Acacia senegal (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Cereal / pulse|
|Carbohydrate / starch|
|Essential oil / exudate|
|Forage / feed|
Acacia senegal (L.) Willd.
- Protologue: Sp. pl. 4(2): 1077 (1806).
- Family: Mimosaceae (Leguminosae - Mimosoideae)
- Chromosome number: 2n = 26
- Mimosa senegal L. (1753),
- Senegalia senegal (L.) Britton (1930).
- Gum tree, gum arabic tree, three-thorned acacia (En).
- Gommier blanc, gommier (Fr).
- Aiti, kikwata, mgunga (Swahili).
Origin and geographic distribution
Acacia senegal is widely distributed in the drier parts of tropical Africa, from Senegal and Mauritania in the west to Eritrea and Ethiopia in the north-east and to South Africa in the south. Of the 4 recognized varieties var. senegal is the most widespread and is found throughout the area of distribution of Acacia senegal except along the west coast of central and southern Africa; outside Africa it occurs in Oman, Pakistan and India and has been introduced into Egypt, Australia, Puerto Rico and the Virgin Islands. This variety is the major source of gum arabic. Var. kerensis Schweinf. occurs in Ethiopia, Somalia, Uganda, Kenya and Tanzania; var. leiorhachis Brenan throughout eastern Africa from Ethiopia to South Africa; var. rostrata (Sim) Brenan in the same area and in Namibia and Angola, and possibly also in Oman.
Gum arabic is the exudate formed in the bark of Acacia senegal, Acacia seyal Delile and a few related species. In Sudan and sometimes in international trade, a distinction is made between ‘gum hashab’ from Acacia senegal and ‘gum talha’ from Acacia seyal. Gum arabic is easily soluble in water and forms solutions over a wide range of concentrations. It has highly valued emulsifying, stabilising, thickening and suspending properties and does not become highly viscous. The food industry uses 60–75% of the world production. In confectionery, gum arabic is used to prevent crystallisation of sugar, as an emulsifier, and as a glaze or topping in bakery products; in soft drinks and alcoholic drinks it is used either as a vehicle for flavouring or as a stabiliser or clouding agent; in frozen dairy products gum arabic is used for encapsulating flavours such as citrus oils. Its pharmaceutical use has decreased, but it is still used as a suspending or emulsifying agent and in tablet manufacture, where it functions as a binding agent or as a coating prior to sugar coating.
Gum arabic is used in the printing industry for coating offset lithographic plates to prevent oxidation, to increase their hydrophilic properties and to make them repellent to ink. It is also a base for photosensitive chemicals. In ceramics, gum arabic helps to strengthen the clay. Other technical applications include pyrotechnics and ink manufacturing. In textiles, paints, paper size and adhesives (including the traditional office glue and postage stamps) its use has decreased to very low levels in recent years. Gum arabic is used locally in special dishes and as chewing gum; it has medicinal applications for both humans and livestock, e.g. to treat skin diseases and inflammation.
Acacia senegal is a multipurpose tree. The foliage and pods are an important fodder source for camels and goats. Seed may be dried and conserved for human consumption mainly as an emergency food. The wood is used for small-scale construction purposes and agricultural implements; it yields a fuelwood of good quality, that can be made into good charcoal. The thorny branches are often used to make ‘dead fences’ to enclose livestock or protect agricultural fields. Being a very drought-resistant tree, it is planted for sand dune fixation, windbreaks and shelter belts in arid regions. Bark, leaves and gum are used as an astringent to treat colds, ophthalmia, diarrhoea and haemorrhages. The flowers are a source of honey. Cordage is made from the roots, either directly or after beating to extract the fibres; its strength makes it suitable for well ropes and fishing nets. The seed contains a fat which is used both in medicine and for soap making.
