Rheum cultorum (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Rheum ×cultorum Thorsrud & Reisaeter

Protologue: Norske pl.: 95 (1948).

Family: Polygonaceae

Chromosome number: 2n= 44 (tetraploid)

Synonyms

• Rheum rhabarbarum L. (1753) sensu auct. mult.,
• R. rhaponticum L. (1753) sensu auct. mult.,
• R. × hybridum Murr. (1775) pro parte.

Vernacular names

• Rhubarb (En)
• Rhubarbe (Fr)
• Indonesia: kelembak, talembak (Madura)
• Malaysia: kelembak.

Origin and geographic distribution

The cultivated rhubarb is of unclear hybrid origin. Most probably several parental species are involved: R. rhabarbarum (occurring naturally in the highlands of northern China and eastern Siberia), R. rhaponticum (occurring naturally in the Rila Mountains in Bulgaria), R. palmatum L. (occurring naturally in the highlands of north-western China), and perhaps others. The genus Rheum L. has its centre of origin in central and eastern Asia. The medicinal use (strong purgative) of several Rheum species (e.g. R. palmatum) is very ancient, but the culinary use of rhubarb in Europe dates from the 18th Century only. It is quite certain that the culinary use in Asia is much older.

The cultivation of vegetable rhubarb has mostly spread in the northern hemisphere, especially in West and Central Europe, the United States, Canada, the former Soviet Union and Japan. In South-East Asia rhubarb is cultivated as a vegetable in the cool mountainous regions of Java in Indonesia, Cameron Highlands in Malaysia and around Baguio in the Philippines. It is grown to a limited extent in the mountains of Central and East Africa, India and the West Indies as well.

Uses

The edible parts of rhubarb are the fleshy petioles which are chopped and stewed with sugar. It is either served as a sweet, used in pies or made into jam. The roots of vegetable rhubarb are of inferior medicinal value, but have furnished local medicines.

Production and international trade

No statistics are available on the production and trade of vegetable rhubarb.

Properties

Per 100 g edible portion, rhubarb contains: water 92 g, protein 0.5 g, fat 0.1 g, carbohydrates 2.9 g, fibre 0.7 g and ash 1.3 g. The energy value is 46 kJ/100 g. The pleasant acid taste is caused by the presence of malic, oxalic and citric acids. Rhubarb leaves are poisonous as they contain a high content of anthraquinone and of free oxalic acid. Some fatalities have been recorded from the ingestion of the leaves. The roots also contain anthraquinone.

Description

• A robust, perennial, tufted herb, up to 1.5 m tall, with a dense rhizome and fleshy roots.
• Leaves in a radical rosette from the centre of which an erect flowering stem emerges near anthesis; leaf-sheath large, white, thinly membranous; petiole fleshy, on the upperside flat, on the underside obscurely sulcate or rounded with sharp margins, up to 1-1.5 m long and often more than 2 cm in diameter, green often tinged with red or pink; leaf-blade broadly ovate or cordate, 20-50 cm × 15-50 cm, base cordate, margins undulate or crispy and irregularly ciliate, apex obtusely rounded, palmately 3-7-veined, pubescent on the veins beneath; cauline leaves gradually shorter and narrower.
• Inflorescence a large panicle, with numerous small, bisexual, greenish-white flowers; tepals in 2 whorls of 3; stamens 9; styles 3.
• Fruit an achene, ovoid, broadly 3-alate, often more than 1 cm long.

Growth and development

Under favourable conditions plants may extend to almost 2 m in diameter, hence they need a fairly large open site. In temperate climates plants enter a dormancy period during winter. Low temperatures have a vernalizing effect on flower and leaf formation. Gibberellin may substitute frost to break dormancy. Harvesting should not be carried out during the first six months to enable plants to develop well and to build up reserves. Plants can be harvested thereafter and for many years. Plant clumps should be divided and replanted about every five years, depending on their vigour, to prevent the development of a high proportion of smaller leaves with thin petioles.

Other botanical information

The identity of rhubarb is still unknown. The name R. × cultorum is proposed for all commercial cultivated hybrid rhubarb, as the identity of the parents is not known. It seems best to use cultivar names for suitable selections, e.g. "Early Red", "Prince Albert", "Victoria", "Linneus", "Oregon Red Giant". The genus Rheum comprises about 50 species; several wild species are used in the same way as the hybrid rhubarb and are occasionally cultivated, e.g. R. rhaponticum, R. rhabarbarum, R. compactum L. (Mongolia, Siberia), R. emodi Wallich (Himalaya, also the rhizomes are used medicinally), R. altaicum A. Los. (Mongolia, Siberia), R. palmatum (mainly for medicinal use of the rhizome), R. ribes L. (Iran, Caucasus), R. wittrockii Lundstr. (also cultivated as ornamental in Europe, originating from Central Asia), R. compactum L. (Mongolia, Siberia), R. undulatum L. (Mongolia).

