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Gnetum buchholzianum Engl.

Protologue
Bot. Jahrb. 40. 519 (1908).
Family
Gnetaceae
Synonyms
Thoa buchholziana (Engl.) Doweld (2000).
Vernacular names
Eru (En). Koko (Fr). Nkoko (Po).
Origin and geographic distribution
Gnetum buchholzianum is restricted to the humid forest zone of Cameroon.
Uses
Fresh leaves of Gnetum buchholzianum and the very similar Gnetum africanum Welw., both called eru (koko in French), are widely used as a vegetable. They are usually cooked with meat or fish and occasionally consumed as a salad. Leaves are shredded into thin strips and are often eaten as part of a mixture in, for example, a groundnut-based stew. To soften this rather tough vegetable, people often mix it with waterleaf (Talinum triangulare (Jacq.) Willd.). Shredded leaves can be dried and preserved for later use. The seeds are also eaten.
The leaves are chewed to mitigate the effects of drunkenness and they are taken as an enema against constipation and to ease childbirth. They are also used to treat boils and fungal infections on the fingers. The supple stem is sometimes used as rope.
Production and international trade
In trade, consignments of Gnetum buchholzianum and Gnetum africanum are often mixed. Traders will pay more for the thick dark green leaves of the former, but much variation is also caused by growing conditions. Most eru is consumed locally, but intensive trade has developed from Cameroon to meet the large demand in Nigeria. Most eru from Cameroon is transported to Idenau, a coastal village in Cameroon, and from there by boat to Nigeria. Estimates for the annual export of eru leaves (both species) to Nigeria range between 2500 t and 4000 t. Other marketing centres in Cameroon are Campo near Kribi for export to Gabon and the Mfoundi market in Yaound้. Dried shredded leaves are exported, mainly from Nigeria to the United States and to a lesser extent from other countries to France and the United Kingdom.
Properties
The composition of Gnetum buchholzianum leaves per 100 g of fresh edible portion is: water 70 g (65.0–84.0), energy 432 kJ (103 kcal), protein 6.0 g (3.6–6.7), fat 1.3 g, carbohydrate 21.6 g, fibre 5.5 g, Ca 130 mg, P 54 mg, Fe 5.6 mg, ascorbic acid 100 mg (Leung, W.-T.W., Busson, F. & Jardin, C., 1968). The dry matter content of fresh leaves is higher than for most other dark or medium green leaf vegetables. This gives a feeling of firmness during preparation, hence certain consumers consider eru as a substitute for meat. The leaves of Gnetum buchholzianum are somewhat thicker and darker green than the thinner and paler leaves of Gnetum africanum. Consequently, the content of micronutrients in the latter might be lower. Eru leaves contain C-glycosylflavones, including 2"-xylosylisoswertisin and 2"-glucosylisoswertisin, compounds that are only known from these two species.
Adulterations and substitutes
The leaves of Gnetum buchholzianum can be replaced by those of the other eru species, Gnetum africanum, or leaves of the shrub Lasianthera africana P.Beauv., which impart a similar taste to the dish.
Description
Dioecious liana up to 10 m long but sometimes longer; branches somewhat thickened at the nodes, glabrous. Leaves decussately opposite, sometimes in whorls of 3, simple; stipules absent; petiole c. 1 cm long, canaliculate above; blade ovate-elliptical to broadly elliptical, 8–15 cm ื 4–8 cm, base rounded to slightly cuneate, apex abruptly acuminate, obtuse or minutely apiculate, entire, very thick-papery, glabrous, dark green above, paler beneath, with 4–8 pairs of strongly curved lateral veins looped near the margin. Inflorescence an unbranched catkin, axillary or terminal on a short branch, solitary but male inflorescences at apex of branches often in groups of 3, up to 8 cm long, jointed, peduncle 1–1.5 cm long, with 2–3 scale-like, triangular bracts; male inflorescence with thick internodes widening towards apex and with whorls of flowers at nodes; female inflorescence with slightly turbinate internodes and 3 flowers at each node. Flowers small, c. 2 mm long, with moniliform hairs at base and an envelope; male flowers with a tubular envelope and hardly exserted staminal column bearing 2 anthers; female flowers with cupular envelope and naked, sessile ovule. Seed resembling a drupe, ellipsoid, 10–15 mm ื 4–8 mm, apiculate, enclosed in the fleshy envelope, orange-red when ripe, with copious endosperm.
Other botanical information
Gnetum comprises approximately 35 species of small trees, shrubs or most often lianas, found in tropical South and Central America (about 7 species), Africa (2 species) and Asia (about 25 species). They look much like dicotyledonous flowering plants (having opposite leaves with a net venation and cherry-like seeds), although in fact they are gymnosperms. The 2 African species, which are very similar, have been classified in section Gnetum, subsection Micrognemones. Gnetum buchholzianum has thick dark green leaves. The male catkins have thick internodes widening towards the terminal part. Gnetum africanum has leaves which are relatively thin and pale green. Its male catkins have slender internodes of equal width from the base to the tip.
It was recently proposed that all Gnetum species be transferred to Thoa, except two Asiatic species, mainly based on seedcoat structure.
Growth and development
