than droplet number or droplet size in determining glyphosate absorption and translocation. Absorption and translocation increased with increasing herbicide concentration regardless of whether droplet size or number was altered in conjunction with herbicide concentration. This relationship explained why low spray volume (increased herbicide concentration) increased herbicide efficacy. The concentration gradient be- tween droplet and leaf, rather than droplet coverage, was the primary mechanism responsible for the observed effect. Large droplets caused localized tissue injury, which may have caused decreased translocation. Nomenclature: Gly- phosate, N-(phosphonomethyl) glycine; aspen, Populus tre- muloides Michx.#3 POPTM. Additional index words. Efficacy, droplet generator, forest