Absorption and translocation of 14 C‐3,6‐dichloropicolinic acid in Cirsium arvense (L.) Scop.

Summary Experiments were conducted in a growth cabinet to investigate the absorption and translocation of 14 C‐3, 6‐dichloropicolinic acid by Cirsium arvense (L.) Scop. (Canada thistle, creeping thistle), a sensitive species. Applications were made, either to the middle four leaves of 12‐cm‐tall vegetative plants grown under low (40%) and/or high (>95%) relative humidity (r.h.), or to four upper or lower leaves of 30‐cm‐tall flowering plants grown under low r.h. Following application to vegetative plants, absorption and translocation of 14 C‐3,6‐dichloropicolinic acid was rapid and was approximately doubled by high r.h. High r.h. increased the amount of radioactivity retained by the treated leaves or translocated to the shoots but did not affect greatly the amount retained in the roots. The herbicide was highly mobile, with over half of that absorbed, translocated out of the treated leaves after two days. The apex accumulated most of the radioactivity, while approximately 8% was recovered from the roots. The absorption and translocation patterns were similar to those reported in the literature for picloram in C. arvense. Absorption of 3,6‐dichloropicolinic acid was greater in vegetative than in flowering C. arvense plants, and placement of herbicide on lower leaves tended to decrease the amount of radioactivity recovered from shoot apex and increase the amount recovered from the roots. Approximately 15% of the applied radioactivity could not be recovered from treated plants by 2 days after treatment.