The advantages of endophyte-infected over uninfected tall fescue in the growth and pathogen resistance are counteracted by elevated CO2.

Atmospheric CO2 concentrations are predicted to double within the next century. Despite this trend, the extent and mechanisms through which elevated CO2 affects grass-endophyte symbionts remain uncertain. In the present study, the growth, chemical composition and pathogen resistance of endophyte-infected (E+) and uninfected (E-) tall fescue were compared under elevated CO2 conditions. The results showed that the effect of endophyte infection on the growth of tall fescue was significantly affected by elevated CO2. Significant advantage of E+ over E- tall fescue in tiller number, maximum net photosynthetic rate and shoot biomass occurred only under ambient CO2. With CO2 concentration elevated, the beneficial effect of endophyte infection on the growth disappeared. Similarly, endophyte infection reduced lesion number and spore concentration of Curvularia lunata only under ambient CO2. These results suggest that the beneficial effect of endophyte infection on the growth and pathogen resistance of tall fescue could be counteracted by elevated CO2. An explanation for the counteraction may be found in a change in photosynthesis and nutritive quality of leaf tissue.