**2. African walnut (***P. conophora***)**

#### **2.1 Description**

*P. conophora* Muell Arg. known as African walnut belong to Euphorbiaceae family [15]. It is a climber that twines around cocoa and kola nut trees for support [16] (**Figure 1**). *P. conophora* is a small tropical flowering shrub, a woody perennial plant 6 m – 18 m long when it attains maturity stage; the stem can be as wide as 16 cm and turns dark gray as it ages, but it is green and glabrous at tender age [17]. The leaf is simple, crenate and having serrated margin. They are spherical at the base with the leaves arranged alternately [18, 19]. The seeds can be boiled and eaten as snacks [20]. Fruits not yet developed have green color but change from dark brown to black at

*Lesser Known African Indigenous Tree and Fruit Plants: Recent Evidence from Literatures… DOI: http://dx.doi.org/10.5772/intechopen.104890*

**Figure 1.** *Staked African walnut plants.*

**Figure 2.** *African walnut capsules.*

maturity [21]. The seed is white when cracked upon shell removal with a thin layer between split halves. After eating the nut, the presence of chemical substances such as alkaloids gives a bitter taste upon drinking water [22]. Walnut seeds are housed in a pod having: one seed (single), two seeds (double), three seeds (triple), four seeds (quadruple) and five seeds (quintuple) [23]. The walnut shells are usually black or brown in color (see **Figure 2**).

#### **2.2 Origin and distribution**

*Plukenetia conophora* originated from tropical western and central Africa and it is available in Nigeria, Cameroon, Congo, Central African Republic, Gabon, Niger, and Sierra Leone [18, 24]. The African walnut is cultivated in Western and Eastern regions of Nigeria [18]. It is found in Uyo, Akamkpa, Akpabuyo, Lagos, Akure, Kogi, Ajaawa, Ogbomosho, Ibadan [25, 26], Ife, Ekiti and Osun State. In the South and cocoa producing States in Nigeria, walnut is available [24, 27]. African walnut thrives on loamy soils that are deep, fertile, moist and well drained. Walnut does well on silty clay and loam soils [28, 29] and for optimum growth, walnut requires high solar radiation.

#### **2.3 Food and economic importance**

*P. conophora* Muell Arg. is a multipurpose crop used for food, nutritional and economic purposes in Africa. This plant is grown purposely for its nuts, boiled in water and eaten as a snack [25]. In Nigeria, Sierra Leone and Ghana, the fruits improve the livelihood of the rural people by providing income [30]. Extracted oil from the nut is used in making wood varnishes, vulcanized oil for rubber, stand oil and leather substitutes [31, 32]. Essential oils are usually extracted and used in food, cosmetics, perfumes, soaps and drinks as flavor. It can also be used in treating skin diseases and as remedy for cancer. Walnut peels (shell) combined with other materials are used as filler in dynamite [33]. The shell can be included in catfish meal with no negative effect on the performance and health status of the fish [34].

#### **2.4 Medicinal value**

*P. conophora* have numerous ethno-medicinal uses among the African rural populace. The leaves, root, bark and fruit are known for their medicinal values. Walnut leaves are used to treat venous insufficiency, hypoglycemia, hemorrhoids, indigestion, constipation, dysentery, diarrhea, syphilis, asthma, thrush, prolonged and constant hiccups, pruritus, eczema, fungal and microbial infections, psoriasis and parasitic skin conditions majorly among children, the elderly and immunosuppressed [35, 36]. African walnut can be used to expel worms; it can also treat rheumatism, kidney pain, cold, gout, cleaning of blood and abnormal menstrual bleeding [37]. The succulent leaves are used as vegetable and for treating cancers growing in the neck. They control inflammation of the gums and throat and mouth when used as tea [33]. Brown dye is extracted from the husk and leaf which is used to manage hiccups [38]. The root is effective in the treatment of piles. It lowers the risk of developing cancer and it controls high blood pressure [37] and it can be used as an antidote to snakebite, tonification of kidneys and strengthening of the back and knees [39]. The bark can be used in tea as laxative, chewed to reduce toothache and to treat high blood pressure while the root is used for frost bite and varicose ulcers [40, 41].

