Poplar (Populus tremula ϫ alba) trees (clone INRA 717-1-B4) were cultivated for 1 month in phytotronic chambers with two different levels of ozone (60 and 120 nL L Ϫ1 ). Foliar activities of shikimate dehydrogenase (EC 1.1.1.25), phenylalanine ammonia lyase (EC 4.3.1.5), and cinnamyl alcohol dehydrogenase (CAD, EC 1.1.1.195) were compared with control levels. In addition, we examined lignin content and structure in control and ozone-fumigated leaves. Under ozone exposure, CAD activity and CAD RNA levels were found to be rapidly and strongly increased whatever the foliar developmental stage. In contrast, shikimate dehydrogenase and phenylalanine ammonia lyase activities were increased in old and midaged leaves but not in the youngest ones. The increased activities of these enzymes involved in the late or early steps of the metabolic pathway leading to lignins were associated with a higher Klason lignin content in extract-free leaves. In addition, stress lignins synthesized in response to ozone displayed a distinct structure, relative to constitutive lignins. They were found substantially enriched in carbon-carbon interunit bonds and in p-hydroxyphenylpropane units, which is reminiscent of lignins formed at early developmental stages, in compression wood, or in response to fungal elicitor. The highest changes in lignification and in enzyme activities were obtained with the highest ozone dose (120 nL L Ϫ1 ). These results suggest that ozone-induced lignins might contribute to the poplar tolerance to ozone because of their barrier or antioxidant effect toward reactive oxygen species.Plants submitted to ozone generally respond with a stimulation of enzymes involved in the phenylpropanoid pathway. Both the activity and transcript level of Phe ammonia lyase (PAL), the first enzyme of the phenylpropanoid pathway, have been reported to rapidly increase under ozone exposure in various herbaceous plants and forest species (Tingey et al., 1976; Heller et al., 1990;Rosemann et al., 1991; EckeyKaltenbach et al., 1994;Sharma and Davis, 1994; Booker et al., 1996;Tuomainen et al., 1996;Pääkkö nen et al., 1998). In poplar (Populus maximorwizzii ϫ Populus trichocarpa), higher levels of PAL activity were found to be associated with a greater ozone tolerance (Koch et al., 1998). Other enzymes of the phenolic secondary metabolism, such as 4-coumarate CoA ligase (Eckey-Kaltenbach et al., 1994; Booker and Miller, 1998), caffeic acid O-methyl transferase (Koch et al., 1998), and cinnamyl alcohol dehydrogenase (CAD), the enzyme involved in the synthesis of the monolignols (Galliano et al., 1993a(Galliano et al., , 1993b Eckey-Kaltenbach et al., 1994; Booker and Miller, 1998;Zinser et al., 1998), have been shown to be ozone stimulated at the activity and/or transcript level (for review, see Sandermann et al., 1998).The activation of the phenylpropanoid metabolism also has been reported for other biotic or abiotic stresses (Dixon and Paiva, 1995) such as wounding, pathogen attack, UV irradiation, heavy metals, or drought. Stress-induc...