Clover Sweden ? A national three-year study of the effects of tropospheric ozone on Trifolium subterraneum, L.

Abstract

In 1992 a cooperative project, Clover Sweden, was initiated. The aim was to study if subterranean clover could be used as a bioindicator in the different climate zones in Sweden by studying the impact of ambient ozone concentrations on this species in different parts of the country during three consecutive summer seasons. Plants of subterranean clover, Trifolium subterraneum, L., were exposed to ambient air at 24 sites from north to south Sweden. The project was designed to be compatible with the international programme, ICP Crops within the UNECE and the Convention on Long Range Transboundary Air Pollution. The results showed that subterranean clover is a useful bioindicator of ozone in all agricultural areas of Sweden, with the exception for very cool and rainy summers resulting in poor growth of the plants. In 1992, and especially in 1994, ozone injury was detected at almost all sites in Sweden, reflecting the higher ozone levels of those summers as compared to 1993, when ozone concentrations were generally low and not much injury was detected. Typical injury was chlorotic and bifacial necrotic lesions on parts of the leaf surface. It is concluded that at mean ozone concentrations of 25 ppb (24 h mean) and 30 ppb (7 h mean) there is a potential risk for injury on 10% of the leaves. When % injured leaves was plotted against AOT (Accumulated exposure Over a Threshold) using different thresholds, it became obvious that a threshold of 20 ppb ozone should be used in order to fully protect from leaf injury under Swedish conditions.