Tropospheric ozone is considered as one of the most important air pollutants affecting crop and forest vegetation. Necrosis may be the result of a long term exposure of plants to low ozone concentrations, but biochemical processes are affected prior to any detectable symptoms of injury. Initially, the critical level of ozone exposure associated to biomass losses was based on the seasonal sum of the external concentrations of the pollutant above 40 nL L-1 (AOT 40). Recently, a novel concept of “effective ozone flux” has been proposed. This concept takes into account the ozone flux in the leaf through stomata and the internal defense capacity of the foliar tissues. Among defense processes, the detoxification reactions play a prominent role and we suggested to also consider some parameters linked to the generation/state of the reducing power which is essential to sustain the functioning of the detoxifying systems. In this report, we will present preliminary results on poplar and wheat species exposed to chronic ozone fumigations. In the leaves of both species, the pool of NAD(P)H appeared quite well maintained. This result could be the consequence of a stimulatory effect on the activity of enzymes implied in the biosynthesis of the reduced pyridine nucleotides. The stimulation of the phosphoenolpyruvate carboxylase (PEPc) activity in these tissues validates this hypothesis: PEPc is involved in the biosynthesis of malate, a mobile form of reducing power in the cell. In addition, we have shown that the activity of enzymes involved in NADPH biosynthesis (malic enzymes, glucose-6-phosphate dehydrogenase, etc.) was stimulated in different plant species exposed to a chronic ozone fumigation. We are running additional experiments to precise their level of regulation (transcriptional, translational or post-translational) and to use some of these key enzymes as indicators to improve the risk assessment for plants exposed to ozone.