Peltigera rufescens

Peltigera rufescens, the field dog lichen or field pelt, is a species of terricolous (ground-dwelling), foliose lichen in the family Peltigeraceae. The common and widespread species has a cosmopolitan distribution. The lichen has a surface that is densely covered with a soft, velvety layer, extending from the edges to the centre. The edges of this lichen typically curl upwards, revealing that can be up to 10 millimetres wide. Beneath, it possesses a network of rhizines, which are root-like structures that merge together into a thick mat, anchoring the lichen to its.

Taxonomy
The lichen was first formally described as a variety of Lichen caninus in 1770 by Friedrich Wilhelm Weiss. At that time, lichens were classified in the eponymous genus Lichen, based on the influence of Carl Linnaeus and his 1753 work Species Plantarum. Alexander von Humboldt transferred the taxon to the genus Peltigera and promoted it to the status of species in 1793. Vernacular names used for the species in North America include "field dog lichen" and "field pelt".

The complete mitochondrial genome sequence of Peltigera rufescens was published in 2021. It has 65,199 base pairs and a cytosine+guanine content of 26.7%. Molecular phylogenetic analysis suggests a close relationship with Peltigera membranacea.

Description
Peltigera rufescens has a grey to brown coloured thallus that is often covered with a heavy tomentum (closely matted or fine hairs). It forms rosettes up to 20 cm in diameter. The lobes comprising the thallus usually measure 5 – wide and have edges that are curled upwards. The undersurface of the thallus is strongly veined, dark in colour with a paler margin. Rhizines on the underside affix the lichen to its substrate; near the centre they are so dense so as to form an almost continuous mat. Fruit bodies, or apothecia, are common in this species. They are saddle shaped and dark red brown in colour. Isidia and soredia are absent in this species. The ascospores usually have between 3 and 5 septa and measure 40–70 by 3–5 μm. Conidiomata are sometimes produced by the lichen; the conidia are 7–10 by 2.5–4.5 μm. The photobiont partner of Peltigera rufescens is cyanobacteria from the genus Nostoc.

No lichen products are associated with Peltigera rufescens, and consequently, the expected results of standard lichen spot tests are all negative.

If grown in a metal-polluted environment, Peltigera rufescens will have a reduction in thallus size and in rhizine length, as well as denser growth of the rhizines, veins that are more profusely branched, and an increase in volume of the medulla. When treated with mercury, cadmium, or nickel, P. rufescens chlorophyll α and carotenoid concentrations will also decrease. As with other lichens, P. rufescens is a bioaccumulator of heavy metals. This may be due to the thallus having a large surface area in contact with the substrate. P. rufescens also has free amino acid concentrations higher than other lichens in similar polluted habitats, comparable to vascular plants, suggesting that this may play a role in heavy metal tolerance.

Reproductive structures
In 1971, Marie-Agnès Letrouit-Galinou and R. Lallemant published a study on the thallus, apothecia, and asci of Peltigera rufescens, using histological techniques and microscopic examination. The thallus was described as having a "cladomian" structure, evolving from axial filaments that are lodged in the veins. These filaments give rise to both short ventral pleuridia (lateral branchlets) and well-developed dorsal ramifications. The formation of apothecia is initiated by a limited number of marginal dorsal pleuridia, leading to a primary thallus resembling those in the families Lecanoraceae and Graphidaceae, with a distinct development process marked by a lengthy angiocarpic stage (a phase in the development where the apothecium is initially formed as a closed structure, enclosing the asci and ascospores). The asci were identified as and 'archaeasce', signifying a complex reproductive structure. Additionally, the research investigated the cladomian, multi-axial structure of the thallus in detail, comparing it to similar structures in algae and highlighting the differentiation between dorsal and ventral pleuridia, which contribute to the thallus's growth and structure.



Habitat and distribution
Peltigera rufescens is a common and widespread lichen with a cosmopolitan distribution. It is most often encountered in dry, sunny habitats. It prefers more or less basic soils. It also grows on limestone and dolomite (rarely on silicate rocks) as well as nutrient-rich silicate soils, including diverse environments like calcareous grasslands and stone structures. Individuals that grow in association with mosses tend to grow more robustly and have a higher amount of chlorophyll α then those that do not. Not only does the moss provides a buffer against extremes in temperature variation, the moss-associated thalli have higher photosynthetic rates, and increased protection against desiccation. Additionally, their thalli are thicker, leading to enhanced water retention. Mosses that have been recorded associating with Peltigera rufescens include Racomitrium heterostichum, Campylopus introflexus, Hypnum cupressiforme, and Polytrichum juniperinum. A study of the high-elevation biological soil crust associated with the volcanic tephra in Hawaii's Haleakalā Crater found that Peltigera rufescens was one of the two important components of this crust (the other was the moss Grimmia torquata) and occurred in about a quarter of soil specimens sampled.

Ecology
Preussia peltigerae, Dinemasporium strigosum, Lichenopenicillus versicolor, Nectriopsis lecanodes, Norrlinia peltigericola, and Scutula didymospora are lichenicolous fungi that use Peltigera rufescens as a host. In the case of Scutula didymospora, the relationship appears to be commensalistic, as the fungus, which develops on the underside of the thallus, does not cause any damage, discolouration or galls.

In one experiment, to test the effect of thallus hydration on metabolic activity, the photosystem II fluorescence of Peltigera rufescens was monitored for a full year. The lichen was inactive for 46.5% of the time, active during daylight for 25.6%, and hydrated at night for 27.9% of the time. Its photosynthetic activity and moisture levels were correlated with environmental conditions, with four distinct activity patterns discerned. Despite previous experimental findings suggesting high light could be harmful when the lichen is hydrated, field observations found little evidence of damage, suggesting an unknown photoprotection mechanism possibly involving certain carotenoids. In a study examining the effects of long-term UV-B radiation on lichen species, Peltigera rufescens, typically found in open meadow spaces, demonstrated increased hydrogen peroxide content and superoxide dismutase activity, indicating a possible adaptive response to oxidative stress caused by UV-B exposure. This lichen species displayed a higher resilience to UV-B compared to Peltigera aphthosa, suggesting a species-specific response to UV-B radiation that likely stems from their typical habitat's light conditions.

Bioactivity
Laboratory experiments suggest that extracts of Peltigera rufescens have insecticidal activity against the maize weevil (Sitophilus zeamais).