Release of picloram from leafy spurge (Euphorbia esula L.) roots

Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) exudation from leafy spurge (Euphorbia esula L.) roots following foliar application was determined with field trials examining the effects of plant growth stage, picloram rate, and time interval after treatment and with laboratory studies examining the effects of picloram rate, picloram plus 2,4-D, and root temperature. Release of 14C from leafy spurge roots in the laboratory was not affected by picloram rates from <0.005 to 0.28 kg/ha or by combining picloram at 0.14 kg/ha plus 2,4-D up to 1.12 kg/ha. No consistent effect was detected for increasing root temperatures from 14 to 32 C. The temperature coefficient (Q10) for picloram release was 1.3 +/- 0.8. Most 14C exudates from leafy spurge roots co-chromatographed with 14C-picloram suggesting that picloram is exuded as the parent acid or a rapidly hydrolyzed metabolite. Most picloram release occurred from the upper 5 cm of the root system. 14C distribution in leafy spurge suggests symplastic movement. Only about 28% of the applied picloram was absorbed, 75% remained in the leaves and stems of the plants. Over 60% of the 14C that entered the plant roots was released to the nutrient solution