ASSESSING EFFECTS OF AMBIENT OZONE ON INJURY AND GROWTH OF TRIFOLIUM-SUBTERRANEUM AT 4 RURAL SITES IN THE NETHERLANDS WITH ETHYLENEDIUREA (EDU)

To assess adverse effects of ambient ozone on injury and growth, EDU ( ethylenediurea) and non-EDU-treated plants of subterranean clover (Tri folium subterraneum cv. Geraldton) were exposed to ambient air at four rural sites in the Netherlands. In each of two successive experiments of eight weeks during the 1994 growing season, an intermediate harves t (cutting) was performed after four weeks of exposure. Concentrations of ambient ozone were high in Experiment 1 (June 21-August 16) and re latively low in Experiment 2 (August 23-October 18). Subterranean clov er displayed characteristic ozone injury at all sites in Experiment 1 and in Experiment 2 until the intermediate harvest (September 20). The proportion. of leaves injured in non-EDU-treated plants was higher th an that in plants treated with the antioxidant. Both the degree of inj ury and the protective effect by EDU differed between sites and were n ot related to the measured ozone levels. The proposed short-term criti cal levels for injury development by ozone would not have protected cl over against ozone injury in Experiment 2. The site-dependent pattern of injury in non-EDU-treated plants was similar for the various harves ts and experiments and a maximal degree of 23% injury was observed aft er four weeks of exposure. The protective effect by EDU was related po sitively to the injury intensity in non-EDU-treated plants, irrespecti ve of site, harvest and experiment. EDU caused variable effects on lea f biomass production under conditions of low ozone in Experiment 2 whe reas EDU generally increased leaf biomass when the plants were exposed to the relatively high concentrations in Experiment 1. The EDU-induce d increase in leaf biomass production in this experiment did not vary between sites and was on average 5-7%, depending on the harvest. The i nvolvement of elevated concentrations of ambient ozone measured at the four sites in the Netherlands in 1994 in the observed increase of lea f biomass by EDU is discussed. (C) 1997 Elsevier Science B.V.