Prota 2: Vegetables/L้gumes
Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
2n = 22, 44
Sword bean, sword jackbean, Japanese jackbean (En). Pois sabre, pois sabre rouge, haricot sabre (Fr). Fava-contra (Po). Mwingasiafu, mbwanda (Sw).
Origin and geographic distribution
Sword bean is only known cultivated and naturalized. Its origin is in the Old World tropics and it was probably domesticated in eastern Asia. The wide dispersal in historic times is thought to be partly due to carrying the remarkable seeds as curios.
In Madagascar the young green fruits and immature seeds of sword bean are used as a cooked vegetable. Sword bean is eaten in Tanzania, where the Swahili expression eating sword bean means being happy. Use of the fruits and immature seeds is also reported from Sri Lanka, India, Indonesia, China, Korea and Japan. Sword bean is further planted as a forage and cover crop. The ripe seeds can be eaten after cooking, but only after removing the seed-coat and several changes of water. The seed is used as feed for cattle and chicken, but if eaten in considerable quantity dry seeds may cause poisoning. Sword bean is grown as an ornamental climber on fences and houses.
Urease is extracted from the seed; it is used in clinical laboratories for the in-vitro determination of urea in human blood. In Korea it is used in the treatment of vomiting, abdominal dropsy, kidney-related lumbago, asthma, obesity, stomach -ache, dysentery, coughs, headache, intercostal neuralgia, epilepsy, schizophrenia, inflammatory diseases and swellings. In Japan it is effective in treating ozena, haemorrhoids, pyorrhea, otitis media, boils and cancers, all kinds of inflammatory diseases and atopic dermatitis. In Korea soap is marketed based on extracts of sword bean; it is used for the treatment of athletes foot and acne.
Production and international trade
No information is available on the trade of sword bean as a vegetable. Seeds of Canavalia are traded internationally for the production of urease, but quantities of seed entering the international market are not known.
The nutritional composition of fresh sword bean fruits per 100 g edible portion is: water 83.6 g, energy 247 kJ (59 kcal), protein 4.6 g, fat 0.4 g, carbohydrate 10.7 g, fibre 2.6 g, Ca 33 mg, P 66 mg, Fe 1.2 mg, vitamin A 40 IU, thiamin 0.2 mg, riboflavin 0.1 mg, niacin 2 mg, ascorbic acid 32 mg. Dry seeds contain per 100 g: water 10.7 g, energy 1453 kJ (347 kcal), protein 24.5 g, fat 2.6 g, carbohydrate 59 g, fibre 7.4 g, Ca 158 mg, P 298 mg, Fe 7.0 mg, thiamin 0.8 mg, riboflavin 1.8 mg, ascorbic acid 1 mg (Rubatzky, V.E. & Yamaguchi, M., 1997). The seed protein is poor in methionine, but rich in lysine.
The seeds of Canavalia species contain several growth-inhibiting and toxic storage proteins, e.g. canavalin (vicilin), con -canavalin A and B and canatoxin. The urease contained in the seed is chemically related to canatoxin. It also contains the toxic non-protein amino acid canavanine, a structural analogue of L-arginine. In both human and animal nutrition dry seeds have the shortcoming that their proteins have a low digestibility and a low biological value, and raw seeds are poisonous in large quantities. The digestibility can be improved by treatment such as heating (prolonged cooking, pressure-cooking or roasting) or fermenting.
There are numerous publications of results of research into the chemical composition of several Canavalia species. Seed protein content and composition of Canavalia gladiata and Canavalia ensiformis (L.) DC. are similar.
Adulterations and substitutes
Alternative sources of urease are various bacteria (e.g. Klebsiella aerogenes), fungi (e.g. Coccidioides immitis) and higher plants (e.g. soya bean).
Perennial trailing or climbing herb up to 10 m long, often grown as an annual; root system deep. Leaves alternate, pinnately 3-foliolate; stipules small, deciduous; petiole 517 cm long; leaflets with 47 mm long stalks, ovate, 7.520 cm ื 514 cm, apex acuminate, shortly pubescent on both sides. Inflorescence an axillary raceme 712 cm long; peduncle 420 cm long. Flowers bisexual, papilionaceous, often resupinate; calyx up to 1.5 cm long, 2-lipped with a large 2-fid upper lip and a much smaller 3-fid lower lip; corolla white, standard c. 3.5 cm long; stamens 10, all joined; ovary superior, style slender, curved, stigma small. Fruit a linear-oblong pod, slightly compressed, sometimes curved, 2040(60) cm ื 3.55 cm, widest near the apex, 816-seeded, spirally dehiscent; each valve with ventral rib and extra rib spaced c. 4 mm. Seeds 23.5 cm ื 1.52 cm, red or red-brown, rarely black, pink or white; hilum 1.52.0 cm long. Seedling with epigeal germination; first 2 leaves simple, opposite, with stipules connate.
