LITERATURE REVIEW

2.1 INTRODUCTION

This chapter gives an insight into various studies conducted by outstanding researchers, as well as explained terminologies with regards to evaluate the phytochemical, nutritional and microbial inhibitory content present in Bombax costatum leaves leaves grown in Nigeria with a little trace of its inherent medicinal constituents. The chapter also gives a resume of the history and present status of the problem delineated by a concise review of previous studies into closely related problems.

2.2 CONCEPTUAL AND EMPIRICAL REVIEW

THE BOMBAX COSTATUM (Pellegr. et Vuillet)

Botanical Description

The Bombax costatum belongs to the family Bombacaceae and is locally called redflowered silk-cotton tree or red kapok tree. It is a deciduous tree growing straight up to about 30 m tall and 100 cm in diameter. The crowns from the young trees are storeyed, developing into irregular and hardy in older trees. The bark is thick, rough, corky, greyish brown in colour, and covered with conical pointed spines on the stem and branches. The leaves are digitally compound, ovate with 5-7 leaflets and the petioles are about 8-15 cm long. At both ends, the leaflets are partly acuminate and partly ovate with lateral nerves of 8-10 pairs. The flowers are solitary, bisexual and 5-7 cm with deep red, orange or yellow, tulip-shaped, glabrous peduncles. The calyx are cup-shaped, 12-17 mm long, truncate, 5- toothed. The plant has 5 oblong-linear petals 4.5 cm x 1.5 cm with a round apex. The fruits are contained in ellipsoidal capsule darkbrown in colour with variable shapes. The fruits are embedded in white floss called kapok and contain several small seeds (Oyen, 2011).

Figure 1.1 Stem bark of Bombax costatum (source: www.worldagroforestry.org).

Ecological and Geographical distribution

Bombax costatum is a characteristic fire resisting tree of the savannas and dry woodlands from

Senegal to central Africa, from Guinea across Ghana and Nigeria to southern chad. It grows on stony and gravelly lateritic soils, and often in cropland and close to settlements. Its thick, corky bark protects it against fire (Oyen, 2011).

TAXONOMY, GEOGRAPHICAL DISTRIBUTION AND ECOLOGY

Bombax is a pantropical genus of the Malvaceae (order Malvales), comprising eight species: two in Africa, five in Asia, and one from Asia to the Solomon Islands (Robyns, 1957; Orwa et al., 2009). Bombax costatum Pellegr. & Vuillet is one of these species popularly known as “red-flowered silk-cotton tree”, “red kapok tree”, and “Gambia silk-cotton tree”. This species is also known by other common and vernacular names: “kapokier à fleurs rouges”, “fromager”, “kapokier rouge”, “faux kapokier”, “kapokier de forêt” (France); “kattupa”, “garablaobe” (Senegal); “huntin” and “forgo” (Benin) (Akoègninou et al., 2006).

Bombax costatum is native and distributed in savanna zones in Western Africa from Chad to Mauritania (Orwa et al., 2009; Ouédraogo et al., 2014; GBIF, 2017). It is a characteristic tree of savannas and dry woodlands of the Sudanian zone of West Africa, extending to the north into the Sahelian zone and to the south into the Guinean zone (Aubréville, 1950; Arbonnier, 2002). The species is occasionally cultivated in West Africa for its edible calyx (Achigan-Dako et al., 2010). Generally, B. costatum occurs in regions with 800-1,500 mm mean annual rainfall. In Burkina Faso, it grows in areas with 600-900 mm annual rainfall (Belem, 2009). It grows on stony and gravelly lateritic soils, and often in cropland, close to settlements (Oyen, 2011) and in parklands (Bayala et al., 1995). Individual trees which can reach 17 m are often observed along temporary stream banks on clay-sandy soils. Bombax costatum is a fire resisting species with tuberous roots acting as water and/or sugar storage during long drought periods (Orwa et al., 2009). In the wild, B. costatum is usually associated with Pterocarpus erinaceus Poir., Daniellia oliveri (Rolfe) Hutch. & Dalziel, Cordyla pinnata (Lepr. ex A.Rich.) Milne-Redh., Terminalia macroptera Guill. & Perr., and Prosopis africana (Guill. & Perr.) Taub. (Orwa et al., 2009). In dry forests of the Guinean zone, B. costatum is replaced with Bombax buonopozense P.Beauv. and in wetter forests with Bombax brevicuspe Sprague (Oyen, 2011). In the literature there is a lack of knowledge regarding the species’distribution and associated environmental conditions.

