European aspen (Populus tremula) is a fast growing tree species, rich in phenolic compounds. Defense theories suggest that soil nitrogen greatly influence plant allocation togrowth and defense; however, the allocation priorities are not well understood. Further,although foliar phenolic compounds are considered defensive, specialist organisms may positively associate with and alter them. There are two classes of phenolics in aspen,condensed tannins (CTs) and salicinoids. They are likely to shape the interactions of themany organisms, for example, herbivorous insects and endophytic fungi and three-way interactions among host genotype, specialist herbivores and endophytic fungi could be greatly altered by aspen geno- and chemotypes’ responses to soil nitrogen.
Firstly, I focused on the allocation of carbon to growth and defense in aspen genotypes with varied tannin content in response to nutrient addition. Nitrogen promoted plant growth and suppressed foliar CT levels. At the molecular level expression of genes of the phenylpropanoid pathway (PPP) decreased under low additions of N (equivalent to 15kg/ha), whereas genes at the beginning and at the end of the pathway increased in response to high levels of N (~150 kg/ha). Aspens high in CTs displayed consistently stronger PPP gene expressions compared to CT-low aspens, and correlations between PPP genes and phenolic products varied with tannin content, as an effect of leaf age, in response to N enrichment, and individually with genotype. More negative correlations (indicative of allocation trade-offs) between PPP gene expressions and phenolic products were found in aspen genets with low tannin levels compared to aspens with inherently high tannin levels.
Secondly, I studied the connection between foliar phenolic compounds and endophytic fungi in the presence and absence of a specialist herbivorous beetle (Chrysomela tremula) and as an effect of soil nitrogen addition. Richness and abundance of fungal endophytes associated with aspen genotypes and phenolic profile, however this specificity disappeared in the presence of the leaf beetles. Herbivory both enhanced endophyte richness andabundance in the leaves and it also increased in response to nitrogen addition.