Modelling hydration and photosystem II activation in relation to in situ rain and humidity patterns: a tool to compare performance of rare and generalist epiphytic lichens

Abstract

A dynamic water and activity model was developed to assess how efficiently lichens can exploit in situ rain and humid air. The capacity to rehydrate and activate photosynthesis [i.e. photosystem II (PSII)] by these water sources was compared among four hydrophilic and one generalist epiphytic lichen. Hydration status, potential (instant activation) and realized (delayed activation) day-light activity were simulated using a model based on species-specific hydration, PSII activation characteristics and in situ water content for Platismatia norvegica in three microclimatic scenarios. The results showed that delayed PSII activation could have profound effects on lichens' ability to exploit environmental water sources. During rain, realized activity was reduced by 19, 34 and 56% compared to simulations assuming instant activation for three hydrophilic lichens in the driest microclimate. During humid air, the reduction was 81% for the most extreme species and scenario, because of slow hydration and low equilibrium water content. Many and brief hydration events may thus hamper species with slow activation and fast desiccation kinetics. No evidence of compensation by a 'water-holding' morphology was observed among studied species. The developed model may provide a tool for identifying suitable habitats for long-term persistence of lichens with physiological constraints.

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

“…For the water retention curve fitted to observations (black line, λ = 0.11%), the duration of wetting was ~4 times that for drying (Fig. b) in agreement with earlier studies reported in the literature (Jonsson et al, ; Jonsson‐Čabrajić et al, ). This ratio between wetting and drying times was larger for more nonlinear water retention curves and ranged between 0.65 for the purely linear and 5.90 for the exaggerated nonlinear curve.…”

“…Wetting would aid transport of nutrients from mycobiont to photobiont, while drying would aid transport of sugars and carbohydrates from photobiont to mycobiont. Additionally, our results that potentials are not constantly uniform between fungus and algae may partially explain the time‐lag between the onset of bulk lichen saturation and the turning on of photosynthesis (Jonsson‐Čabrajić et al, ; Lidén et al, ). Bulk saturation best represents the moisture status of the mycobiont, since it makes up most of the thallus, while photosynthetic potential depends only on the hydraulic status of the photobiont.…”

“…Under this framework, the turning off of lichen photosynthesis under desiccation (Lange et al, ) could be explained by the reduction in maximum photosynthetic potential ( V cmax and J max in Farquhar et al, ) under highly negative matric water potentials (Vico & Porporato, ) or under excess sugar concentrations (Hölttä, Lintunen, Chan, Mäkelä, & Nikinmaa, ) in the photobiont. We believe that this type of photosynthesis model in coordination with a model framework that distinguishes between photobiont and mycobiont as separate water reservoirs may also be able to describe the time‐lag between the onset of bulk lichen saturation and the turning on of photosynthesis during wetting (Jonsson‐Čabrajić, Lidén, Lundmark, Ottosson‐Löfvenius, & Palmqvist, ; Lidén, Jonsson‐Čabrajić, Ottosson‐Löfvenius, Palmqvist, & Lundmark, ). Simulating dynamic lichen photosynthesis in response to fluctuating water availability will require differentiating the hydration status of mycobiont and photobiont to describe the transport of water and gasses through various pathways within the thallus and to describe the various phenomenon in lichen photosynthesis, namely (1) the suppression of photosynthesis at low water contents, (2) suppression of photosynthesis at supersaturated water contents where gas diffusion is limited, and (3) the time‐lag for photosynthetic activity to peak after initial wetting.…”

Estimating global carbon uptake by lichens and bryophytes with a process-based model

“…A further property of the relation between water content and metabolic activity is that in some species, the metabolic activity corresponding to a certain water content is only reached after a time delay (Jonsson et al, 2008;JonssonČabrajić et al, 2010;Lidén et al, 2010). The delay is not only species-specific, but it also depends on the length of the preceding dry period (Ried, 1960;Gray et al, 2007;Proctor, 2010).…”

Photosynthetic traits of freshwater lichens are consistent with the submersion conditions of their habitat

“…The present differences between sub-and hyper-hydrophilic freshwater lichens were smaller than those of species-specific patterns of PSII activation timelags and water-holding capacity which allowed Liden et al (2010) to explain habitat restriction. However, following Jonsson-Cabrajic et al (2010), we consider that slightly slower activation (6 min against 1 min) and higher sensitivity of PSII to desiccation may be important factors to explain the confinement of the most freshwater-related species to habitats that provide sufficiently long hydration periods. Indeed, small differences in activation/deactivation time-lag could strongly affect the lichen's long-term performance if hydration/desiccation events are brief and frequent.…”