Production and international trade
The term ‘gum arabic’ was coined by European traders, who imported it from Arabian ports such as Jeddah and Alexandria. Sudan dominates the world market, accounting for about 80% of the volume produced. Around 1970 annual trade amounted to 70,000 t, but 2 serious droughts brought trade down to 20,000–24,000 t in 1992. During the periods of shortage, many companies invested in alternative manufacturing equipment using other hydrocolloids, resulting in an irreversible loss of market share for gum arabic. From 1988 to 1994, annual exports from Sudan averaged nearly 20,000 t of ‘gum hashab’ whereas Nigeria exported over 4800 t of gum arabic during the same period. Part of the production of Sudan is illicitly exported via other countries. The European Union is by far the largest market for gum arabic and imported an average of 28,000 t/year for the period 1988–1993. The price per t (ex Port Sudan) from 1980 to 1992 fluctuated as a result of years of shortage and years of surpluses from US$ 1500 (1980) to US$ 5000 (1983–1984) and US$ 2300 (1988–1991) to US$ 7000–9000 in 1992. The 1994–1995 prices (FOB, Port Sudan) of the best quality ‘gum hashab’ were US$ 5000/t. Gum arabic from Sudan is traded in several quality classes; the most important ones (with their indicative 1994–1995 prices FOB, Port Sudan) are: ‘hand-picked selected’ (US$ 4850), ‘cleaned and sifted’ (US$ 4200), ‘kibbled’ (US$ 5000), ‘dust’ (US$ 2760). In 1994, the price of Nigerian gum arabic Grade 1 was US$ 3500/t. The United States imports mainly processed gum arabic from Europe.
Gum arabic is a pale to orange-brown solid, which breaks with a glassy fracture. The best grades have the shape of whole, round tears, orange-brown in colour and with a matt surface texture; after processing to the broken or ‘kibbled’ state the pieces are much paler and have a glassy appearance. Unlike many other vegetable gums, gum arabic dissolves very well in water (up to 50%). The solution is colourless and free of taste and does not readily interact with other chemical compounds. Chemically, gum arabic is a slightly acidic complex composite of glycoproteins and polysaccharides and their calcium, magnesium and potassium salts. The main polysaccharide is arabic acid, a branched polysaccharide with a (1,3)-linked D-galactose backbone with (1,6)-linked ramified branches composed of L-arabinose, L-rhamnose and D-glucuronic acids. The proteins are characterized as hydroxyproline-rich arabinogalactan proteins. Commercial samples of gum arabic showed the following composition: arabinose 24–29%, galactose 32–41%, rhamnose 12–18%, uronic acid 14–17% and protein about 2%. The molecular weight is 47,000–3,000,000, representing a number of basic monomeric sugars of 290–18,500. In human nutrition, gum arabic has less than 1 cal/g.
As gums similar to gum arabic are obtained from several acacias and even from non-Acacia species, official specifications to identify true gum arabic are important. However, these specifications have a confusing history in which the term ‘gum arabic’ is sometimes used exclusively for gum from Acacia senegal, and sometimes for gums from several Acacia species. The Joint FAO/WHO Expert Committee on Food Additives (JECFA) indicated in its Compendium on food additive specifications - Addendum 7 (1999) that no distinction is made between gum from Acacia senegal and Acacia seyal; gum from other Acacia species is not admitted. Some of the further specifications are: 1 g should dissolve in 2 ml water and be acidic in a litmus test, but be insoluble in ethanol; mannose, xylose and galacturonic acid should be absent; the optical rotation of an aqueous solution of gum from Acacia senegal should be laevorotatory, that from Acacia seyal dextrorotatory; the loss on drying should be not more than 15% for granular material and not more than 10% for spray-dried material; gum arabic should be free of starch and dextrin, tannins, Salmonella spp. and Escherichia coli.
Export consignments from Sudan receive a certificate giving the analytical data such as moisture content, acid-insoluble matter and optical rotation. Commercial gum arabic samples have a moisture content of 9–14%, an optical rotation of ([a]20D) –26° to –34° and a nitrogen content of 0.27–0.39% . Gum of Acacia seyal is dextrorotary (+54°) and can as such be distinguished from Acacia senegal gum. In Europe, gum arabic is admitted as food additive under No E414; in the United States it has FDA GRAS (generally recognized as safe) status. ‘Gum hashab’ from Sudan is the highest quality and sets the standard. Within Sudan, gum arabic from the Kordofan region has the highest reputation, and traders and end-users in importing countries often refer to ‘Kordofan gum’ when indicating their preferences. Nigerian gum arabic is of very variable quality. A major problem is the inconsistent and heterogenous nature of consignments: gum of varying degrees of cleanliness and colour is present, which reflects the less rigorous methods of harvesting and post-harvest handling when compared to ‘gum hasbab’ from Sudan. Mixing gum from different Acacia spp. adversely affects the quality. Officially, Nigerian gum arabic Grade 1 is pure gum of Acacia senegal, Grade 2 may include gum from Acacia seyal and other Acacia species, while Grade 3 may contain gum from species other than Acacia.
