Variation in arbuscular mycorrhizal fungi colonization modifies the expression of tolerance to above-ground defoliation

P>1. Plant association with arbuscular mycorrhizal fungi (AMF) has been considered a factor increasing plant tolerance to herbivory. However, this positive effect could decrease with colonization density if the benefit : cost ratio of the AMF-plant association changes. We measured plant performance and tolerance to defoliation across a gradient of commercial AMF (Glomus sp.) inoculum concentration. 2. Six genetic families of Datura stramonium were grown under greenhouse conditions and subjected to five increasing levels of AMF inoculum concentration and to defoliation treatments, i.e. the presence/absence of 50% artificial damage, following a full-factorial design. 3. AMF colonization increased linearly with inoculum concentration while foliar area, root mass, flowering phenology and seed production expressed nonlinear functions. Plant genetic variation in the benefit function of AMF colonization was also detected. We show a negative interaction between AMF concentration and plant tolerance to defoliation. 4. Synthesis. The negative correlation between plant tolerance and AMF concentration suggests that defoliation can reduce AMF benefits and that natural variations in AMF can limit the evolution of optimum levels of tolerance. Moreover, genetic variation in the shape of the reaction norms to AMF in the presence/absence of defoliation suggests that plants may evolve in response to variation in densities of AMF and herbivores.