Experimental studies with foliose macrolichens: fungal responses to spatial disturbance at the organismic level and to spatial problems at the cellular level during drought stress events

Abstract

Improved culture methods and the application of cryotechniques in light and electron microscopy have allowed new insights at the organismic and cellular levels in mycobiont–photobiont relations of selected lichen species. Growth, regenerative capacity, and the ability to cope with spatial disturbances were tested in the foliose macrolichens Xanthoria parietina and Parmelia sulcata. These mycobionts can sense and partly correct an inverted position and thus insure adequate illumination for the photobiont cell population. In both taxa, the apical/marginal pseudomeristem and adjacent elongation zone bend backwards. Parmelia sulcata continues this bending process and grows arch-like until its margin reaches the substratum. Xanthoria parietina stops bending and growing but regenerates new primordial stages along the marginal rim and cut edges. These primordia develop into new lobes with a normal upside up topography. Its remarkable regenerative capacity permits X. parietina to develop a new rosette from an assembly of minute thallus fragments that had been fixed to a ceramic substratum. Cryotechniques applied at the light and electron microscopy levels allow us to visualize drought stress induced structural alterations at the cellular level in lichen bionts. Neither partner of the symbiosis plasmolyses under extreme drought stress. The globose to ovoid green algal or cyanobacterial photobiont cells and their walls shrivel dramatically but the fungal cell wall cannot be deformed to such a high degree during stress events. Ascomycetous lichen mycobionts of three different orders formed a large, intracellular gas bubble of unknown ontogeny and content when their turgor pressure dropped to very low values. This cytoplasmic "air bag" allows the protoplast to shrink and, at the same time, to keep the cell membrane in contact with the wall. In the desiccated state many lichens tolerate extreme stress conditions. Thallus fragments of dry X. parietina grew normally after having been cryofixed in subcooled liquid nitrogen, sputter coated, and examined in a low temperature scanning electron microscope. Fully hydrated samples did not survive this treatment. Key words: culturing experiments, regenerative capacity, drought stress, LTSEM, freeze substitution, intracellular gas bubble.