Low Resource Availability Differentially Affects the Growth of Host Grasses Infected by Fungal Endophytes

Grasses infected with endosymbiotic ascomyceteous fungi (family Clavicipitaceae) often show increased growth and vigor; however, when resources are limited, the costs of being infected may outweigh the advantages. We grew multiple genotypes of two grasses species (Lolium perenne and Lolium arundinaceum) infected (or uninfected) with Neotyphodium sp. under limited light or water availability and measured both biomass and relative growth rates. There was a significant interaction between water stress and infection in L. perenne, while in L. arundinaceum, there were significant differences between infected and uninfected plants but no significant interaction. In both species, there were significant interactions between light level and infection. At full sun, infected plants were larger, while under reduced light, uninfected plants produced as much biomass as or more than infected plants. Although infected plants have increased vigor under high resource levels, infection will not necessarily result in increased host growth when resources are limiting. The costs and benefits of symbiotic associations should be assessed in a variety of environments to better understand the potential range of interactions.