Intraspecies variation in mycorrhizal response of Medicago sativa to Rhizophagus irregularis under abiotic stress.

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Plant partnerships with arbuscular mycorrhizal fungi (AMF) improve plant resilience to stress by increasing the plant's access to and uptake of essential nutrients and water, as well as regulating the plant's stress response. The magnitude and direction of AMF effects during the relationship depend on multiple factors including plant identity and environmental context. To investigate how AMF influence plant responses to environmental stresses, we assessed the effects of drought and salinity on growth, final biomass, and reproduction of nine alfalfa (Medicago sativa) cultivars inoculated with Rhizophagus irregularis or grown alone. In absence of stress, the fungus increased nutrient content, but caused declines in biomass through a reduction in initial growth that was not overcome by a later growth spurt. Mycorrhizal fungus inoculation also magnified stress effects on growth in most scenarios, but this depended on the stress type and cultivar. For salinity, this stress increase in inoculated plants was mediated by increased salt accumulation. Flowering of each cultivar was affected by both inoculation and stress type, albeit erratically, whereas seed production was only affected by inoculation when drought stressed. We found no clear pattern distinguishing differences in mycorrhizal fungus effects on stress among cultivars; however, our results show that mycorrhizal fungus effects on plant stress responses are contingent on the plant performance metric and stress type, highlighting the complexity of responses to mycorrhizas.