Both African Gnetum species are lianas with two different types of stems. The orthotropic ones have small, scale-like leaves and rapidly grow vertically, reaching the main branches of a tree where they produce plagiotropic stems with fully developed leaves. The orthotropic stem continues climbing until it reaches the canopy where it branches into several leafy stems. Female plants often show more vigorous growth with stronger stems than male plants. This is less obvious in Gnetum buchholzianum than in Gnetum africanum.
Eru continues to grow during the dry season and new shoots may develop where the stem has been cut or where side shoots have been removed. New shoots are also formed from rhizomes that spread along the forest floor. The distinctly coloured drupe-like seeds are probably dispersed by birds and other animals.
Ecology
Eru can be found in rainforest from sea-level up to 1200 m altitude, and prefers an annual rainfall of about 3000 mm. It is usually found with other climbers on middle- and under-storey trees, frequently forming thickets. It can also be found in riverine forest in areas that would otherwise be too dry for the species. Gnetum buchholzianum is mainly found in primary forest, especially near openings created by fallen trees, and is today less common than Gnetum africanum which mostly occurs at the periphery of primary forest and in secondary forest.
Management
Eru is still mainly collected from wild stands, but farmers often retain it when clearing fields. If cultivated, farmers need to provide support, e.g. by using commercial plantations of rubber trees, oil palm and other tree crops. Fences were only found to be successful when there is enough shade, and they are generally too expensive. Fully exposed plants do not grow well; their leaves are thin and pale green, and traders reject them. In experiments, nutrients, especially nitrogen, have shown a positive effect on growth and rate of leaf development.
Propagation and planting
Experimental plantings for domestication are being made with both species. Nurseries are now concentrating their efforts on Gnetum buchholzianum because it is preferred by traders and is more vigorous. Moreover, male vines of Gnetum africanum are less appreciated because of their smaller, thinner and paler leaves, and because of their less vigorous growth. For Gnetum buchholzianum there is no need to harvest only female plants. However, the field trials might show that Gnetum buchholzianum is more difficult to cultivate than Gnetum africanum because the former probably requires more shade than the latter. In experiments in Cameroon, propagation by seed was difficult because the seed is reluctant, germination taking one year or more. It is assumed that seeds need pretreatment, such as passing through the intestines of a bird, fruit bat, squirrel or other animal, before they germinate. Seed is normally found only in the tree canopy. Seed collection is thus far from easy, a further reason why eru is hardly cultivated.
Methods of vegetative propagation using leafy stem cuttings have recently been developed. It is recommended that leaf blades of cuttings be trimmed in half. Nursery beds under shade and made of well-decomposed sawdust or fine river sand can be used for propagation. Ectomycorrhizae assist the roots in absorption of nutrients; the most common species reported is Scleroderma sinnamarense. After about 6 weeks the rooted cuttings are transferred to polythene sleeves, bamboo pots or other containers where they remain for a further 2–3 months. The soil mixture for these containers consists of 25% sand and some compost, supplemented with forest soil. Field planting, preferably next to a young tree or shrub, takes place at the beginning of the rainy season.
Diseases and pests
Mealy bugs are the main pest in the nursery. When eru is grown along dead poles attacked by termites, these insects will damage adjacent leaves. Diseases have not been found to reduce productivity of eru.
Harvesting
The current method of harvesting, especially for export trade, is to pull the stems or branches from trees. This leads to large-scale destruction of natural stands. Occasionally, trees have to be cut to reach leafy stems in the canopy. This is mainly done during the dry season when the forest is more accessible and when there is little work on the farm. Controlled harvesting, in which only side shoots or parts of stems are collected, is clearly better than destructive harvesting. After controlled harvesting, new shoots may develop where a stem has been cut or where side shoots have been removed. Preliminary observations indicate that 3–4 harvests per year are possible, still allowing for substantial regrowth. More frequent harvesting will result in thin leaves that are considered inferior. The first harvest may take place 6–9 months after planting. The total lifespan of eru is estimated at over 10 years.
Yield
Preliminary observation indicates that in cultivation during the first harvest year the fresh leaf yield may reach 20 t/ha. This may double in subsequent years.
Handling after harvest
Leafy stems remain fresh for at least a week. Stems collected from the forest are brought to collecting points from where they are either sold in the local market or exported. For this trade, whole leafy stems are packed in large bales. Selection takes place for size and texture of the leaves, and is mainly determined by the species. Gnetum buchholzianum is more popular with consumers and more expensive because its leaves are generally thicker than those of Gnetum africanum. Leaves are shredded before consumption or prior to drying.
