Could the differences in O3 sensitivity between two poplar clones be related to a difference in antioxidant defense and secondary metabolic response to O3 influx?

Increasing global background concentrations of tropospheric ozone (O3) are expected to affect both crops and forest ecosystems negatively. The phytotoxic effects of O3 are mainly associated with the O3-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 betweenO3 uptake and cellular antioxidant potential that determines O3 effects on vegetation. The greater sensitivity to ambient O3 exposure (60 nl l-1 O3, 15 days, 5 h a day) of poplar clone Eridano (Populus deltoides × maximowiczii) compared with clone I-214 (P. x euramericana) was reflected in a lower photosynthetic efficiency, higher stomatal conductance and hydrogen peroxide (H2O2) accumulation and more pronounced leaf tissue membrane injury in Eridano than in I-214. We checked if the differences in clonal responses to O3 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 H2O2 accumulation and the consequent oxidative stress. This was probably because the higher influx of O3 into Eridano leaves compared with I-214 leaves resulted in a lower potential detoxification capacity per unit of O3 influx.