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Vitellaria paradoxa (C.F.Gaertn.) is a multi-purpose tree species distributed in a narrow band across sub-Saharan Africa. The species is integrated into cropping and agroforestry systems as a nutritional and economic resource, which provides a range of environmental services. Integration of the species into land-use systems provides an essential source of livelihoods and income for local populations. The economic potential of the shea butter tree derives from its edible products, which also serve cosmetic and pharmaceutical applications.
- agroforestry
- biodiversity conservation
- domestication
- genetic resources
- livelihoods
- molecular marker
- sub-Saharan Africa
1. Introduction
Non-timber forest products (NTFPs) contribute to the livelihood improvement of local communities [1]. They play a significant role in poverty alleviation among rural farmers by providing food security and reducing malnutrition [2,3]. Today, many crops and trees on our planet are utilized for human consumption, providing nutritional and economic resources in smallholder farming systems [4]. Many of these edible plants are indigenous local fruit tree species adapted to their ecosystem, with potential for domestication. Local communities face difficulties due to overexploitation of forest and woodland resources, which are rapidly depleting [5,6,7]. With growing populations, and demand for food expected to double over the next 30 years, solutions are required to address future food and nutritional security in smallholder agricultural systems, including aspects of nutrition, plant science, genomics, and agroforestry [8,9].
Trees play a significant role in African agricultural landscapes and agroforestry parklands [10], contributing substantially to rural livelihoods [11]. Tree genetic resources can improve soil fertility, among other environmental services, and provide sources of livelihood through products such as timber, fruits, and medicines [12]. Other ecosystem services provided by trees include conservation of associated biodiversity, providing food and shelter for living organisms, regulating temperature, shade, and carbon sequestration [13].
The African Orphan Crop Consortium (AOCC) has been working with World Agroforestry (the International Centre for Research in Agroforestry, CIFOR-ICRAF) on genomic resources for the improvement of underutilized species which have hitherto received little attention from the scientific community, with a particular focus on trees. The objective of this consortium is to modernize and improve the efficiency of plant breeding practices to improve yield, climate resilience, and nutrition, thus improving the livelihoods and quality of life of smallholder farmers [14,15,16,17]. Target species provide vitamins, minerals, and essential micronutrients in the diets of producer communities. Early results on two Artocarpus species, jackfruit (Artocarpus heterophyllus) and breadfruit (Artocarpus altilis), have resulted in their genome sequencing and publication [18]. Some genomic data have also been generated for white Acacia (Faidherbia albida), marula (Sclerocarya birrea), and moringa (Moringa oleifera) [19]. These species are classified as crucial African orphan crops with strong potential to improve the livelihoods of farmers [20].
One such species, the shea (butter) tree (Vitellaria paradoxa C.F.Gaertn.), known as karité in French, is an indigenous fruit tree belonging to the Sapotaceae family. There are two primary sub-species, namely nilotica, and paradoxa. It is found in a belt of Sudanic vegetation extending south of the Sahel within the western, central, and eastern regions of sub-Saharan Africa (SSA) [21,22,23]. The shea tree is an essential source of income and livelihood, generating ecological and environmental benefits in the traditional parkland agroforestry system [24]. It is a multipurpose tree, yielding nutritious fruit pulp and kernels and a range of other derived products with edible and medicinal applications. The kernel lipids, known as shea butter, are consumed by rural households, and sold on local markets across the zone as an edible fat of great importance [24]. It is also used as an industrial feedstock in global supply chains serving the confectionery, cosmetic, and pharmaceutical industries. Aside from these value-added applications, the wood of the shea tree is used to produce charcoal of very high quality in terms of its density and performance [25]. Shea butter is referred to as ‘women’s gold’ due to the livelihood benefits that women farmers derive from shea production and processing across the value chain [26]. It is one of the priority species in the larger tree domestication and improvement program implemented on a long-term basis by World Agroforestry.
