The Absence of a Role of Absorption, Translocation, or Metabolism in the Selectivity of Picloram and Clopyralid in Two Plant Species

Experiments were conducted to determine whether differences in sensitivity of sunflower (Helianthus annuusL.), and rapeseed (Brassica napusL.) to14C-picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) and14C-clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) can be attributed to differences in absorption, translocation, or metabolism of the herbicides. Within 24 h, more than 97% of the radioactivity from picloram or clopyralid was absorbed by the foliage of both plant species. Acropetal transport of both herbicides was similar in the two plant species, with approximately 60% or more of the recovered radioactivity from picloram and clopyralid treatments moving acropetally out of the treated leaf 144 h after application. Less than 6% of the14C-label from either herbicide treatment moved basipetally in both species. Therefore, differences in absorption and translocation did not account for intra- or inter-species sensitivity differences to the two herbicides. Significantly more14C-picloram and14C-clopyralid was converted to water-soluble metabolites in rapeseed than in sunflower plants. However, in rapeseed plants, the pattern of metabolism of both herbicides was similar, indicating that the difference between the metabolism of picloram and clopyralid did not account for the sensitivity difference within this species. It appears that only one picloram and clopyralid metabolite was formed in both plant species as determined by TLC. The metabolite of picloram or clopyralid was a water-soluble conjugate that yielded the carboxylic acid amide of the respective herbicide upon ammonolysis.