Benefits of a fungal endophyte in Leymus chinensis depend more on water than on nutrient availability

• The benefits of the endophyte to the native host depended on water availability.
• The beneficial effect of endophyte infection was less dependent on fertilizer supply.
• EI plants allocated higher fractions of N to photosynthetic machinery than EF plants.
• EI plants had higher photosynthetic N use efficiency than EF plants when fertilizer was limited.

Symbiotic relationships with microbes may influence how plants respond to environmental change. In this study, we tested the hypothesis that symbiosis with the endophyte promoted drought tolerance of the native grass and hypothesized the drought tolerance was affected by nutrient availability. In the greenhouse experiment we compared the performance of endophyte-infected (EI) and endophyte-free (EF) Leymus chinensis, a dominant species native to the Inner Mongolia steppe, under altered water and nutrient availability. The results showed that the benefits of the endophyte to the host depended on water availability. Under well-watered conditions, total biomass was not affected by endophyte infection. Under drought stress conditions, however, EI had significantly more total biomass than EF plants. In contrast to expectations, the beneficial effect of endophyte infection was less dependent on fertilizer supply. In the well-watered treatment, there were no significant differences in total biomass between EI and EF plants regardless of fertilizer levels, and their differences occurred only in biomass allocation. Under drought conditions, EI produced significantly more biomass than EF plants regardless of fertilizer levels. Endophyte infection tended to reduce leaf nitrogen (N) concentration of host leaves but made the host allocate significantly higher fractions of N to photosynthetic machinery, and thus EI plants had higher photosynthetic N use efficiency and shoot biomass than did EF plants when fertilizer was limited. Our results support the idea that the endophyte–grass interactions are dependent on available resources – but in this case they depended more on water than on nutrient availability.