Host plant benefit from association with arbuscular mycorrhizal fungi: variation due to differences in size of mycelium

Plants differ in the benefit they receive from an association with arbuscular mycorrhizal fungi (AMF) but it is unclear what factors determine host response. We tested whether this difference is due in part to differences among AMF families (Glomaceae, Gigasporaceae, and Acaulosporaceae) in the size of their mycelium. For example, an AMF with a large mycelium might be better able to transfer nutrients to its host. Alternatively, the large mycelium might present a large carbon drain for the host. It might also be the location of AMF mycelium. that is important: whether it occurs in the root or in the soil. In a greenhouse experiment, four different host plant species were inoculated with each of 21 AMF isolates representing different mycelial. sizes and we measured both host plant biomass and foliar phosphorus concentration over 12 weeks. AMF family significantly affected host biomass with larger internal mycelia conferring greater host benefit. However, both intra-family variation in AMF effect and host identity also influenced host response. For foliar P, AMF family did not affect host response significantly. Our results show that differences in mycelial size among AMF families may contribute to variation in host responsiveness to AMF, but other factors are also important.