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Global Food Security

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Under-exploited wild Vigna species potentials in human and animalnutrition: A review

Difo Voukang Harounaa,c,⁎, Pavithravani B. Venkataramanab,c, Patrick A. Ndakidemib,c,Athanasia O. Matemua,c

a Department of Food Biotechnology and Nutritional Sciences, Nelson Mandela African Institution of Science and Technology (NM-AIST), P.O. Box 447, Arusha, TanzaniabDepartment of Sustainable Agriculture, Biodiversity and Ecosystems Management, Nelson Mandela African Institution of Science and Technology, P.O. Box 447, Arusha,Tanzaniac Centre for Research, Agricultural Advancement, Teaching Excellence and Sustainability in Food and Nutrition Security (CREATES, FNS), Nelson Mandela AfricanInstitution of Science and Technology, P.O. Box 447, Arusha, Tanzania

A R T I C L E I N F O

Keywords:Wild Vigna speciesWild food cropsWild legumesHuman nutritionAnimal nutritionWild Vigna genetic resources

A B S T R A C T

Food insecurity, protein-energy malnutrition, and food-feed competition have motivated the search for alter-native food and feed sources for human and animal nutrition. According to the FAO, only four crop speciesprovide half of the plant-based calories in the human diet. This review, with an inquisitive focus on investigatingalternative potential food and feed sources, has revealed that the Vigna genus (an important group of legumes)possesses more than a 100 species from which only 10 have been domesticated and are being given betterattention. Thus, more than 90 species are still under-exploited despite their probable huge potential to alleviatefood insecurity either by adding food varieties (domestication) or by providing information for breeding pur-poses. The review further demonstrates that the utilization of the wild Vigna species for both human food andanimal feed is still very limited because of the unawareness of their potentials over some improved varietieswhich are facing challenges. An increased scientific effort towards exploring the potentials of wild legumes isrecommended in planning the future food strategies.

1. Introduction

Ensuring a stable food supply for human and animal nutrition hasbeen a major challenge for many nations which has resulted in thedomestication of many wild plants for thousands of years since thebeginning of human civilization. Crop plants have been manipulated inorder to possess new and desirable characters. Thanks to plant breedingsciences and technologies which evolved from the artificial selectionprocesses based on phenotypes over the centuries (Acquaah, 2007),new species and/or varieties have been derived. Those new varietieshave become genetically different from their original progenitors overtime. In some cases, such domesticated varieties have received verylimited economic importance in the global market, hence the term or-phan or underutilized crops attributed to them (Cullis and Kunert,2017).

Food insecurity, protein-energy malnutrition (PEM), hidden hunger,increase demand for food and food-feed competition are among themajor challenges for the developing countries (Bhat and Karim, 2009;Riley, 2016). These challenges, coupled with the negative effect of

mono-cropping and climate change, increase the necessity for cropimprovement. In addition, the low genetic diversity of crops whichhinders crops domestication and artificial selection, presents a potentialchallenge for crop improvement.

It is generally known that legumes have very important nutritionalvalue for both humans and animals and are referred to as the “poorman's meat’’. The two widely consumed types of legumes belong to twodifferent genera namely the Phaseolus and the Vigna commonly referredto as common beans and peas, respectively (Garcia et al., 1997; Gepts,2001). Among these two genera of legumes, there exist many domes-ticated edible species as well as under-exploited wild species. This re-view focuses on the genus Vigna.

The genus Vigna is categorized into seven subgenera and sixteensections (Fatokun et al., 1997). The seven genera include Ceratotropis,Haydonia, Lasiocarpa, Macrorhycha, Plectotropis, Sigmoidotropis andVigna (Boukar et al., 2013). All domesticated and cultivated Vignavarieties belong to only three subgenera, namely Ceratotropis, Plecto-tropis, and Vigna (Pandiyan et al., 2012). The subgenus Ceratotropis iswell known as Asian Vigna and the subgenus Vigna commonly called

https://doi.org/10.1016/j.gfs.2018.06.002Received 15 February 2018; Received in revised form 7 June 2018; Accepted 12 June 2018

⁎ Corresponding author at: Department of Food Biotechnology and Nutritional Sciences, Nelson Mandela African Institution of Science and Technology, P.O. Box 447, Arusha,Tanzania.

E-mail addresses: harounad@nm-aist.ac.tz, difovoukang@yahoo.fr (D.V. Harouna).

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African Vigna are the most known subgenera containing most popularlegumes like cowpeas, black gram and green gram (Boukar et al., 2013;Pandiyan et al., 2012). Studies have shown differences between thethree subgenera and also revealed that Vigna vexillata is an intermediatespecies between Asian and African Vigna (Boukar et al., 2013). Suc-cessful crosses between members of the three subgenera have beenreported, though some failure of inter-specific hybridization involvingmembers of the Asian and African Vigna have also been reported andattributed to postfertilization events (Fatokun et al., 1997).

It has been documented that the genus Vigna (in the familyFabaceae) is comprised of more than 100 wild species within whichvery few species have been domesticated (Tomooka et al., 2014). Cropspecies with little attention or completely ignored by agricultural re-searchers, plant breeders, and policymakers which are wild or semi-domesticated varieties and non-timber forest species adapted to parti-cular local environments are defined as neglected and underutilizedspecies (Padulosi et al., 2013). The term under-exploited wild Vignaspecies in this context denote those Vigna species which have not yetbeen domesticated. They do not possess commercial names since theyhave not got a common popular use by people or group of people. Thus,they should be differentiated from some domesticated Vigna speciessuch as Bambara groundnuts (Vigna subterranea), considered as under-utilized crops. They are regarded as wild and under-exploited species ofVigna which are collected from their natural agro-ecological environ-ment and kept in research genebanks for breeding purposes. It is cur-iously noted that neglected and underutilized species present tre-mendous opportunities for fighting poverty, hunger and malnutrition(Padulosi et al., 2013). In addition, it is also reported that wild plantrelatives present uncontestable potential genetic resources for cropimprovement and an avenue for exploring alternative production sys-tems (Dwivedi et al., 2008; McCouch, 2004).

