THE MORPHOLOGICAL, ANATOMICAL, CYTOLOGICAL, PHYTOCHEMICAL AND PROXIMATE CHARACTERISTICS OF SEVEN VARIETIES OF VIGNA UNGUICULATA (L) WALP
Background of the study
Vigna unguiculata (L.) Walp is a member of the Vigna (peas and beans). It is commonly called cowpea. Unguiculata is Latin for "with a small claw", which reflects the small stalks on the flower petals (Ernest, 2009). All cultivated cowpeas are found within the universally accepted V. unguiculata subspecies unguiculata classification, which is then commonly divided into four cultivar groups: Unguiculata, Biflora, Sesquipedalis, and Textilis (Padulosi et al.,1997 and Perrino et al.,1993). Some well-known common names for cultivated cowpeas include black-eye pea, southern pea, yardlong bean, catjang, and crowder pea (Timko et al., 2007). The first written reference of the word 'cowpea' appeared in 1798 in the United States (Ernest, 2009). The name was most likely acquired due to their use as a fodder crop for cows. Black-eyed pea, a common name used for the unguiculata cultivar group, describes the presence of a distinctive black spot at the hilum of the seed. Black-eyed peas were first introduced to the southern states in the United States and some early varieties had peas squashed closely together in their pods, leading to the other common names of southern pea and crowder-pea. Vigna unguiculata comes first ahead of other arable food legumes in the sub-Saharan Africa. Cowpeas can either be short and bushy (as short as 20centimetres (7.9m)) 0r act like a vine by climbing supports or trailing along the ground (to a height of 2metres (6.6ft) (Sheahan, 2012 and NRC 2006). The tap root can penetrate to the depth of 2.4 metres after eight weeks (Davis, 1991).The size and shape of the leaves varies greatly making this an important feature for classifying and distinquishing cowpea varieties (Pottorff et al., 2012) The recent annual global production of cowpea approximates to 3.3 million tons; Central and West African regions being the major areas of its cultivation (CGIAR, 2014). While it is chiefly a vegetable and grain crop for human who values it as a nutritional supplement to cereals and an extender of animal proteins, it provides a very safe fodder for livestock animals. In semiarid regions it is mainly grown by subsistence farmers, who sell the fresh or dried seeds, fresh pods and leaves as vegetables and the green or dried leftover parts of the plant. Leaves and stems (haulms) can be used as fodder for livestock (Inaizumi et al., 1999). Cowpea has vast utility in the food culture of both man and animal (Tarawali et al., 2002; Diouf and Hilu, 2005).
According to Faris (1964), Vigna spp belong to the genus Phasoeleae together with Dolichos spp and phaseolus spp. The Vigna spp include subspecies V.luteola, V.lutea, V.sinensis and V.vexillata. Vigna sinensis now commonly refered to as V. unguiculata include both the wild and the cultivated cowpea and V.sesquipedalis called yardlong bean and V.cylindrica, the Cajan bean are also known as subspecies of Vigna. Vavilor (1951) reported that cowpea must be originated from Asia. However, Faris (1964) provided evidence largely based on the abundance of many wild species to show that the centre of origin of this crop is in Africa. In India, which is one of the centres of diversities, selection is being made for the yardlong beans (Vigna sesquipedalus) which are small seeded and has long pods which the large grain small pod types were being positively selected in West Africa.