BOTANICAL DESCRIPTION AND STRUCTURAL CHARACTERISTICS

Bombax costatum is a deciduous tree that grows between 10 and 25 m height in relatively wet conditions (Ouédraogo et al., 2014), but hardly reaches over 6 m in the Sahel (Orwa et al., 2009). With a straight bole, it can reach 60 cm in diameter, and is sometimes buttressed. The crown structure of young trees is storeyed, becoming irregular and sturdy in older trees. The bark is thick, grey brown and corky, with typical conical, stout, sharp-pointed spines on the stem and branch, and with a light red brown slash (Orwa et al., 2009).

Leaves are digitate compound, with 5-7 leaflets, 8-15 cm long (Oyen, 2011) and 10-18 cm long with petiolules up to 5 mm long (Orwa et al., 2009). Leaflets are partly ovate, and acuminate at both ends, with 8-10 pairs of lateral nerves. Flowers are 5-6 cm long and solitary, deep red, orange or yellow, tulip shaped, on long, glabrous peduncles with the calyx cup-shaped. The fruit, an ellipsoidal capsule, is dark brown to brown, and composed of 5 valves, dehiscent, 8-16 cm long and 3-6 cm wide, of variable shape (Orwa et al., 2009). The fruit contains white floss, called kapok and several small seeds. Seeds are globose to pear-shaped, 4-5 mm in diameter, dark brown to black, embedded in the white floss (Oyen, 2011).

The spatial distribution of B. costatum is generally characterized by scattered individuals in both protected and anthropogenized natural stands (Ouédraogo et al., 2014). In the Sudanian zone, the natural stands are sometimes large with a density of 6-10 trees per 1,000 m2. Reportedly, natural stands of this species in drier conditions tend to show relatively many large stems and few smaller stems (Ouédraogo, 2006).

Traditional Uses

A macerated powder of the root is used to treat epilepsy (Orwa et al., 2009). Medicinal stem bark preparations promote wound healing (Orwa et al., 2009). The stem bark and roots, in Sierra Leone and Senegal, are said to possess diuretic properties. The stem bark is also utilized to treat trichomoniasis, amoebiasis and dysentery in other forms. A powdered preparation of the stem bark is used to treat headache. The stem bark can also be prepared in a bath to treat insanity (Orwa et al., 2009). The bark is also used for the treatment of skin diseases. In Mali, a decocted preparation of the bark, leaves and other parts are used to solve menstrual problems. In children, decocted preparations of the leaves may be used to ameliorate fever. The leaves are also used in the treatment of hookworm and the flowers active against taenia. Natural preparations of the leaves and other medicinal plants are used to treat leucorrhoea and diarrhea (Orwa et al., 2009). An extract formulation of the leaves is used to treat convulsions and other problems during childbirth. A decocted formulation of the leaves, roots or stem bark is used to treat severe cases of oedema. Additionally, the leaves and twigs are decocted to treat jaundice and rickets in children. Various plant parts are used for promoting lactation and as a tonic against fatigue. Mixture of the leaves and shea butter is used to attenuate leprosy of the skin (Orwa et al., 2009).

In veterinary medicine, leaves are given to sheep bitten by a snake. In Burkina Faso, a macerated leave preparation is given to animals with retained placentae. In Niger, a cold decocted formulation of leaves and twigs is given to animals with piles (Paré et al., 2010). A study conducted on Bombax costatum by Atawodi et al., (2012) had shown that the plant possesses anti-trypanosomal activity.

USES, NUTRITIONAL, PHYTOCHEMICAL PROPERTIES AND SOCIOECONOMIC IMPORTANCE

Food uses

Bombax costatum is among the most used and important wild vegetable species (Mertz et al., 2001; Lykke et al., 2002). The calyx is the plant organ most frequently used especially for food (Ouédraogo et al., 2014; Assogba et al., 2017). This also appears as the most common use of B. costatum across its natural range. Leaves and the fleshy calyx of the flowers are cooked and eaten as a vegetable (Lykke et al., 2002). Bombax costatum is recorded as the second most preferred sauce species in rural Burkina Faso (Lykke et al., 2002). Unripe fruits and flowers are added to sauces to thicken them (Oyen, 2011). The young fruit is harvested around August and September, dried and used for the preparation of meals (Orwa et al., 2009). The young, unripe fruit is also used in the preparation of a beverage while the seed oil is suitable for consumption (Oyen, 2011).

