Plants interact with both their belowground and aboveground environments, but the interaction between these different domains is under-studied. The belowground environment can directly and indirectly affect plant aboveground interactions via changes in nutrient acquisition, biomass accumulation, defense against antagonists, and floral traits. We examined how plant interactions with the belowground biotic and abiotic environment scale up to alter floral traits and, in turn, pollinator disease. Soil-dwelling mycorrhizal fungi are considered plant mutualists, although the outcome of this relationship can depend on environmental conditions such as nutrients. In a 2x2 factorial design, we asked whether Nicotiana tabacum (tobacco) plants grown with or without arbuscular mycorrhizal fungi (AMF) in high or low nutrients affect nectar and pollen alkaloids previously shown to reduce bumblebee disease. To answer how plant interactions affect bee health, we fed pollen and nectar from our treatment plants, and a control of wildflower pollen and artificial nectar, to the native bumblebee Bombus impatiens infected with the gut parasite Crithidia bombi.
Results/Conclusions
Our results show that mycorrhizae and fertilizer interact to influence flowering phenology and floral alkaloid concentrations. Plants grown with high nutrients had higher pollen alkaloid concentrations, and AMF reduced pollen nicotine concentrations under high, but not low, nutrient conditions. Furthermore, bees consuming diets from plants grown under different fungal and nutrient conditions experienced different levels of infection; bees eating diets from plants grown without AMF and low fertilizer experienced lowest infection, and bees eating diets from plants grown without AMF and high fertilizer experienced highest infection. These differences in infection did not correspond with differences in alkaloid concentrations, indicating that another mechanism underlies outcomes for bee pathogen infection. These results show that soil environment can affect aboveground pollinators via changes in plant floral traits, and underscore the importance of integrative studies to predict ecological outcomes relevant to managing pollinator health.