Invasive plant species are destructive and costly, yet we do not fully understand barriers to invasion. Species interactions certainly play a role, but belowground mutualists have been understudied despite their important role in providing nutrients to plants. Additionally, mutualistic bacteria and fungi may increase the fitness of plants when faced with abiotic stress. Genetic variation in the ability to associate with rhizobia may affect the invasion success of some legumes. Here we address: 1) Is there genotypic variation in an invasive plant’s resistance to water stress and its association with rhizobia? and 2) Do rhizobia affect genotype-specific responses to drought? We used 40 genotypes of an invasive legume, Medicago polymorpha, to manipulate water conditions and the presence of rhizobia. We measured differences in root to shoot ratio and root nodule density, as well as plant biomass as a fitness proxy.
We found genotypic variation in the response of Medicago to water stress. Genotypes also varied in their response to rhizobia and the number of rhizobia nodules affected their response to drought. Fitness responses to drought were dependent upon whether plants previously evolved in native or invasive ranges. Medicago genotypes from the native range had higher biomass when rhizobia were present in low water conditions, while invasive Medicago genotypes derived a larger benefit from rhizobia when water was plentiful. The proportion of resources devoted above or belowground in response to water stress was also mediated by both rhizobia and origin. These results suggest that mutualistic rhizobia may play a role beyond a simple resource mutualism, but may also protect plants from stressful abiotic conditions. Moreover, the variation in genotypes that we observed in this species that has successfully invaded many parts of the world suggests that genetically determined traits, such as drought tolerance or rhizobia association, may play a role in invasive species success and range expansion.