Distribution of [14C]glyphosate in mature glyphosate-resistant cotton from application to a single leaf or over-the-top spray

Distribution of [14C]glyphosate was examined in mature glyphosate-resistant cotton plants at the 13-node stage in the absence of phytotoxicity. Initial experiments employed manual application of glyphosate to individual leaves within a relatively immature (Node 9) or mature (Node 5) sympodium (i.e., fruiting node) in the plant. We measured glyphosate export out of the treated leaf to the fruiting structures and foliage in the sympodium as well as out of the sympodium into the plant. Application to the Stem leaf, Leaf 1 or Leaf 2 in Node 9 showed 30-37% glyphosate export by 14 days after treatment. While Stem leaf exported mainly to the plant, Leaves 1 and 2 exported equally between the plant and the sympodium. Within the sympodium, glyphosate was distributed almost entirely to the fruiting structures with Boll 1 containing the highest level irrespective of the treated leaf. In Node 5, application to the Stem leaf or Leaves 1-3 showed overall lower glyphosate export (20-27%) with Stem leaf and Leaf 3 exporting mainly to the plant, and Leaves 1 and 2 exporting mainly to the sympodium. Within Node 5, the subtending boll of the treated leaf was not the main distribution target, but instead the boll at the next higher sympodial position relative to the treated leaf. Subsequent studies were conducted using over-the-top spray application of [14C]glyphosate at field use-rates in mature plants at the 13-node stage. Analysis of open bolls at full maturity showed high residues in seeds and fiber of Bolls 1 and 2 in the mature sympodia. Our results suggest that glyphosate distribution in mature plants was affected by sympodial age, position of the treated leaf, and position and sink strength of the bolls.