Plasticity in the growth habit prolongs survival at no physiological cost in a monocarpic perennial at high altitudes.

Monocarpic plants are those that flower, produce seeds and then die. Although most monocarpic plants are annual or biennial, some of them are perennial. However, relatively little is known of the biology of monocarpic perennials. Pyrenean saxifrage (Saxifraga longifolia Lapeyr.) is a monocarpic perennial that is well adapted to high-mountain ecosystems. Here, we evaluate altitudinal changes in clonality in various populations growing in their natural habitat with particular emphasis on the physiological costs of clonal growth.We assessed the percentage of clonal plants in nine populations growing in their natural habitat, as well as the plant stress response of clonal and non-clonal plants, in terms of photoprotection and accumulation of stress-related phytohormones, in a three-year study at Las Blancas (2,100 m a.s.l.). We also evaluated the influence of plant size on the activation of defensive responses to biotic and abiotic stresses.We found that 12% of Pyrenean saxifrage plants growing at the highest altitudes (2,100 m a.s.l.) produced lateral rosettes which survived the flowering of the main rosette and shared the same axonomorphic root, thus escaping monocarpic senescence. This clonal growth did not worsen the physiological performance of plants growing at this altitude. Furthermore, increased plant size did not negatively affect the physiology of plants, despite adjustments in endogenous stress-related phytohormones. In contrast, maturity led to rapid physiological deterioration of the rosette, which was associated with monocarpic senescence.This study shows that the evolution of clonality has allowed Pyrenean saxifrage to survive harsh environmental conditions and it provides evidence that harsh environments push plant species to their limits in terms of life form and longevity.