Psophocarpus tetragonolobus (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Psophocarpus tetragonolobus (L.) DC.

Protologue: Prodr. 2: 403 (1825).

Family: Leguminosae

Chromosome number: 2n= 18

Synonyms

• Dolichos tetragonolobus L. (1759),
• Botor tetragonolobus (L.) O. Kuntze (1891).

Vernacular names

• Wing(ed) bean, asparagus pea, goa bean (En)
• Pois carré, haricot ailé (Fr)
• Indonesia: kecipir
• Malaysia: kacang botor
• Papua New Guinea: asbin, bin (Motu)
• Philippines: kalamismis, sigarilyas, kabey
• Burma: pe-saung-ya, hto-pong, ku-bemya
• Cambodia: prâpiëy
• Laos: thwàx ph'uu
• Thailand: thua-phu
• Vietnam: dậu rồng, dậu khế.

Origin and geographic distribution

East Africa, north-eastern hill tracts of India and Papua New Guinea have been variously suggested as centres of origin, but Indochinese-Indonesian and East African centres of origin have also been proposed. Centres of greatest diversity are located in Indonesian and Papua New Guinea islands. Burma and Papua New Guinea appear to be the foci of domestication. Here winged bean is cultivated on field scale. Before the recent recognition of its potential it was grown as a horticultural plant in East Africa, parts of India, Sri Lanka, Thailand, Indo-China, Malaysia, Indonesia, Philippines and a few Pacific islands. More recently it has been introduced to almost all tropical areas of the world as well as to some subtropical areas.

Uses

Traditionally, green pods have been widely used as a vegetable in South-East Asia. In Papua New Guinea tubers are used in the highlands; young leaves and flowers are eaten as a salad. Tubers are also used in Burma. More recently the nutritional value of dry seeds has been recognized: it is similar to soya beans (Glycine max (L.) Merrill) and they can be put to similar uses. Winged bean flour can be used as protein supplement in bread-making. Seeds can also be utilized for making edible oil, milk, and traditional South-East Asian delicacies such as tempeh, tofu and miso. The whole plant as well as processed seeds offer excellent animal feed.

Production and international trade

Traditionally, winged bean has only been grown for domestic consumption and local markets on a small scale. No national or international production data are available. Large-scale planting of trellised winged bean in Thailand seems promising.

Properties

Immature green pods contain 1-3% protein and compare favourably with other leguminous vegetables of the tropics. They are a rich source of calcium, iron and vitamin A. Leaves are nutritionally important containing 5-7% protein and large quantities of vitamins A and C, and minerals.

The mature seed is nutritionally the most interesting part, containing per 100 g edible portion approximately: water 11 g, protein 33 g, fat 16 g, carbohydrates 32 g, fibre 5 g and ash 3 g. The energy value averages 1697 kJ/100 g. It compares favourably with soya bean. The amino-acid spectrum is also similar to that of soya bean, although it may be a little higher in lysine and leucine. The sulphur-containing amino-acids methionine and cystine are the first limiting amino-acids, tryptophane and valine the second. The oil resembles that of groundnut (Arachis hypogaea L.). Oleic and linoleic acids make up about 67% of the total fatty acid component and saturates make up 29%. The saturated/unsaturated fatty acid ratio is 1 : 3. The oil is reasonably stable and tocopherol content is high. The oil is easily refined. Amongst minerals, phosphorus and zinc occur in favourable quantities and calcium content is similar to soya bean. Thiamine and riboflavin are present in amounts comparable with other grain legumes.

The 1000-seed weight is about 250 g.

Tubers contain 8-10% protein on fresh weight basis. However, essential amino-acids occur in low proportions. They are rich sources of carbohydrates (30%), calcium and phosphorus.

