Ozone exposure-response relationships for mixtures of perennial ryegrass and white clover depend on ozone exposure patterns

The effect of different ozone (O3) exposure regimes on yield and species composition of frequently cut mixtures of ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) was studied in order to assess the importance of intermittent periods with low O3 and to evaluate the need to use an exposure index that emphasizes peak concentrations. Exposure-response relationships were used to test the performance of different indices characterising the accumulated exposure. Potted plant mixtures were exposed in open-top chambers and cut repeatedly after four-week growth periods (GP). Yield data were related to O3 exposure expressed as AOT40 (accumulated exposure above 40 ppb during daylight hours) for the GP immediately preceding the harvest, or for all GPs combined. Data for relative clover yield showed that the effect determined after several GPs represented the combined effect of all preceding GPs, and significant correlations were found between clover yield and AOT40 calculated over all GPs. During GPs with low O3, clover recovered to some extent from injury produced during previous exposures to high O3. By comparing the effects of treatments with similar AOT40 values but different peak concentrations, i.e. a treatment with a more constant O3 exposure vs a treatment with episodic O3 stress, no difference in the effect on clover yield was found, but a stronger effect on total forage yield by the episodic treatment. The strength of the correlation between total yield and AOT increased with increasing threshold concentration suggesting that peak concentrations are more effective in eliciting the yield response than are lower concentrations. It is concluded that AOT40 calculated over all GPs is a suitable exposure index to describe the effect of O3 on the species composition of this type of mixture, but to describe the effect on total forage yield, peak concentrations of > 110 ppb must be emphasized by using an AOT with higher threshold concentrations. A comparison with Swiss O3 monitoring data revealed, however, that in ambient air such high concentrations rarely occur.