The nutritive value for livestock of young shoots, green pods and seeds respectively is as follows: crude protein 20%, 22% and 39%, crude fibre 28%, 39% and 21% and nitrogen-free extract 42%, 31% and 26%. The leaves contain 10–13%, the pods 15% digestible protein. The wood is dark to almost black, heavy and hard and can be polished well. It is termite resistant. The energy value of the wood is about 13,400 kJ/kg.
Adulterations and substitutes
Gums from other acacias, and sometimes from Albizia and Combretum, are also marketed as gum arabic. Although regulations for the admission of gum arabic no longer distinguish between gums from Acacia senegal and Acacia seyal, and although the gum of Acacia seyal is most often marketed as gum arabic, its properties are inferior to those of the gum from Acacia senegal. In exports from Sudan, the distinction is clearly made: gum from Acacia senegal is marketed as ‘gum hashab’, while gum from Acacia seyal is sold under the name ‘gum talha’. In Zimbabwe, gum from Acacia karroo is locally traded as gum arabic. Synthetic substitutes for gum arabic are the ‘modified starches’, such as xanthan and gellan, which increasingly replace gum arabic as food hydrocolloids.
Armed, deciduous shrub or small to medium-sized tree up to 15 m tall; bark yellowish-brown to purplish-black, rough or smooth, papery and peeling off in strips or not, deeply fissured and blackish on old trees; crown slightly rounded or flattened and somewhat spreading, or slender and spindly with irregular virgate branches; branchlets glabrous to densely pubescent, with prickles just below the nodes, either in 3s with the central one hooked downwards and the lateral ones curved upwards, or with laterals absent, young prickles reddish, later blackish. Leaves alternate, bipinnate; stipules minute or absent; petiole, rhachis and rhachillae sparingly to densely clothed with spreading hairs, rarely glabrous; pinnae in (2–)3–8(–12) pairs; leaflets in 7–25 pairs, linear to elliptical-oblong, 1–5(–9) mm × 0.5–2(–3) mm, sparingly appressed-pubescent on both surfaces or glabrous. Inflorescence an axillary spike up to 12 cm long, axis densely pubescent or glabrous. Flowers bisexual, white or cream in colour; calyx 2–3(–3.5) mm long, glabrous to somewhat pubescent; corolla 3–4 mm long; stamens numerous, up to 7 mm long; ovary superior, shortly stalked, glabrous. Fruit an oblong pod, (2–)4–19 cm × 1–3.5 cm, rounded to acuminate apically, venose, sparsely to densely appressed-pubescent or puberulous, yellowish- or greyish-brown to brown, dehiscent, up to 7-seeded. Seeds subcircular-lenticular, 8–12 mm in diameter, areole 2.5–6 mm × 2.5–5 mm, impressed, horseshoe-shaped.
Other botanical information
Acacia senegal is classified in the subgenus Aculeiferum, which is based chiefly on characters of seed and seedlings, on the absence of stipular spines (but prickles present) and pollen characters. Subgenus Aculeiferum accommodates all African Acacia species lacking spinescent stipules. Generally 4 varieties are distinguished in Acacia senegal: var. senegal, var. kerensis Schweinf., var. leiorhachis Brenan and var. rostrata (Sim) Brenan. It is doubtful whether var. kerensis is a good taxon as it only represents the shrubby growth form of Acacia senegal; all shrubby specimens in north-east Africa are referred to as var. kerensis. Var. senegal can be distinguished from the other 3 varieties by the following combination of characters: tree with a single central stem and a usually dense flat-topped crown, bark without any papery peel, rough, peduncle pubescent (very rarely glabrous) and pods usually rounded to somewhat pointed, but never rostrate or acuminate at the apex. None of the varieties kerensis, leiorhachis or rostrata appear to produce much gum in their native range.
Gum arabic is formed in cysts in the inner bark of the branches and not in the wood. The gum cysts are formed first in the parenchyma of the phloem. In case of wounding, the gum is transported to the wounded site via new channels formed by lysis of cells. The following wood description is based on South African samples of the varieties leiorhachis and rostrata.
- Macroscopic characters:
- Sapwood pale to creamy yellow, heartwood pale to dark brown.
