Effect of picloram on resistant and susceptible yellow starthistle (Centaurea solstitialis): the role of ethylene

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Abstract
The noxious weed yellow starthistle is commonly controlled by the auxinic herbicide picloram. Induction of ethylene synthesis, epinasty, and reduction in shoot growth are typical symptoms of picloram treatment. Picloram did not induce ethylene evolution in the resistant accession RDW-1, though it caused a 250% increase in ethylene evolution in the susceptible wildtype SCI-1. The ethylene synthesis inhibitor aminoethoxyvinylglycine reduced the amount of ethylene induced by picloram in SCI-1 to control levels, but only reduced epinasty by 20% after 6 d. Aminoethoxyvinylglycine did not affect the reduction in shoot weight caused by picloram. The ethylene-releasing compound ethephon induced only a small amount of epinasty and had little effect on shoot weight in either accession. These results suggest that ethylene induced by picloram in wildrype plants plays only a minor role in the herbicidal effects of picloram. Furthermore, the resistance of the RDW-1 accession is not due to the lack of ethylene biosynthesis following picloram application to this accession.