The persistence of mutualisms suggests that participants are capable of sanctioning “cheaters,” or non-beneficial partners. The symbiotic relationship between arbuscular mycorrhizal fungi and plants is common and widespread. Mycorrhizal fungi can strongly benefit plants in nutrient-poor soils, but they can parasitize plants in nutrient-rich soils. Sanctions against non-beneficial fungi, by plants reducing their carbon allocations to ‘cheaters,’ are expected to reduce the parasitism in this relationship. These sanctions are also expected to reduce mycorrhizal fungal biomass in nutrient-rich soils. Plant species differ in the degree to which they benefit from fungi (“mycorrhizal responsiveness”) even in favorable (nutrient-poor) conditions. Plants are also likely to vary in their ability to sanction non-beneficial fungi and consequently in their susceptibility to parasitism in nutrient-rich soils. However, the relationships among plant benefit, parasitism, sanctions, and soil nutrients are not well understood. I conducted a greenhouse experiment to test the hypothesis that C4 prairie grasses would benefit more strongly from mycorrhizal fungi in low phosphorus (P) soils, suffer more extreme parasitism in high P soils, and less effectively sanction fungi in high P environments than C3 pasture grasses.
As predicted, the two C4 species I tested (Andropogon gerardii and Schizachyrium scoparium) gained more benefit from mycorrhizal fungi than the two C3 species (Bromus inermis and Elymus repens) when P was limiting. However, a species mycorrhizal responsiveness in low P did not predict the degree of parasitism in high P treatments. Only Andropogon showed evidence of strong parasitism; Bromus, Elymus, and Schizachyrium did not. Accordingly, fungal abundance also depended on the identity of the plant species and P availability. This result suggests that plant species differ in the degree to which they can sanction cheating mycorrhizal fungi and thus control the degree to which they are parasitized by a potential symbiont. Plant species differences in vulnerability to parasitism may explain the loss of strongly mycorrhizal responsive species from natural systems undergoing eutrophication. It also raises interesting questions about the maintenance of the symbiosis in rapidly changing environments.