Allocation of carbon to growth and secondary metabolites in birch seedlings under UV-B radiation and CO2 exposure

In plants, the allocation of carbon to secondary metabolites has been shown to be determined by both the availability of resources (e.g., CO2 concentr ation) and by specific stress factors (e.g., ultraviolet [UV]-radiation). I t has been suggested that, in combination, CO2 and UV-B radiation may diffe rentially affect plant growth and morphogenic parameters, and elevated CO2 may ameliorate the effects of UV-B radiation, In the present work, the effe cts of increased atmospheric CO2 concentration and UV-B radiation on growth and the accumulation of different types of secondary metabolites were stud ied in silver birch (Betula pendula Roth), Seedlings were exposed to 350 an d 700 pmol mol(-1) of CO2 in a greenhouse. At both CO2 levels, additional U V-B was either present (8.16 kJ m(-2) day(-1) of biologically effective UV- B irradiance) or absent. The time course of accumulation of individual seco ndary compounds and the shifts in allocation of carbon between biomass and the secondary metabolites (phenolic acids, flavonoids, condensed tannins) w ere studied during a 1-month-long exposure. Additionally, the activities of enzymes (L-phenylalanine ammonia-lyase [PAL], EC 4.3.1.5; peroxidase, EC 1 .11.1.7; polyphenol oxidase, EC 1.10.3.1) were determined for leaves, UV-B radiation significantly increased biomass, PAL activity, and the accumulati on of phenolic acids and flavonoids in seedlings. Elevated CO2 concentratio n increased the activities of all the enzymes studied and the accumulation of condensed tannins in leaves, especially with UV-B radiation. Because the observed UV-B induction of flavonoids was smaller under a high CO2 concent ration, it was suggested that the excess of carbon in the atmosphere may mo derate the effect of UV-B by increasing the metabolic activity of leaves (h igh enzyme activities) and by changing the allocation of internal carbon be tween different primary and secondary metabolites in the plant. Our results demonstrate the significant increase in the allocation of carbon to second ary metabolites without any large change in growth due to the elevation of CO2 concentration and UV-B radiation. There also was a stronger impact of C O2 than UV-B on the phenolic metabolism of birch seedlings.