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

Abstract

Diclofop-methyl (DM) sprayed onto 6-8-week-old plants of leafy spurge (Euphorbia esula L.) caused senescence and abscission of older leaves, while the young leaves and apex remained attached. The phytotoxicity of DM was reversed by the antioxidant, α-tocopherol (vitamin E), in leafy spurge and DM-susceptible oat (Arena sativa L. cv. Gary). DM and 2,4-dichlorophenoxyacetic acid (2,4-D) increased ethylene evolution in mature leaves of leafy spurge. Vitamin E reduced the DM-induced ethylene by approximately 50%, but had 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 ethylene evolution over controls in DM-resistant tobacco. 2,4-D appears to act at a site different from that of DM in the pathway of ethylene formation. Ethylene evolution increased in DM-treated susceptible biotypes of annual ryegrass (Lolium rigidum L.) and wild oat (Avena fatua L.), but not in resistant biotypes of these species. DM reduced root and shoot formation and dry weight in hypocotyl segments of etiolated leafy spurge seedlings grown in vitro. Organogenesis and dry weights were increased by the combination of DM + antioxidants. Vitamin E was a more effective antioxidant than ascorbic acid. These results support the hypothesis that DM induces oxidative stress in susceptible plant tissues and that antioxidants reduce the damaging action of the phytotoxic free radicals.