Does a foliar endophyte improve plant fitness under flooding?

Abstract

Although endophytic fungi are ubiquitous in plants, their full range of ecological effects has yet to be characterized, particularly in non-agronomic systems. In this study, we compared the responses of two congeneric bluegrass species to flooding. Both plant species co-occur in subalpine zones of the Rocky Mountains. Marsh bluegrass (Poa leptocoma) commonly hosts a vertically transmitted fungal endophyte (Epichloe sp.) and naturally grows in wetter conditions than does nodding bluegrass (Poa reflexa), which lacks an epichloid endophyte. We investigated the novel hypothesis that endophyte symbiosis promotes host fitness under flooded conditions, contributing to niche differentiation between the two bluegrass species. We used a factorial greenhouse experiment to test whether endophyte presence improved survival, growth, or reproduction of P. leptocoma under flooded versus non-flooded edaphic conditions by experimentally removing the endophyte from half of the plants. We compared P. leptocoma responses to those of the endophyte-free congener. In contrast to expectations generated from the natural distributions of the two plant species, endophyte presence was more beneficial to P. leptocoma under ambient soil moisture than under flooding. Increased benefits of symbiosis in drier soils are consistent with studies of other grass endophytes. Flooded soils also unexpectedly improved the growth of P. reflexa more than that of the wet habitat specialist, P. leptocoma. While our results demonstrate an overall benefit of fungal symbiosis in this system, ecological factors other than flooding per se likely underlie the observed geographical distributions of these congeneric grasses in nature.