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Resistance to the auxin-like herbicide picloram has been reported in a yellow starthistle population growing in Washington. In addition, this population is cross resistant to clopyralid, another auxin-like herbicide. To understand the mechanism of resistance to clopyralid, studies were was conducted to determine uptake and translocation and to characterize clopyralid-induced ethylene production in the susceptible (S) and resistant (R) biotypes. R and S yellow starthistle plants were grown under ambient greenhouse conditions until full rosette stage and then transferred to a growth chamber (14-hour photoperiod, 25°C, 200 mol·m –2 ·s –1 ) 48 hours before treatment. Radiolabel solutions were prepared from  12 C and  14 C-clopyralid. Each treatment in the uptake experiment contained 0.009 Ci (20,000 dpm) and in the translocation experiment 0.225 Ci (500,000 dpm). Clopyralid 11.7 m m  (420 g a.e./ha) solutions were applied as six 0.5-L droplets to the adaxial surface of completely expanded leaves using a microsyringe. Radioactivity was quantified by Liquid Scintillation Spectrometry. Uptake was determined at specified times after treatment. Almost all clopyralid uptake occurred within the first 2 hours, with no significant differences between the two biotypes. The amount of picloram translocated was 2.4%, 40.2%, and 50.7% of that absorbed at 2, 24, and 96 hours after treatment, respectively, but was not different between biotypes. Clopyralid induced about ten times greater ethylene production in S than in R. Ethylene production was followed by epinasty and chlorosis but appears to play only a small role in the resistance mechanism.

Clopyralid Uptake, Translocation, and Ethylene Production in Resistant and Susceptible Yellow Starthistle (Centaurea solstitialis) Plants