Water distribution in foliose lichen species: Interactions between method of hydration, lichen substances and thallus anatomy

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 effects of hydration method, lichen substances and thallus anatomy on the water distribution of hydrated thalii. 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 film 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 aerohygrophytic 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. Differences between X. parietina and Neofuscelia in the polarity and distribution of their lichen substances agreed with species differences in the presence and distribution of free water both as a film over the surface and inside the thallus. Lichen substances appear to play a role in the maintenance of air-filled intrathalline spaces in species whose anatomy, habitat, or both, favour water-logged conditions. (C) 2000 Annals of Botany Company.