Rheum (plant)


Rheum is a genus of about 60 herbaceous perennial plants in the family Polygonaceae. Species are native to eastern Europe, southern and eastern temperate Asia, with a few reaching into northern tropical Asia. Rheum is cultivated in Europe and North America. The genus includes the vegetable rhubarb. The species have large somewhat triangular shaped leaves with long, fleshy petioles. The flowers are small, greenish-white to rose-red, and grouped in large compound leafy inflorescences. A number of cultivars of rhubarb have been domesticated both as medicinal plants and for human consumption. While the leaves are slightly toxic, the stalks are used in pies and other foods for their tart flavor.

Description

Rheum species are herbaceous perennials growing from fleshy roots. They have upright growing stems and mostly basal, deciduous leaves growing from short, thick rhizomes. They have persistent or deciduous ocrea. The inflorescences are terminal and panicle-like with pedicels. The hermaphrodite flowers consist of a whitish green to pinkish green, hairless and campanulate perianth, composed of six tepals. The outer three tepals are narrower than the inner three and all are sepal-like in appearance. The flowers have nine stamina inserted on the torus at the base of the peranthium, they are free or subconnate at their base. The anthers are yellow or pinkish green, elliptic in shape. The ovary is simple and triangular shaped with three erect or deflexed styles. The stigmas are head-like. The fruits are a three-sided achene with winged sides, and the seeds are albuminous with a straight or curved embryo.

Taxonomy

The genus Rheum was erected in 1753 by Carl Linnaeus, initially for three species: R. rhaponticum, R. rhabarbarum and R. ribes. Linnaeus did not explain the origin of the genus name. Rheum is usually derived from the Greek rheon, mentioned by Dioscorides as an alternative name for medicinal rhubarb; the word rheon is itself thought to be derived from the Persian rewend. Dioscorides calls the plant rha, but mentions the Romans call it rha ponticum, and it was also called ria or rheon. It is theorised the Ancient Greek word rha was derived from an ancient Scythian name for the Volga River in Russia, Rā, near from where the plant was supposedly brought.
In 1936 Agnia Losina-Losinskaja in Vladimir Leontyevich Komarov's Flora SSSR recognised 22 native species for the USSR, and furthermore two introduced species, one variety, and one form. The 1989 Plants of Central Asia, dealing with a larger geographical remit, has Alisa E. Grabovskaya-Borodina recognising only 12 species, synonymising a great number. The Vascular plants of Russia and adjacent states of 1995 accepted 17 species for the states of the former USSR, re-recognising many of the taxa as species. In the Flora of China in 2003 Borodina and Bao Bojian recognise 38 species in China, including a number Borodina considered synonyms in 1989.

Intergeneric relationships

Rheum is placed in the family Polygonaceae, subfamily Polygonoideae. Within the subfamily, it is in the tribe Rumiceae, along with the two genera Oxyria and Rumex. It is most closely related to Rumex.

Infrageneric classification

In the 1998 Flora Republicae popularis Sinicae A. R. Li proposed classifying the Chinese representatives of the genus into five sections. These sections are distinct morphologically, but as of 2010 studies in karyotypy, pollen morphology or molecular data have failed to elucidate interspecific relationships.
Losinskaja used a slightly different classification in the Flora SSSR in 1936:
The genus is represented by about 50–60 extant species. The many cultivars of culinary rhubarb more usually grown for eating are recognised as Rheum × hybridum in the Royal Horticultural Society's list of recognised plant names. The drug rheum is prepared from the rhizomes and roots of another species, R. officinale or medicinal rhubarb. This species is also native to Asia, as is the turkey rhubarb, R. palmatum. Another species, the Sikkim rhubarb, R. nobile, is limited to the Himalayas.
The centre of diversity for this genus is found in Central Asia.
The following is a partial list of species names, some of which, according to some authorities, are considered synonyms or not fully resolved or accepted:
Rheum species have been recorded as larval food plants for some Lepidoptera species such as the buff ermine, Spilarctia luteum, as well as Arctia caja, Hydraecia micacea and Xestia baja.
Rheum species are often the host plants for myrmecophilous caterpillars of the butterfly genus Callophrys; Callophrys titanus feeds on R. maximowiczii in southern Kazakhstan, C. mystaphia on R. ribes in eastern Turkey, and C. mystaphioides on R. persicum in southwest and central Iran. The caterpillars of the related Lycaena violacea from southeastern Siberia are only known to feed on R. rhabarbarum.
R. ribes leaves are food for the moth Xylena exsoleta in eastern Turkey. Beetles which are specialised herbivores of this plant species in eastern Turkey are a Petrocladus sp. weevil, the jewel beetle Capnodis marquardti, and the leaf beetle Labidostomis brevipennis.
In the Taldy-Bulak valley in the Talas Alatau of Kyrgyzstan, the emerging leaves of R. maximowiczii are an important food source for Himalayan brown bear awakening from hibernation in April.

Uses

Many Rheum species have food and medicinal uses. Some of these uses originated in Asia more than 2,000 years ago. All parts of the plant contain slightly poisonous oxalic acid, but its concentration in the leaf stems or petioles used in food preparation is very low, and their tart flavor instead is caused by nontoxic malic acid. The plants also produce other compounds, including citric acid and anthraquinone glycosides, and the raw or cooked leaf blades are poisonous to humans and livestock if consumed in large enough amounts. Plants in cultivation are propagated by cutting up the crowns of larger plants and by seeds.
Some species are grown for their ornamental qualities, including R. acuminatum, R. alexandrae, R. australe, R. kialense, R. palmatum, R. rhabarbarum and R. ribes.
The roots of R. macrocarpum are exploited in the Tian-Shan to make a dye.