Picloram Absorption by Broom Snakeweed (Gutierrezia sarothrae) Leaf Tissue

Abstract

Foliar absorption of picloram by broom snakeweed, a rangeland shrub, was investigated. Picloram uptake into leaf, axillary bud, and stem tissues was similar. In addition, picloram uptake by leaf tissue from greenhouse- and field-grown broom snakeweed did not differ. Picloram accumulated rapidly and absorption saturated between 15 min and 1 h of application; no further absorption occurred through 72 h with maximum uptake ca. 15% of applied picloram. Picloram content increased linearly with increasing external picloram concentration, implying that movement of the herbicide across the cuticle is via diffusion. Absorption was dependent on relative humidity and temperature with the greatest uptake at 94% relative humidity and 35 C, respectively. Absorption was pH dependent; picloram absorption was greatest at pH 4 and least at pH 8. In addition, picloram absorption was less at pH 3 compared to pH 4. These results provide evidence that picloram is absorbed across the cuticle via simple diffusion and absorption is dependent on environmental conditions and solution pH at and following application.