Mycorrhizal inoculation mitigates damage from an intermediate, but not severe, frost event for a cool-season perennial bunchgrass

Extreme cold events can damage plant tissues, altering growth and reproduction. Soil fungi may help plants tolerate environmental stressors, but the role these microbes play during episodes of severe cold warrants further examination. Using the bunchgrass Elymus canadensis L., we tested how inoculation with mycorrhizal fungi alters plant tolerance to freezing temperatures (tested at –8 °C and –16 °C). We found that, regardless of mycorrhizal inoculation, E. canadensis exposed to –16 °C exhibited greater tissue damage, less tiller growth, and fewer reproductive tillers than plants exposed to the control or –8 °C conditions. Plants exposed to –8 °C and –16 °C displayed greater levels of visible damage compared with the control plants. Mycorrhizae reduced damage to tillers in the –8 °C treatment, but had less effect on tiller damage in the control or –16 °C treatments. Inoculation with arbuscular mycorrhizal fungi limited the tiller number for E. canadensis, but only at the control temperature, suggesting that mycorrhizae may impose costs on E. canadensis under benign thermal conditions. Our study demonstrates that extreme temperatures can affect multiple components of growth in E. canadensis, and that the costs and benefits of arbuscular mycorrhizal fungi, where found, depend upon the thermal environment. Our findings reinforce the overarching importance of historically rare, but increasingly common, environmental extremes in shaping the growth of plants.