Water immersion effects on seed germination and <i>Epichloë</i> endophyte viability in <i>Festuca arundinacea</i> : A comparison between a naturalized population and a cultivar

Abstract Tall fescue ( Festuca arundinacea Schreb.) is a perennial, cool‐season forage grass that typically hosts the fungal endophyte Epichloë coenophiala , which enhances stress tolerance. In Flooding Pampa grasslands (Argentina), tall fescue has become highly dominant, with increasing prevalence of endophyte‐symbiotic plants. Concurrently, commercial cultivars continue to be introduced and established in the region. Given the recurrent flooding of these grasslands, it was hypothesized that naturalized populations with wild‐type endophytes have undergone adaptation and plant–endophyte coevolution, resulting in greater tolerance to water excess at the seed‐to‐seedling stage than commercial cultivars. Here, a comparison was made between two tall fescue origins − a naturalized population with its wild‐type endophyte and a commercial cultivar hosting the selected endophyte strain AR584 − under immersion treatments. Over 5 weeks, seed germination and endophyte viability were assessed across water and temperature combinations. Immersion progressively reduced seed viability with increasing time and temperature, but this decline was more pronounced in the cultivar–AR584 combination. The inhibitory effect of immersion was strongest at the optimal germination temperature (20°C) and weakened at sub‐ and supra‐optimal temperatures. Inhibition was generally stronger in endophyte‐symbiotic than in endophyte‐free seeds in both origins, a pattern that persisted at higher temperatures (25 and 30°C). Seed mortality increased under immersion with higher temperature, with no consistent effect of endophyte symbiosis. The naturalized population maintained high endophyte viability across treatments. These results indicate that the naturalized population−wild‐type endophyte association displayed traits conferring tolerance to water excess not observed in the cultivar−AR584 combination, supporting the hypothesis of adaptation to local conditions.