INFLUENCE OF NUTRIENT SUPPLY AND PLANT-GROWTH REGULATORS ON PHYTOTOXICITY OF IMAZAMETHABENZ IN WILD OAT (AVENA-FATUA L)

The influences of nutrient supply and plant growth regulators on the phytotoxicity of imazamethabenz in wild oat (Avena fatua L.) were evaluated in the greenhouse. Wild oat plants supplied with half-strength rather than one-eighth-strength Hoagland solution were more susceptible to imazamethabenz, showing greater growth reduction in main shoot and tillers. The improved herbicide efficacy at higher nutrient levels appeared related to increased herbicide interception by the greater leaf surface available. Leaves develop at either nutrient level did not differ significantly in epicuticular wax, so differential absorption appeared unlikely. Wild oat plants supplemented with nutrient, switching from low to high levels at the time of herbicide application, were as susceptible to imazamethabenz or even more so than plants growing with a constant high level of nutrition. The wild oat pure-line Montana 73, a strongly tillering line, was more susceptible to imazamethabenz than the limited-tillering line, Crop Science 40. Both 2,4-D and GA3 reduced imazamethabenz-induced tillering. Imazamethabenz efficacy was increased by GA3 but not by 2,4-D. These results support the hypothesis that lowering apical dominance of wild oat increases imazamethabenz activity in tillers, and that increased tillering following sublethal doses of imazamethabenz treatment is associated with the release of apical dominance.