Penetration and Translocation of Picloram in Canada Thistle

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Abstract
Added surfactant and high relative humidity enhanced penetration of 4-amino-3,5,6-trichloropicolinic acid (picloram) into leaves of Canada thistle (Cirsium arvense(L.) Scop.). Leaves and roots of Canada thistle readily absorbed picloram after which it was translocated in both phloem and xylem. Picloram tended to accumulate in young, growing leaves following both foliar and root uptake. In field-grown plants, injury symptoms appeared in young shoots as far as 1 m from treated shoots. The results are in harmony with a “source-to-sink” pattern of picloram translocation in the phloem. Leaves, but not roots, retained a substantial portion of the picloram absorbed. Following foliar application, small amounts of picloram were exuded by the roots into surrounding soil.