- Microscopic characters:
- Growth rings consisting of flattened marginal parenchyma or thick-walled fibres. Vessels solitary, in pairs or radial groups, 70–200 µm; perforation plates simple; intervessel pits alternate, vestured, vessel-ray pits similar to intervessel pits. Fibres with simple pits. Axial parenchyma confluent (var. leiorhachis) or banded (var. rostrata). Rays 1–5-seriate, homogenous, average height 270 µm (var. leiorhachis) to 420 µm (var. rostrata). Prismatic crystals in chambered axial parenchyma cells.
Growth and development
Under favourable conditions Acacia senegal can start flowering when 3 years old. It flowers on branches formed during the previous rainy season. Flowers open from the base to the tip of the inflorescence in 24 hours. They show weak protogyny, with the styles protruding above the stamens in open flowers. In Sudan, flowering is in June–July, in western Africa the flowering peak occurs around July–September and fruits are mature between the end of November and early February. In South Africa, flowering is in December–January(–April) and fruits are ripe in October. Pollination is presumably by insects. Good seed years are relatively infrequent. In the first growing season the taproot can grow up to 4 m long. Later a fairly dense lateral root system develops which can reach up to 13 m from the taproot and is able to collect rain water from an area of up to 500 m².
In southern Ethiopia, 5-year-old trees have a wood volume increment per year and per ha of 5.4–5.9 m³, a mean height of 5.0 m and a mean diameter at breast height of 7.4–7.7 cm on sites at 1580–1650 m altitude which receive an annual rainfall of 625–690 mm. Generally, wood production of natural stands is estimated at 4–7 m³ per ha per year. In gum plantations wood production is only 0.5–1 m³ per ha per year.
The relationship between the degree of vigour of trees and their ability to exude gum and the contingent role of pathogenic organisms in the induction of gummosis are still not known. A negative correlation between soil water availability and relative air humidity on the one hand and gum yield on the other has been observed in Senegal, but this fact needs confirmation from other observations.
Nodulation has been observed in Acacia senegal but the amount of nitrogen fixation is fairly low compared to other Acacia species. Mycorrhizal associations have been observed in the form of vesicular-arbuscular mycorrhizae. South Africa and Australia have policies against further introduction of Acacia senegal var. rostrata and var. leiorhachis as both varieties may become noxious weeds there.
Acacia senegal grows in tropical and sub-tropical, arid and semi-arid regions and is very drought resistant. Trees survive in the most adverse conditions, subject to hot winds and sandstorms on the poorest soils of rock and sand. Acacia senegal occurs naturally in areas with an annual rainfall of (100–)200–400(–800) mm with 7–11 dry months/year. However, in some highland sites in Rwanda and Kenya it may receive as much as 1000 mm of rain. Average annual temperatures commonly vary from 25–30°C, although it can withstand mean maximum temperatures of 45°C. The mean maximum temperature in the hottest month is 30–41°C, the mean minimum temperature in the coldest month is 4–15°C. Over most of its natural range Acacia senegal is sensitive to frost, although in Asia it occurs in areas with minimum temperatures as low as –2.5 to –5°C. The altitude range of Acacia senegal is 0–2000 m. It is associated with a variety of vegetation types ranging from semi-desert grassland to Anogeissus woodland. Sandy soils are preferred, particularly those of fossil dunes in the Sahel, but it will also grow on loamy sand, on rocky hill slopes and even on clay plains, provided they are well drained and rainfall is at least around 600 mm/year, compensating for the lower available soil moisture. It will not grow on mineral soils or strongly leached ferrous soils. A coarse texture is preferred. There is no correlation of soil organic matter content with abundance of Acacia senegal. Free drainage is essential and waterlogging is not tolerated at all. The pH of the soil may range from slightly acid to moderately alkaline.