Considerable progress has been made during the past years on germplasm collection, characterization, evaluation, ecogeographical studies and taxonomy of cowpea and its wild relatives. The efforts have greatly contributed towards a better understanding of species diversity ecogeographical distribution and evolution of V.unguiculata. The genus Vigna family Fabaceae, formerly Leguminosae (peas or beans) is composed of more than 200 species that are native to the warm regions of both the old world and new world. It is one of the several species of the widely cultivated Vigna; belonging to the kingdom Plantae, family Fabaceae called Papillionaceae and sub-family, Fabiodeae and order Fabales, tribe Phaseoleae, subtribe Phaseolinae, and section Catiang (Verdcourt,1970; Marechal et al., 1978). Vigna is a pantropical genus with several species, whose exact number varies according to authors: 184 (Philips, 1951).170 (Faris, 1965 between 170 and 150) (Summerfield and Roberts, 1985) 150 Verdcourt,
1970) 154 (Steele, 1976), and about 84 (of which some 50 and about 84 (of which some 50 species are indigenous to Africa (Marechael et al. 1978,). In their revision of the genus Vigna Marechal et al., (1978) subdivided the genus described earlier by Verdcourt (1970) into seven subgenera. In this classification V.unguiculata and V.nervosa markotter constitute the section Catianj, one of the six sections of the subgenus Vigna. Species of the section Catianj are characterized by spurred Stipules below the attachment point of the leaf stalks and canoe shaped keel with beak. Vigna is one of the most ancient human food sources and has probably been used as a crop plant since Neolithic times. Vigna unguiculata (L.) Walp (cowpea) one of the species of the genus vigna is an annual dicotyledonous legume and a warm weather crop, well adapted to drier regions of the tropics like Nigeria, where other food legumes do not thrive well (Sankie et
al., 2012). The planting area is more than 12.5million hectares worldwide, with an annual production of more than 3 million tons (Singh et al., 1997). Nigeria is the largest producer and consumer, accounting for about 45 percent of its world‘s production (Ndong et al., 2012, Lowenberg-Deboer and Ibro, 2008) while the whole of Africa accounts for about 75% (Brissibe et al., 2011). Four subspecies are recognized; of which three are cultivated (more exist, including textilis, pubescens, and sinensis).Cowpea is also refered to as Southern pea, blackeye, crowder pea, lubia, niebe, caupi or frijole. All of these are the species of Vigna unguiculata (L.)Walp, which in older references may be identified as Vigna sinensis.
A lack of archaeological evidence has resulted in contradicting views supporting Africa, Asia and South America as its origin. It is believed that cowpea originated from West Africa by some workers, because both wild and cultivated species abound in the region. Others believed that it originated from Southern Africa. Some literatures indicate that cowpea was introduced from Africa to the Indian subcontinent, approximately 2000 to 3500 years ago. (Faris, 1964). This was the same time as the introduction of sorghum and millet, while others stated that before 300 BC, cowpeas had reached Europe and possibly North Africa from Asia. Speculations are that the Northern part of the Republic of South Africa (former Transvaal region) was the centre of speciation of Vigna unguiculata, owing to the presence of most primitive wild varieties. They further hypothesized that the species moved northwards from the Transvaal to Mozambique and Tanzania, where the subspecies pubescens evolved. (Singh et al., 1997). Cowpea is now grown throughout the tropics and subtropics and has become a part of the diet of about 110 million people. Its production has spread to East and Central Africa, India, Asia, South and Central America. The drier savanna and the sahelian regions of west and central Africa produces about 70% of worldwide cowpea production with, Nigeria, Niger and Brazil being the largest producers. (Singh et al., 2002). Cowpea fixes atmospheric nitrogen through symbiosis with nodule bacteria (Shiringani and Shimeles, 2011).When used with cereal crops, it can help restore soil fertility (Sanginga, 2001) therefore cowpea can play an important role in the development of agriculture. Cowpea is a popular leguminous crop in Africa which is known as beans in Nigeria and ‘niebe’ in the Francophone countries .The largest production is in the moist and dry savannah of sub-saharan Africa (SSA) where it is intensively grown as an intercrop with other cereal crops like millet, sorghum and maize as well as rice fallows (Ishiyaku et al., 2010). Though it is grown in other parts of the world, Nigeria remains the largest producer and
consumer of cowpea in the world. According to FAO data (2001-2010), Nigeria produces an average of 2.58±0.31 million metric tones. Demand deficeit is met by Niger and Burkina Faso.
