Uptake of picloram by roots of alfalfa and barley

Uptake of [14C] picloram (4-amino-3,5,6-trichloropicolinic acid) from culture solution by alfalfa (Medicago saliva L.) and barley (Hordeum vulgare L.) seedlings was investigated to determine to what extent passive and (or) active mechanisms are involved in the uptake of the herbicide by roots of intact plants. The roots of both species had similar absorption patterns consisting of rapid initial uptake lasting 1 to 2 h, followed by continuing absorption at a markedly lower rate during the rest of a 12-h treatment period. Initial uptake both by roots and whole plants was not significantly reduced by 1.0 mM NaN3, whereas continuing absorption was depressed by this inhibitor. Q10 values were 1.4 for alfalfa and 1.0 for barley for the first 1–2 h of uptake, whereas for subsequent periods the values were consistently greater than 2.0. Evidently, uptake during the initial phase of entry is governed by passive processes, while continuing absorption is controlled, at least in part, by active mechanisms. Much of the picloram taken up by alfalfa and barley roots during a 4-h treatment period was released rapidly to unlabelled nutrient solution. Uptake by both species was decreased in the dark and enhanced in the presence of sucrose. The results of inhibitor studies suggest that picloram absorption by alfalfa plants is more dependent on a supply of metabolic energy than is uptake by barley plants. In most instances, total accumulation of picloram in 4 h was much greater in alfalfa plants than in barley plants of nearly equivalent mass. This difference may account for at least part of the difference in susceptibility of alfalfa (sensitive) and barley (tolerant) to the herbicide. Alfalfa roots always contained a much greater proportion of the total amount of picloram in the plant than did barley roots.