Background
The authors introduce their study by stating that "ozone is one of the most powerful oxidants known," reporting that "in plants, primary damage is largely confined to the leaf mesophyll, where ozone [O3] dissolves into the wet surface of the exposed cell walls (Kangasjarvi et al., 1994)."  They also note that "reactions of ozone with water and solutes in the apoplasm lead to the formation of reactive oxygen species (ROS) including hydrogen peroxide (H2O2), hydroperoxide, superoxide, hydroxyl radicals and singlet oxygen (Foyer et al., 1994; Kangasjarvi et al., 1994; Wohlgemuth et al., 2002)," and that "it is generally assumed that damage by ozone is mainly the result of ROS formation, promoting oxygen toxicity in the plant cell (Podila et al., 2001)."  What is more, they report that "about 25% of the global forests are currently at risk from damaging ozone concentrations during the growing seasons, and this is predicted to further expand to 50% by year 2100 (Fowler et al., 1999)."

What was done
At the Aspen FACE site near Rhinelander, Wisconsin, USA, saplings of three aspen (Populus tremuloides) genotypes and seedlings of paper birch (Betula papyrifera) that had been exposed to ambient and elevated (1.5x ambient) ozone concentrations and ambient and elevated (ambient + 200 ppm) CO2 concentrations, singly and together since 1998, were assessed for O3-induced leaf damage and the degree to which that damage was or was not ameliorated by the experimental enrichment of the air with CO2.

What was learned
Oksanen et al. say they "were able to visualize and locate ozone-induced H2O2 accumulation within leaf mesophyll cells, and relate oxidative stress with structural injuries in aspen and birch."  However, they say that "H2O2 accumulation was found only in ozone-exposed leaves and not in the presence of elevated CO2," adding that "CO2 enrichment appears to alleviate chloroplastic oxidative stress."

What it means
In this study, it would appear that the detrimental effects of the damage-dealing addition of ozone to the ambient air were completely countered by the beneficial effects of the damage-healing addition of CO2 to the air; and in light of the tremendous potential for already-elevated and still-increasing ozone concentrations to adversely affect all types of vegetation [see Ozone (Effects on Plants) in our Subject Index], we can be thankful the air's CO2 concentration is continuing to rise in tandem with its ozone concentration.

References
Foyer, C.H.  1996.  Oxygen processing in photosynthesis.  Biochemical Society Transactions 24: 427-433.

Foyer, C., Lelandais, M. and Kunert, K.  1994.  Photo-oxidative stress in plants.  Physiologia Plantarum 92: 224-230.

Kangasjarvi, J., Talvinen, J., Utriainen, M. and Karjalainen, R.  1994.  Plant defense systems induced by ozone.  Plant, Cell and Environment 17: 783-794.

Podila, G.K., Paolacci, A.R. and Badiani, M.  2001.  The impact of greenhouse gases on antioxidants and foliar defense compounds.  In: Karnosky, D.F., Ceulemans, R., Scarascia-Mugnozza, G.E. and Innes, J.L. (Eds.).  The Impact of Carbon Dioxide and Other Greenhouse Gases on Forest Ecosystems.  CABI Publishing, Vienna, Austria, pp. 57-125.

Wohlgemuth, H., Mittelstrass, K., Kschieschan, S., Bender, J., Weigel, H.-J., Overmyer, K., Kangasjarvi, J., Sandermann, H. and Langebartels, C.  2002.  Activation of an oxidative burst is a general feature of sensitive plants exposed to the air pollutant ozone.  Plant, Cell and Environment 25: 717-726.