Water Distribution in Foliose Lichen Species: Interactions between Method of Hydratation, Lichen Substances and Thallus Anatomy

Abstract

Three lichens (Neofuscelia pokornyi, N. pulla and Xanthoria parietina) from a semi-arid habitat were examined using low-temperature scanning electron microscopy to evaluate the e ects of hydration method, lichen substances and thallus anatomy on the water distribution of hydrated thalli. In the Neofuscelia species, extracellular water within the thallus was observed in association with cracks in its otherwise impervious upper cortex, while X. parietina showed abundant extracellular water between medullary hyphae. Spraying the thalli followed by maintenance for 14±20 h in a water-saturated atmosphere led to the disappearance of the external water ®lm in X. parietina but not in the Neofuscelia species. Surface water was abundant in specimens of all species immediately after spraying for 15 min. No extracellular water was observed inside the thallus 14±20 h after spraying, but after rinsing with acetone its presence was detected in all three species. Hydric strategy correlated with cortex hygroscopicity: X. parietina, an aero- hygrophytic species, had a more hygroscopic upper cortex than the Neofuscelia species, which are substrate- hygrophytic. The hygroscopicity of the upper cortex was linked with the amount of extracellular water in the thalline interior. Di erences between X. parietina and Neofuscelia in the polarity and distribution of their lichen substances agreed with species di erences in the presence and distribution of free water both as a ®lm over the surface and inside the thallus. Lichen substances appear to play a role in the maintenance of air-®lled intrathalline spaces in species whose anatomy, habitat, or both, favour water-logged conditions.