Propagation and planting
Acacia senegal var. senegal can be propagated by seed and by tissue culture. One kg contains 7000–19,000 seeds. The hard-coated seeds are orthodox and remain viable for up to 7 years when stored under cool and dry conditions. Fresh seed with a soft seed coat can be sown immediately without pretreatment, but pretreatment is required for seed which has been stored for several months. Treatment with concentrated sulphuric acid for 3–15 minutes or immersion in boiling water for 5 seconds is satisfactory. Older seed may be treated with concentrated sulphuric acid for 40 minutes or soaked in water for 12–24 hours. Polythene bags 30 cm high and 6 cm in diameter with light and moist soil are used for sowing. No compost is applied to avoid overheating by fermentation. Per bag 2–4 seeds are sown 1 cm deep. One kg of seed will produce 4000–6000 seedlings. After 4–6 weeks the seedlings are thinned to 1 per pot. Removed seedlings cannot withstand transplanting to other bags. After 14–18 weeks, seedlings attain a height of 30 cm and can be planted out in the field. Planting is done in pits of 30 cm × 30 cm × 30 cm spaced at 4 m × 4 m. For direct sowing, 5–8 seeds are sown in similar pits at the same spacing. Mechanised seeding has been successful on the clay plains in Sudan. In compact soils larger planting holes of up to 60 cm × 60 cm × 60 cm may be required, or subsoiling should be done up to 60–70 cm deep. The latter method is often too expensive. Application of 150 g of NPK fertiliser per planting hole not only assures faster growth in the first year but also increases drought resistance of the planted seedlings. Weeding is essential after planting, 2–3 times during the first year and for another 2 growing seasons. The young plants need protection from livestock for the first 3 years until they have grown out of their reach. Planting at a wide spacing of 10 m × 10 m or direct sowing allows for interplanting with annual crops such as millet, beans or groundnuts. In vitro micropropagation of Acacia senegal has been successful, but it is probably only economic to multiply high-yielding mother trees. Natural regeneration is very unpredictable due to the irregularity of rains and the frequent attack of the seeds by insects and rodents. However, regrowth occurs often from coppice.
In Sudan, gum trees were traditionally grown in a controlled bush fallow system with a 20–25 year rotation; 4–5 years of cropping was alternated with a fallow period with gum trees for 15–20 years. The gum trees needed 5 years to become established and could be tapped for the remaining fallow period. When gum production declined, all gum trees were cut at 1.5 m height or sometimes killed by fire. Cutting at ground level will also kill the tree. After the rotation period with crops, gum trees were either regenerated or were sown at the same time as the last crops. Animals were allowed to graze during the fallow period after tapping the gum trees and crop harvesting. The tree density in a productive stand used to be 500–2000 trees/ha. Unfortunately, this fallow system has collapsed due to the combination of the Sahel drought, especially the drought from 1979 to 1986, pressure for cultivated land, low prices received by tappers and the high prices obtained for fuelwood. Only in southern parts of Kordofan province have farmers maintained their gum gardens due to better revenues which were obtained by smuggling gum arabic to Chad. Government-owned plantations, which were left untapped during the drought to reduce stress on the trees, grew vigorously again after the rains returned. By 1996 the Sudanese Forest Service had established 12,500 ha of new gum tree plantations inside forest reserves to act as buffer plantations. These are now rented out to gum tappers on a sharecropping basis.
Diseases and pests
Acacia senegal trees are mildly susceptible to root knot nematodes. Locusts (Acridium melanorhodon) can defoliate vast areas overnight, but trees normally recover. The buffalo treehopper (Stictocephala bubalus) destroys 15–85% of fallen seed.
In Sudan, tapping usually starts when trees are 4–5 years old and begins when they are starting to shed their leaves or when the grass beneath them starts to dry out, usually about the end of October or beginning of November. The start of tapping coincides with the tree’s largest accumulation of carbohydrate reserves from the previous rainy season. All main branches are tapped, but the main stem is not tapped, as the bark would soon become too thick and fibrous. In Sudan a special tool with a metal head known as a ‘sunki’ is used. A tangential cut is made penetrating just below the bark; then a longitudinal strip of bark of 2–3 cm wide and 1 m long is pulled off from the wood. Damage to the wood should be minimal. Several branches are treated in a similar manner in one tapping round. In subsequent years, other branches, or the reverse side of the previously treated branches, are tapped. After the superficial injury, gum exudes intermittently, forming nodules with a hard but slightly elastic skin. As more gum exudes the outer skin expands or cracks and the nodule grows in size to 3–5 cm in diameter. After 4–6 weeks the skin becomes too hard to expand any further, the nodule ceases to grow and is ready for picking. Collections are made from the same trees at intervals of approximately 2 weeks until February. Tapping wounds are overgrown by the end of the next rainy season. In Sudan, tapping is the work of men; women are responsible for harvesting, cleaning, storage and transport of the produce. The tears of gum are preferably picked by hand from the branches, which implies that tapping should only be done from the lower branches. When the tears are knocked off, they pick up dirt which seriously impairs the quality. The collected tears are carried in open baskets; the use of plastic bags increases the risk of moisture retention and mould formation. A tapper may tap up to 100 trees per day, covering up to 30 ha per season. During the rainy season, gum exudation does not occur, while some trees that are tapped do not produce gum at all. Older trees produce a darker gum containing more tannins from the bark. In Mali, harvesting periods are December–January and May–June, the latter being the more productive one.
