Background
Many plants respond to elevated CO2 concentrations by reducing their stomatal conductance.  This phenomenon decreases the indiscriminate uptake of ozone (O3) and other noxious gasses, which can cause oxidative cellular damage and reduce potential plant growth and development.  Thus, at elevated CO2 concentrations, there typically are less oxidants entering plants, so there generally is a reduced need for synthesizing and activating enzymes that function as antioxidants.  Because sugar maple does not reduce its stomatal conductance in response to elevated CO2, it is unclear how it will respond at the cellular level to ozone pollution when enriched with atmospheric CO2.

What was done
One-month-old sugar maple (Acer saccharum) seedlings were grown in pots within environmental chambers for approximately six weeks at ambient (350 ppm) and elevated (700 ppm) atmospheric CO2 concentrations in combination with and without an extra 200 ppb ozone to study the effects of elevated CO2 and ozone on the functioning of several foliar antioxidant enzymes.

What was learned
Elevated CO2 and ozone, alone and in combination with one another, increased the activity of ascorbate peroxidase, which is the first line of enzymatic defence against ozone that has diffused into plant tissues, by an average of 150%.  Similarly, elevated CO2, ozone, and the combinations thereof, increased the activity of catalase, which breaks down toxic hydrogen peroxide into benign water and oxygen molecules, by 80%.  Thus, atmospheric CO2 enrichment increased the activities of these two enzymes that function to keep cells from experiencing oxidative damage, particularly to their membranes.

In contrast, however, the combination of elevated CO2 and ozone significantly reduced the activities of two other antioxidizing enzymes, glutathione reductase and superoxide dismutase, by 34 and 47%, respectively.  Thus, atmospheric CO2 enrichment decreased the activities of two enzymes responsible for reducing oxidative damage within cells.

What it doesn't mean
As the CO2 content of the air rises, there is no compelling reason to believe that sugar maple will experience increased oxidative stress from ozone, as implied by the authors who concluded that "elevated CO2, concomitant with O3, does not play any positive role on the activity of the studied antioxidant enzymes."  In this particular study, the activities of four distinct enzymes were examined with respect to elevated CO2 and ozone, and the observed responses were clearly split down the middle between positive and negative.  However, if the mean response of all four enzymes is calculated, a positive response results.  Nonetheless, these results indicate that further studies are necessary before a concrete conclusion can be reached regarding antioxidant activities in sugar maple exposed to elevated CO2 and ozone.

On another note, this study did not report any growth responses, which could have been used to assess whether or not oxidative stress was truly occurring in the seedlings.  Without evidence to show that a particular factor negatively impacted growth, which is the bottom line for plants, one can not label that factor as a bonafide stress.

What it means
As the CO2 content of the air rises, it is still unclear as to how antioxidizing enzymes in sugar maple seedlings will respond to the uptake of ozone, as some specific antioxidants exhibit increases in their activities while others display decreases.  Likewise, without any photosynthetic or growth data, it is difficult to determine whether or not changes in antioxidizing enzymes are truly occurring to the benefit or the detriment of the plant.  Thus, further research is needed to reach any solid conclusions on this topic with respect to sugar maple seedlings.  However, elevated CO2 has been demonstrated to decrease oxidative stress in other species (see our Journal Reviews on the Effects of Lifetime Exposure to Elevated CO2 on Antioxidative Enzymes in Mature Oak Trees and the Effects of Elevated CO2 and Nitrogen Supply on Antioxidative Enzymes in Beech Seedlings).