Increasing global background concentrations of tropospheric ozone (O(3)) are expected to affect both crops and forest ecosystems negatively. The phytotoxic effects of O(3) are mainly associated with the O(3)-induced production of reactive oxygen species (ROS) in excess of the ability of the plant to maintain ROS below the tolerance threshold. It is the balance between O(3) uptake and cellular antioxidant potential that determines O(3) effects on vegetation. The greater sensitivity to ambient O(3) exposure (60 nl l(-1) O(3), 15 days, 5 h a day) of poplar clone Eridano (Populus deltoides x maximowiczii) compared with clone I-214 (P. x euramericana) was reflected in a lower photosynthetic efficiency, higher stomatal conductance and hydrogen peroxide (H(2)O(2)) accumulation and more pronounced leaf tissue membrane injury in Eridano than in I-214. We checked if the differences in clonal responses to O(3) fumigation were related to differing capacities for antioxidant defense and phenylpropanoid metabolism and found that the increases in foliar ascorbate and phenolic concentrations and phenylpropanoid metabolism in Eridano were insufficient to counteract H(2)O(2) accumulation and the consequent oxidative stress. This was probably because the higher influx of O(3) into Eridano leaves compared with I-214 leaves resulted in a lower potential detoxification capacity per unit of O(3) influx.