Translocation and Complex Formation of Picloram and 2,4‐D in Rape and Sunflower

Uptake, translocation and complex formation of 14C‐labelled 4‐amino‐3,5,6‐trichloropicolinic acid (picloram) and 2,4‐dichlorophenoxyacetic acid (2,4‐D) in seedlings of rape (Brassica napus L. cv. Nilla) and sunflower (Helianthus annuus L. var. uniflorus) were studied. Sunflower is susceptible both to 2,4‐D and picloram, while rape is susceptible to 2,4‐D but more tolerant to picloram. The uptake of the herbicides through the leaves was almost complete in both species. Translocation of 2,4‐D into the roots took place more readily than that of picloram. In sunflower about 50 per cent of the applied 2,4‐D was extruded through the roots into the nutrient solution after 9 days. In the picloram‐treated sunflower most of the activity was found in the aerial parts, while in picloram‐treated rape most of the activity still occurred in the treated leaf after 9 days. No activity at all was found in the roots or in the nutrient solution of the picloram‐treated rape seedlings. While the major part of 2,4‐D always was found in the state of free herbicide, a large fraction of picloram was rapidly bound into water‐soluble complexes. This binding was especially pronounced in rape. Separation by paper chromatography showed that different radioactive compounds were formed. Most of these could be hydrolyzed, thereby releasing free herbicide. The results support the hypotheses that complex formation could counteract herbicide translocation and toxicity of auxin herbicides.