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Effects of Lifetime Exposure to Elevated CO2 on Antioxidative Enzymes in Mature Oak Trees
Schwanz, P. and Polle, A.  1998.  Antioxidative systems, pigment and protein contents in leaves of adult mediterranean oak species (Quercus pubescens and Q. ilex) with lifetime exposure to elevated CO2New Phytologist 140: 411-423.

What was done
The authors measured various parameters in leaves of mature holm and white oak trees that had been growing near natural CO2 springs in central Italy for 30 to 50 years in order to determine the effects of elevated CO2 on their antioxidative systems.

What was learned
Elevated CO2 significantly decreased the activities of superoxide dismutase, which detoxifies highly reactive oxygen species, by approximately 30 and 47% in leaves of holm and white oak trees, respectively, when compared with activities measured in leaves of trees growing some distance away from the CO2-emitting springs.  Trees of both species growing near the springs also exhibited lower activities of catalase and other enzymes involved in the degradation of hydrogen peroxide (H2O2), which is produced during photorespiration.  Thus, atmospheric CO2 enrichment generally decreased the activities of protective enzymes that reduce oxidative stress brought about by unfavorable environmental factors such as drought, high light intensity, high air temperature, and aerial pollutants.

In order to determine whether or not the CO2-induced decreases in the antioxidative machinery of the CO2-enriched trees increased their susceptibility to oxidative damage, the authors evaluated the degree of lipid peroxidation within their leaves.  They determined that trees growing near the CO2-emitting springs did not display increased levels of lipid peroxidation in their leaves, and in some cases, they actually exhibited significant reductions in their amounts of lipid peroxidation.

What it means
As the CO2 content of the air increases, most plants will likely experience an amelioration of unfavorable environmental growing conditions, which often lead to oxidative stresses within leaves.  This study is the first to identify changes in leaf physiology that have persisted for decades in response to elevated CO2.  Specifically, it shows that reductions in antioxidative enzymes, which have been observed in seedlings, can persist indefinitely as trees mature.  Furthermore, because these enzymes remove reactive compounds that can cause cellular damage, their reduced activities at high CO2 implies that as the amount of CO2 in the air increases, plants experience less intrinsic oxidative stress and produce fewer harmful oxidants.  Thus, this beneficial consequence of atmospheric CO2 enrichment should allow plants to increase their productivity, growth, and yield as the air's CO2 content continues to climb.

Reviewed 15 May 1999