Some species of wild under-exploited Vigna genus have been re-ported with good agronomic characteristics such as disease resistance(Oyatomi et al., 2016), important nutrients and mineral elements (Difoet al., 2015; Macorni et al., 1997) as well as nutraceuticals (Bhat andKarim, 2009). On the other hand, the challenges faced by the cultivatedlegumes varieties is beginning to raise serious concerns to the scientificcommunity (Ojiewo et al., 2018). It is revealed in a recent report thatdomesticated legume crop production is challenged by a number ofbiotic (diseases and pests) and abiotic stresses (heat, frost, drought andsalinity), edaphic factors (associated with soil nutrient deficits) andpolicy issues (where less emphasis is put on legumes compared topriority starchy staples) (Ojiewo et al., 2018). This might be one of thekey motivating factors that led to many attempts in breeding and bio-fortification (Márquez-Quiroz et al., 2016). Therefore, studies on thegenetic diversity and biochemical constituents of the wild and domes-ticated wild relatives of the Vigna species are necessary in order toprovide useful information that will help to improve the cultivatedvarieties or to domesticate the wild ones.

In an attempt to contribute in bringing possible solutions to thesechallenges and information that expose the knowledge gaps in this area,this review focuses on the genetic, agronomic, nutritional, biochemicalpotentials and various utilization of the under-exploited wild Vignaspecies in human and animal nutrition with reference to the domes-ticated ones.

2. An overview of domesticated Vigna species with their potentials

Earlier reports have shown that about 100 species of the genus Vigna(Leguminosae plant family) exist and are widely distributed in thetropical and subtropical areas of the world (Tomooka et al., 2002). Sofar, only nine (Tomooka et al., 2014) or ten (Takahashi et al., 2016)species of the genus Vigna have been domesticated and being widelyutilized as human food though some are still considered domesticatedbut under-utilized. In this section, a brief review of the ten domes-ticated species with respect to their level of production, yield,

availability, utilization as human foods and the level of genetic di-versity is given.

Azuki bean (Vigna angularis var. angularis) is recently considered asthe second most important legume in Japan after soybean (Kaga et al.,2008). It is a widely consumed dietary legume crop in Eastern Asia withan annual cultivation area estimated to be 670,000; 120,000; 30,000,and 20,000 ha for China, Japan, the Korean peninsula, and Taiwan,respectively (Kang et al., 2015; Rubatzky and Yamaguchi, 1997). Thisclearly shows how much important it is for human nutrition and con-sequently the impact of its domestication on food security. The cropaverage seed yields is estimated to be in the range of 1–2.5 t/ha(Ecocrop, 2007). It is estimated to be 1450 kg/ha in Taiwan, 1900 kg/ha in Japan, 500–600 kg/ha in Kenya and 1340–2240 kg/ha in NewZealand (Ecocrop, 2007; Jansen, 2006a). It is assumed that a variety ofazuki bean with a yield of 2160 kg/ha could be able to uptake 74 kg N,18 kg P and 50 kg K per ha (Jansen, 2006a). The real geographical lo-cation (country) where this bean was domesticated is not well known.However, the wide distribution of its presumed wild ancestor, Vignaangularis var. nipponensis in Japan suggests Eastern Asia as its domes-tication place. It is generally present there as a crop complex where thecultivated, wild, and weedy forms are widely distributed and en-countered (Kaga et al., 2008; Tomooka et al., 2002). The genetic di-versity of its seed coat and seed size (Fig. 1A) further demonstrate itsimpact on the global food security level by adding food varieties.

Bambara groundnut (Vigna subterranea (L.) Verdc) is documented tooriginate in West Africa with a considerable genetic diversity(Mohammed et al., 2016). It is an important food legume grown widelyin semi-arid Africa which is closely related to cowpea (Vigna un-guiculata) with which it shares much of its area of cultivation and ori-gins of genetic diversity (Basu et al., 2007). In many parts of Africa, itrepresents the third most important legume after groundnut (Arachishypogaea) and cowpea (Basu et al., 2007). The seed yield varies anddepends mainly on rainfall. It can reach up to 4 t/ha under field con-ditions. However, an average seed yield ranging from 300 to 800 kg/kais commonly reported under farmers conditions (Brink et al., 2006).Nutrition wise, it is rich in carbohydrate and protein content, making itan important benefit to the diets of people who cannot afford expensiveanimal protein (Hillocks et al., 2012). Botanical wise, it consists of twotaxonomic forms; var. spontanea comprising the wild forms, which arerestricted to an area from Nigeria to Sudan, with a center of diversityaround Cameroon, and var. subterranea comprising the cultivatedforms, found in many parts of the tropics and particularly sub-SaharanAfrica (Basu et al., 2007). The relatively high genetic identity betweenwild and domesticated forms suggests that wild form (Vigna subterraneavar. spontanea) is likely to be the true progenitor of cultivated forms(Pasquet et al., 1999). The higher genetic diversity in wild materials ascompared with domesticated forms makes the spontanea forms im-portant potential sources of beneficial genes for Bambara groundnutbreeding and improvement (Pasquet et al., 1999). The most importantand frequently used part of the crop as human food is the seed. Thoseseeds possess diverse identifiable morphological features, such as seedtexture, color, seed shape, seed eye, and hilum color and pattern.Fig. 1B illustrates some of the morphological features of seed geneticdiversity. The morphological features of Bambara groundnut can beutilized for its genetic improvement upon classification into homo-genous seed material (Mohammed et al., 2016). Thus, future researchshould focus on the pre-breeding and breeding of this crop to its geneticpotential, followed by the dissemination of seed of improved varietiesto farmers.

