Print PageThe presence and identity of mycorrhizal fungi can strongly affect plant growth, survival, and reproduction in natural and managed systems. Although mycorrhizal fungal densities vary within and among environments, the effect of mycorrhizal abundance on plant response and trophic consequences has rarely been examined. Using predictions from mutualism theory, we hypothesized that increasing density of arbuscular mycorrhizal fungi in the soil would increase both plant nutrition and carbon costs. However, at high fungal density, carbon costs will overtake nutrient benefits, resulting in a nonlinear effect of mycorrhizal fungi on plant growth, defense, and caterpillar growth response. We germinated Asclepias syriaca seedlings from five genetic families and planted them into soil inoculated with mycorrhizal fungi Glomus etunicatum, Scutellospora fulgida, or a mix of the two species in one of ten levels of fungal density for a total of 2000 seedlings (N=10/treatment). When plants were three months old, Danaus plexippus (monarch) eggs were introduced on a subset of the plants and larvae allowed to eat for five days before harvest, after which caterpillar growth rate was assessed. Plant biomass, mycorrhizal colonization, and plant defense traits including trichome density, latex, toughness, phosphorus content and cardenolides were quantified.
Results/Conclusions
Increasing density of fungi strongly affected A. syriaca growth and defense. Inoculation with either G. etunicatum or S. fulgida alone caused nonlinear, but divergent responses in biomass, whereas increasing inoculation of both species together linearly increased plant biomass. Plant nutrition and defense traits also responded strongly to fungal density, but the shape of the response curve varied depending on the trait and fungal species examined. Mycorrhizal density treatment explained between 10-70% of variation in plant growth and defense traits, whereas plant genotype and caterpillar herbivory explained much less, from 0-20% of variation in plant growth traits. Monarch caterpillar growth rate also responded strongly to increasing fungal inoculation density—caterpillars grew larger on plants with high levels of fungal inoculation--although this increase saturated at high fungal abundance. Although not all traits responded as hypothesized, we found some evidence to suggest that mutualism theory can aid predictions of plant defense expression. We conclude that the abundance and identity of mycorrhizal fungi can interact to strongly influence plant growth, defense traits, and the growth rate of primary consumers.
