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 p hytotoxicity of imazamethabenz in wild oat (Avena fatua L.) were evalu ated in the greenhouse. Wild oat plants supplied with half-strength ra ther 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 app eared related to increased herbicide interception by the greater leaf surface available. Leaves develop at either nutrient level did not dif fer significantly in epicuticular wax, so differential absorption appe ared unlikely. Wild oat plants supplemented with nutrient, switching f rom low to high levels at the time of herbicide application, were as s usceptible 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 tha n the limited-tillering line, Crop Science 40. Both 2,4-D and GA3 redu ced imazamethabenz-induced tillering. Imazamethabenz efficacy was incr eased by GA3 but not by 2,4-D. These results support the hypothesis th at lowering apical dominance of wild oat increases imazamethabenz acti vity in tillers, and that increased tillering following sublethal dose s of imazamethabenz treatment is associated with the release of apical dominance.