Black gram (Vigna mungo var. mungo (L.) Hepper) also known as urd,urad, or mash; is another important grain legumes mainly grown andconsumed in South and Southeast Asian countries, like Afghanistan,Bangladesh, India, Pakistan, Nepal, etc. (Kaewwongwal et al., 2015). Itoriginated from central Asia and now found in many tropical areas ofAsia, Africa and Madagascar while in the USA and Australia it is mostlycultivated as a fodder crop (Jansen, 2006b). Its global growing area is

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expected to be higher than 5 Mha with India being the largest producer(about 3 Mha), followed by Myanmar (about 1 Mha) and Pakistan (0.5Mha) (Kaewwongwal et al., 2015). The average dry seed yield is about350–800 kg/ha but under good management it can reach1500–2500 kg/ha (Jansen, 2006b). The seeds contain about 25% pro-tein and 65% carbohydrates and are mainly consumed as soup. Theseed flour is used as a major ingredient for several kinds of foods, suchas cakes, biscuits, snacks, cookies, and doughnuts. Sprouts producedfrom black gram are also consumed as a vegetable source of vitaminsand minerals. It is believed to have been domesticated in India about4500 years ago from its wild progenitor, Vigna mungo var. silvestris(Fuller and Harvey, 2006). Many breeding programs for this crop existin India, Pakistan, and Thailand, however, to increase its potential asfood and feed, it is necessary to study and exploit its genetic diversity.There has been less research on this crop, especially in terms of mole-cular genetic diversity as compared with cowpea and mung beanthough some efforts have been noticed (Ghafoor et al., 2001; Ghafoorand Ahmad, 2005; Souframanien and Gopalakrishna, 2004). However,these studies have provided little information on the extent of geneticdiversity in this crop because most of them, employed less than 150accessions, and the germplasm used originated from only a singlegeographical region (country). Though very well known as ‘’black’’gram, the genetic variations (diversity) of the seeds based on color, sizeand texture for this crop exist (Fig. 1C) confirming that many cultivatedvarieties do exist (Vyas et al., 2016) and hence their potential is pro-mising.

Cowpea (Vigna unguiculata (L.) Walp.) is one of the most importantgrain legume crops in the world with a larger zone of occurrence andcultivation which covers the semiarid regions of Africa, Asia, Southern

Europe, Southern United States, and Central and South America (Diouf,2011; Singh, 2006; Timko et al., 2007). Its cultivation covers 14.5million hectares with an annual production of 5.5 million tons world-wide. Presently, Nigeria is the largest producer of cowpea followed byNiger, Burkina Faso, Myanmar, Cameroon, and Mali (Simon et al.,2007). It is cultivated not only for its seed but also for its leaves whichserves both as a human food as well as animal feed. It is assumed thatleaves yield of about 400 kg/ha can be obtained without noticeablereduction of seed yields (Madamba et al., 2006). The world averageyield is estimated at 240 kg/ha and 500 kg/ha for its dry seed andfodder (air-dried leafy stems), respectively (Madamba et al., 2006). Itsaverage yield of dry seeds under subsistence agriculture in tropicalAfrica is estimated at 100–500 kg/ha (Madamba et al., 2006). This cropplays a crucial role in human nutrition due to its grain protein content(23–32%) of high quality (rich in lysine, tryptophan) and high nutri-tional value. The grains also contain a substantial amount of mineraland vitamins (like folic acid and vitamin B) and the hay is used forfeeding animals during the dry season in many parts of West Africa(Badiane et al., 2014). In the poorer areas, cowpea is a valuable sourceof protein cheaper than animal protein (fish, meat, or poultry), thushelping to fight malnutrition for the low-income farmers. Both theleaves and the grains of this crop have found various usages especiallyas human food since they are processed into various foods such asAkara, Moin-moin, Koki, Couscous, Red-Red Stew, Ndambe, ThiebouKathiakh, Cake, Bread, and Cookie and used as ingredients in com-plementary food formulation for children (Badiane et al., 2014;Hallensleben et al., 2009; Mamiro et al., 2011). Although mainly cul-tivated and consumed in West Africa, it is believed it was first domes-ticated in East Africa and then transported to West Africa (Badiane

Fig. 1. Illustration of domesticated Vigna species depicting the diversity in seed morphology.Source: Images compiled from A (Isemura et al., 2011), B, C, D, E, G, H, I (NARO Genebank, 2017), F, J (Norihiko et al., 2011).

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et al., 2014). The seed also present a certain range of genetic diversitywith phenotypic attributes of color (white, cream, green, buff, red,brown, or black), texture (smooth, rough, or wrinkled) and uniformity(solid, speckled, to patterned) (Badiane et al., 2014). Much researchattention is being given to this crop as compared with other legumes ofthe genus Vigna and this may be due to its wider distribution and uses.Evaluation of genetic diversity, variation, and genetic distance incowpea genotypes has been conducted in several studies according tomorphological and physiological markers (Ntundu et al., 2006; Siiseand Massawe, 2013), and molecular markers such as Amplified Frag-ment Length Polymorphism (AFLP) (Coulibaly et al., 2002; Tosti andNegri, 2002). Nevertheless, much is still to be done to cover the existinggaps concerning this crop. Morphological identification of cowpeaseeds is illustrated in Fig. 1D.

Creole bean (Vigna reflexo-pilosa var. glabra = Vigna glabrescens) isthe only tetraploid species in genus Vigna and the little-known culti-vated species of the subgenus Ceratotropis (Pandiyan et al., 2011). Forthat reason, information on its area of production and yield data arevery scanty and almost not documented. The ancestral species thatmake up this allotetraploid species have not conclusively been identi-fied, although previous studies suggested that a donor genome for thiscrop is V. trinervia (Chankaew et al., 2014). It was first found used as aforage crop in West Bengal, Mauritius, and Tanzania, while it was usedas a pulse in the same ways as mung bean in Vietnam and Philippines(Tomooka et al., 2002). The crop is considered to have been domes-ticated in Southeast Asia from its possible wild ancestor, Vigna reflexo-pilosa var. reflexo-pilosa (Egawa and Tomooka, 1994). The wild formhas a wide geographical distribution ranging from East, Southeast andSouth Asia, and across the islands from the west to the north PacificIslands. The cultivated form is very close to mung bean as it was re-cognized in the past as a glabrous variety of mung bean, V. radiata var.glabra which was later treated as a distinct species, V. glabrescens(Chankaew et al., 2014). It is distinguished from its wild progenitormainly through the thick glabrous stem and erect-growth habit(Tomooka et al., 2002). It has been reported that this crop possesses ahigh potential as a gene source for breeding other Vigna crops(Chankaew et al., 2014). It also presents morphological variations fromits seeds as shown in Fig. 1G.

