Effect of shoot decapitation on the translocation of 2,4-D in Cirsium arvense

When 14C-labelled 2,4-D [(2,4-dichlorophenoxy)acetic acid] was applied to the basal leaves of Canada thistle (Cirsium arvense L.), prior removal of approximately the upper two-thirds of the flowering shoot resulted in a 20 to 30% increase in total 14C exported from the treated leaves and a twofold to threefold increase in the amount in the roots. Removing only the apical region of the shoot had the opposite effect, markedly reducing 14C export from the treated leaves and translocation to the roots. Provision of a higher N level more than doubled 14C translocation from the treated leaves and caused a threefold increase in 14C content of the roots of undecapitated plants. The reduction in basipetal transport produced by apical decapitation was attributed to the inhibiting influence of subapical lateral shoots on basal branching and root bud growth. When apical decapitation was combined with removal of the axillary buds in the subapical region, basal branching and root bud growth were promoted and 14C translocation into the roots was significantly increased.