Medicinal uses

Several parts of the tree are used in traditional medicine against a great variety of illnesses. Various parts are used against fever or to promote lactation in women and as tonic for tiredness (Orwa et al., 2009). In Senegal and Sierra Leone, diuretic properties are attributed to the bark of stem and roots (Oyen, 2011). The powdered root is macerated and eaten in a sauce or applied as a bath against epilepsy (Oyen, 2011). Roots and bark are much used for the treatment of dizziness, dysentery and malaria in Burkina Faso (Belem et al., 2008). The bark is used for the treatment of skin diseases, yellow fever and headache. Bark preparations are applied to wounds to facilitate healing. A bath in an extract of the stem bark is taken against insanity (Oyen, 2011). The latter argued that the powdered stem bark is used in a medicine applied as a fumigant against headache. The bark is also used for the treatment of skin diseases. In Benin, this species bark is used against fever, skin disease, pain, abscess, oedema, hernia, and epilepsy (Assogba et al., 2017). In Mali, a decoction of the bark and leaves and parts of other plants is taken against menstrual problems (Inngjerdingen et al., 2004). A warm bath in a decoction of leaves may be prescribed to feverish patients, especially children. The leaves are often prescribed with other drugs derived from plants against leucorrhoea and diarrhoea in Senegal (Oyen, 2011). An extract of ground leaves is drunk against problems during childbirth and a bath in this extract is taken repeatedly for the treatment of convulsions. The tea derived from dried leaves is drunk or applied to the body against measles. The leaf decoction is also given to children as a drink for the treatment of rickets. The leaves and immature fruit are used as emollient. A decoction of leaves, root or stem bark is drunk in cases of serious oedema (Oyen, 2011). Leaves mixed with shea butter are rubbed onto affected skin against leprosy. Leaves ground in water are given to sheep against snake bites. Maceration of pounded leaves is given to animals with a retained placenta in Burkina Faso, while a cold decoction of leaves and twigs is given to animals with haemorrhoids in Niger (Oyen, 2011).

5.3. Timber and other uses

The wood has acoustic properties making it suitable for sound boxes of drums and xylophones (Oyen, 2011). It is also used for making match-sticks, domestic appliances, door and window frames and fuel wood. The bark is used for making whistles. Stakes are planted to establish live fences in Sierra Leone. Containing tannins, the bark produces a brown dye. The white floss in the fruits of B. costatum is widely used for stuffing mattresses, pillows and cushions (Oyen, 2011). Flowers and leaves are valuable fodder for domestic animals, while the tree provides shade for grazing animals (Oyen, 2011). The species is an important bee forage (Orwa et al., 2009). Bombax costatum provides ecosystems services such as erosion control, soil improvement, fencing material and intercropping attesting to its agroforestry significance (Orwa et al., 2009).

Nutritional and phytochemical properties

Suitability of B. costatum for human and animal consumption is supported by its high protein content, low content of toxic substances and the amount of polyunsaturated fats in its seed oil (Orwa et al., 2009). In Niger the fruit was found to contain 20-25% protein (Oyen, 2011). The seeds have high oil content (20%). Iron and zinc were also found in high concentrations in flowers: 1.5 mg.g-1 iron and 67.1 µg.g-1 zinc (Maiga et al., 2005), suggesting that this species could be an interesting source of iron for humans in case of haemoglobin deficiency. Although the species is characterized by a high nutritional value (Kristensen & Lykke, 2003), there is still a lack of detailed information on the nutritional content of this species especially in terms of human nutrition.

Atcholi et al. (2015) studied the species as a potential bio adhesive source in the development of formaldehyde-free particle board. In this study, mucilage and pectin fractions of the flower sepals were analyzed for monosaccharides, their identification and quantification. The aqueous extraction obtained and ethanolic precipitation, followed by ionic chromatography showed interesting qualitative results. Bombax costatum is an untapped reservoir of phytocompounds which may play a supportive role in the pharmaceutical field (Refaat et al., 2013). The fatty acid of its oil is composed of caproic acid (3%), caprylic acid (7%), palmitic acid (8%), stearic acid (3%), oleic acid (49%), linoleic acid (13%), rachidic acid (3%), lignoceric acid (1-2%), and others (13%) (Oyen, 2011; Refaat et al., 2013).

Socioeconomic importance of Bombax constatum

Bombax costatum has a great socioeconomic importance (Ouédraogo et al., 2014) with interesting commercial value (Kristensen & Lykke, 2003). The calyx is sold on local markets and hence constitutes an additional source of income in rural communities (Kristensen & Lykke, 2003; Belem et al., 2007; Assogba et al., 2017). The monetary value of the calyces varied with their diameter and with the season. For instance in Burkina Faso, over the same period, their annual value per tree varied from 2.57 US$ (tree of diameter size 10 to 25 cm) to 26.10 US$ (tree of diameter size 50 cm and above) (Ouédraogo et al., 2014). Prices predictably increase during the rainy season when stocks become low and demand high (Mertz et al., 2001). Oyen (2011) estimated that about 1,000 t of kapok from natural Bombax costatum stands were exported annually from francophone West Africa, suggesting high potential for business.