Minni payaru (Vigna stipulacea) is a newly recognized as a domes-ticated Indian Vigna species and therefore has very limited publishedinformation (Pandiyan et al., 2011). The name V. stipulacea has notbeen used in the Indian literature and this species seems to have beenincluded in the description of V. trilobata (Pandiyan et al., 2011). It wasfirst known to be a wild Vigna species until recently when a survey wasconducted in India; researchers realized that there was a semi-domes-ticated form of V. Stipulacea in Tamil Nadu (India) and many otherareas around that region (Tomoka, 2008). For many farmers, it hasdifferent utilization and was cultivated for different purposes such asanimal feeding, manure production as well as an ingredient for cakebaking (Pandiyan et al., 2011). Further studies are still needed to unveilthe possible potential of this crop. Fig. 1J shows the seed structure ofthe crop as no much on literature was found showing different seedvariations.

Moth bean (Vigna aconitifolia (Jacq.) Marechal) is a minor legumecrop and considered to be the most drought and heat tolerant cultigenamong Asian Vigna (Norihiko et al., 2011). It is widely grown in Indiaand the Far East and has been qualified as a possibly more significantfood source for the future (Adsule, 1996). It is considered to have beendomesticated in India, Pakistan, Myanmar or Ceylon (NARO Genebank,2017). India's driest state, Rajasthan, is the major moth bean growingstate contributing almost 86% area of the country's cultivation (Guptaet al., 2016). Its average seeds yield is estimated to be in the range of70–270 kg/ha, though yields of up to 2600 kg/ha have been recorded inthe United States and Australia with experimental seeds (Brink andJansen, 2006). The Yield of its green matter for forage has also beenestimated to be at about 37–50 t/ha and that of its hay at about

7.5–10 t/ha (Brink and Jansen, 2006). It is generally also known bycommon names such as dew bean, dew gram, moth, mat, mat bean andmatki. Its nutritional content is also well appreciable for human con-sumption as it possesses very important nutrients. The nutritionalcontent consists of 24.3% protein, 68.0% carbohydrates, 3.9% lipids,3.8% ash, 133 (mg/100 g) calcium, 356 (mg/100 g) phosphorus, 183(mg/100 g) magnesium, 11 (mg/100 g) iron, 0.50 (mg/100 g) thiamine,0.10 (mg/100 g) riboflavin and 1.7 (mg/100 g) niacin (Adsule, 1996).The wild ancestral form and cultivated form have not been dis-tinguished taxonomically. However, the existence of a putative wildancestral form during Tamil has been recognized (Tomoka, 2008).Researchers have found that there is a substantial genetic variationbetween moth bean germplasms which could be used in crop im-provement (Gupta et al., 2016). Few accessions of moth beans kept atthe National Institute of Agro-biological Sciences genebank, Japan is ashown in Fig. 1H. Further research on these accessions is needed to shedmore light on their genetic potential in legume improvement programs.

Mung bean (Vigna radiata (L.) Wilczek) is another important grainlegume, especially in Asia (India, South East-Asia, and East Asia) butalso eaten in Southern Europe and in the Southern USA (Heuzé et al.,2015). It is mainly produced in Asia (90%) with India being the largestproducer (more than 50% of world production) and consumer of itsentire production. China also produces large amounts, representing19% of its legume production, but Thailand remains the main exporterof its production which increased by 22% per year between 1980 and2000 (Lambrides and Godwin, 2007). Although this crop is also pro-duced in many African countries, it is known as a minor crop there(Mogotsi, 2006). Reliable statistics for its production are difficult toobtain, as it is often lumped together with that of other Vigna andPhaseolus spp., though it is reported that China produced 891,000 t(19% of her total pulse production) from 772,000 ha in 2000 (Mogotsi,2006). Its average yields are estimated at 300–700 kg/ha, though yieldsof 1.25 t/ha were obtained under irrigation in Kenya (Mogotsi, 2006).Such productivity also attests its importance as food for human nutri-tion and food security impact. It is rich in crude protein (25–31%), iron(4–6mg/100 g), crude fiber (1–5%) and many other biochemical con-stituents (Anwar et al., 2007). It is believed that the seeds represent aninvaluable source of digestible protein for humans in places where meatis lacking or where people are mostly vegetarian (Heuzé et al., 2015). Itcan be cooked fresh or dry and be eaten whole or made into flour,soups, porridge, snacks, bread, noodles and ice-cream (Heuzé et al.,2015). The crop is known to have originated from India. Based on thearchaeobotanical evidence, both south-eastern India and western Hi-malayan foothills are probably the places where domestication of thiscrop could have occurred (Fuller and Harvey, 2006). Its presumedprogenitor is the wild form (Vigna radiata var. sublobata (Roxb) Verd-court), which is widely distributed across the world tropics from wes-tern Africa to northern Australia and Papua New Guinea (Tomookaet al., 2002). The cultivated form was introduced to southern andeastern Asia, Africa, Austronesia, the Americas and the West Indies. It isnow widespread throughout the tropics and found from sea level up toan altitude of 1850m in the Himalayas (Lambrides and Godwin, 2007;Mogotsi, 2006). Both the cultivated and the wild forms of this crop alsopossess a very large pool of genetic diversity which is conserved ingenebanks around the world as genetic resources (Tomooka et al.,2002). The World Vegetable Center with about 5600 accessions pos-sesses the largest collection of genetic resources for this crop. Thenumber of wild genetic resources (var. sublobata) has significantly in-creased due to the particular interest by Australian and Japanese sci-entists for the crop recently (Lawn and Watkinson, 2002; Vaughanet al., 2006). The use of genetic resources of wild and cultivatedgermplasm efficiently for research and breeding through both mor-phological and molecular approaches is continuously gaining interestsnowadays. This crop can easily be identified through its various seedsmorphological traits as shown in Fig. 1E.