Henshaw (2008) classified cowpea varieties into size categories based on their 100-seed weight. Varieties with seeds 10-15g are described as small, 15.1-20g as medium-sized seeds while large seeds have 20.1-25.0g. Seeds weighing over 25g are described as very large seeds. These different varieties of cowpea vary greatly in their growth habits, seed colour, from white, cream, yellow, and red, purple, brown to black and may be smooth or wrinkled according to Arthur (2009). These seeds differ in all physical properties which include hilum colour and seed coat texture. Seed shapes vary from the typical kidney shape for beans to globose, ovoid and rhomboid shapes (Henshaw, 2008). Cowpea (Vigna unguiculata) is of major importance to the livelihoods of millions of relatively poor people in less developed countries of the tropics (FAO, 2002).This is why cowpea is called ‘poor man’s meat (Diouf and Hilu,2005) because the seed protein content ranges from 23% to 32% of seed weight rich in lysine and tryptophan and a substantial amount of mineral and vitamins (folic acid and vitamin B) necessary for preventing birth defect during pregnancy stage (Neilson et al., 1993). Some varieties are rich in protein up to 30% and in addition contain micronutrients such as zinc, iron which are necessary for healthy living (Bouka et al., 2010). It is for this reason that societies endowed with cowpea have evolved different ways of utilizing the grain for food. Perhaps the coinage of naman talaka (poor man’s meat) by the Hausas of west and central Africa points to the perception about the nutritional attributes of cowpea grains. All parts of the cowpea are used for food.The leaves, greenpods, green peas and the dry grains dishes are consumed differently.These parts are nutritious providing protein,vitamins and minerals especially micronutrients.The grains are rich in Amino acids ,lysine and tryptophan making it better than cereals. In many parts of West Africa, cowpea hay is also critical in the feeding of animals during the dry season (West and Francois, 1982). There is a large morphological diversity found within the crop, and the growth habit ranges from indeterminate to fairly determinate with the non vining types tending to be more determinate. Plant types are often catergorized as erect, semi-erect, prostrate (trailing), or climbing. Conditions and grower preferences for each variety vary from region to region. (Singh et .al., (2002). (Porter et al., 1974) reported that morphological variability in cowpeas (Vigna unguiculata (L.) Walp abounds in the tropics. Purple –hull pea, southern pea, sowpea, yard long bean, cream pea, pea bean asparagus bean, cartjang are seeds of cowpea. They are often called
‘black eyed pea’ due to its black or brown-ringed hilum. Poor concern for available but unutilized cowpea variants may promote the losses of diversity in the species. Morphological characters of plants are commonly used in the description of varietal differences in leaves, the photosynthetic organs of the plant growth and seed yield. The flat leaf blade is specialized for catching sunlight. Leaves vary greatly from plant to plant and are useful in classification and identification. Leaf appearance in crop plants is an important process involved in canopy development. There is wide range among the plant genotypes in shape, size and number of leaves produced by plants. In cowpea, the leaf shape is important for taxonomic classification and also for distinguishing cowpea varieties. However, there isn’t a central naming convention for neither cowpea leaves nor detailed descriptions of the leaf shapes; thus, many researchers name the leaf shapes differently. The two largest cowpea germplasm agencies are the International Institute of Tropical Agriculture (IITA) and the United States Department of Agriculture (USDA). IITA, which houses 14,500 cowpea accessions from 65 different countries, classifies cowpea leaf shapes into four categories, sub-globose, sub-hastate, globose and hastate/lanceolate.The hastate leaf shape was reported to be dominant to the ovate leaf shape in several studies (Oluwatosin, 2002). This may indicate that the hastate shape is ancestral to the ovate leaf shape and the preponderance of the latter in most cultivated cowpea is due to direct or indirect selection by humans over time.
The flowers of cowpea are zygomorphic (bilaterally symmetrical). The perianth is made up of a standard which is usually large and purple in colour especially in most of the wild varieties.
There is a prominent pollen guide which is usually yellow in colour. There are two wings which are smaller and lighter in colour than the standard and there are two joined petals making up the keel. Flower colour varies (NRC, 2006), the purple colour is dominant over the white which is likely to be more recent on the evolutionary scale. However, there are several shades of purple colour and these may be due to multiple allelic series or to the effect of a modifier. Similar explanation may be given for the inheritance of flower with yellow petals recently noticed in one of the wild form. The flower is also found to be scented and possesses well developed nectaries which are added attractions for the insect pollinator. The surfaces of the pollen grains are reticulate with raised exine (De leonardis et al.,1993). Interspecific crosses made between the two species have not been successful (Mithen, 1987; Ng and Apeji, 1988 Ng, 1995). On the basis of a study on isoenyme variation in generation Phaseolus and Vigna. Jaaska and Jaaska, (1988)
proposed to raise the section Catinj to the rank of subgenus. Some varieties secrete a large amount of nectar especially from the inflorescence of abscissed flowers. Bees, which had been reported as the major insect pollinators (Leleji, 1974), are usually found in large numbers on cowpea fields. The stickiness of the pollen grains and the reticulate design on its exine are peculiar to insect-pollinated plants. Another distinquishing feature of cowpeas is the long 20-50 penduncles which hold the flowers and seed pods. One penduncle can support four or more seed pods.Wild cowpea are very small while cultivated varieties can have between 10 and 110 centimetres (3.9 and 43.3 in) (Davies et al., 1991). Pods can have 6-13 seeds that are usually kidney shaped, although the seeds become more spherical the more restricted they are within the pod (Sheahan, 2012 and Davies et al., 1991). Their texture and colour is very diverse .They can be smooth or rough coat and speckled, mottled or bloctchy. Colours include white, cream, green, red, brown and black or various combinations (Davies et al., 19991)
Some of the varieties as reported by the IITA include; TVX 3236 (Danknarda), IT82 E-60 (Ezorowo), Ife Brown L-25, Vita 4, ER-7, CA-I, II, III, IV (Henshaw, 2008). Others are Texas Cream 40, IFE BPC, Kanannado, Moola, L-80, and IT86D-1010.
