Foliar Uptake and Transport of 2,4-D and Picloram by Drummond's Goldenweed (Isocoma drummondii)

Abstract

Detached leaves of Drummond's goldenweed [Isocoma drummondii(T. & G.) Greene] absorbed the potassium salt of 2,4-D [(2,4-dichlorophenoxy)acetic acid] from aqueous solutions more slowly than did sunflower (Helianthus annuusL.), and both species absorbed less of the potassium salt of picloram (4-amino-3,5,6-trichloropicolinic acid) than 2,4-D. However, there was no difference in absorption of the herbicides by Drummond's goldenweed leaves after 6 h of exposure to solutions containing 0.5% (v/v) surfactant. Attached Drummond's goldenweed leaves absorbed about 50% of available 2,4-D (diethylamine) and 25% of available picloram (potassium salt) within 5 days after spraying in the field during July or November. Herbicide accumulation in Drummond's goldenweed taproots after spray applications was generally slow, regardless of season, but translocation to taproots was substantially greater after application in November than after treatment in March or July. Accumulation of picloram in taproots was faster and more extensive than accumulation of 2,4-D. However, based on mortality at 6 months after treatment, both herbicides were translocated in quantities adequate for control. The greater effectiveness of picloram, despite its low foliar uptake, than 2,4-D is attributed to its greater mobility and root uptake in Drummond's goldenweed after broadcast sprays.