Erysimum, or wallflower, is a
genus of
flowering plants in the cabbage family,
Brassicaceae. It includes more than 150
species, both popular garden plants and many wild forms.[1][2][3][4][5][6] The genus Cheiranthus is sometimes included here in whole or in part. Erysimum has since the early 21st century been ascribed to a monogeneric
cruciferoustribe, Erysimeae, characterised by
sessile,
stellate (star-shaped) and/or
malpighiaceous (two-sided)
trichomes, yellow to orange flowers and multiseeded
siliques.
Morphology
Wallflowers are
annuals,
herbaceousperennials or
sub-shrubs. The perennial species are short-lived and in cultivation treated as
biennials. Most species have stems erect, somewhat winged,
canescent with an
indumentum of bifid hairs, usually 25 ± 53 cm × 2–3 mm in size, and t-shaped
trichomes. The leaves are narrow and sessile. The lower leaves are linear to oblanceolate pinnatifid with backwardly directed lobes, acute, 50–80 mm × 0.5–3 mm. Stem leaves are linear, entire, all canescent with 2-fid hairs; 21–43 mm × 1.5–2 mm.
Inflorescences are produced in racemes, with bright yellow to red or pink bilateral and
hermaphrodite,
hypogynous and
ebracteateflowers. Flowering occurs during spring and summer. One species, Erysimum semperflorens,
native to Morocco and Algeria, has white flowers. The floral
pedicel ranges from 4 to 7 mm. Four free
sepals somewhat
saccate, light green, 5–7 mm × 1.5–2 mm.
Wallflowers are native to temperate Eurasia, North Africa and
Macaronesia, and North America south to Costa Rica. Many wallflowers are
endemic to small areas, such as:
E. teretifolium (endangered – inland sandhills of Santa Cruz County, California)
Cultivation
Most wallflower garden
cultivars (e.g.
Erysimum 'Chelsea Jacket') are derived from E. cheiri (often placed in Cheiranthus), from southern Europe. They are often attacked by fungal and bacterial disease, so they are best grown as
biennials and discarded after flowering. They are also susceptible to
clubroot, a disease of
Brassicaceae. Growth is best in dry soils with very good drainage, and they are often grown successfully in loose wall mortar, hence the vernacular name. There is a wide range of flower color in the warm spectrum, including white, yellow, orange, red, pink, maroon, purple and brown. The flowers, appearing in spring, usually have a strong fragrance. Wallflowers are often associated in spring
bedding schemes with
tulips and
forget-me-nots.[11]
Erysimum is found in a range of habitats across the northern hemisphere, and has developed diverse morphology and growth habits (herbaceous annual or perennial, and woody perennial). Different Erysimum species are used as food plants by the
larvae of some
Lepidoptera (
butterflies and
moths) species including the
garden carpet (Xanthorhoe fluctuata). In addition, some species of weevils, like Ceutorhynchus chlorophanus, live inside the fruits feeding on the developing seeds. Many species of beetles, bugs and grasshoppers eat the leaves and stalks. Some
mammalian herbivores, for example mule deer (Odocoileus hemionus) in North America, argali (Ovis ammon) in Mongolia, red deer (Cervus elaphus) in Central Europe, or Spanish ibex (Capra pyrenaica) in the Iberian Peninsula, feed on wallflower flowering and fruiting stalks. Erysimum crepidifolium (pale wallflower) is toxic to some generalist vertebrate herbivores.[14][15]
Most wallflowers are pollinator-generalists, their flowers being visited by many different species of bees, bee flies, hoverflies, butterflies, beetles, and ants. However, there are some specialist species. For example, Erysimum scoparium is pollinated almost exclusively by Anthophora alluadii.
Defensive compounds
Like most
Brassicaceae, species in the genus Erysimum produce
glucosinolates as defensive compounds.[16][17] However, unlike almost all other genera in the Brassicaceae, Erysimum also accumulates
cardiac glycosides, another class of
phytochemicals with an ecological importance in insect defense.[18][19] Cardiac glycosides specifically function to prevent insect
herbivory[20] and/or
oviposition[21] by blocking
ion channel function in muscle cells.[22] These chemicals are toxic enough to deter generalist,[23] and even some specialist[24] insect herbivores.
Cardiac glycoside production is widespread in Erysimum, with at least 48 species in the genus containing these compounds.[19][25] Accumulation of cardiac glycosides in Erysimum crepidifolium, but not other tested species, is induced by treatment with
jasmonic acid and
methyl jasmonate,[26][25] endogenous elicitors of chemical defenses in many plant species.[27] Molecular
phylogenetic analysis indicates that Erysimum diversification from other
Brassicaceae species that do not produce cardiac glycosides began in the
Pliocene (2.33–5.2 million years ago),[28][1] suggesting relatively recent evolution of cardiac glycosides as a defensive trait in this genus.
Escape from herbivory
The evolution of novel chemical defenses in plants, such as
cardenolides in the genus Erysimum, is predicted to allow escape from herbivory by specialist herbivores and expansion into new ecological niches.[29] The crucifer-feeding specialist Pieries rapae (white cabbage butterfly) is deterred from feeding and oviposition by cardenolides in Erysimum cheiranthoides.[30][31][32][33][34] Similarly, Anthocharis cardamines (orange tip butterfly), which oviposits on almost all crucifer species, avoids E. cheiranthoides.[35]Erysimum asperum (western wallflower) is resistant to feeding and oviposition of Pieris napi macdunnoughii (synonym Pieris marginalis, margined white butterfly).[36][37] Two crucifer-feeding beetles, Phaedon sp. and Phyllotreta sp., were deterred from feeding by
cardenolides that were applied to their preferred food plants.[38][39] Consistent with the hypothesis of enhanced speciation after escape from herbivory, phylogenetic studies involving 128 Erysimum species indicate diversification in Eurasia between 0.5 and 2 million years ago, and in North America between 0.7 and 1.65 million years ago.)[1] This evolutionarily rapid expansion of the Erysimum genus has resulted in several hundred known species distributed throughout the northern hemisphere.[1][2][3][4][5][6]
Ethnobotanical uses of Erysimum
Erysimum species have a long history of use in traditional medicine. In Naturalis Historia by
Pliny the Elder (
c. 77), Erysimum is classified as a medicinal rather than a food plant. Erysimum cheiri is described as a medicinal herb in De Materia Medica by
Pedanius Dioscorides (
c. 70), which was the predominant European medical
pharmacopeia for more than 1,500 years. Other
medieval descriptions of medicinal herbs and their uses, including the Dispensatorium des Cordus by
Valerius Cordus (1542), Bocks Kräuterbuch by
Hieronymus Bock (1577), and Tabernaemontanus' Neuw Kreuterbuch by
Jacobus Theodorus Tabernaemontanus (1588), also discuss applications of E. cheiri. In traditional Chinese medicine, Erysimum cheiranthoides has been used to treat heart disease and other ailments.[40] Although medical uses of Erysimum became uncommon in
Europe after the
Middle Ages,[41]Erysimum diffusum, as well as purified erysimin and erysimoside, have been applied more recently as Ukrainian ethnobotanical treatments.[42]
^
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