What was done
Three-year-old beech seedlings were grown for two years in open-top chambers with 355 or 700 ppm atmospheric CO2 and weekly applications of nutrient solutions of low, medium, or high nitrogen content to study the interactive effects of these variables on oxidative stress within leaves.

What was learned
Elevated CO2 significantly decreased the activity of catalase, which is the primary enzyme responsible for detoxifying hydrogen peroxide (H2O2) produced during photorespiration, regardless of nutrient supply.  Similarly, elevated CO2 decreased the activity of superoxide dismutase, which detoxifies another reactive oxygen compound.  In this case, however, decreases in this enzyme were only significant for the low and medium nutrient treatments.  Since the presence of these enzymes is triggered by the presence of reactive compounds that can cause cellular damage, their reduced concentrations at high CO2 implies that growth in elevated CO2 causes less oxidative stress and production of harmful oxygen compounds than typically occurs in ambient CO2.

What it means
As the CO2 content of the air increases, most plants will likely experience concomitant decreases in oxidative stresses, which trigger the production of reactive compounds that can cause cellular damage.  Thus, plants should require reduced concentrations of the enzymes that detoxify these harmful substances; and this phenomenon should allow valuable resources to be invested elsewhere in the plant, which may (to some extent) actually keep low nutrient-induced oxidative stresses from occurring in the first place.