The AOCC has prioritized the shea tree as one of the 101 local plant species identified as crucial to develop livelihood options for many families across SSA [16]. However, the species has a weak regeneration ability in some areas (particularly in Cameroon) and is facing survival challenges due to deteriorating ecological conditions, poor soil, poor pollination, parasites, and anthropic pressures [25]. Domestication using traditional and advanced technologies, such as genomics, offers a means of selection for the development of new plant varieties that can help farmers. Scientists have been using phenotypic studies and environmental characterization in genetic variation to assess variation in compositional properties [2,27,28,29,30]. The shea tree can be considered a high-priority genetic resource of SSA countries [8,31]. The main objective of this review paper is to summarize the available information from the existing literature, identify research gaps relevant to the domestication potential of the shea tree, and consider potential for development of improved genetic material to support the needs of smallholder farmers. Specifically, the paper focuses on the genetic diversity, compositional and morphological variation, and population structure of V. paradoxa populations across their natural range.
2. Nutritional Composition
2.1. Nutritional Composition of Fruit Pulp
Across its range, shea tree fruit pulp is of high dietary and nutritional importance [72,100]. Different authors have reported variable nutritional composition of shea fruit pulp with different methods used for estimation. This variability is perhaps due to agroecological differences within the populations, or genetic differences (Table 3).
Table 3. Nutritional and mineral composition of shea butter tree (Vitellaria paradoxa) raw pulp.
Some researchers have estimated the moisture content of the dry shea fruit pulp to range from 8.8% to 72.4% [77,101]. A more significant moisture percentage of 67–80.3% has also been recorded. The variation of moisture content can be attributed to processing, storage methods, and seasonality. The following figures regarding shea tree fruit pulp are compared with Dacryodes edulis fruit. The fruit pulp contains the following macromolecules: crude fibre, 3.06 ± 0.16 mg/100 g dry weight (dw) compared with 42.2 mg/100 g dw for D. edulis; crude protein, 19.23 ± 0.76 mg/100 g dw compared with 4.2 mg/100 g dw for D. edulis; and carbohydrates, 15.03 ± 0.45 mg/100 g dw compared with 8.1 mg/100 g dw for D. edulis [77,102]. Other compounds, including calcium and iron, can be found in the shea tree fruit [72,103]. Therefore, it is an essential local fruit to improve the livelihood of local populations [104].
A crude protein content has been reported in the range from 15.2 g/100 g dw [73] to 3.5 g/100 g dw [106]. A crude lipid content of 4.2 g/100 g dw and a crude fibre content of 42.2 g/100 g dw have been described [77,101,107]. An ash content has been reported from 4.7 to 5.4 g/100 g dw [79,101]. The information presented could help to have an overview of the vital energy and nutritional source derived from the shea fruit pulp.
2.2. Biochemical and Phytochemical Composition of Shea Kernels and Butter
The average moisture content of dried shea kernels has been reported as 6.8% and 1.4% for the kernel and butter, respectively (Table 4). Kernels contain nearly 50% of fats that, after processing to butter, rise to 75%. It has a moderate mineral content of magnesium and calcium both in kernels and butter. The whole kernels are not edible due to their high levels of antinutritional factors, including tannins and catechins.
Table 4. Composition of shea kernels and butter. Adapted from Honfo (2015).
The components of shea butter that affect its physicochemical properties are triglycerides and a large fraction of unsaponifiable compounds that are recognized as active principles in cosmetic products [120]. An unsaponifiable compound is a fraction identified in shea butter that does not dissolve in acetone. It could also be designated as a highly unsaturated compound that consists of a mixture of different polyisoprenes [121,122]. The average unsaponifiable content differs, depending on the authors, from 1.2% to 17.6% [112,123]. The acid value in the butter determines the way glycerides are decomposed by lipase; therefore, it assesses the impact that heat and light cause on the components. The results also help to indicate the oil quality. The value varies with an average of 8.1 mg KOH/g [124]. The free fatty acid (FFA) percentage has also been evaluated, ranging from 1% to 10.7% [125].
