Can the DSE Fungus Exserohilum rostratum Mitigate the Effect of Salinity on the Grass Chloris gayana?

Dark septate endophytes (DSEs) are commonly found in saline environments, such as the Flooding Pampas (Argentina), where the forage grass <i>Chloris gayana</i> has been introduced. This study evaluated the effect of salinity on the DSE fungus <i>Exserohilum rostratum</i>, isolated from <i>C. gayana</i>, and its contribution to the grass's salinity tolerance. Two greenhouse experiments were conducted under three salinity levels (0, 40, and 80 meq Na·L^-1), with and without fungal inoculation. Fungal growth, root colonization, functional traits, plant biomass, chemical composition, and salinity tolerance indices were assessed. The fungus tolerated salinity and colonized roots, showing qualitative evidence of enzyme production and phosphate solubilization. In both experiments, shoot and root biomass decreased with increasing salinity. Inoculation significantly enhanced shoot biomass only under non-saline conditions in the first experiment, whereas in the second experiment no inoculation effect was observed on shoots. For roots, no effect of inoculation occurred in the first experiment, but a positive interaction between salinity and inoculation was recorded in the second experiment, where moderate salinity increased root biomass in inoculated plants. The K/Na and Ca/Na ratios decreased under salinity regardless of inoculation, indicating limited influence on ionic balance. These results suggest that although <i>E. rostratum</i> tolerates salinity and expresses functional traits, its ability to enhance plant performance under stress is context-dependent and restricted to specific conditions.