Do secondary substances in the thallus of a lichen promote CO 2 diffusion and prevent depression of net photosynthesis at high water content?

Abstract

Many lichens show seriously depressed net photosynthesis (NP) at high thallus water contents due to increased carbon dioxide diusion resistance through blockage of diusion pathways by water. The soil lichen Diploschistes muscorum, however, shows no depression and NP is close to maximal even at the highest thallus water content. We investigated whether lichen substances (lecanoric and diploschistesic acids) in the cortex and medulla contributed to this ability to maintain high NP. Dry thalli were extracted with water-free acetone and, after this treatment, were found to be fully viable to the extent of continued growth after replanting in the ®eld. No dierences were found in the response of NP to thallus water content between the normal and extracted thalli, in fact the response curves were often nearly identical. Thus, in this species it seems that lichen substances did not maintain the water-free diusion pathways and some other explanation, possibly structural, needs to be sought.

Interfacial Self-Assembly of Fungal Hydrophobins of the Lichen-Forming Ascomycetes Xanthoria parietina and X. ectaneoides

“…A Xanthoria phenotype with easily wettable hyphae in the thalline interior is predicted to hardly be functional under natural conditions since it would become waterlogged and photosynthates, as normally passively translocated within the apoplastic continuum underneath the hydrophobic wall surface layer, would leak out. As in artificially waterlogged (supersaturated) thalli the gas exchange rate and thus the productivity of the photoautotrophic partner would decrease (Lange et al, 1996(Lange et al, , 1997. Finally, the ever-present microbial epibionts would invade the thalline interior.…”

“…Numerous taxa of lichen-forming ascomycetes secrete large amounts of phenolic secondary metabolites which crystallize either within the extracellular matrix of the tissue-like peripheral cortex (cortical lichen compounds) or on the wall surfaces of the loosely interwoven aerial hyphae in the thalline interior (medullary compounds) and partly even on the algal wall surface (Honegger, 1986). These deposits of mycobiont-derived, crystalline secondary metabolites render the wall surfaces remarkably hydrophobic (Armaleo, 1993;Lange et al, 1997). However, large numbers of lichen-forming fungi do not secrete secondary metabolites in their medullary and algal layers, yet have water-repellent wall surfaces (Honegger and Peter, 1994).…”

Lichen palatability depends on investments in herbivore defence

“…Unlike the secondary compounds of plants, lichen compounds are deposited as numerous tiny crystals outside living cells of fungal hyphae in the upper cortex, and/or the upper part of the medulla where the photobiont is located. Pure acetone can extract such lichen compounds and does not harm desiccated lichens, which means that lichen compounds can be completely extracted from living lichens Gauslaa 1996, 2001;Lange et al 1997). As long as the viability of a lichen is unaffected, which should always be checked (Solhaug and Gauslaa 2001), acetone only extracts compounds located extracellularly.…”

Light screening in lichen cortices can be quantified by chlorophyll fluorescence techniques for both reflecting and absorbing pigments

“…Atranorin may possibly function as a hydrophobic substance preventing water from entering air spaces in the cortex and thereby raising the reflection. A hydrophobic function has been tested for compounds located outside the medullary hyphae, but with negative results (Lange et al 1997). However, medullary hyphae are covered with the strongly water-repellents hydrophobins that are apparently not present in cortical hyphae (Scherrer et al 2002).…”

“…However, as atranorin crystals screen throughout the entire spectrum, the technique with two wavelengths will not function. In our study, we assume that photosynthesis will not be adversely affected by removal of atranorin with acetone, as the photosynthesis earlier has been shown to be insensitive to acetone rinsing when done with air-dry lichens (Lange et al 1997). This implies that U PSII will be the same in both pieces of each thallus pair with equal light level hitting the chloroplasts.…”