Context-dependent responses of Festuca rubra subsp. pruinosa to salinity, nutrient availability, and Epichloë festucae.

Festuca rubra subsp. pruinosa is a maritime grass native to sea cliffs, a habitat characterized by high salinity and low nutrient availability. This species forms symbiotic associations with Epichloë festucae, a vertically transmitted endophytic fungus that colonizes aerial tissues. Two experiments evaluated whether E. festucae influences the salinity tolerance of its host. In Experiment 1, symbiotic and non-symbiotic plants were irrigated with saline solution or tap water, whereas Experiment 2 additionally included fertilization. In both experiments, unfertilized symbiotic plants exhibited the highest leaf biomass under saline conditions, whereas in the absence of salinity they showed the lowest biomass. Fertilized symbiotic plants produced less biomass than non-symbiotic ones. Spectral vegetation indices further suggested that symbiosis effects on plant performance under salinity depended strongly on nutrient availability and that its occurrence enhanced fertilized plants resilience to salinity. Overall, these results indicate that the symbiosis between Festuca rubra subsp. pruinosa and Epichloë festucae is context-dependent, conferring benefits under saline and nutrient-poor conditions characteristic of its natural habitat. Regardless of endophyte presence, plant growth was either enhanced or unaffected by salinity, demonstrating the high salt tolerance of Festuca rubra subsp. pruinosa. Salinity increased foliar concentrations of Na, photosynthetic pigments, proline, glycine betaine, glucose, fructose, and sucrose. Additionally, salinity increased the leaf concentration of the fungal alkaloid ergovaline, which was also detected in roots. Anatomical observations revealed a nearly tubular leaf morphology with a thick epicuticular wax layer and stomata confined to the inner adaxial surface, traits likely associated with osmotic stress tolerance.