Rice bean (Vigna umbellata (Thunb.) Ohwi & Ohashi) is a

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multipurpose legume as well as a neglected and under-utilized legume(Joshi et al., 2008). It was found naturally in India, central China and inSoutheast Asia and was introduced to Egypt, the East Coast of Africaand the islands of the Indian Ocean. It is now cultivated in tropical Asia,Fiji, Australia, tropical Africa, the Indian Ocean Islands as well as in theAmericas (USA, Honduras, Brazil, and Mexico) (Rajerison, 2006).Compared with cowpea (Vigna unguiculata), adzuki bean (Vigna angu-laris) and mung bean (Vigna radiata), it is a less important crop. How-ever, it represents a locally important contributor to human nutrition inparts of India and South-East Asia (Joshi et al., 2008; Tomooka et al.,2011). All parts of the plant are edible and used in culinary prepara-tions (Heuzé et al., 2016). It is also used as an important fodder and agreen manure (Tomooka et al., 2002). The dry seeds are boiled andeaten with rice or sometimes replace rice in stews or soups. In Mada-gascar, they are ground to make nutritive flour included in the chil-dren's food (Heuzé et al., 2016). The annual productivity and area ofproduction for this crop is not documented enough (Rajerison, 2006).This may be due to its limited utilization and consumption despite itsimportance in some parts of the world. Its average seed yield is only200–300 kg/ha which is assumed to be low due to its very short cycle,though experimental yields of up to 2500 kg/ha have been obtained inIndia (Rajerison, 2006). Fresh fodder yields of up to 35 t/ha has alsobeen obtained (Rajerison, 2006). It is believed that it originated fromSoutheast Asia and was probably domesticated in Thailand andneighboring regions (Norihiko et al., 2011). Though it can better tol-erate harsh conditions (drought, waterlogging and acid soils etc.) thancowpea, it is still considered as an underutilized legume. There is a verylimited number of breeding programs to improve this crop, thereforecompelling farmers must rely on landraces rather than on cultivars(Joshi et al., 2008). Recently, the genetic diversity in cultivated andwild forms of this crop from Thailand, India, and Nepal using molecularmarkers was studied (Muthusamy et al., 2008; Seehalak et al., 2006).However, studies of genetic diversity using many cultivated and wildaccessions from many countries have not been conducted, and the leveland geographic cline of genetic diversity remain unknown. A view ofthe seed morphological diversity of this crop can be seen in Fig. 1I.

Tuber cowpea (Vigna vexillata) is another recently recognized asdomesticated Vigna species as it was found cultivated in Bali and Timor,Indonesia (Karuniawan et al., 2006). That domesticated form was dis-covered with some important agronomic characteristics such as pro-minent seed size increase, loss of pod shattering and loss of seed dor-mancy ( Tomooka et al., 2011). It is cultivated for its tuber but the seedsare also used as human food. Its root protein content is 15% which isabout 2.5 times higher than that of yam (6%), 3 times higher than thatof potato (5%) and sweet potato (5%) and 5 times higher than that ofcassava (3%) (Tomooka et al., 2011). Earlier reports have described theuse of wild Vigna vexillata as edible tuber and sometimes edible seeds inAfrica (Senegal, Ethiopia, Sudan, and South Africa), East and North EastIndia, northern Australia and Southeast Asia (Norihiko et al., 2011).The limited use of this other domesticated legume can explain its lim-ited productivity and commercialization and therefore the limitedavailability of documented information. However, forage dry matteryields ranging from 300—1100 kg/ha have been obtained in northernAustralia while dry matter yields of 2780 kg/ha have been achieved inZambia (PROTA, 2018). Seed yields of 500—1250 kg/ha have also beenreported while fresh tuber yields of 1.44 t/ha have been obtained inNigeria (PROTA, 2018). The wild form is an extremely polymorphicspecies and several taxonomic varieties exist (Norihiko et al., 2011). Italso presents a considerable diversity in terms of seed morphology asshown below (Fig. 1F)

From the above description of the utilization of the domesticatedVigna species, it can be noticed how much the domestication has gainedpopularity and usages especially in terms of human as well as animalnutrition. However, a greater impact may be possible with more do-mesticated Vigna species existence as foods and feeds. Due to the lack ofinformation and intensive research on the wild Vigna species, it is

important to acknowledge the efforts of some international organiza-tions in trying to genetically classify them and catalog them in gene-banks.

3. Wild under-exploited Vigna species, diversity, genetic resourcesand agronomic characteristics

The genus Vigna comprises about 100 species among which only tenare known to be domesticated while the rest are under-exploited wildspecies. The world wild Vigna genetic resources, as well as cultivars,landraces, breeding populations, are maintained and cataloged byseveral international and national research programs and genebanks.Most genebanks possess accessible databases, making the wild Vignagenetic resources available to breeders searching for new sources ofgenetic diversity, e.g., resistance or tolerance to abiotic stresses(drought, heat, waterlogged soils, acidic soils, zinc-deficient soils, andsoils with toxic levels of boron) and biotic stresses (diseases, insects,and weeds).

The Genetic Resources Center of the International Institute ofTropical Agriculture, (IITA), Ibadan-Nigeria possess the largest re-servoir for the wild Vigna species originated from Africa (IITA, 2017).The passport data of the wild Vigna accessions cataloged by the GeneticResources Center of the International Institute of Tropical Agriculture,(IITA), Ibadan- Nigeria is easily accessible through the institutionalwebsite (http://my.iita.org/accession2/). According to that passportdata, about 1871 accessions consisting of more than 60 wild Vignaspecies have been collected and conserved for researchers worldwidebased on their interest through a formal request upon signature of theSMTA (Standard Material Transfer Agreement) by the researcher. Agood number of wild Vigna accession can also be obtained from theAustralian Grains Genebank (AGG) through the same process. A requestfrom that genebank showed that four accessions of Vigna racemosa,three accessions of Vigna reticulata and many wild and landrace speciesof other Vigna are available for distribution. The National Bureau ofPlant Genetic Resources (NBPGR), India also contain a very largenumber of the Asian wild Vigna species accessions (Bisht et al., 2005).The genebank of the National Institute of Agrobiological Sciences(NIAS), Japan mainly possesses wild Vigna species of domesticated re-latives.