Description

• A perennial vine, usually grown as an annual.
• Roots numerous, with long lateral roots running horizontally at shallow depth, and some becoming thick and tuberous.
• Stems twining, 2-4 m long, ridged and glabrous.
• Leaves trifoliolate; leaflets generally (broadly) ovate, 8-15 cm × 4-12 cm, entire, acute, with small, 2-parted stipules.
• Flowers 2-10, in axillary 5-15 cm long racemes; calyx campanulate, with 5 short, rounded teeth, green to dark purple; corolla mauve or white, various mixtures of mauve, cream, blue and red, standard 2.5-4 cm long; stamens with 9 filaments fused, 1 free, enclosing the pistil.
• Fruit a legume (pod), oblong to linear, 6-40 cm × 2.5-3.5 cm, more or less square with 4 smooth to serrated wings 0.3-1 cm wide, rough or smooth, yellow-green, green or, less commonly, cream, occasionally with red flecks.
• Seed 5-21 per pod, subglobose, about 0.6-1 cm long, brown, yellow, dark tan, white, uniform or variously mottled, glabrous, with a small aril.

Growth and development

Seed emergence under field conditions occurs between 5-7 days after sowing. Temperatures around 25 °C appear most suitable for germination and growth.

About 2.5 months after sowing at equatorial latitudes, plants start flowering, although some genotypes require as long as 5 months. Pod development is not affected greatly by environmental conditions. Maximum pod length and pod ripening occur about 20 days and 65 days after pollination, respectively.

The fibrous root system with large nodules (up to 1.5 cm in diameter) grows in proportion to the shoots until about 3 months after planting. Then root growth either levels off as the reproductive sinks sequester photosynthates, or accelerates in cultivars and under conditions which favour continued vegetative growth and initiation of tubers. In tuberous cultivars, increases in root dry weight continue beyond the 6th month after planting. Tuberization is known to be enhanced by pruning young shoots and flowers. By the 7th or 8th month, when the shoots senesce, tubers are ready for harvest.

Other botanical information

Three other species of the genus Psophocarpus DC. may be of interest:

• P. grandiflorus Wilczek is naturally distributed from Zaire to Uganda and Ethiopia. This species is possibly the closest to the winged bean.

• P. palustris Desv. is naturally distributed from Senegal to Sudan. Recently, it has been introduced to many other parts of Africa and in Asia. It is a traditional food item for some tribes in Zaire. It can be used as a cover crop as well.

• P. scandens (Endl.) Verdc. is widely distributed in tropical Africa. It has been suggested as a cover crop and reported to be resistant to diseases which cause great damage to winged bean (e.g. Synchytrium psophocarpi).

Ecology

Winged bean appears to be best adapted to equatorial climates. In Papua New Guinea and Burma it grows at altitudes up to 2000 m but does not tolerate night frost. Day temperatures in the region of 27 °C and nights warmer than 18 °C are optimal for growth and reproductive development. The tuber initiation is favoured by cooler conditions. It requires about 1000 mm or more annual rainfall but it is intolerant to waterlogging.

Winged bean is a quantitative short-day plant. Flower induction requires short-day conditions with a critical daylength around 12 hours. The response to daylength varies with genotype, temperature and light intensity. Induction of tubers also requires short days. It thrives on a range of soil types provided that adequate drainage is ensured, and the pH is not lower than 5.5.

Propagation and planting

Winged bean is propagated by seed. The crop is grown as a sole crop or intercropped with cereals.

Three broad patterns of cultivation and use can be discerned:

• Field crop tuber production (Burma): the crop is planted on ridges and is not staked. Tubers are harvested before the crop reaches the mature-seed stage.

• Multipurpose horticultural crop (Papua New Guinea): winged bean is the most important leguminous crop grown in the settled valleys of the Papua New Guinea Highlands, at altitudes of 1400-2200 m. 10-20% of the cultivated valley land may be used to grow the crop during June - December. It may be grown as a sole crop in rotation with the staple crop (sweet potato), or in separate blocks within mixed gardens. The plants are staked. All parts of the plant are eaten. When grown for tubers, the flowers and young fruits are removed.

• Minor garden vegetable (South-East Asia): throughout much of lowland South and South-East Asia winged bean plays a minor role as an occasional vegetable in home gardens. One or two plants are planted at the base of fences, stumps or trees over which they sprawl, sometimes to a height of 3-7 m.

