Secondary metabolites and nutrient concentrations in silver birch seedlings under five levels of daily UV-B exposure and two relative nutrient addition rates

Changes in secondary metabolites in silver birch (Betula pendula) seedlings are presented in response to ultraviolet-B radiation (UV-B) and nutrient addition. Concentrations of individual secondary metabolites and nutrients were measured in leaves of greenhouse-grown silver birch seedlings exposed to five time-integrated irradiances of biologically effective UV-B (I-BE(UV-B)) and fertilized with two relative nutrient addition rates. Several phenolics were not only induced by UV-B, but their concentration was dependent on UV-B daily time-integrated irradiance. Relative nutrient addition rate also affected the concentration of phenolics but had little effect on the UV-B daily time-integrated irradiance-responses. The mineral nutrient concentration of leaves was affected by nutrient addition but not by increasing I-BE(UV-B). Possible threshold-exposures for the accumulation of secondary metabolites or nutrients were not detected. UV-B and relative nutrient addition rate have mainly an additive (rather than synergistic) effect in silver birch, and leaves respond to increasing UV-B by synthesizing metabolites (e.g. flavonols), which are important UV-B filters. This study reports the first UV-B-exposure-response curves for individual secondary metabolites and nutrients in leaves of a woody tree species.