Authors have surveyed the fatty acid content of the butter. It is characterized by a total of 16 saturated and unsaturated fatty acids, with 5 of them (oleic, stearic, palmitic, linoleic, and arachidic) showing relatively high levels [100]. Oleic acid is dominant, from 37.2% to 60.7% [117,126], followed by stearic acid, varying from 29.5% to 55.7% [75,103,117]. The quantity and quality of fatty acids differ from one region to another. Authors have revealed that oleic acid is more dominant in butter coming from Uganda, while stearic acid is more dominant in butter coming from West African provenances [51,95,100]. Palmitic acid varies from 3.3% [100] to 7.5% [103]. The linoleic acid content is between 4.3% and 8.0%. This fatty acid is essential in nutrition because it is a component of the cell membrane [110,118,127]. Arachidic acid varies between 0.8% and 1.8% [51,100,103]. The data provided reveal a significant variation in the composition of kernels and butter.
An assessment of the phytochemical diversity and quality of shea kernels has revealed that they are rich in anti-inflammatory phytochemical constituents [88]. Triterpenoids in kernels (α-amyrin and β-amyrin) have anti-inflammatory activities [128]. Terpenoids are credited as having the inhibitory potential of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) to reduce inflammation and counteract cancer [129]. Shea kernels contain triterpene acid, triterpene glycoside, steroid glucosides, and other phenolic compounds, as active principles. There are also positive aspects of extracting specific compounds from defatted shea kernels that could be used as antioxidants and anti-inflammatory agents [35]. An extract of the nutshells has been investigated as a potential source of medicines for treating diabetes, based on the presence of bioactive compounds including protocatechuic acid, trihydroxycoumarin, taxifolin, and quercetin. These chemicals have been investigated via the molecular docking method for high-performance liquid chromatography (HPLC) fingerprints and bioactivity evaluation [130]. According to the authors, nutshells could provide a source of cheap, natural antidiabetic compounds and are especially rich in (2R, 3R)-(+)-taxifolin, which would help to reduce waste of the currently discarded shells and increase the profitability of tree growing.
The fatty acid composition of triterpene alcohol fractions of the non-saponifiable lipids of shea tree kernels has also been studied. The dominant fatty acids are stearic (28–56%) and oleic (34–61%) acids. Triterpene contains a range of components: α-amyrin, β-amyrin, lupeol, and butyrospermol as the major constituents [35,117,120,131,132].
Secondary metabolites are present in the shea tree’s plant parts (leaves, bark, stems, roots, and fruits) [133]. The phytochemical analysis of leaves and bark extract of shea tree has revealed compounds, such as alkaloids, flavonoids, tannins, steroids, phenols, phlobatannins, glycosides, and carbohydrates [76,116,134,135,136].
3. Importance of V. paradoxa in the Context of Agroforestry
The shea tree is considered one of the principal tree species primarily found in agroforestry parklands in sub-Saharan African countries, where the species occurs naturally [137]. It constitutes a high percentage of standing biomass, contributes to reducing soil degradation, and possesses a significant carbon sequestration ability, which can be used for climate change mitigation strategies [59,138,139,140]. The shea tree is the most common in Mali and has a relatively high density of 30–50 trees/ha [53].
Growing shea trees in agroforestry plantations impact the microclimate positively. Due to trees shading effects, there is an essential effect on the soil moisture content, leading to higher crop yields [87]. Some researchers have observed higher fruit yields in cultivated fields with shea trees compared with naturally fallow land. Cultivated fields with annual crops growing where shea trees were present provided better flowering conditions, fruiting, and better yields [3,141,142]. In Burkina Faso, 51% of agricultural land was identified as suitable for growing the species [143]. The cultivation of crops such as maize and soybean is being used to generate income and to enhance food security in shea tree parklands. The authors of a study carried out in Uganda suggest that different responses have been observed regarding soybean and maize yields [144]. Due to the competition for light and nutrients, intercropping with mature shea trees led to very low maize and soybean yields compared with intercropping with young shea trees. Another investigation has been carried out regarding the influence of the shea tree and P. biglobosa trees on sorghum production in Burkina Faso [144]. The authors found that sorghum yield was affected negatively by 50–70% with P. biglobosa. According to some studies, as shea trees age, there is a reduction in the yield of adjacent crops. The yield variability could be explained by the shading and competition for water and nutrients, but the cultivation of soybean in shea tree parklands is a possible recommendation as lower reduction in yields were reported compared with cereals [144,145,146].
This entry is adapted from the peer-reviewed paper 10.3390/f12121740
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