Is Ozone Flux Inside Leaves Only a Damage Indicator? Clues from Volatile Isoprenoid Studies

Tropospheric ozone is increasing at a fast pace and causes large losses of production in cultivated plants and forests (UNECE, 2004). Ozone risk assessment has been based for a long time on exposure to external ozone concentration. Doses above a selected threshold (generally 40 nL L−1) have been considered injurious, and the cumulated exposure to these concentrations has been the basis of the level I approach to determine and predict ozone damage to vegetation (Fuhrer et al., 1997). In the attempt to improve ozone dose/effect predictions, a more mechanistic approach has been recently developed. This approach is based on the flux of ozone through stomata and on the consequent stomatal uptake by leaves (Emberson et al., 2000). The flux-based level II approach has been recommended as a suitable indicator of ozone damage in Europe (UNECE, 2004). Yet, in only a few cases has a clear improvement of risk assessment for vegetation when estimating actual ozone flux into leaves been reported (e.g. Uddling et al., 2004).