Ozone threshold doses and exposure-response relationships for the development of ozone injury symptoms in wild plant species

The relative ozone sensitivities of 25 German native herbaceous plant species, representative of arable field margins or disturbed habitats, were examined over three consecutive growing seasons. Plants were grown from seed, potted into natural soils and exposed in open-top chambers for the entire season to different ozone-exposure regimes covering a range of concentrations from <5 to 48 ppb (seasonal 8 h daily mean). The assessment of ozone effects was carried out by recording the first day of visible symptom appearance and the percentage of injured leaves at the end of vegetative growth. Species exhibited contrasting patterns of symptom expression under ozone stress, with either ozone-specific symptoms or ozone-enhanced foliar pigmentation and senescence. Classifications of species according to their ozone susceptibility varied depending on whether measurement was of the total extent of visible injury, ozone threshold doses for the incidence of symptoms, or modelled exposure-response relationships. The most sensitive species exhibiting ozone-specific symptoms were Cirsium arvense and Sonchus asper, which responded to accumulated ozone exposures <1500 ppb.h (AOT40). For these and three other species, an AOT-CO peak of a single day was found to be responsible for the incidence of ozone-specific symptoms, i.e. injury occurred rapidly within a few days of the day with the highest AOT40, while other species responded only to longer-term ozone exposures. The relative ozone sensitivity of the species was calculated by combining the different sensitivity criteria, and possible systematic trends (taxonomic or evolutionary features) are pointed out. The results suggest it may be possible to use a particular group of native herbaceous plant species with contrasting patterns of ozone sensitivity as a biomonitoring system in the field. This allows plant responses to be related either to peak values or to prolonged ozone exposure, making it possible to distinguish between short- and long-term effects of ozone.