Physiological responses of resistant and susceptible plants to diclofop-methyl and its antagonism by 2,4-D and antioxidants

Diclofop-methyl (DM) sprayed onto 6-8-week-old plants of leafy spurge (Euph orbia esula L.) caused senescence and abscission of older leaves, while the young leaves and apex remained attached. The phytotoxicity of DM was rever sed by the antioxidant, alpha-tocopherol (vitamin E), in leafy spurge and D M-susceptible oat (Avena sativa L. cv, Gary), DM and 2,4-dichlorophenoxyace tic acid (2,4-D) increased ethylene evolution in mature leaves of leafy spu rge. Vitamin E reduced the DM-induced ethylene by approximately 50%, but ha d no effect on the 2,4-D-induced ethylene, DM did not increase ethylene in DM-resistant pea or tobacco, but 2,4-D induced a 3-fold increase in ethylen e evolution over controls in DM-resistant tobacco. 2,4-D appears to act at a site different from that of Dill in the pathway of ethylene formation. Et hylene evolution increased in DM-treated susceptible biotypes of annual rye grass (Lolium rigidum L,) and wild oat (Avena fatua L,), but not in resista nt biotypes of these species. DM reduced root and shoot formation and dry w eight in hypocotyl segments of etiolated leafy spurge seedlings grown in vi tro, Organogenesis and dry weights were increased by the combination of DM + antioxidants. Vitamin E was a more effective antioxidant than ascorbic ac id. These results support the hypothesis that DM induces oxidative stress i n susceptible plant tissues and that antioxidants reduce the damaging actio n of the phytotoxic free radicals.