OZONE POLLUTION MODIFIES THE RESPONSE OF SUGAR-BEET TO THE HERBICIDE PHENMEDIPHAM

Experiments were conducted in which sugarbeet plants (Beta vulgaris L. cv. Saxon) with 2 to 3 leaves were exposed to a simulated 2 day ozone episode (100 nl l(-1), 7 h d(-1)). Three days later; the plants were sprayed with field rate phenmedipham (1.14 kg a.i. ha(-1)) and growth analysis conducted 7 days later indicated an antagonistic interaction was occurring Physiological and biochemical studies were undertaken to determine the nature of this antagonism. Treatment with phenmedipham increased the ratio of transpiration to photosynthetic rates within 2 days of spraying, whilst exposure to ozone had no effect. When die two treatments were combined, water use efficiency was not significantly different from that when phenmedipham was applied alone. In contrast, trends in the membrane permeability after treatment, indicated that th e response of plants exposed to ozone followed by treatment with the h erbicide, was intermediate between that of the herbicide (high pamrabi lity) and ozone (low permeability). Furthermore, when the two treatmen ts were combined the results of antioxidant enzyme assays indicated gr eater than expected activities of enzymes which am mainly cytosolic, e g. guaiacol peroxidase, as well as a similar increase in the activity of the mainly chloroplastic superoxide dismutase. Treatment with ozone alone and phenmedipham alone only slightly increased superoxide dismu tase. Ozone may therefore induce the activities of these protective en zymes. Thus, when another oxidative stress,such as the photosystem II inhibitor phenmedipham was applied the plants could then respond more quickly and showed less herbicide visible damage.