PHENMEDIPHAM-OZONE POLLUTION INTERACTIONS IN SUGAR-BEET (BETA-VULGARIS L CV SAXON) - A PHYSIOLOGICAL STUDY

Actively growing sugarbeet is treated with the post-emergent herbicide phenmedipham at times when ozone pollution episodes are likely to occ ur. There is a possibility of an interaction occurring between ozone a nd phenmedipham as both treatments produce similar effects in suscepti ble plants, such as a reduction in growth and photosynthesis and an in crease in the activities of endogenous antioxidant enzymes. To investi gate this likelihood, laboratory experiments were conducted in which t wo- to three-leaf sugarbeet plants (Beta vulgar is L. cv. Saxon) were exposed to a simulated two-day ozone episode (100 nl litre(-1), 7 h da y(-1)) followed three days later by treatment with field rate phenmedi pham (1.14 kg AI ha(-1)). Growth analysis indicated that an interactio n was occurring in which plants treated with ozone and phenmedipham ha d less reduction in shoot fresh weight than expected. Exposure to phen medipham alone or ozone followed by phenmedipham reduced net photosynt hesis by over 50% and transpiration rate by 30%. The activities of ant ioxidant enzymes such as catalase, guaiacol peroxidase and superoxide dismutase were stimulated by both treatments individually, but to a gr eater extent when ozone and phenmedipham were combined. For example, t hree days after herbicide treatment, the activity of superoxide dismut ase increased by 20% in plants treated with ozone alone, 20% in plants treated with phenmedipham alone and 85% in plants that were treated w ith ozone followed by phenmedipham. We conclude that ozone pollution m ay predispose sugarbeet to tolerate the herbicide phenmedipham by enha ncing the activity of the endogenous antioxidant detoxification enzyme system.