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Russula virescens is a basidiomycete mushroom of the genus Russula, and is commonly known as the green-cracking Russula, the quilted green Russula, or the green brittlegill. It can be recognized by its distinctive pale green cap covered with darker green patches, its crowded white gills, and its white stem. A popular edible fungus with a mild or nutty taste, its distribution encompasses Britain, Europe, and Asia, where it occurs solitary or scattered on the ground in both deciduous and mixed forests, forming mycorrhizhal associations with hardwood trees like oak and European beech. Its distribution in North America has not been clarified, due to confusion with the similar species Russula parvovirescens and Russula crustosa.[1][2] The ribonuclease enzyme of R. virescens has been studied and shown to have a unique biochemistry compared to other edible mushrooms.
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Russula virescens was first described by Jacob Christian Schaeffer in 1774 as Agaricus virescens. Subsequently the taxon was transferred to the genus Russula by Elias Fries in 1836.[3][4] It is the type species of subsection Virescentinae in the Russula, a grouping of mushrooms characterized by a cap surface that breaks into patches of bran-like (furfuraceous) particles.[5]
The specific epithet virescens is Latin for "becoming green".[6] The characteristic pattern of the cap surface has earned the species common names such as green-cracking Russula, the quilted green Russula,[7] and the green brittlegill.[8] In the mid-Atlantic United States, it is also known locally as the moldy Russula.[9]
The cap is at first dome or barrel-shaped, becoming convex and flattened with age with a diameter of up to 15 cm (5.9 in). The cuticle of the cap is green, most profoundly in the center, with patches of the same color dispersed radially around the center; the distinct pattern is called areolate.The green color of the cuticle is often of variable shade, ranging from gray to verdigris to grass-green. The extent of the patching of the cuticle is also variable, giving specimens with limited patches a resemblance to other green-capped species of Russula, such as Russula aeruginea. The green patches of the cap lie on a white to pale green background. The cap, while frequently round, may also exhibit irregular lobes and cracks. The cap cuticle is thin, and can be readily peeled off the surface to a distance of about halfway towards the cap center.[8] The gills are white to cream colored, and fairly crowded together; most of the gill is fused to the stem, a so-called adnate attachment. The gills are interconnected at their bases by veins.[10] The stem is cylindrical, white and of variable height, up to 8 cm (3.1 in) tall and 4 cm (1.6 in) wide;[11] it is roughly the same thickness at both the top and the base. Then top portion of the stem may be farinose—covered with a white, mealy powder.[12] It may turn slightly brown with age, or when it is injured or bruised from handling.[13] Like other species in the Russulales order of mushrooms, the flesh is brittle, owing to the sphaerocyst cytoarchitecture—cylindrical cells that contrast with the typical fibrous, filamentous hyphae present in other orders of the basidiomycota.[14]
The spores of R. virescens are elliptical or ellipsoid with warts, translucent (hyaline), and in deposit produce a white, pale or pale yellow spore print;[7][15] the spore dimensions are 6–9 by 5–7 µm.[7] The spore-bearing cells, the basidia, are club-shaped and with dimensions of 24–33 by 6–7.5 µm; they are colorless, and each hold from two to four spores. The pleurocystidia (cystidia located on the face of a gill) are 40–85 by 6–8 µm and end abruptly in a sharp point.[16]
Russula virescens is an edible mushroom considered to be one of the best of the genus Russula, and is a popular edible in Europe.[9][15][17][18] Its smell is not distinctive, but its taste has been described as mild, or nutty.[2][10] When collecting Russula virescens for consumption, caution is of vital importance to avoid confusion with the dangerously poisonous Amanita phalloides (known as the death cap), a mushroom that can be most easily identified by its volva and ring.[19]
Another green-capped Russula is R. aeruginea, but this species may be distinguished from R. virescens by its smaller size and smooth cap.[20] Russula crustosa, like R. virescens also has an areolate cap, but the cap becomes sticky (viscid) when moist, and its color is more variable, as it may be reddish, yellowish, or brown.[7][8] Also, the spore print of R. crustosa is a darker yellow than R. virescens.[8] R. redolens has a cap that is "drab-green to blue-green", but unlike R. virescens, is smooth. Russula redolens also has an unpleasant taste and smells of parsley.[21]
Russula virescens can be found fruiting on soil in both deciduous forests and mixed forests,[8] forming ectomycorrhizal symbiotic relationships with a variety of trees,[22] including oaks (Quercus) and European beech (Fagus sylvatica). Fruiting bodies may appear singly or in groups,[23] reappear in the same spots year after year, and are not common. In Britain and Europe, fruiting occurs mainly during the months of summer to early autumn.[12] A Mexican study of the seasonal occurrence of several common mushroom species in subtropical forests in Xalapa showed that the fruiting period of R. virescens occurred in April, before the onset of the rainy season.[24]
The distribution of R. virescens in North America is subject to debate, where a number of similar species such as R. parvovirescens and R. crustosa are also recognized.[1][2] One author suggests that R. virescens "is strictly a European species",[25] citing Buyck and collaborators (2006), who say "the virescens-crustosa group is much more complex than suspected and embraces at least a dozen taxa in the eastern US".[1] In addition to Britain and Europe, Russula virescens has also been collected from Malaysia,[26] Korea,[27] and China.[28]
Ribonucleases (or RNases) are enzymes that catalyze the hydrolysis of ribonucleic acid (RNA), and collectively they play a critical role in many biological processes. A RNase from R. virescens was shown to be biochemically unique amongst seven edible mushroom species in several ways: it has a co-specificity towards cleaving RNA at poly A and poly C, compared to the monospecific RNases of the others; it can be adsorbed on chromatography columns containing DEAE–cellulose as the adsorbent; it has a pH optimum of 4.5, lower than all other species; and, it has a "distinctly different" N-terminal amino acid sequence.[29]