Genetic resources
Eru is hardly cultivated at all at present, but there is massive exploitation of the remaining natural stands, which have almost disappeared in Nigeria and are becoming scarce in Cameroon, Gabon and the Central African Republic. Gnetum buchholzianum, which seems to be endemic to Cameroon and prefers primary forest, is even more endangered than Gnetum africanum. There is an urgent need to collect and preserve the diversity found within the two African Gnetum species, preferably throughout their natural range. Accessions need to be evaluated for their agronomic potential and for their ability to germinate without the need for interventions. A small collection is currently held at the Limbe Botanic Garden, Limbe, Cameroon.
Prospects
Alternatives to destructive harvesting of eru should be found. Once the new methods of propagation and cultivation have been adopted, there will be scope for development of eru as a new crop, for which there is already a high demand and for which an attractive price could be paid. Diversity found between accessions is considerable, offering scope for improvement of both quality and productivity. Research work is currently being done at Limbe Botanic Garden, Limbe, Cameroon.
Major references
• Keay, R.W.J., 1954. Gnetaceae. In: Keay, R.W.J. (Editor). Flora of West Tropical Africa. Volume 1, part 1. 2nd Edition. Crown Agents for Oversea Governments and Administrations, London, United Kingdom. p. 33.
• Leung, W.-T.W., Busson, F. & Jardin, C., 1968. Food composition table for use in Africa. FAO, Rome, Italy. 306 pp.
• Schippers, R.R., 2000. African indigenous vegetables. An overview of the cultivated species. Natural Resources Institute/ACP-EU Technical Centre for Agricultural and Rural Cooperation, Chatham, United Kingdom. 214 pp.
• Schippers, R.R. & Fereday, F. (Editors), 1998. Opportunities and constraints in the subsistence production and marketing of indigenous vegetables in East and Central Africa. Technical report. Natural Resources Institute, Chatham, United Kingdom. 53 pp.
• Shiembo, P.N., 1997. Domestication of Gnetum spp. by vegetative propagation techniques. In: Schippers, R.R. & Budd, L. (Editors). Proceedings of a Workshop on African Indigenous Vegetables, Limbe, Cameroon, 13–18 January 1997. Natural Resources Institute/IPGRI, Chatham, United Kingdom. pp. 31–35.
• Shiembo, P.N., 1999. The sustainability of eru (Gnetum africanum and Gnetum buchholzianum): over-exploited non-wood forest product from the forests of Central Africa. In: Sunderland, T.C.H., Clark, L.E. & Vantomme, P. (Editors). Current research issues and prospects for conservation and development. Based on the outcome of the International Expert Meeting on Non-Wood Forest Products in Central Africa, held at the Limbe Botanic Garden, Limbe, Cameroon, 10–15 May 1998. FAO, Rome, Italy. pp. 61–66.
Other references
• Asaha, S., Tonye, M.M., Ndam, N. & Blackmore, P., 2000. State of knowledge. Studies on Gnetum africanum and Gnetum buchholzianum. Report for the Central African Regional Programme for the Environment. Limbe Botanic Garden, Limbe, Cameroon.
• Burkill, H.M., 1994. The useful plants of West Tropical Africa. 2nd Edition. Volume 2, Families E–I. Royal Botanic Gardens, Kew, United Kingdom. 636 pp.
• Carlquist, S. & Robinson, A.A., 1995. Wood and bark anatomy of the African species of Gnetum. Botanical Journal of the Linnean Society 118: 123–137.
• Doweld, A.B., 2000. Rehabilitation of the genus Thoa Aublet (Gnetaceae). Turczaniniwia 3(4): 28–36.
• Friedman, W.E. & Carmichael, J.S., 1998. Heterochrony and developmental innovation: Evolution of female gametophyte ontogeny in Gnetum, a highly apomorphic seed plant. Evolution 52: 1016–1030.
• Lowe, J., 1984. Gnetum in West Africa. Nigerian Field 49(1–4): 99–104.
• Markgraf, F., 1930. Monographie der Gattung Gnetum. Bulletin du Jardin Botanique Buitenzorg S้rie 3, 10: 444–448.
• Ouabonzi, A., Bouillant, M.L. & Chopin, J., 1983. C-Glycosylflavones from Gnetum buchholzianum and Gnetum africanum. Phytochemistry 22(11): 2632–2633.
• Stevels, J.M.C., 1990. L้gumes traditionnels du Cameroun: une ้tude agrobotanique. Wageningen Agricultural University Papers No 90-1. Wageningen University, Wageningen, Netherlands. 262 pp.
Sources of illustration
• Keay, R.W.J., 1954. Gnetaceae. In: Keay, R.W.J. (Editor). Flora of West Tropical Africa. Volume 1, part 1. 2nd Edition. Crown Agents for Oversea Governments and Administrations, London, United Kingdom. p. 33.
Author(s)
• R.R. Schippers
De Boeier 7, 3742 GD Baarn, Netherlands


Editors
• G.J.H. Grubben
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, R้sidence 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
Illustrator
• Achmad Satiri Nurhaman
PROSEA Network Office, P.O. Box 332, Bogor 16122, Indonesia
Photo Editor
• E. Boer
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article:
Schippers, R.R., 2004. Gnetum buchholzianum Engl. In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/L้gumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
Distribution Map wild


1, branch with male inflorescences; 2, male inflorescence; 3, young infructescence; 4, female flower.
Redrawn and adapted by Achmad Satiri Nurhaman



commercial crop (Gnetum africanum or G. buchholzianum)


leaves packed for export (Gnetum africanum or G. buchholzianum)