Advantages and disadvantages of being poikilohydric in the context of lichen sensitivity to adverse habitat factors

Piotr Osyczka

Lichens represent poikilohydric organisms that cannot actively regulate water content inside the thallus and their hydration status is regulated almost completely passively. Therefore, lichens are constantly exposed to an alternating desiccation and hydration cycles, while the regularity and duration of these cycles are largely determined by the climate of a given geographical region, local weather conditions and specificity of the habitat. The lack of advanced water‐holding mechanisms is usually considered in the context of a feature that causes lichens largely metabolically inactive, which in turn affects their performance in a habitat. The prolonged period of desiccation and metabolic inactivation may pose a critical problem for sensitive hygrophilous species. On the other hand, desiccation tolerance and the ability to anhydrobiosis enable lichens to survive unfavorable or extreme climatic conditions, such as high temperatures and high insolation. In their natural habitat, lichens have to find a proper balance between being in a state of hydration or dehydration depending on the current conditions in the environment. This specificity of lichens raises many interesting problems to be investigated and discussed.