The vegetative morphology and important agronomic character-istics of some of these wild Vigna species present a wide range of di-versity which could be exploited in domestication and crop improve-ment. For example, it was revealed that most wild Vigna species underthe subgenus Ceratotropis (also known as Asian Vigna) and some othersubgenera of the genus Vigna present epigeal germination and sessilefirst and second leaves making these characters key distinguishingfeatures for the wild Vigna species (Bisht et al., 2005). Some of thedistinguishing features of the wild Vigna species are comprehensivelysummarized and illustrated in Fig. 2. The illustration is based on seedsof wild Vigna species requested from genebanks and few seeds collectedfrom the wild and identified.

4. Potentials of under-exploited wild Vigna species as promisingfood crops for the future

4.1. Agronomic, environmental and climatic potentials

The wild Vigna species present a very wide range of variability bothin terms of important agronomic characteristics and genetic diversitywhich makes it an important source of information for crop improve-ment and an important food and animal feed source for the future. In anearlier study about the genetic diversity of the Vigna species, it wasdemonstrated that the cultigens (domesticated forms) of the con-spe-cific wild Vigna species present better agronomic characteristics thantheir wild relatives (Bisht et al., 2005). It was shown that the domes-ticated accessions were more robust in growth, with large vegetative

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Fig. 2. Diversity and distinguishing features of the wild Vigna species in terms of vegetative morphology I: Flower color, II: Germination habit, III: Attachment ofprimary leaves at two leaves stage, IV: Pod pubescence, and V: Growth habit and pod attachment.Source: Images taken and compiled by the authors based on the seeds requested from the Australian Grains Genebank (AGG) (b, d, g, k, l, m, p, q, and s) and theGenetic Resources Center, International Institute of Tropical Agriculture, (IITA), Ibadan- Nigeria (a, c, e, f, h, i, j, n, o, and r).

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parts and often of erect growth type with three- to five-fold increase inseed size and seed weight, except V. aconitifolia, which has still retainedthe wild-type morphology to a greater extent. This can be normallyunderstood because domesticated crops are improved crops to suithuman preferences through selection and breeding processes. On theother hand, some researchers still suggest that some desirable agro-nomic characters found in wild Vigna species should be explored forgenetic improvement of domesticated Vigna species such as cowpea(Popoola et al., 2015). This means that the domesticated Vigna speciesstill need some beneficial characters offered by the wild Vigna speciesdespite the improvement done so far. Such characters are hairiness,abundant pod production, a high number of locules and seeds per pod,longer lifespan and extensive branching habit which could lead tohigher seed yield (Popoola et al., 2017). These observations have beenreported for Vigna unguiculata (cowpea), Cajanus cajan (pigeon pea) andSphenostylis stenocarpa (African Yam Bean) (Popoola et al., 2011). It isalso believed that there is a high level of genome homology among thevarious cultivated varieties of cowpea for example, due to the non-ex-ploitation of the genomes of the cowpea's wild relatives during thedevelopment of such cultivated varieties (Fatokun et al., 1997). Thiscould present another challenge in individual recognition and char-acterization of individual varieties and their specific challenges in orderto improve them. In addition, continuous efforts are being madethrough crossing processes to address susceptibility to some insect pestslike pod borer (Maruca vitrata), sucking bug complex and storage pestswhich can cause high yield losses in some cultivated cowpea varieties(Baker et al., 1989; Fatokun et al., 1997).

Apart from the agronomic characteristics which have been ad-dressed in a considerable level to suit human desires, the improveddomesticated Vigna species now face climatic and environmental chal-lenges. It is remarkably noticed that under harsh climatic and en-vironmental conditions (biotic and abiotic stress), the domesticatedVigna species find it difficult to resist and perform well while their wildunder-exploited relatives find no problem to such conditions. It is re-ported that wild Vigna species are adapted to various habitats includingharsh environments such as sandy beaches, acid soils, limestone rocks,deserts and wetlands (Tomooka et al., 2014). For example, V. marinawas found growing well in a saline land, V. vexillata in an acidic land, V.exilis in alkaline lands, V. trilobata in drought-prone lands, V. luteola inflood-prone lands and V. stipulacea in a pest and disease-prone en-vironment (Tomooka et al., 2014).

Therefore, studies aiming at clarifying the mechanisms of adapta-tion to these extreme environments represent great potential towardsenhancing world food production. Moreover, reports have shown thepotentials of the under-exploited Vigna as a disease-resistant geneticresource. It is the case for example for some wild Vigna species whichwere identified as highly resistant to S. gesnerioides though most of themwere not members of section Catiang, where cultivated cowpea (V.unguiculata) belongs (Oyatomi et al., 2016).

It is more appropriate to domesticate these wild species welladapted to environmental stress than improving stress resistance ofexisting domesticated species, due to the low levels of resistance toenvironmental stress (Tomooka et al., 2014). Unfortunately, there arevery limited numbers of studies attempting the domestication of thesewild species. Furthermore, it is also noted that very few or almost nilstudies that address individual species with an attempt of relating theiragronomic features and genetic diversity with their biochemical andnutritional constituents as well as their acceptability by farmers andutilization as feed ingredients. Hopefully, this concern may attract theinterest of the scientific community in the nearest future because of thepositive impact made by other neglected underutilized species on im-proving rural livelihood (Gotor et al., 2013; Padulosi et al., 2014).

Most of the recent studies are directed towards elucidating the ge-netic diversity of the wild Vigna species especially through morpholo-gical and molecular means (Illakkiam et al., 2014; Pandiyan et al.,2012). It is also supported that the taxonomic affinities and genetic

diversity among the Vigna species are of great importance because theypresent a great potential utilization as nutritious human food, fodderfor ruminant animals, cover crop for rotational farming and more im-portantly genetic improvement of cowpea (Popoola et al., 2015). Theseed size, color, and shape of these wild Vigna species are also diverse insuch a way that they can represent other important selection criteria forconsumers’ acceptances and preferences as well as for crop improve-ment programs.

Furthermore, researches focusing on food and feed experiments arenecessary to reveal their potentialities in animal nutrition and palat-ability studies for consumer acceptability of cooked seeds. Consideringthe level of food and feed challenges coupled to the unpredictable cli-matic and environmental constraints in some parts of the world, it isnecessary to also screen and investigates some types of food crops thatcan adapt to such unpredictable conditions. There is a very limitedsource of information on the human or animal uses of these wild spe-cies. In addition, most of the previous studies are carried out on alimited number of accessions and species of the wild Vigna genus whichlimit the available information on the genetic diversity, agronomicfeatures and biochemical constituents. The limited uses, perception,and knowledge of these wild species by researchers and local commu-nities (consumers and farmers) can explain the limited documentedinformation about some of them.

