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) 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). In this correspondence, it is shown that ozone uptake is largely driven by stomatal opening, but evidence is also presented that ozone uptake is not necessarily related to ozone sensitivity, as isoprenoids, and probably other ozone scavenging molecules (e.g. ascorbate; Eller and Sparks, 2006), may contribute to increased ozone uptake by leaves. Because isoprenoids have an important antioxidant action (Loreto and Velikova, 2001; Affek and Yakir, 2002), it is concluded that a part of the ozone uptake may not be related to damage, at least in isoprenoid-emitting species, and may indicate the activation of defensive mechanisms scavenging reactive oxygen molecules and an overall reduced ozone damage in leaves. OZONE UPTAKE BY LEAVES DEPENDS ON STOMATAL CONDUCTANCE