Sowing is ideally done at the begining of the rainy season. As a vegetable crop, sowing may be done at any time provided adequate soil moisture is available. Adequate drainage is essential and under wet conditions raised beds may be necessary. Seeding rates depend on the plant vigour. For seed production, about 20 000 plants/ha for vigorous South-East Asian cultivars and 150 000 plants/ha for Papua New Guinea type growth habit should be aimed at. For a tuber crop, the plant density should be about 250 000 plants/ha.

Husbandry

Weed control in the first 4-6 weeks is important as the early growth is slow. A well-grown crop should be able to smother weeds. One or two weedings should be done before the plant support system is erected.

Staking plants is necessary. Yield may be reduced to less than half if plants are allowed to trail on the ground. Two meter high stakes or similar supporting systems appear suitable. For a crop raised solely for tuber production, shorter supports may be adequate. Such a crop may also require vegetative and reproductive pruning after the first flowers appear, in order to encourage tuber production.

Under rainfed conditions, irrigation is only needed during periods of prolonged drought. Successful crops have been raised through overhead irrigation in the dry season in Papua New Guinea, when virtually no rainfall occurs.

Winged bean is known to nodulate profusely in symbiotic association with Rhizobium strains of the cowpea group. Such strains occur widely in the tropical areas but should problems with nodulation occur, seeds might be inoculated with an appropriate strain. In a well-nodulated crop, nitrogen application is not necessary. However, in soils low in nitrogen, small quantities of ammonium sulphate or urea may be applied. Response to applied phosphorus occurs in most soils and response to applied potassium has been reported in Brazil. Application of P and K in the ratio of 2:1 has been suggested.

Mulching may be needed in dry conditions. There is some evidence that mulching encourages tuber production through reduction of soil temperature.

Winged bean may serve as an important leguminous crop in crop rotations. It may precede a non-leguminous tuber crop or cereal. In Burma, sugar cane following winged bean has been reported to yield twice as much. In the Papua New Guinea Highlands a rotation of sweet potato (wet season) with winged bean (dry season) is commonly practised.

Diseases and pests

A number of mycoplasma and virus diseases have been observed. Of these ring spot mosaic virus and necrotic mosaic virus have been positively identified in Ivory Coast. Avoiding sowing seeds from infected crops and locating plots away from the vicinity of other known hosts of viruses may be helpful.

Bacterial blight (Pseudomonas solanacearum) has been reported in South-East Asia. Resistant cultivars appear to be the only control measure.

Amongst fungal diseases, false rust or orange gall (Synchytrium psophocarpi) is perhaps the most widespread and damaging. Cultivar resistance has also been reported. Dark leaf-spot (Pseudocercospora psophocarpi) is also common throughout South-East Asia and Papua New Guinea, and particularly serious in hot and humid areas. Benomyl spray at fortnightly intervals has been found to give good control. Powdery mildew (Erysiphe cichoracearum) may be potentially important in relatively cooler areas with dry season but high humidity.

Root knot nematodes (Meloidogyne spp.) cause galling of infected roots and stunting and yellowing of leaves. Up to 50% yield may be lost. Flooding for 30-40 days has been recommended as control method in the Philippines.

Amongst insect pests, bean pod-borer (Maruca testulalis), Mylabris afzelli, Mylabris pustulata, Heliothis armigera and Icerya purchasi have been reported.

Harvesting

Fresh and tender pods are harvested as a vegetable when they reach about four-fifths of their full length; this may continue for several weeks. Harvesting a seed crop is complicated because pod ripening occurs over a long period. Mature pods should be collected at regular intervals because pods split and shatter seeds when they remain too long on the plants.

Optimum time for tuber harvesting varies widely. In the Papua New Guinea Highlands, tubers are dug out at first sign of senescence in the crop.

Yield

Green pod yields range from 10-15 t/ha but up to 34 t/ha has been reported. Seed yield estimates from farmers' crops are not available. About 1 t/ha is easily obtained. Experimental yields above 2 t/ha have been frequently reported; 4.5 t/ha was recorded in an experiment in Malaysia. Tuber yield in the Papua New Guinea Highlands in farmers' plots was estimated to be 5.5-11.7 t/ha.

