Perennial ryegrass (Lolium perenne) is a cool-season grass species that is often infected by the leaf-inhabiting endophyte Neotyphodium lolii. This particular endophyte is asexual and has the potential to impact host survival, growth and reproduction. The objective of this research was to assess the potential costs and/or benefits of endophyte infection on drought stress tolerance of perennial ryegrass populations from its native range in the Mediterranean region. Four accessions of Lolium perenne originally collected from Italy, Morocco, Tunisia, and Turkey were tested for the effects of their fungal endophyte on plant growth. Sixty infected (E+) individuals from each accession were planted in a greenhouse and half of the individuals were treated with a systemic fungicide to eliminate the endophyte (E-). There were two drought periods where water was withheld for 10-14 days and then a one week recovery period. In the control group all plants were watered as needed, in the drought treatment water was withheld and soil moisture was as low as 0%. Response variables recorded for the experiment were number of tillers, tiller length, and dry mass of roots and shoots at harvest.
Under drought, E+ plants had more tillers, greater tiller lengths, total mass and root mass, than E- plants, suggesting a positive effect of endophyte infection on host growth. For the accessions from Morocco and Tunisia, the number of tillers and total tiller length measured after the second drought period showed significant treatment*population*infection interactions. Endophyte infected individuals from these accessions grew better under drought than noninfected individuals. The effects of endophytes were specific for particular host populations as indicated by a significant population*infection interaction. E+ grasses from Turkey, showed significantly greater total mass and shoot mass that was alive and photosynthetic at harvest. This work is one of the few that documents the benefits of endophyte infection for a grass species from its native distributional range. The results demonstrate that endophyte infection can increase plant performance under abiotic stress such as drought and there may be a selective advantage for grasses from certain Mediterranean regions to have mutualistic interactions with endophytes.