Ecology

Rhubarb is well adapted to high rainfall conditions provided drainage is good. It cannot stand waterlogged conditions. The crop is cultivated more frequently in temperate climates, but in the tropics cultivation above 1000 m altitude is possible.

The optimum temperature for growth is from 15-20°C. Diurnal variations in temperature are beneficial (in the traject 4-24 °C). Most cultivars are sensitive to high temperatures which may cause the production of spindly, weak petioles. Above 30 °C, cultivars that normally exhibit a pink or red petiole colour, usually become green. Plants rarely flower under short-day conditions and at high temperatures. Soils should have a high content of organic material and minerals if yields are to be satisfactory. A moisture-retaining, but well-drained soil is preferable. Although rhubarb tolerates acid conditions, growth is optimal in the pH range 6.5-7.0.

Agronomy

Rhubarb is propagated by division of the rhizome into pieces which each have at least one good bud. A new planting should only be started from vigorous healthy mother plants, preferably of named cultivars. Rhizomes should preferably be divided and planted during the dormant phase, in temperate climates in late autumn, in the tropics at the beginning of the rainy season. Propagation from seed is possible but not recommended as the resulting plants are likely to be variable and high quality cannot be guaranteed. Before planting the soil should be deeply cultivated. Planting material is planted in trenches or furrows, about 25-30 cm deep, partially filled with organic manure or compost, 75-90 cm apart, with the buds just showing at the soil surface. If necessary, drainage can be improved by planting into raised beds or ridges. Plants should be kept well watered and free from weeds, particularly while establishing. Flowering stems, which are rare in the tropics, should be removed as they weaken the plant. To stimulate growth, regular applications of NPK are required. Little is known about diseases and pests in the tropics. Rhubarb is susceptible to virus diseases, downy mildew, Cercospora leaf-spot, and root rot. Harvesting can be carried out by pulling petioles upwards so that they separate from the rhizome cleanly without snapping. To sustain the plant, 3-4 leaves should always be retained per plant. The basal sheath of the petiole may remain attached or be trimmed, the leaf-blade is removed. One plant may yield 1.5-3 kg of petioles per season or per year. Petioles can be stored for up to 3 weeks at 0-1 °C temperature and high humidity.

Genetic resources and breeding

A germplasm collection of rhubarb species is present in the Department of Plant Breeding, Swedish University of Agricultural Sciences, Uppsala. Breeding possibilities are good, as different species hybridize easily and promising selections can be propagated vegetatively. For tropical areas breeding objectives are thick straight red petioles that are not stringy and without a strong acid taste, tolerance to drought and waterlogging. Farmers in the highlands of Indonesia use local selections derived from old introductions from Europe. These have small petioles, possibly because of degeneration through virus infections.

Prospects

As vegetable rhubarb is only of local importance in the tropics there is no high prioirity for research or breeding. Cultivars from the temperate regions may continue to serve as a source of planting material. Local virus-free propagation of planting material is needed.

Literature

• Buishand, T. & Karsten, J.E., 1971. Teelt van rabarber [Cultivation of rhubarb]. Consulentschap in Algemene Dienst voor de groenteteelt in de vollegrond in Nederland, Alkmaar, the Netherlands. 47 pp.
• Commonwealth Bureau of Horticulture and Plantation Crops, 1970. Annotated bibliography on rhubarb: uses, composition and biochemistry, 1935-1969. Query file no. 5641. East Malling, United Kingdom. 4 pp.
• Huxley, A., Griffiths, M & Levy, M. (Editors), 1992. The new Royal Horticultural Society dictionary of gardening. Vol. 4. MacMillan, London, United Kingdom. pp. 25-26, 81.
• Libert, B. & Englund, R. 1989. Present distribution and ecology of Rheum rhaponticum (Polygonaceae). Willdenowia 19: 91-98.
• Marshall, D.E., 1988. A bibliography of rhubarb and Rheum species. United States Department of Agriculture, National Agricultural Library and Agricultural Research Service Bibliographies and Literature of Agriculture No 62. 377 pp.

Authors

• I.I.M. Huibers-Govaert