Environmental Stress Tolerance and Intraspecific Variability in Cortaderia selloana: Implications for Invasion Risk in Mediterranean Wetlands

Cortaderia selloana is an invasive grass spreading rapidly and becoming a serious environmental concern in many areas of the world. The species expanded to the Iberian Peninsula, including its eastern coast, where it increasingly occupies diverse ecosystems. This is the first evaluation of C. selloana’s tolerance to salinity and water deficit, combined with heat stress, during two key developmental stages: germination and early vegetative growth. Experimental trials were conducted using seeds and juvenile plants from two populations. Elevated temperature reduced germination, biomass accumulation, and shoot elongation, particularly when combined with water or salt stress. Drought exerted the strongest inhibitory effect on photosynthetic pigments, whereas salinity mainly affected carotenoid content, mostly in one of the populations analysed. Proline accumulation increased under drought and salinity, reaching up to 70 µmol·g−1 DW, but to a lesser extent when combined with a heat treatment, suggesting enhanced proline catabolism at high temperature. Total soluble sugars tended to increase under water deficit (from ~75 to >100 mg equivalent of glucose g−1 DW), indicating a potential osmoprotective shift from proline to carbohydrates. These results highlight intraspecific variability in stress tolerance and emphasise that C. selloana’s success in Mediterranean environments depends on its capacity to withstand transient but not prolonged combined stresses.