Clopyralid Uptake, Translocation, Metabolism, and Ethylene Induction in PicloramResistant Yellow Starthistle (Centaurea solstitialis L.)

Resistance to the auxinic herbicide picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) has been reported in a population of yellow starthistle (Centaurea solstitialis L.) growing in a nonarable pasture in Dayton, Washington. In addition, these plants are crossresistant to clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) and other auxinic herbicides. To understand the mechanism of cross-resistance to clopyralid, uptake, translocation, and metabolism of clopyralid were compared between the picloram-resistant (R) and the susceptible (S) accessions of this weed species. Radiolabeled clopyralid was applied to individual leaves of rosette-stage plants under controlled environmental conditions. Both accessions absorbed about 75% of the total applied within the first 2 h following treatment. Uptake increased linearly with clopyralid concentration, but was not different in accessions in the range of concentrations studied. Also, there was no difference between accessions in clopyralid partitioning into epicuticular wax. About 50% of the total absorbed radioactivity moved out of the treated leaf toward both upper and lower shoots 96 h following treatment in both accessions. Within 2 h of treatment, clopyralid was converted by R and S plants to similar quantities of metabolites. After 96 h, R plants metabolized more clopyralid than S plants, suggesting a possible role of herbicide metabolism in cross-resistance. However, the clopyralid-induced symptoms of ethylene production and epinasty appeared in S plants within 5 h, implying that mechanism(s) of cross-resistance involves process(es) other than metabolism.