Fate of Picloram in Canada Thistle, Soybean, and Barley

Autoradiography and radioassay results indicated that14C-picloram (14C-4-amino-3,5,6-trichloropicolinic acid) was absorbed and translocated rapidly in Canada thistle [Cirsium arvense(L.) Scop.], soybean [Glycine max(L.) Merr. ‘Harosoy 63’], and barley (Hordeum vulgareL. ‘Parkland’) following foliar or root application. Foliar absorption was much faster and more complete in soybean and Canada thistle than in barley. The radioactivity from14C-picloram accumulated in shoot meristems in Canada thistle and soybean, whereas in barley it was distributed throughout the plant following uptake by foliage or roots. Decarboxylation of14C-picloram by foliarly-treated Canada thistle, soybean, and barley plants did not occur in appreciable amounts. Chromatographic analysis of ethanol extracts of plants treated with14C-picloram up to 20 days revealed no evidence of picloram metabolism by these plant species. It is concluded that differences in absorption and in distribution patterns of picloram after shoot or root uptake contribute greatly to the expression of its selective action in the species studied.