Cowpeas have also been grouped into the following market classes based on seed type and colour: Black eye and purple eye—the immature pods shell easily because the hull (pod wall) is pliable and the seeds come out of the pod clean and free. The shelled peas are attractive, mild flavoured and suitable for processing. The white hilum is surrounded by black, pink, or light-red colour.
Brown eye—Pods vary in colour from green to lavender and have a wide range of lengths. The immature seeds, when cooked, are a medium to dark brown colour, very tender, and have a delicate flavour.
Crowder—Seeds are closely crowded in the pods and tend to be globular in shape. Cream—Seeds of these types are generally cream coloured and have no noticeable "eye" (the hilum is inconspicuous).
Clay—these are generally older varieties that are medium to dark brown in colour and kidney shaped. They are no longer commonly grown.
White acre—the peas are kidney shaped with a blunt end. This type is a semi-crowder, generally tan in colour and somewhat small. Pods are quite stiff. (Marsh et al., 1991).
Scientifically cowpea (Vigna unguiculata) is classified as follows:
Species: Vigna unguiculata
Binomial name: Vigna unguiculata (L.) Walp.
Some of the common cultivars of cowpea (Vigna unguiculata) are; Group Common name
Unguiculata Crowder –pea, Southern pea, Black eyed pea Biflora Catjang, cowpea.
Sesquipedails Yardlong bean, asparagus bean Chinese long bean. Textilis
Among the various cultivars of Cowpea. (V. unguiculata) cultivated in Nigeria include; large Kano white (iron beans), small Kano white (Potiskum beans), Ife Brown beans, Sokoto guzo, Oloka, Kafanji and Crowderpea (black beans).
Statement of the Problem
Various morphological variations are found in the genus Vigna and have resulted in the disorder in the identification and characterization of the species and this has led to inadequate information on the systematic studies of the plant.
Justification of the Study
This study could be justified from the economic importance of cowpea as food and drug, and the classification of Vigna unguiculata which has been an issue in plant taxonomy.This study therefore aimed at exploiting the various taxonomic parameters that will produce relevant and comprehensive information in the proper classification of the species thus reinforcing the accuracy of cowpea classification to advance the taxonomy of cowpea.
Significance of the Study
The various uses of cowpea, Vigna unguiculata (L.) Walp, and the potential they hold in agriculture as leguminous plants, the gross inadequacy of basic systematic information on the species and their ethnobotanical characteristics, were among the bases for the study of these Varieties namely; ‘Kafanji’, ‘Ifebrown’, ‘Potiskum’, Iron beans, ‘Sokoto guzo’, Crowderpea and ‘Oloka’.
Aim and Specific Objectives of the Study
The aim of this research was to use the morphological, anatomical, cytological, phytochemical and proximate characters of seven varieties of Vigna unguiculata (L.) Walp Available in Anambra State to develop an acceptable taxonomic tool for the identification and characterization of seven varieties of Vigna unguiculata (L.) Walp.
The specific objectives include;
1. To determine various morphological parameters that will aid in the identification of the different varieties of Vigna unguiculata (L.) Walp.
2. To assess the anatomical features of different organs of the plants that will provide relevant information in the classification of these varieties.
3. To examine the cytological information on the mitotic studies of these varieties that will aid in solving taxonomic problems in the genus Vigna.
4. To evaluate the phytochemical and proximate constituents of the seven varieties for optimum utilization as food and in Ethnomedicine..