Atmospheric change may affect plant phenolic compounds, which play an important part in plant survival. Therefore, we studied the impacts of CO2 and O3 on the accumulation of 27 phenolic compounds in the short-shoot leaves of two European silver birch (Betula pendula Roth) clones (clones 4 and 80). Seven-year-old soil-grown trees were exposed in open-top chambers over three growing seasons to ambient and twice ambient CO2 and O3 concentrations singly and in combination in central Finland.
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, but decreased the flavone aglycons (−7%) by growth dilution. Elevated O3 increased the concentration of 3,4′-dihydroxypropiophenone 3-β-d-glucoside (+22%), chlorogenic acid (+19%) and flavone aglycons (+4%) by inducing their accumulation possibly as a response to increased oxidative stress in the leaf cells. Nevertheless, this induction 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. On the other hand, 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. The effects of the chamber and elevated CO2 on some compounds changed over time in response to the changes in the leaf traits, which implies that the trees were acclimatizing to the altered environmental conditions. Although the two clones used possessed different composition and concentrations of phenolic compounds, which could be related to their different latitudinal origin and physiological characteristics, they responded similarly to the treatments. However, in some cases the variation in phenolic concentrations caused by genotype or chamber environment was much larger than the changes caused by either elevated CO2 or O3.