It is well admitted that to apprehend the critical level of ozone susceptible to damage vegetation, the effective ozone flux, corresponding to the non-detoxified ozone molecules, needs to be determined (Musselman et al., 2006; Wieser and Matyssek, 2007; Dizengremel et al.2008). This concept firstly integrates the stomatal conductance to appreciate the amount of ozone entering in the leaf. Additionally it needs to consider the capacity of the cell to neutralise ROS which could be estimated by the levels of antioxidant molecules but also by the activity of enzymes generating NADPH, useful to regenerate ascorbate and glutathione (Dizengremel et al., 2009). In a preliminary experiment, we have selected 3 Populus x euramericanagenotypes (Carpaccio, Cima and Robusta) for their distinct response to chronic ozone exposure (extent of necrosis and leaf loss). Subsequently several metabolites (ascorbate, glutathione, carotenoids, polyphenols) as well as the activity of NADPH delivering enzymes (malic enzyme, isocitrate dehydrogenase, glucose-6-P-dehydrogenase, glyceraldehyde-3-P-dehydrogenase) have been measured in poplar genotypes exposed to 120 ppb ozone for 21 days. The results which were expressed relative to the instantaneous ozone flux through stomata, underlined intraspecific variability of these poplar genotypes in their capacity to detoxify