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 a nd thallus anatomy on the water distribution of hydrated thalii. In the Neo fuscelia species, extracellular water within the thallus was observed in as sociation with cracks in its otherwise impervious upper cortex, while X. pa rietina showed abundant extracellular water between medullary hyphae. Spray ing the thalli followed by maintenance for 14-20 h in a water-saturated atm osphere led to the disappearance of the external water film in X. parietina but not in the Neofuscelia species. Surface water was abundant in specimen s of all species immediately after spraying for 15 min. No extracellular wa ter was observed inside the thallus 14-20 h after spraying, but after rinsi ng with acetone its presence was detected in all three species. Hydric stra tegy correlated with cortex hygroscopicity: X. parietina, an aerohygrophyti c species, had a more hygroscopic upper cortex than the Neofuscelia species , which are substrate-hygrophytic. The hygroscopicity of the upper cortex w as linked with the amount of extracellular water in the thalline interior. Differences between X. parietina and Neofuscelia in the polarity and distri bution of their lichen substances agreed with species differences in the pr esence and distribution of free water both as a film over the surface and i nside the thallus. Lichen substances appear to play a role in the maintenan ce of air-filled intrathalline spaces in species whose anatomy, habitat, or both, favour water-logged conditions. (C) 2000 Annals of Botany Company.