#### **2.5 Nutritional qualities**

The seed and leaf of *Plukenetia* are good sources of nutrients that can ensure food security and remedy malnutrition of the populace. Olajide *et al.* [42] evaluated the variability in proximate quality traits of 10 accessions of *Plukenetia conophora* from Southwestern Nigeria. They found that the seeds contained proximate contents and established that location of seed collection significantly affected ash, crude fat, crude

#### *Lesser Known African Indigenous Tree and Fruit Plants: Recent Evidence from Literatures… DOI: http://dx.doi.org/10.5772/intechopen.104890*

fiber, dry matter, moisture content and nitrogen free extract, which suggests the need for selection and also gives way to improvement program. Agbo and Baiyeri [43] also reported variability in proximate and mineral qualities of five accessions of African walnut, which probably suggests genetic diversity or more probably, could be environmentally induced. Olajide *et al.* [42] also found that fresh and boiled walnut is an excellent food material with the ability to combat food insecurity in rural populace. They reported that the proximate composition in the nut include ash (6.40 and 6.34%), crude protein (17.04 and 19.20%) crude fat (40.77 and 39.74%), crude fiber (11.76 and 11.28%), dry matter (90.93 and 90.94%), moisture content (9.07 and 9.06%) and nitrogen free extract (14.97 and 14.40%) for fresh and boiled nuts respectively. Boiling positively influenced proximate qualities of African walnut as it increased protein content and dry matter while it reduced ash, crude fat, crude fiber, moisture content and nitrogen free extract. The proximate composition of *P. conophora* shows that it contains carbohydrate (4.17%), ash (3.32%), protein (29.14%), fat (54.14%) and various vitamin contents [44]. Suara *et al.* [45] reported 6.86% for moisture content, 11.78% for protein, 8.57% for total ash, 20.12% for crude fiber, 1.56% for total fat and 51.8% for total carbohydrate. In addition, Olajide *et al.* [46] evaluated the nutritional differences in 10 accessions of African walnut obtained from Southwestern Nigeria as affected by collection center and processing and the results suggested sufficient genetic variability in seeds of walnut obtained from Southwestern Nigeria, emphasizing the possibility for selection. The nutritional assessment of the seed revealed that the fresh and boiled seed contains iron (16.82 and 24.59 mg/kg), potassium (10781.0 and 10420.0 mg/kg), magnesium (5076.0 and 4621.0 mg/kg), phosphorus (162.7 and 229.7 mg/kg), zinc (65.2 and 54.54 mg/kg) and sodium (729.2 and 718.2 mg/kg). The results showed that processing had positively effect on iron, magnesium, phosphorus and zinc contents. Fresh seeds possessed higher quantity of zinc, potassium, magnesium and sodium compared to the boiled seeds. Conversely, iron and phosphorus were more in boiled seeds. Enujiugha [47] reported that walnut seeds contain mineral (465.95 mg/100 g of phosphorus, 57.37 mg/100 g of magnesium, 1.55 mg/100 g of iron and 6.84 mg/100 g of zinc). Phytochemical contents revealed 0.243 and 0.31% of phenol, 0.0142 and 0.0179% of phytate, 0.0851 and 0.0784% of tannin, 0.5419 and 0.5547% for alkaloid and 0.1396 and 0.1577% of glycoside for fresh and boiled nuts, respectively [48]. Concentration of phytate, alkaloids, phenol and glycosides were more pronounced in boiled seeds in comparison with the fresh nuts. On the other hand, higher value for tannin was obtained in fresh seeds. Ekwe and Ihemeje [19] found tannins of 0.89 mg/100 g, oxalate of 1.28 mg/100 g, phytic acid of 3.105 mg/100 g, trypsin inhibitors of 1.84 mg/100 g, saponin of 985.0 mg/100 g and alkaloid of 40.91 mg/100 g. However, higher concentration of alkaloid of 2.670 mg/kg and lower tannin of 0.56 mg/kg were recorded. Ayoola *et al.* [49] found that nutritional and elemental components are more in the nuts than in the leaves. The phytochemical contents observed in the seeds were also available in walnut leaves. The nut contained oil of 48–50%, the oil color is golden yellow, the taste and odor resembles that of linseed oil [50].

#### **2.6 Climatic requirement**

*P. conophora* is native to West Africa or Central Africa. It is abundant in Nigeria, Congo, Ghana and Cameroon. It occurs in the rain-forest region and plantations found at elevations from 250 to 1400 m [51]. The plant thrives on fertile, well—drained loam soils and can also grow on silt clay loam soils [29]. Generally,

subsistence farmers grow *P. conophora* around gardens and backyards in humid and hot zones of tropical Africa [16]. It is also found in bottomlands, coves, rich woodlands and abandoned agricultural fields [52]. As a climber, the plant twines round the host plant to the apex in order to trap sufficient sunlight.

#### **2.7 Cultivation**

The plant thrives on deep, fertile, moist and well-drained loam soils [29]. The African walnut is majorly grown for subsistence consumption in the humid and hot regions of tropical Africa [16]. It does well in rich woodlands, fallow fields, bottomlands and coves [52]. The plant twines round the host plant to settle at the apex to receive more light from the sun, it may join trees to each other and hold a dead tree in position until it decays. Flowering occurs from November to early January and fruiting starts in February till September with the highest yield in July [16]. Walnut seed takes about 4–6 months to reach maturity stage [53].
