Rheum (plant)

Genus of flowering plants

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. 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.

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 (bell shaped) 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 (sometimes six) 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.

The genus Rheum was erected in 1753 by Carl Linnaeus, initially for three species: R. rhaponticum, R. rhabarbarum and R. ribes. 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. (See .) 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 (of which 19 are endemic) in China, including a number Borodina considered synonyms in 1989.

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.

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. 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 (Ursus arctos ssp. isabellinus) awakening from hibernation in April.

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.