Background
"Phenolics," in the words of the authors, "seem to have evolved in response to abiotic and biotic challenges and, thereby, have enabled plant survival under different growth conditions."  As a general rule, therefore, self-created phenolic compounds are some of a plant's best friends; and, as Peltonen et al. report, "part of the extra carbon assimilated as a consequence of increased photosynthesis under CO2-enriched atmospheres is directed to the synthesis of phenolic compounds."

What was done
To see how the beneficial suite of CO2-enhanced phenolic synthesis activities may be affected by concurrent increases in the air's ozone concentration, the three Finnish researchers studied the impacts of doubled atmospheric CO2 and O3 concentrations on the accumulation of 27 phenolic compounds in the leaves of two European silver birch (Betula pendula Roth) clones (V5952 and K1659) in seven-year-old soil-grown trees that were exposed in open-top chambers for three growing seasons to ambient and twice-ambient atmospheric CO2 and O3 concentrations singly and in combination in central Finland.

What was learned
Peltonen et al. report that "elevated CO2 increased the concentration of the phenolic acids (+25%), myricetin glycosides (+18%), catechin derivatives (+13%) and soluble condensed tannins (+19%) by increasing their accumulation in the leaves of the silver birch trees."  Elevated O3, on the other hand, increased the concentration of one glucoside by 22%, chlorogenic acid by 19%, and flavone aglycons by 4%.  However, Peltonen et al. say that this latter O3-induced production of antioxidant phenolic compounds "did not seem to protect the birch leaves from detrimental O3 effects on leaf weight and area, but may have even exacerbated them."  Last of all, in the combined elevated CO2 and O3 treatment, they found that "elevated CO2 did seem to protect the leaves from elevated O3 because all the O3-derived effects on the leaf phenolics and traits were prevented by elevated CO2."

What it means
Once again, a carefully conducted experiment demonstrates that the negative effects of a doubling of the air's ozone concentration all may be overcome by a doubling of the air's carbon dioxide concentration.  For more on this point, see the several sub-headings under Growth Response to CO2 With Other Variables (Ozone) in our Subject Index.