Intraspecific variation in plant–fungal interactions across tidal elevation in a salt marsh

Interspecific interactions vary depending on environmental and genetic factors, with intraspecific variation potentially altering these relationships. Specifically, intraspecific variation within host plants and their endosymbionts may affect above- and belowground interactions depending on environmental conditions. We examined how intraspecific variation within the salt marsh foundation species Spartina alterniflora and a common fungal root endosymbiont (Lulworthia sp.) affected their interactions. We conducted a glasshouse experiment comparing interactions between Spartina growth forms (tall-form from low marsh and short-form from high marsh) and Lulworthia isolates from different marsh locations, and a laboratory experiment exposing Lulworthia isolates from both endpoints of a tidal elevation gradient to different salinities. Intraspecific variation affected fungal characteristics, plant traits, and plant-fungal interactions. Lulworthia isolates distinguished by a single-nucleotide polymorphism had distinct morphologies and different salinity-dependent growth rates. Spartina origin zone affected plant responses. Fungal treatment also affected plant performance: one isolate reduced Spartina density, height, biomass, and respiration - particularly for tall-form plants - whereas the other isolate had neutral to positive effects relative to the control. Our results highlight how intraspecific variation in both fungal isolates and plant genotypes shapes plant-fungal interactions and mediates the capacity of host plants to respond to changing conditions depending on plant ecotype.