Morphology drives water storage traits in the globally widespread lichen genus Usnea

Water and vapor transport in algal‐fungal lichen: Modeling constrained by laboratory experiments, an application forFlavoparmelia caperata

“…There exist laboratory methods to estimate the hydraulic and mechanical properties of fungus, algae, storages of external water, and their interfaces. We summarize these methods as: Volume–pressure relationships (void‐ratio and water‐ratio) of individual fungus and algae cells can be developed similar to how others have measured the osmotically driven expansion of guard cells in higher plants (Franks et al, ; Franks, Cowan, & Farquhar, ; Meidner & Edwards, ; Zimmermann & Steudle, ). Eriksson et al () have distinguished between contributions of internal (fungus and algae) and external (extracellular pore‐space) compartments, which may allow for development of pressure–water content relationships of the pore‐space component in the model. Ten Veldhuis et al () have differentiated between the evaporation rates of top and bottom surfaces of the thallus by sealing ends with tape. We believe this method can be extended to interpret each side's bulk conductance to water vapor. If the vapor conductance of nonlichenized fungal filaments can be determined experimentally, then the conductance of pores can be estimated as the difference in bulk lichen and fungal conductances.…”

“…• Eriksson et al (2018) have distinguished between contributions of internal (fungus and algae) and external (extracellular pore-space) compartments, which may allow for development of pressurewater content relationships of the pore-space component in the model.…”

“…We conceptualize lichen as a multiple‐porosity, swelling porous media system comprised of three main compartments for water storage: (1) fungal filaments that store liquid water, (2) algae that store liquid water and exist solely within the photobiont layer, and (3) the variably saturated extracellular pore‐space between bundles of filaments and cells that contains both liquid water and vapor. Adopting the terminology of Eriksson et al (), fungal filaments and algae cells both represent stores for “internal” water, while the liquid water and vapor in the extracellular pore‐space represents the “external” water that suppresses photosynthesis when supersaturated by slowing diffusion of gasses to the photobiont (Green et al, ; Lange et al, , ). We conceptualize the fungal and pore storages as vertically continuous domains that are laterally differentiated from one another and which span the four main morphological layers (upper cortex, photobiont layer, medulla, and lower cortex), bound between the interfaces of lichen, atmosphere, and the substrate below.…”

“…A similar approach would provide deeper insights into the fundamental biology of lichen. Applying a photosynthesis model like Farquhar et al (1980) to lichen would require first estimating the carbon dioxide concentration in the chloroplasts in the photobionts and consequently the thallus's resistance to the carbon dioxide transport, which depends on the saturation of "external" pathways (apoplast; Eriksson, Gauslaab, Palmqvista, Ekström, & Esseen, 2018), because of the inherently slower diffusion of gasses in water than in air (Green, Sancho, & Pintado, 2011;Lange et al, 1993Lange et al, , 2001. Estimating the photobiont's carbon dioxide concentration is further complicated by the possibility of gas recycling between mycobiont and photobiont.…”

“…3) the variably saturated extracellular pore-space between bundles of filaments and cells that contains both liquid water and vapor. Adopting the terminology ofEriksson et al (2018), fungal filaments and algae cells both represent stores for "internal" water, while the liquid water and vapor in the extracellular pore-space represents the "external" water that suppresses photosynthesis when supersaturated by slowing diffusion of gasses to the photobiont(Green et al, 2011;Lange et al, 1993, …”