Vigna vexillata (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Vigna vexillata (L.) A. Richard

Protologue: Hist. fis. polit. nat. de Cuba (Spanish ed.) 11: 191 (1845).

Family: Leguminosae - Papilionoideae

Chromosome number: 2n= 22, (20)

Synonyms

• Phaseolus vexillatus L. (1753),
• Plectrotropis angustifolia Schumach. & Thonn. (1827),
• Vigna tuberosa A. Richard (1847).

Vernacular names

• Wild mung bean, zombi pea (En).
• Pois zombi, pois poison (Fr).
• Indo-China: qua
• Laos: thwàx phi:
• Vietnam: qua (southern).

Origin and geographic distribution

V. vexillata most probably originated in the Old World tropics. There are two centres of genetic diversity, one in Africa (from Tanzania to South Africa) and one in South-East Asia (from Yunnan (China) to Indonesia). It is now pantropical and occasionally it is also cultivated.

Uses

V. vexillata is grown as a green manure, cover crop, erosion-controlling plant, and as a forage. The tuberous roots are eaten like sweet potatoes in north-eastern and southern India, Australia, Ethiopia, South Africa and the Sudan. In South Africa, the young leaves and young pods are also eaten as a vegetable. Seeds are eaten as a pulse e.g. in India.

Properties

Per 100 g dry matter, an actively growing, flowering plant contains: crude protein 20.3 g, ether extract 6.1 g, nitrogen-free extract 37.2 g, crude fibre 26.4 g and ash 10.1 g. The corresponding digestibility values are approximately: dry matter 69%, crude protein 81%, crude fibre 62%, ether extract 74% and nitrogen-free extract 73%. A study conducted at this growth stage in south-eastern Queensland found it had one of the highest recorded dry matter intakes by sheep for tropical legumes, while N retention was second only to Lotononis bainesii Baker. Tubers contain per 100 g dry matter: protein 14.5 g, fat 0.9 g, carbohydrates 58.2 g, ash 3.8 g, Ca 58.5 mg, P 88.7 mg and Fe 1.0 mg. The seed contains large amounts of the dipeptide para-aminophenylalanine, a compound giving resistance against bruchids.

Description

• A polymorphic, perennial, climbing or trailing herb, up to 6 m long, with sparse to dense, brownish hairs, usually with a fusiform tuberous rhizome.
• Leaves trifoliolate, very variable, with silky hairs on both surfaces; petiole 1.5-11.5 cm long; stipules lanceolate-cordate, 0.5-1.3 cm long; rachis up to 3 cm long, petiolule 2-4 mm; leaflets very variable, ovate-lanceolate, elliptical or linear-oblong, 2-17 cm × 0.5-8.5 cm, apex acute or acuminate, base rounded, cuneate or truncate, usually entire but rarely slightly lobed.
• Inflorescence a 2-6-flowered, axillary raceme; peduncle 4.5-36 cm long; rachis very short, usually bearing 3 extrafloral nectaries; pedicel 1-2 mm long.
• Flowers about 2.5 cm long, purple or pink, sometimes yellow or white; calyx tubular, tube 5-7 mm long, bristly with long brown and short white hairs, ending in 5 lanceolate lobes up to about 2 cm long; standard asymmetrical, subcircular, 2.5-3.5 cm × 2-4 cm, emarginate, pale violet inside, yellowish outside; wings 2.5 cm long, purplish; keel asymmetrical, white or pale lilac, prolonged into a 180° incurved beak, twisted to one side; stamens 10, 1 free, 9 connate at base for 6-8 mm; pistil with upper half of style bearded and lateral stigma.
• Pod linear-cylindrical, 4-15 cm × 2.5-9 mm, 10-18-seeded, with brown bristly hairs.
• Seed globose to oblong reniform, 2.5-5 mm × 2-5 mm, plain brownish-green to black, or dark red with black speckles; aril hardly developed.
• Seedling with epigeal or hypogeal germination, depending on source of seed and variety.

Growth and development

Seed takes about 8-10 days to germinate. V. vexillata produces excellent leaf growth and covers the ground quickly. Roots develop abundant N-fixing nodules. Flowering occurs 3-4 months after planting, and pods ripen 1 month later. In Costa Rica and South Africa, Xylocarpa bees play a role in the pollination. When the fruits are ripe, the tubers are also ready for harvesting.

Other botanical information

Due to its great variability and wide distribution, many varieties and forms of V. vexillata have been described. The variability has been studied best for Africa; a thorough study for South-East Asia remains to be done. Three varieties seem most important in South-East Asia:

• var. angustifolia (Schumach. & Thonning) Baker: plants glabrescent or with sparse bristly hairs; leaflets very narrow, 4-8 cm × 0.5-1.5 cm; calyx lobes 2-8 mm long; occurring from Africa and India throughout South-East Asia to Australia, especially in areas with a long dry season.

• var. macrosperma Maréchal, Mascherpa & Stainier: stems rather thick, robust, leaflets ovate-elliptical, hirsute; pods 12-15 cm × 7-9 mm; seeds subglobose, 3.5-5 mm in diameter, yellow-green; widespread pantropically.

