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Rheum ×hybridum Murray

Protologue
Novi Comment. Regiae Sci. Gott. 2(5): 50, t. 11 (1775).
Family
Polygonaceae
Chromosome number
2n = 44
Synonyms
Rheum ×cultorum Thorsrud & Reisaeter (1948), Rheum rhabarbarum auct. non L.
Vernacular names
Rhubarb (En). Rhubarbe (Fr). Ruibarbo, rabárbaro (Po).
Origin and geographic distribution
Cultivated rhubarb is of unclear hybrid origin, one of the parents probably being Rheum rhabarbarum L. from Mongolia and neighbouring territories. The medicinal use of several Rheum species in China dates from 4500 years ago, whereas the culinary use of rhubarb in Europe dates from the 18th century only, but is older in Asia. Rhubarb is cultivated as a vegetable in many temperate regions, but can be found occasionally in tropical highlands, e.g. in East and southern Africa.
Uses
The fleshy leaf stalks are chopped and stewed with sugar. The produce is eaten either as a side dish with a staple food and meat, served as a sweet dessert, used in pies or made into jam. Some users add lime to temper the acidity, but then the taste also fades. Wild Rheum species, but also rhubarb, are used in traditional medicine in Asia for lowering blood pressure and cholesterol, inhibiting progression of diabetic nephropathy, and curing cerebral injuries.
Production and international trade
Although rhubarb is a fairly important vegetable in temperate areas, there are few statistical data on production and trade. In tropical Africa it is grown on a small scale, mainly for European clientele, mostly in Zimbabwe.
Properties
The nutritional composition of raw rhubarb per 100 g edible portion is: water 93.6 g, energy 88 kJ (21 kcal), protein 0.9 g, fat 0.2 g, carbohydrate 4.5 g, fibre 1.8 g, Ca 86 mg, Mg 12 mg, P 14 mg, Fe 0.2 mg, Zn 0.1 mg, vitamin A 102 IU, thiamin 0.02 mg, riboflavin 0.03 mg, niacin 0.30 mg, folate 7 μg, ascorbic acid 8 mg (USDA, 2002). The pleasantly acid taste is caused by the presence of malic, oxalic and citric acids and saturated and unsaturated C6 aldehydes and acids. Rhubarb contains free oxalic acid. The oxalate content varies between cultivars and samples. In 71 rhubarb genotypes the mean content of water-soluble oxalate on a dry matter basis was 3.1% (1.6–6.0%), that of total oxalate 5.9% (3.2–9.2%); the mean content of malate was 21.0% (12.2–29.2%). If consumed in modest portions and not more than a few times per week, health hazards are unlikely. Anthocyanins in the juice of the leaf stalk are responsible for the red colour. A common antinutritional component in Rheum species is the mutagenous anthraquinone, but no evidence of mutagenicity has been detected in rhubarb leaf stalks. Anthraquinone concentrations are higher in the leaf blades and roots.
Description
Robust, perennial, tufted herb up to 1.5 m tall, with a woody rhizome and fleshy roots. Leaves in a rosette, simple, large, alternate and gradually smaller on flowering stem; stipules united to form a large, whitish, membranous sheath; petiole up to 1 m long and often more than 2 cm in diameter, fleshy, on the upper side flat, on the underside obscurely grooved or rounded with sharp margins, green often tinged with red or pink; blade broadly ovate or cordate, 20–50 cm × 15–50 cm, base cordate, apex obtusely rounded, margins undulate or crispy and irregularly ciliate, palmately 3–7-veined, pubescent on the veins beneath. Inflorescence a large panicle, many-flowered. Flowers bisexual, small, greenish-white; tepals in 2 whorls of 3, free; stamens 9; ovary superior, 1-celled, styles 3. Fruit an ovoid nut c. 1 cm long, with 3 membranous wings.
