Chemical and Physical Effects of the Accumulation of Glyphosate in Common Milkweed (Asclepias syriaca) Root Buds

Distribution of14C-glyphosate [N-(phosphonomethyl)glycine] in chemically treated and physically manipulated common milkweed [Asclepias syriaca(L.) ♯ ASCSY] was studied in greenhouse and growth chamber experiments. Pretreatment with glyphosate at 0.6 and 1.1 kg ae/ha 3 days prior to14C-glyphosate application to leaves reduced the concentration of14C recovered from shoots and leaves above the14C-glyphosate-treated leaves but had no influence on the concentration of14C in proximal root buds. Partial removal of the shoot and root prior to the application of14C-glyphosate increased bud respiration and the concentration of14C in the proximal root buds. Proximal root buds treated with 1 mM of 6-benzyl-aminopurine (BAP) for 6 days (3 days prior to application of14C-glyphosate) contained seven times more14C/mg than root buds of BAP-untreated plants, suggesting that dormant buds could be chemically stimulated to accumulate higher concentrations of glyphosate. BAP-treated buds were killed by foliar applications of glyphosate at 1.1 kg/ha, indicating that proximal root buds can be stimulated to acquire lethal concentrations of glyphosate.