• var. vexillata: plants very densely ferruginous pubescent or bristly; leaflets ovate to lanceolate; calyx lobes up to 2 cm long; widespread pantropically; it is the most common form whose tuberous roots are used as food.

Ecology

V. vexillata thrives in a wide range of conditions, e.g. in grassland, in disturbed areas and as a weed of cultivation. In India it flourishes from 1200-1500 m altitude in the foothills of the Himalayas and in the hills of eastern and north-eastern India. In Australia it grows in the far northern monsoon region with heavy summer rainfall (1250-1500 mm) followed by a very long, pronounced dry season, on very poor, lateritic, acid soils rich in aluminium. V. vexillata thrives during the rainy season and can withstand at least 6 months of waterlogged conditions. Although it is susceptible to frost and fire, it is one of the first plants to sprout after drought or fire, making the location of the tubers immediately apparent.

Agronomy

Propagation is usually by seed, but is also possible by stem cuttings. Seed must be scarified to improve germination. Inoculation with appropriate Rhizobium and mixing with phosphate fertilizer prior to sowing improves establishment and growth. Seedlings grow vigorously. Planting densities vary according to the use of the crop. When grown for its tubers, it can be planted and treated much like Irish potatoes. V. vexillata is very sensitive to herbicides. When grown together with angleton grass (Dichanthium aristatum (Poiret) C.E. Hubbard), pangola grass (Digitaria eriantha Steudel) or para grass (Brachiaria mutica (Forssk.) Stapf), V. vexillata increases the N-content of the forage mixture significantly. V. vexillata develops more slowly in mixture with grasses, but maintains a high N-content longer than Desmodium intortum (Miller) Urban or Pueraria phaseoloides (Roxb.) Benth. In northern Australia, forage dry matter yields ranging from 300-1100 kg/ha have been obtained, depending on fertilization and soil type. In Zambia, dry matter yields of 2780 kg/ha have been achieved. Seed yields of 500-1250 kg/ha have been reported. Fresh tuber yields of 1.44 t/ha have been obtained in Nigeria.

Genetic resources and breeding

Germplasm collections are being maintained at the Cowpea Breeding Program of the International Institute of Tropical Agriculture, Ibadan, Nigeria and at the Southern Regional Plant Introduction Station, Griffin, Georgia, the United States. The primary distinguishing attribute among the various varieties is the leaflet shape which appears to be simply inherited. Single, completely dominant gene action appears to control anthocyanin stem pigmentation, silver leaflet midrib, leaflet hirsuteness, presence of long hairs on the calyx lobes, black pod colour, speckled seed testa and twining stem habit. Width of the floral standard, peduncle length, number of seeds per pod, seed weight, tuberous root dry weight, total plant dry weight, and harvest indices for seed and tuberous roots all appear to be quantitatively inherited. In spite of the great variability, no difficulties have been encountered in obtaining F₁ and back-cross progenies involving crosses between accessions from Australia and Africa. No fertile interspecific crosses have been obtained so far.

Prospects

With its wide ecological adaptability, ability to thrive on poor soils, tolerance of waterlogging, and spreading vigour, V. vexillata is an excellent pioneer for poor land as a cover crop, green manure or erosion-controlling crop. It may also prove to be a valuable tropical forage. The high protein content in the tuber makes it a promising protein food crop.

Literature

• Bhattacharyya, P.K., Ghosh, A.K., Sanyal, B. & Ray, G.D., 1984. Grow Vigna vexillata for protein rich tuber cum pulse crop in the north-eastern hill region. Seeds & Farms 10: 33-36.
• Duke, J.A., 1981. Handbook of legumes of world economic importance. Plenum Press, New York, United States. pp. 306-307.
• Maréchal, R., Mascherpa, J.-M. & Stainier, F., 1978. Etude taxonomique d'un groupe complexe d'espèces des genres Phaseolus et Vigna (Papilionaceae) sur la base de données morphologiques et polliniques, traitées par l'analyse informatique [Taxonomic study of a complex group of species of the genera Phaseolus and Vigna (Papilionaceae) on the basis of morphological and pollen data, analysed by computer]. Boissiera 28: 202-205.
• National Academy of Sciences, 1979. Tropical legumes, resources for the future. National Academy Press, Washington, D.C., United States. pp. 34-36.
• Nguyên Van Thuân, 1979. Leguminosae - Papilionoideae, Phaseoleae. In: Vidal, J.E. & Vidal, Y. (Editors): Flore du Cambodge, du Laos et du Viêtnam. Vol. 17. Muséum National d'Histoire Naturelle, Laboratoire de Phanérogamie, Paris, France. pp. 186-189.
• Pienaar, B.J. & Kok, P.D.F., 1991. The Vigna vexillata complex (Fabaceae) in southern Africa. South African Journal of Botany 57: 236-245.
• Skerman, P.J., Cameron, D.G. & Riveros, F., 1988. Tropical forage legumes. 2nd Edition. FAO Plant Production and Protection Series No 2. Food and Agriculture Organization of the United Nations, Rome, Italy. pp. 482-483.
• Vandenborght, T., 1989. Some observations on seedlings of Vigna vexillata (L.) Rich. Bulletin du Jardin Botanique National de Belgique 59: 179-187.

Authors

• K.C. Wong