Propagation, Growth and Development

The best way to propagate B. costatum is by direct seeding, although wildings may also be used (Oyen, 2011). Before sowing, the fibrous material covering the seed should be removed and the seed should then be immersed in boiling water and soaked in cold water for 24 h (Oyen, 2011). It is easy to produce the seedlings of B. costatum in nursery, but seeds seem difficult to keep for several years. However, it is unclear how long the seeds are viable and how long they can be conserved for future use as planting material. Seedlings are difficult to transplant despite their vigorous rooting ability (Oyen, 2011). According to this author, when the seedlings are adequately protected against fire and livestock, natural regeneration is easy and abundant. Assessing the regeneration pattern of the species in Burkina Faso, Ouédraogo & Thiombiano (2012) observed an irregular structure in most of the studied populations.

In addition, high mortality was observed between the first and second year, followed by reduced mortality. It was concluded that “B. costatum seedlings showed potential for use in silviculture as evidenced by the fact that it showed high post-transplantation rates of survival and growth” (Ouédraogo & Thiombiano, 2012). When natural regeneration is not easy, the stimulation of suckering under the mature trees can be recommended (Belem et al., 2008). The suckering represents the main and most suitable way of recruitment for B. costatum (Ouédraogo, 2006). Belem et al. (2008) investigating the suckering potential of the species, showed that the suckering can be induced by injuries to its roots at the beginning of the rainy season, and therefore lack of seeds is not a limiting factor for the species regeneration. While Belem et al. (2008) developed techniques to stimulate, control and optimize the suckering potential of B. costatum, Ouédraogo & Thiombiano (2012) showed that B. costatum can regenerate about 95% by suckering in a relatively young stand. These show up favourable prospects for in situ conservation of the species.

Bombax costatum flowers after leaf fall in November to February (Orwa et al., 2009) and new leaves unfold from April onwards (Oyen, 2011). Under favourable conditions, trees start bearing fruit from the 6th year onwards and fruit ripening begins in August-September (Orwa et al., 2009; Oyen, 2011). However, Oyen (2011) stated that trees often do not produce fruits due to fire-damage at the critical time of flowering in the dry season. Fruits mostly open while still on the tree and the lightweight seeds are spread by gravity and wind. A tree bears up to 1,500 fruits, each with 5-8 g of kapok. Under favourable conditions, 3-5 kg kapok per tree can be obtained from the 10th year onwards (Orwa et al., 2009; Oyen, 2011).

Threats and Conservation

Bombax costatum is threatened presumably because of its poor natural regeneration (Ouédraogo et al., 2006b), and also by the intensive harvesting of its flowers for domestic and commercial use as a vegetable (Belem et al., 2008). In Burkina Faso, the species is among the most preferred by the local population for its uses (Mertz et al., 2001) and is threatened because of the overexploitation of its different organs, mainly the calyx (Ouédraogo et al., 2014). Considering these threats and its great socioeconomic importance for local populations, B. costatum could be regarded as a species of high conservation priority (Kristensen & Lykke, 2003; Sop et al., 2012). In spite of existing efforts towards its conservation, B. costatum is still under serious threat as a result of degradation of natural stands through:

flower harvesting which impedes the fructification;

seedling destruction by farmers during the weeding in farmlands;

lopping of mature trees for handicrafts; – grazing of seedlings by the livestock, etc.

Yet it is important to note that neither the impact of the overharvesting of flowers nor the combined effect of all sources of threats has been explicitly quantified.

Nevertheless, several studies have reported that its rejuvenation in the degraded savanna parkland is inadequate to sustain production of flowers, fruits and leaves. In general, rural communities from sub-Saharan Africa domesticate multipurpose tree species in home gardens (Gautam & Watanabe, 2004). This conservation option would be more appropriate for B. costatum as the way the species’conservation is now initiated in West Africa does not include its cultivation. While it is not clear whether its genetic diversity is threatened, it is under threat as an economic resource with no formal and active management strategy (Oyen, 2011). In any case, the genetic diversity of the species needs to be investigated to better inform conservation strategies and future genetic improvement and breeding.

Bombax costatum does not have yet an official conservation status on the IUCN Red List of Threatened Species. However, it is reported to be disappearing from the Sahel and Guinean woodland areas of its range. In Benin, it has been included on the National List of Benin Food forest vegetable resources (Codjia et al., 2003). Hahn-Hadjali & Thiombiano (2000) mentioned that B. costatum is quoted as a species in regression in the east of Burkina Faso. According to Ouédraogo (2006), B. costatum has unstable population structures characterized by few seedlings in anthropogenized zones. Bombax costatum is classified as a vulnerable species and is among the species which could be highly threatened if adequate measures are not taken to ensure their conservation (Garzuglia, 2006).