Handling after harvest

Freshly harvested green pods store poorly and should be marketed within 24 hours. Tubers have been shown to keep in a fresh state for up to 2 months but they are normally consumed or sold soon after harvesting.

Seeds not intended for sowing store better than most grain legumes due to their resistance to common storage insect pests. Seed for sowing should be stored as briefly as possible, since loss of viability may occur in the humid tropics. Seed viability is considerably enhanced by storage at low temperatures and by reducing the moisture content of the seed.

Genetic resources

The Indonesian Archipelago and Papua New Guinea are considered to be the centres of greatest genetic diversity. Germplasm has been collected from most parts of South and South-East Asia. However, particular attention is needed for less explored areas such as Burma, Vietnam, Laos and Cambodia. Collections are currently available at the National Bureau of Plant Genetic Resources, New Delhi (India), the Thailand Institute of Scientific & Technological Research (TISTR), Bangkok (Thailand), the Centre for Research & Development in Biology, Bogor (Indonesia), the Pallekele Research Centre, Kundasale (Sri Lanka), the Papua New Guinea University of Technology, Lae (Papua New Guinea), the University of the Philippines, Los Baños (the Philippines), and Southampton University (United Kingdom). Over 1000 accessions may be available in the above collections.

Breeding

Winged bean is a semi-domesticated plant and breeding would play a vital role in its development. The breeding objectives depend on the product for which it is raised. As a green vegetable crop, early flowering, high pod yield, pod production over a longer period of time, green pod colour, less fibrousness (reduced parchment layers) and a better taste are desirable. As a pulse crop, important objectives are early flowering, synchronization of pod maturity, senescence at the end of the growing season, high seed yield, low shelling percentage, high protein and oil content depending upon the processing needs, and white seed colour. For a tuber crop, selection for low pod yield, vigorous vegetative growth, high tuber yield, tuber quality factors including high protein, low fibre content and acceptable flavour are relevant. Cultivars for cover cropping should show vigorous vegetative growth, high leaf area index, perennial growth habit, high rate of nodulation and nitrogen fixation, tolerance of waterlogging and ability to compete with weeds.

Psophocarpus scandens and P. palustris may be sources for insect resistance but they appear to be genetically distant from winged bean. Attempts to realize interspecific hybridization have been unsuccessful so far.

Prospects

Winged bean offers a valuable source of food and feed in subsistence farming. However, large-scale cultivation of winged bean is limited at this stage due to its requirement for plant supports, its tendency to perennial growth, and the occurrence of pod splitting and seed shattering. Although potentially a very useful plant, it should be regarded as semi-domesticated. Active plant breeding with a view to evolve agronomically more acceptable cultivars is crucial for future development. A self-supporting type, or at least one which shows minimum loss of yield when grown without support, is the most urgent breeding objective.

Literature

• Eagleton, G.E., Khan, T.N. & Erskine, W., 1985. Winged bean (Psophocarpus tetragonolobus (L.) DC.). In: Summerfield, R.J. & Roberts, E.H. (Editors): Grain legume crops. Collins, London, United Kingdom. pp. 624-657.
• Erskine, W., 1981. Heritability and combining ability of vegetative and phenological characters of winged bean (Psophocarpus tetragonolobus (L.) DC.). Journal of Agricultural Science, Cambridge 96: 503-508.
• Khan, T.N., 1976
• Papua New Guinea: a centre of genetic diversity in winged bean (Psophocarpus tetragonolobus (L.) DC.). Euphytica 25: 693-706.
• Khan, T.N., 1982. Winged bean production in the tropics. FAO Plant Production and Protection Paper 38, FAO, Rome, Italy. 217 pp.
• Masefield, G.B., 1973. Psophocarpus tetragonolobus - a crop with a future? Field Crop Abstracts 26: 157-160.
• PCARR, 1978. The winged bean. Papers presented at the 1st International Symposium on Developing the Potentials of the Winged Bean. Los Baños, the Philippines. 448 pp.

Authors

• T.N. Khan