Penetration of clopyralid and related weak acid herbicides into and through isolated cuticular membranes of Euonymus fortunei

A model system consisting of chemically isolated cuticular membranes placed on agar was used to study the penetration of various formulations of ¹⁴C-labelled clopyralid, fluroxypyr, triclopyr, picloram, and 2,4-D into and through cuticular membranes. Clopyralid, commercially formulated as the acid, or 1-decyl ester was rapidly absorbed after 12 h by isolated cuticles of Euonymus fortunei. There was less absorption of the monoethanolamine and potassium salt formulations when compared to the acid and 1-decyl ester. However, in terms of the absorbed ¹⁴C-activity that partitioned into the agar, there was no difference between the acid and salt formulations with approximately 90% being partitioned after 48 h. Conversely, the 1-decyl ester formulation of clopyralid was retained in the cuticle; less than 5% of the absorbed fraction was recovered in the agar after 48 h. When the acid forms of clopyralid, fluroxypyr, triclopyr, picloram, and 2,4-D were compared, there was little or no difference in absorption among the herbicides. However, the ¹⁴C-activity from clopyralid partitioned the most (90%) and triclopyr the least (50%) into the agar. When ester formulations of clopyralid, fluroxypyr, and triclopyr were compared, at least 95% of the ¹⁴C-activity was absorbed 24 h after application. However, of the amount absorbed, significantly more of the butoxyethyl ester of triclopyr (36%) partitioned into the agar than did the 1-decyl ester of clopyralid (6%) or the 1-methylheptyl ester of fluroxypyr (5%). Differential retention of various herbicide formulations in this model system may explain, in part, the differences in absorption and translocation among radiolabelled clopyralid formulations observed in previous research (Kloppenburg & Hall, 1990).