EFFECT OF PICLORAM ON RESISTANT AND SUSCEPTIBLE YELLOW STARTHISTLE (CENTAUREA-SOLSTITIALIS) - THE ROLE OF ETHYLENE

The noxious weed yellow starthistle is commonly controlled by the auxi nic 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 aminoetho xyvinylglycine reduced the amount of ethylene induced by picloram in S CI-1 to control levels, but only reduced epinasty by 20% after 6 d. Am inoethoxyvinylglycine did nor affect the reduction in shoot weight cau sed by picloram. The ethylene-releasing compound ethephon induced only a small amount of epinasty and had little effect on shoot weight in e ither accession. These results suggest that ethylene induced by piclor am in wildtype plants plays only a minor role in the herbicidal effect s of picloram. Furthermore, the resistance of the RDW-1 accession is n ot due to the lack of ethylene biosynthesis following picloram applica tion to this accession.