Natural exudation, particularly known from northern Kenya where trees are not tapped, is generally provoked by wounding, for example, from particles carried by the wind, drought, browsing animals, insects or parasitic plants.
Yields of gum arabic from individual trees are very variable and few reliable data are available. A yield of 250 g/tree per season is generally cited as average, but individual trees may yield several kg. A figure of 100 g/tree is reported for Mauritania. Yields from cultivated trees are said to increase up to the age of 15 years, when they level off and then begin to decline after 20 years. In Mali, the best yields from Acacia senegal are said to be produced between ages 7 and 15 years.
Handling after harvest
In Sudan, the minimum guaranteed gum price is fixed by the government; the price is lower in years when buffer stocks accumulate to discourage maximum production, whereas in years of shortage the opposite applies. Sudanese merchants buy the gum competitively at auction, and are obliged to pay the supplier immediately after the auction. The Gum Arabic Company will buy at the official base price any gum that remains unsold. The gum is then cleaned and graded into the following grades: Hand-Picked Selected (HPS), Cleaned and Sifted (CAS), Cleaned Natural, Red Gum, Gum Siftings, and Gum Dust. Cleaning and grading involves sorting the gum into whole tears and smaller pieces, while separating any dark gum and removing pieces of bark and other foreign matter. All gum for export is subsequently purchased by the Gum Arabic Company, cleaned again and packed for export in jute bags. A 40% export tax is levied by the government. This system, introduced in 1967–1968 to ensure the quality of Sudanese production, is being abused by smuggling of gum to neighbouring countries to avoid export duty.
Nigerian gum arabic is sorted into 3 grades; Grade 1 is gum produced from Acacia senegal, Grade 2 is gum from other Acacia species, and Grade 3 is much darker and very mixed in quality and may consist of gum from species other than Acacia (e.g. Albizia and Combretum). From the 1993/1994 production season, Sudan began producing kibbled gum; 2500 t were produced. Previously, all kibbling was done in consumer countries. Kibbling entails passing whole tears or large lumps of gum through a hammer mill and screening it to produce smaller granules of more uniform size. These pieces are more easily dissolved in water. Powdered gum may be produced from kibbled gum but it may also be produced by spray-drying. The gum is dissolved in water, filtered and/or centrifuged to remove impurities and the solution, after pasteurization to remove microbial contamination, is sprayed into a stream of hot air to promote evaporation of water. Although spray-drying adds around US$ 1000/t to the price of gum arabic, it is preferred by most food processing manufacturers, who do not want to stock raw gum arabic, of which the microbiological quality may not be good.
Isozyme studies indicate that West African provenances of Acacia senegal var. senegal show little variation. Although small collections of germplasm exist, they do not cover the full geographic and genetic variability, as collection activities have concentrated on high gum-yielding provenances of Acacia senegal var. senegal.
The relationship between the degree of vigour of trees and their ability to exude gum and the contingent role of pathogenic organisms in the induction of gummosis are still not known. Before selection of high-yielding strains can be made, more physiological and anatomical information is needed on the mode of gum production.
The lost market share of gum arabic due to droughts and high prices in the 1980s is unlikely to be regained as industries consuming gum arabic have changed to other hydrocolloids, mainly modified starches, in their production processes. However, the 1992 trade of 20,000–24,000 t is still considerable. Increased production and consequent favourable prices, and the marketing asset of a natural product, may lead to better prospects in the future.
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Correct citation of this article
Boer, E., 2002. Acacia senegal (L.) Willd. [Internet] Record from PROTA4U. Oyen, L.P.A. & Lemmens, R.H.M.J. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. <http://www.prota4u.org/search.asp>.
Accessed 3 September 2018.
- See the Prota4U database.