4.2. The nutritional and biochemical potentials of under-exploited wildVigna species

Very few reports in this line of research have been noticed. So far,only eight wild Vigna species have been quantitatively evaluated interms of chemical composition out of more than a hundred speciesexisting. These species are Vigna vexillata, Vigna vexillata macrosperma,Vigna luteola, Vigna oblongifolia, Vigna unguiculata dekindtiana, Vignaracemosa, Vigna reticulate and Vigna ambacensis (Macorni et al., 1997).The flavonoid content of some species has also been qualitatively as-sessed through HPLC method (Lattanzio et al., 1997). The biochemicalparameters so far accessed through published literature are summarizedin Table 1.

Table 1 shows that a very few species and accessions as comparedwith the existing number of wild Vigna specie have been evaluated forfew biochemical parameters. From the eight species quantitativelyevaluated, V. vexillata present the highest protein content of 29.3% andall accessions of the studied species present a high sulfur amino acidscontent ranging from 2.05 to 3.63 g per 16 g N with a resistant starchcontent ranging from 64 to 75% (Macorni et al., 1997). This is an ex-ceptionally important potential that could be exploited in cowpea bio-fortification for essential amino acids. In comparison with other do-mesticated Vigna species on the same Table, the wild V. vexillata re-ported here presents a slightly higher protein content of about 5% overblack gram (23.6%) and green gram (24.5%). Similar comparison withother domesticated edible crops shows that the lowest protein value inTables 1, 22.2% (V. luteola) is two times higher than the protein contentof maize (10.2%) and wheat (14.3) and three times higher than that ofrice (7.6%) (FAO, 2013). This further justify the importance of theVigna genus as a source of protein. There exists a wide variability interms of trypsin inhibitor, tannin and lectins contents with V. luteolahaving the highest levels while V. unguiculata dekindtiana, V. reticulata,and V. ambacensis present very low levels (Macorni et al., 1997). ThePhytic acid content varies from 1.17% to 0.56% for Vigna unguiculatadekindtiana and Vigna ambacensis, respectively (Table 1), and the valuesare within the range of phytic acid concentrations in most legumes(1–3%) (Arendt et al., 2013).

It is very important to note here that information on studies pur-posely focusing on the chemical composition of wild Vigna species ac-cessions is scanty or not well documented. The proximate composition,fatty acid composition, total phenolic content, antioxidant activity andamino acid profile of an unknown accession of Vigna racemosa were

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reported in a recent study (Ade-Omowaye et al., 2015). Chemicalchanges during open and controlled fermentation of Vigna racemosaseeds collected from their natural environment, regardless of their ge-netic specification were also reported (Difo et al., 2015). Other studiesfocusing on qualitative evaluation of bioactive compounds of Vignakirki, Vigna marina, Vigna gracilis, Vigna heterophylla, Vigna parkeri, Vignahosei, Vigna adenantha, Vigna venusta, Vigna minima, Vigna glabrascens,and Vigna triphylla have revealed the presence of biochemicals such asrobinin, kaempferol-3-rutinoside, isorhamnetin-3- rutinoside, hypero-side, delphinidin and cyanidin (Bravo et al., 1999; Lattanzio et al.,1997; Macorni et al., 1997).

The mineral content, the protein fractions, the lipid profile andfunctional potentials of the wild Vigna species need further investiga-tions. Such information might be breakthrough in crop improvement(bio-fortification) activities leading to nutrients (such as minerals,proteins, lipids, and vitamins) increase in legumes which is highly so-licited nowadays in fighting hidden hunger in developing countries.Many other biochemical parameters of these wild Vigna species need toalso be investigated to enhance their usages. It is also noted that onlyfew accessions of these legumes are studies. The chemical compositionof the other parts of the Vigna species such as leaves and steams havenot yet been given attention by scientific community either as roughagein animal nutrition or as human food and this may be due to theirlimited utilization (cultivation) and attention.

5. Potential uses of wild under-exploited Vigna species andconstraints

From the very few existing reports, it results that only very few non-domesticated wild Vigna species have been used as human food in someparts of the world namely Australia (Vigna marina) (Norihiko et al.,2011), Southern Asia (Vigna vexillata) (Karuniawan et al., 2006), andNigeria (Vigna racemosa) (Folashade et al., 2017). The increasing in-terest in preserving the genetic resources of these wild Vigna species ingenebanks may suggest their important use as gene sources for theimprovement of domesticated legumes. Unfortunately, the process ofcrop improvement is usually very limited by the phenomena of cross-species compatibility and low genetic diversity. Therefore, an emphasison the domestication of new species will be of greater importance toglobal food security than the improvement of the domesticated species.New species domestication of crops presents a tremendous advantage of

diversifying food choices and uses for the benefit of human nutritionwhile keeping at the same time the species biodiversity conserved.

It is suggested that further studies are necessary in order to in-vestigate if these underutilized/wild species could also be explored fortheir effective uses as animal feeds (ruminants and non-ruminants) or asa protein supplement in fish farming (Bhat and Karim, 2009). However,an attempt to exploit the wild Vigna species in animal feed experimentsis very scanty. On a curious note, soybean, a legume with an un-contestable importance in human nutrition is also highly used in theanimal feed industry, creating a competitive relationship which couldbe corrected by introducing wild Vigna species in the industry.

