Root-associated mycorrhizal fungi affect plant growth and resource allocation. Our major aim was to explore the plant-internal mechanisms behind the effects of mycorrhizal colonization on leaf chemistry of willow (Salix spp.). Combinations of 2 willow varieties (Loden, Tora) and 2 ectomycorrhizal fungal species (Hebeloma fastibile, Tricholoma cingulatum) were grown under controlled conditions. Host plant variety and fungal species effects on host resource allocation (biomass and leaf chemistry) varied in a complex way. Shoot biomass growth and allocation was mostly affected by host plant variety, whereas leaf and root biomass allocation were strongly affected by mycorrhizal treatment. Leaf biomass production was affected by willow variety, mycorrhizal treatment, and the interaction between them. The results indicate a strong effect of mycorrhizal colonization on host plant biomass allocation, which can mediate mycorrhizal effects on leaf chemistry. For example, leaf biomass allocation was inversely correlated with foliar concentrations of salicylic acid, suggesting a functional link between the two. However, the apparent complexity of the host plant variety-fungal species interaction makes any prediction of their possible outcome very difficult, especially in an ecological context. Still, fungal species identity seems to be more important for the responses of Salix to mycorrhizal colonization than the amount of fungal biomass.