Recovery from drought stress in Lolium perenne (Poaceae): are fungal endophytes detrimental?

Perennial ryegrass (Lolium perenne) is a cool-season, perennial species widely used for forage and turf. It is often infected by a clandestine, endophytic fungus (Neotyphodium lolii) that has the potential to affect host growth responses to abiotically stressful conditions. In some species, the grass-endophyte symbiosis is mutualistic, but the relationship is reported to be contingent on environmental conditions and host genotype in L. perenne. The objective of this research was to determine the potential effects of endophyte infection on recovery from severe drought stress in variable genotypes of a perennial ryegrass cultivar. Sixteen infected (+E) and 16 uninfected (-E) ramets were planted in the greenhouse for each of 10 ryegrass genotypes. Eight +E and eight -E plants per genotype were exposed to three sequential droughts where water was withheld for 11-14 d, resulting in <5% soil moisture; the others (control) were watered as needed. Response variables were tiller numbers 1 wk and 4 wk after drought, and leaf area and dry mass of shoots and roots 7 wk after drought. In both control and drought, -E plants had more tillers, and greater leaf area and total mass, than +E plants, suggesting a detrimental effect of endophytic fungi. Fungal hyphae survived the drought and were abundant in post-drought, +E plants. The effects of endophytes were specific for particular host genotypes, as exemplified by significant genotype × endophyte interactions. Root : shoot ratio and percent of mass allocated to tiller bases (a rough measure of resource storage) showed genotype × endophyte × drought interactions. There was plasticity for root : shoot ratio and genetic variation in the ability to restore root growth during recovery from drought. For 7 of 10 genotypes, -E plants showed an equal or greater allocation to tiller bases than +E plants following drought recovery, illustrating a cost to endophyte infection for some genotypes. The symbiotic relationship between L. perenne and its endophyte primarily benefits the fungus, not the host, under many environmental conditions.