As mentioned earlier, the wild Vigna species present a diverse po-tential in terms of agronomic, ecological and biochemical character-istics. However, they present a very limited utilization in the crop im-provement programs as they are cross incompatible with thedomesticated species. Secondly, demographic explosion, coupled withhigh food demand and production (yield) challenges have acceleratedthe need for crop breeding and improvement to suit human desires andensure food security by promoting the conservation, supply, and dis-tribution of bred and improved crops (Mba et al., 2014). This mighthave gradually led to the abandoning of the wild relatives and land-races of modern crops by the local peoples to the extent that they couldbe later not recognized and considered as weeds to the newly createdplants in the farm leading to the disappearance of the original crops.This could explain why scientists are now realizing that the wild re-latives of modern crops deemed crucial for food security are beingpushed to the brink of extinction (Briggs, 2017). Thirdly, the con-sumption of these wild legumes could be limited because of the pre-sence of some toxic biochemicals in their most edible parts (seeds). Thepresence of antinutrients which could be removed or deactivated byproper processing techniques has been reported as the greatest im-pediment to the consumption of most under-utilized legumes (Bhat andKarim, 2009). Lastly, the rapid evolution, distribution and spreading ofimproved bred crop varieties in order to respond to food securitychallenges are being associated with the disappearance of wild croprelatives (Briggs, 2017; Mba et al., 2014). From that perspective, in-formation about the origin, awareness, believes and preferences of theconsumed modern crops is important. These may explain the stigmaabout the consumption and even the existence of these wild legumesand therefore their rejection as food while they have been used as suchin the past.

Table 1Some reported biochemical constituents of wild Vigna species.

Vigna Species Protein (g/100 g)

TrypsinInhibitors (TIU/mg protein)

Lectin(HA)

Phytic acid(g/100 g)

Tanin (g/100 g)

Starch fractions Bioactive compounds

TS (g/100 g)

RDS(%TS)

SDS (%TS) RS (%TS)

Vigna mugo (blackgram)

23.6 – – – 3.98 37.9 – – 3.40 Daidzein

Vigna radiata(green gram)

24.5 – – – 2.25 39.9 – – 4.18 Rutin, Kaempferol-3-rutinoside

Vigna unguiculatadekindtiana

26.5 65 231 1.173 0.829 41.39 2.995 32.13 64.92 Hyperoside (Quercetin-3-galactoside), Robinin(Kaempferol-3-robinoside-7-rhamnoside)

Vigna vexillata 27.9 126 574 1.009 2.025 29.72 1.48 25.20 73.35 RutinVigna vexillata

macrosperma25.5 151 517 1.01 0.359 41.62 7.90 22.06 70.04 Rutin

Vigna Oblongifolia 26.6 39 250 1.07 1.073 33.24 5.26 16,67 78.07 Rutin (Quercetin -3-rutinoside),

Vigna racemosa 23.5 58 3433 0.778 1.215 33.72 14.68 12.90 72.42 Rutin, RobininVigna reticulata 23.1 29 5233 1.012 1.630 49.69 4.99 20.04 74.76 –Vigna ambacensis 23.1 27 967 0.564 0.957 46.87 5.38 30.72 63.90 Kaempferol-3-rutinosideVigna luteola 22.2 213 16,000 0.827 2.993 28.92 1.45 24.34 74.24 Rutin, Robinin

Note: "-" Means not found values from literature, TS means total starch, RDS: Rapidly digestible starch, SDS: Slowly digestible starch and RS: Resistant starch (RS) =TS - (RDS + SDS) for the wild Vigna species. Source: Compiled by the authors from (Bravo et al., 1999; Lattanzio et al., 1997; Macorni et al., 1997).

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According to the Food and Agriculture Organization of the UnitedNations (FAO), 70% more food is needed over the next four decades tonourish adequately the human population projected to exceed 9 billionby the year 2050 (FAO, 2009). Besides this fact, humankind now de-pends on a reduced amount of agricultural biological diversity for itsfood supplies due to agricultural modernization, changes in diets andpopulation density. A rapid reduction of the gene pool in both plant andanimal genetic resources is observed as only a dozen species of animalsprovide 90% of the animal protein consumed globally and just four cropspecies provide half of the plant-based calories in the human diet (FAO,2009). Therefore, the domestication, adoption and industrial utilizationof the under-exploited wild Vigna species could be of utmost importancein contributing to achieving the FAO expectation to increase foodproduction by 70% more by the year 2050. It is known that maize andsoya beans are two widely produced crops for both humans and ani-mals. These crops are highly cultivated and solicited globally, especiallyfor food and feed formulations making them a subject of competitionbetween humans and animals. This raises the issue of whether it isnecessary to feed animals with the same crops used by humans. Theexploration and exploitation of some wild Vigna species in animal feedformulation could orient the policy makers towards the right cropspecies to direct to animal and human uses according to their pre-ferences. This could help to shape the future better and contributegreatly to ensuring global food security.

6. Conclusion

The genus Vigna (wild and domesticated species) present a very highgenetic diversity in terms of seed morphology, physiological andagronomic characteristics. Though very limited in number as comparedwith wild species, the domesticated species have demonstrated en-ormous impact in both human and animal nutrition especially in partsof the world where meat is less affordable. Considering how fast thedomesticated species have become popular, these wild Vigna may re-present a promising tool to contribute in reducing food insecurity andhidden hunger and a good food and/or feed material for the future.Based on the potentials presented by their genetic diversity, it is nowstrongly believed that wild Vigna species can be the ‘new model plantspecies’ for the genetic study of natural adaptation to stresses such assalt, acid, alkali, drought, flood, pests and diseases. Therefore, moreattention should be given to the wild species since they present hugeopportunities for crop bio-fortification, food variety addition (domes-tication), food value addition, crop improvement and biodiversityconservation. For it is worth thinking about the future food and feedmaterials to be used in case of complications that may arise from un-predictable situations such as climate change, to avoid more surprisingdifficult global food challenges.

Acknowledgments

The authors acknowledge the center of excellence known as Centrefor Research, Agricultural Advancement, Teaching Excellence andSustainability in Food and Nutrition Security (CREATES- FNS) throughthe Nelson Mandela African Institution of Science and Technology (NM-AIST) for the processing of legal documents to import seeds of wildVigna species included in this review from genebanks. The authors alsoacknowledge the Genetic Resources Center, International Institute ofTropical Agriculture (IITA), Ibadan- Nigeria as well as the AustralianGrains Genebank (AGG) for providing detail information and seedmaterials for illustration. The authors are thankful to ProfessorChicgoua Noubactep of the Department of Applied Geology, Georg-August-Universität Göttingen, Germany for his enormous contributionto improving the quality of Figures.

Conflicts of interest

None.

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