Clover as an indicator plant for phytotoxic ozone concentrations: visible injury in relation to species, leaf age and exposure dynamics.

Visible ozone injury on leaves of three clover species was investigated in relation to species, leaf age and exposure dynamics. It was shown that ozone episodes in south-west Sweden cause visible injury to subterranean clover (Trifolium subterraneum L.), white clover (Trifolium repens L.) and red clover (Trifolium pratense L.). Trifolium subterranean was most sensitive to ozone, whilst T. pratense was least sensitive. Application of the anti-ozonant, ethylenediurea (EDU), reduced the extent of visible ozone injury, but did not give complete protection with the concentrations used. Similarly, filtration of the air reduced the extent of visible injury in T. subterraneum enclosed in open-top chambers. EDU-treated plants of T. subterranean accumulated more biomass than the non-EDU treated plants after a period with rather high ozone concentrations, while the opposite occurred for T. pratense. Experiments with T. subterraneum in open-top chambers also showed that older leaves were more sensitive to ozone than younger leaves and that a shorter period with higher ozone concentrations produced more ozone injury than a longer period with lower ozone concentrations, although the two periods had the same number of ppb-hours.