Water relations in lichens visualized with cryo-techniques: an ultrastructural approach

Abstract

Desiccation tolerance, i.e. reviviscence after having reached air dryness (= water contents <20% ' dw^-1) and the very rapid re-establishment of metabolic activities upon rehydration are most fascinating biological properties of lichen-forming fungi and their green algal or cyanobacterial photobionts. This paper aims at summarizing recent ultrastructural data on thalline water relations, especially on the fate of mycobiont and photobiont cells under extreme drought stress. These aspects have been investigated at the electron microscopy level by means of cryotechniques, i.e. by ultra-rapid cryoimmobilisation of the specimens at any level of hydration, followed by either low temperature scanning electron microscopy (LTSEM) of the frozen-hydrated samples or by freeze-substitution, embedding and ultrathin sectioning for transmission electron microscopy (TEM). The location of free water and the routes of solute translocation within the lichen thallus were visualized with LTSEM techniques. In all species so far examined were the photobiont and medullary layers gas-filled at any level of hydration since free water was restricted to the symplast and to the apoplast, the latter being delimited by a proteinaceous, hydrophobic cell wall surface layer. Mycobiont and photobiont cells of all lichen species so far investigated were dramatically deformed under drought stress. Their protoplast was strongly condensed, most organelles shrivelled but the cellular membrane systems could be surprisingly well resolved. Plasmolysis was observed in neither cell type. A rapidly expanding, cytoplasmatic gas bubble of unknown origin and contents allows drought-stressed fungal protoplasts to shrink and, at the same, to keep their plasmamembrane in close contact with the cell wall. This cytoplasmic“air bag”has a smooth lining but is not membrane-bound. Such drought-stress-induced cytoplasmic gas bubbles are reversible and disappear rapidly during the rehydration process. Biological aspects of desiccation tolerance of lichens are compared with published data on other pokilohydric organisms among proand eukaryotic microorganisms and bryophytes and on drought stress tolerant cell types (pollen, spores, seeds) or organs (shoots of so-called ‘resurrection plants’) among vascular plants.

Key Words:

• cryofixation
• cytoplasmic gas bubble
• desiccation tolerance
• drought stress-induced cell shrinkage
• freeze substitution
• hydrophobic cell wall surface layer
• LTSEM
• poikilohydric water relations
• TEM