Endophyte symbiosis benefits a rare grass under low water availability

1. Symbiotic relationships with microbes may influence how plant species respond to environmental change. Here, we investigated how a fungal endophyte symbiosis affects the growth and survival of a rare, mid-western United States grass species under altered water availability. In a 12-week long greenhouse experiment, we compared the performance of endophyte-infected and endophyte-disinfected grove bluegrass (Poa alsodes, Poaceae) subjected to two levels of water availability. Gravimetric water measurements indicated that the low water treatment had 24% less soil moisture than the high water treatment. 2. In the high water treatment, the performance of endophyte-infected plants and disinfected plants was similar. However, under low water, endophyte-infected plants had 17% more total biomass than disinfected plants. Despite localization of the endophyte in above-ground plant tissues, effects of the endophyte were stronger below-ground: Under low water, endophyte-infected plants produced 24% more root biomass, but just 14% more shoot biomass than disinfected plants. When water-limited, disinfected plants had 29% greater leaf senescence than endophyte-infected plants. 3. We investigated several plant traits that may underlie benefits of symbiosis. Under low water, disinfected plants had significantly higher relative leaf water content than endophyte-infected plants, consistent with the hypothesis that endophytes cause plants to up-regulate water conservation mechanisms faster in response to drought. There were no strong differences between endophyte-infected and disinfected plants in root morphology, leaf area or water use efficiency, although plants with the endophyte had slightly shorter leaves. 4. Our results suggest that endophyte symbiosis may ameliorate the negative effects of drought stress for grove bluegrass. The heritable transmission of endophytes from plants to seeds provides opportunities for environmental change to influence selection on native grass-endophyte symbioses.