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A yellow starthistle (Centaurea solstitialisL.) accession from a site near Dayton, Washington is resistant to picloram applied to roots and/or leaves. The level of resistance to picloram varied among experiments, ranging from 3- to 35-fold, depending on the site(s) of application and growth conditions. Whole-plant studies indicated cross-resistance of the resistant accession (R), relative to the susceptible accession (S), to clopyralid, fluroxypyr, and dicamba, but not to triclopyr or 2,4-D. Resistance to root- or foliar-applied picloram was not due to differential metabolism of picloram. Uptake of foliar-applied picloram was reduced by half in R compared to S, 72 hr after treatment. Root uptake and acropetal translocation of picloram were reduced in R compared to S. This reduced acropetal translocation showed specificity for picloram since transpiration and acropetal translocation of atrazine were similar in R and S. Picloram …