English plantain deploys stress tolerance mechanisms at various organization levels across an altitudinal gradient in the Pyrenees.

High-mountain plants must withstand high solar irradiation and low temperatures during winter. Furthermore, climate change is increasing drought events, which pose an additional threat to plants. Here, we studied the stress tolerance mechanisms at various levels of biological organization in English plantain (Plantago lanceolata L.), focusing on photoprotective and antioxidant responses. The response of populations from three different altitudes in the Eastern Pyrenees (1030, 1380, and 1660 m. a.s.l.) was compared during both autumn and winter. Results showed that plants not only suffered from photoinhibition due to very low temperatures at the highest elevation during winter, but also from mild drought stress at the lowest altitude during autumn. Individuals growing at the highest elevation showed reductions in the maximum photochemical efficiency of PSII (Fv /Fm ratio), which might be caused by the lack of an increased induction of tolerance mechanisms at the highest elevation compared to the intermediate one. Although most leaves died at the highest elevation, plants could withstand stress at the organism level by generating new leaves once the stress ceased. Drought at the lowest elevation during autumn caused mild stress with small decreases in the Fv /Fm ratio, along with an increase in abscisic acid and jasmonic acid content. This study underlines the great capacity of English plantain to adapt to high elevation by activating not only photo- and antioxidant protection mechanisms and adjustments in stress-related phytohormones, but also by fully regenerating its aboveground biomass through renewed growth once the stress has ceased.