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

Abstract

Summary: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).