Water relation parameters of six Peltigera species correlate with their habitat preferences

Water relation parameters were measured in six congeneric lichen species with different requirements for water availability and with green algae (Peltigera aphthosa, Peltigera leucophlebia, Peltigera venosa) or cyanobacteria (Peltigera horizontalis, Peltigera praetextata, Peltigera rufescens) as main photobionts. Pressure-volume analysis was performed with a dewpoint hygrometer and integrated with anatomical analyses. The Peltigera species typical of arid environments were characterized by relatively lower osmotic potential (π0) and turgor loss point (ΨTLP), and higher values of bulk modulus of elasticity (ɛ). Both π0 and ΨTLP were correlated with the size of medullary cells, while ɛ was negatively correlated with cell dimensions. The adaptive value of low ΨTLP might reside in the capability to maintain cell turgor for longer time intervals under dry conditions. High ɛ might allow xerophilous lichens to regain cell turgor more promptly even for small amounts of water uptake, thus enlarging the cumulative period of positive carbon balance in environments with fluctuating water availability. The influence of the photobiont type is also discussed.