Other botanical information
Rheum comprises about 50 species; in Asia several wild species are used in the same way as rhubarb and are occasionally cultivated as a vegetable, but more often for medicinal uses. The name Rheum ×hybridum designates all commercially cultivated hybrid rhubarb, as the identity of the parents is not known. Some well-known cultivars are ‘Early Red’, ‘Prince Albert’, ‘Victoria’, ‘Linneus’, ‘Oregon Red Giant’ and ‘Crimson’.
Growth and development
Rhubarb plants grow fast and in the tropics normally without dormancy. They stay vegetative and continuously produce new leaves. Harvesting should not be carried out during the first 6 months to enable plants to become well established and to build up reserves. Rhubarb can be harvested for many years, but needs a period of rest after each harvesting season. The plant clumps are divided and replanted in fresh soil about once every 5 years to prevent the development of a high proportion of small leaf stalks with low commercial value.
Ecology
In temperate climates plants enter a dormancy period during winter, but in tropical conditions dormancy usually does not occur. Dormancy is induced by very short days (< 10 hours) rather than by low temperatures. Elongating daylength, combined with higher temperatures and irrigation after drought will break the dormancy. Rhubarb is well adapted to high rainfall conditions provided drainage is good. It cannot stand waterlogging. Cultivation in the tropics is mostly above 800 m altitude. The optimum temperature for growth is 10–25ºC. Diurnal variations in temperature are beneficial for growth.
For flowering rhubarb requires a vernalization temperature below 6ºC for several months. Consequently, rhubarb rarely flowers in the tropics. Most cultivars are sensitive to temperatures above 30ºC, causing the production of spindly, weak leaf stalks and reducing the desired red colour. Soils should have a high content of organic material and nutrients for satisfactory yields. 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.
Propagation and planting
Rhubarb is propagated by division of the rhizome into pieces with at least one well-developed bud. A new planting should only be started from vigorous healthy mother plants. 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 only recommended for a few commercially available cultivars; in other cultivars 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 placed in trenches or furrows about 30 cm deep, 75–100 cm apart, with 1.5–2 m between rows, partially filled with organic manure or compost, the buds just showing at the soil surface. Under favourable conditions plants may expand to 1.5 m in diameter. If necessary, drainage can be improved by planting on raised beds or ridges.
Management
Rhubarb 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. The mineral uptake is quite high and regular applications of NPK are required to stimulate growth.
Diseases and pests
Rhubarb is susceptible to virus diseases, e.g. arabis mosaic virus (ArMV), turnip mosaic virus (TuMV) and strawberry latent ringspot virus (SRLSV), which cause gradual degeneration. It may act as permanent source of infection for other crops. In temperate areas, virus-free planting material is obtained by meristem culture. A main problem is root rot caused by soil fungi (Pythium, Phytophthora) or bacteria and worsened by impaired drainage. Ramularia rhei and Cercospora may attack the leaf blades causing leaf spot. In Zimbabwe chrysomelid leaf beetles (Altica sp.) and aphids (Macrosiphum euphorbiae, Myzus persicae) have been observed as pests. Rhubarb is moderately susceptible to root-knot nematodes (Meloidogyne spp.)
Harvesting
Harvesting is done by pulling and twisting the leaf stalks upwards so that they separate cleanly from the rhizome. Adult plants can be harvested weekly or each time when sufficient new leaves have developed, leaving at least 4 leaves for photosynthesis and new production. Usually the tough old leaves and small young ones are left. After a harvest period of some months a rest period is recommended in order not to exhaust the plants.
Yield
In temperate areas an annual yield of 1.5–3 kg of leaf stalks per plant is obtained, but in the tropics the plants usually have less vigour and yield less.
Handling after harvest
After harvesting the leaves, the blades are removed and the basal sheath of the stalk may remain attached or be trimmed. The produce can be stored for up to 3 weeks at 0–2ºC and >90% relative humidity.
Genetic resources
A germplasm collection of Rheum species is maintained at the Nordic Gene Bank, Swedish University of Agricultural Sciences, Uppsala. Rhubarb found in Africa usually results from old introductions and often has poor growth and small leaf stalks, possibly because of degeneration through virus infection.
