Picloram Release from Leafy Spurge Roots

Picloram (4-amino-3,5,6-trichloro-2-pyridineacarboxylic acid) release from leafy spurge (Euphorbia esula L.) roots was not affected by application rate, root system temperature, or addition of 2,4-D [(2,4-dichlorophenoxy)acetic acid]. Release of 14C from leafy spurge roots was detected 12 hours after 14C-picloram foliar application and increased linearly over a 120-hour period. Over all experiments, 72% of the recovered 14C remained unabsorbed on the treated leaf and 22% remained in the leaves and stems. Less than 7% of the recovered 14C was in the root zone (roots plus nutrient solution), but over 60% of this portion was in the nutrient solution. Adding 2,4-D at rates up to 1.1 kg/ha to 14C-picloram at 0.14 kg/ha did not affect 14C release from leafy spurge roots. The experimentally determined temperature coefficient (Q10) for 14C-picloram release from leafy spurge roots was 1.3+/-0.8. A linear rate of picloram release with time and a Q10 of 1.3 support the hypothesis of passive release of picloram from leafy spurge roots.