Other botanical information
Canavalia comprises about 60 species, most of these of American origin. There are many reports in the literature of the occurrence of the sword bean in tropical Africa, but there are few herbarium specimens from Africa. Many of the reports likely refer to the widespread indigenous Canavalia africana Dunn (synonym: Canavalia virosa auct. non (Roxb.) Wight & Arn.). In the interior of tropical Africa at higher altitudes (over 300 m) Canavalia africana is the most common indigenous Canavalia species. It is occasionally cultivated as a cover crop and green manure. The seeds are eaten in Ethiopia and Tanzania, but often only as a famine food. In Tanzania the leaves are used in a recipe for treating smallpox. In India, where Canavalia africana occurs as well, researchers have named plants collected in the wild Canavalia gladiata. Jackbean (Canavalia ensiformis) is considered a native of the New World and is only known in cultivation. It is widely grown in Africa but mainly as a cover crop, forage, green manure and ornamental. The use of its fruits and immature seeds seems fairly common in Asia but substantiated reports of use as a vegetable in Africa are lacking. The 3 above-mentioned taxa are considered by some as a single species as they cross freely and their uses and chemical composition are similar. Also, an analysis based on DNA data designed to distinguish legume species failed to find differences between Canavalia ensiformis and Canavalia gladiata. However most floras separate the 3 species as follows:
Canavalia gladiata: standard c. 3.5 cm long, white; fruit 2040(60) cm ื 3.55 cm; seeds red or red-brown, rarely white, 23.5 cm long, hilum 1.52 cm long.
Canavalia ensiformis: standard c. 2.5 cm long, pink to purple; fruit 1535 cm ื 33.5 cm; seeds ivory or white, 1.52 cm long, hilum 0.51 cm long.
Canavalia africana: standard c. 3 cm long, white with mauve veins; fruit 1017 cm ื 2.53 cm; seeds brown or red-brown, 1.52 cm long, hilum 11.5 cm long.
Cultivars of Canavalia ensiformis vary widely, particularly in the degree of twining, the size of the fruits and the number and colour of the seeds.
Growth and development
Sword bean seed germinates readily and the plant is relatively fast growing. Flowers are pollinated by insects and 20% or more cross-pollination occurs.
Sword bean requires temperatures of 2030ฐC and is cultivated from sea-level up to 1000 m altitude. It is tolerant of drought once established and also tolerant of waterlogging, shade and salinity, making it one of the most hardy tropical legumes. It prefers an evenly distributed annual rainfall of 9001500 mm. It grows well even on nutrient depleted soils and on acid soils, even with a pH as low as 4.5.
To avoid a build up of pests and diseases it is recommended that sword bean be treated as an annual crop or retained at most for 2 years.
Propagation and planting
Sword bean is usually grown by smallholder farmers near houses and allowed to climb on walls, fences and trees. Seeds are sown at a depth of 57.5 cm. As a field crop, it is usually sown at a spacing of 7590 cm between rows and 4560 cm within the row, at a seed rate of 2540 kg/ha.
Diseases and pests
Sword bean is fairly resistant to diseases and pests. The most serious fungal disease is root rot caused by Colletotrichum lindemuthianum. Sword bean is a host of tomato spotted wilt virus (TSWV). Canavalia is known to reduce nematode populations. However, it is susceptible to the soybean cyst nematode (Heterodera glycines) that has not yet been recorded in Africa. Major pests are fall army worm (Spodoptera frugiperda) and beetle grubs that bore into the stems. Sword bean seeds are fairly resistant to storage pests.
Young sword bean fruits can be harvested from 34 months after sowing when they are 1015 cm long, before they swell and become fibrous and tough. Mature seed can be harvested after 510 months. As the fruits shatter their seeds when ripe, harvesting should be done timely.
Yields of green fruits can be up to 4 t/ha. Forage yields of up to 60 t/ha have been reported. Seed yields of up to 5.4 t/ha are possible, but a seed yield of 1.5 t/ha is more common.
Worldwide collection of Canavalia germplasm is urgently needed. Small germplasm collections of Canavalia are maintained at the Australian Tropical Crops & Forages Genetic Resources Centre, Biloela, Queensland. A few accessions are available in Ethiopia (ILRI), Nigeria (IITA), South Africa, Brazil, China, Colombia and India.
Sword bean is not known from the wild and must have undergone selection during centuries. Selection has favoured increased pod and seed size but has not resulted in a reduction in biochemical toxins. This would be consistent with selection for use as fodder or as a green fruit vegetable rather than as a pulse crop. Breeding is difficult as the flowers are very sensitive to damage during emasculation and emasculated flowers usually abscise; therefore bud pollination is recommended. In South-East Asia sword bean cultivars have been developed with reduced toxicity. Hybrids of Canavalia gladiata with both Canavalia africana and Canavalia ensiformis have occurred from natural crosses. Breeding programmes should use this wide base of germplasm.
To increase the use of Canavalia green fruits and young seeds as a vegetable in tropical Africa, improved cultivars should be made available either by introducing Asian cultivars or by breeding. Major limitations for increased use of the dry seeds of Canavalia in human nutrition are the poor taste, the unappealing texture and antinutritional factors that make laborious preparation necessary. In these respects Canavalia faces the same acceptability problems in Africa as soya bean. However, as Canavalia is a very tough and resilient crop it could play a larger role if seeds were produced in quantity and processed on an industrial scale. Breeding and selection could play a role in developing cultivars with reduced toxicity. Sword bean will remain important as a green manure and cover crop and as fodder. A critical review of the taxonomy of the genus Canavalia is overdue.
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Sources of illustration
Kooi, G., 1993. Canavalia gladiata (Jacq.) DC. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 134136.
Correct citation of this article:
Bosch, C.H., 2004. Canavalia gladiata (Jacq.) DC. In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/L้gumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, part of flowering stem; 2, inflorescence with young fruits; 3, seeds.