Breeding
Breeding is relatively easy as different species hybridize easily and promising selections can be propagated vegetatively. Seed companies have little interest in rhubarb, although several seed-propagated cultivars are available. Breeding criteria for cultivars adapted to the tropics are vigour, large leaf stalks free of strings, red colour, low acidity and oxalate content, and tolerance to high temperatures, drought and waterlogging.
Prospects
In tropical Africa rhubarb is likely to remain of local importance only and there is no high priority for research or breeding. Adapted virus-free planting material should become available, or seed of adapted cultivars. The medicinal properties of rhubarb need more attention.
Major references
• Choudhury, B., 1967. Vegetables. India, the land and the people. National Book Trust, New Delhi, India. 214 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.
• Huibers-Govaert, I.I.M., 1993. Rheum ×cultorum Thorsrud & Reisaeter. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 237–239.
• Rubatzky, V.E. & Yamaguchi, M., 1997. World vegetables: principles, production and nutritive values. 2nd Edition. Chapman & Hall, New York, United States. 843 pp.
• Tindall, H.D., 1968. Commercial vegetable growing. Oxford University Press, United Kingdom. 300 pp.
• USDA, 2002. USDA nutrient database for standard reference, release 15. [Internet] U.S. Department of Agriculture, Beltsville Human Nutrition Research Center, Beltsville Md, United States. http://www.nal.usda.gov/fnic/foodcomp. Accessed June 2003.
Other references
• Dregus, M., Barta, J. & Engel, K.H., 2003. Investigation of volatile constituents of rhubarb (Rheum rhabarbarum L.). In: Le Quéré, J.L. & Etievant, P.X. (Editors). Flavour research at the dawn of the twenty first century. Proceedings of the 10th Weurman flavour research symposium, Beaune, France, 25–28 June, 2002. Editions Tec & Doc, Paris, France. pp. 580–583.
• Hiller, L.K., Kossowski, M. & Kelly, W.C., 1974. Factors influencing flowering of rhubarb. Journal of the American Society for Horticultural Science 99(2): 125–127.
• 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, Beltsville, United States. 377 pp.
• Paneitz, A. & Westendorf, J., 2000. Anthranoid contents of rhubarb (Rheum undulatum L.) and other Rheum species and their toxicological relevance. European Food Research and Technology 210(2): 97–101.
• Rumpunen, K. & Henriksen, K., 1999. Phytochemical and morphological characterization of seventy-one cultivars and selections of culinary rhubarb (Rheum spp.). Journal of Horticultural Science and Biotechnology 74(1): 13–18.
• Wells, P.D. & Going, W.D., 1983. Rhubarb. Zimbabwe Agricultural Journal 80(4): 165–167.
Sources of illustration
• Huibers-Govaert, I.I.M., 1993. Rheum ×cultorum Thorsrud & Reisaeter. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 237–239.
Author(s)
G.J.H. Grubben
Boeckweijdt Consult, Prins Hendriklaan 24, 1401 AT Bussum, Netherlands
Based on PROSEA 8: ‘Vegetables’.

Editors
G.J.H. Grubben
Boeckweijdt Consult, Prins Hendriklaan 24, 1401 AT Bussum, Netherlands
O.A. Denton
National Horticultural Research Institute, P.M.B. 5432, Idi-Ishin, Ibadan, Nigeria
Associate editors
C.-M. Messiaen
Bat. B 3, Résidence La Guirlande, 75, rue de Fontcarrade, 34070 Montpellier, France
R.R. Schippers
De Boeier 7, 3742 GD Baarn, Netherlands
General editors
R.H.M.J. Lemmens
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
L.P.A. Oyen
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
Photo editor
E. Boer
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article:
Grubben, G.J.H., 2004. Rheum ×hybridum Murray. In: Grubben, G.J.H. & Denton, O.A. (Editors). Prota 2: Vegetables/Légumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, plant habit; 2, leaf stalk; 3, inflorescence branch; 4, flower; 5, fruit.
Source: PROSEA



plant habit


plant habit