Determining Heterotic Response, General Combining and specific Combining Ability for Yield and Yield Contributing Traits in Cowpea (Vigna unguiculata (L). Walp.)

Cowpea’shigh protein content, its adaptability to different types of soil and intercropping systems, its resistance to drought, and its ability to improve soil fertility and prevent erosion makes it an important economic crop in many developing regions. Cowpea belongs to the genus Vigna and species unguiculata under the subfamily Faboideae (Papilionoideae) of the family fabaceae (Leguminosae) with a chromosome number of 2n=22. Vigna is a pantropical genus with 22 species in India and Shoutheast Asia, 120 species in Africa and few in America and Australia. The speices Vigna unguiculata has one subspeices unguiculata which has three cultigroup, namely cv.gr. Unguiculata, cv.gr. Biflora and cv.gr. Sesquipedalis. Legumes represent the second largest family of higher plants, second only to grasses in agricultural importance (Doyle and Luckow, 2003). Cowpea, (Vigna unguiculata (L.) Walp.), is an important food legume cultivated extensively in the tropics and sub-tropics of Africa, Asia, Brazil and some southern states of the United States of America (Smart, 1976). An estimate of 14.5 million hectares of land is planted to cowpea each year worldwide. Global production of dried cowpeas in 2010 was 5.5 million metric tons; Africa was responsible for 94% of the total production (CGIAR, 2010). The experimental yields of the improved genotypes have been reported around 15 quintals per hectare. In India, cowpea is grown in almost all the states but the major cowpea growing states are Gujarat, West Bengal, Tamil Nadu, Andhra Pradesh, Kerala and Orissa.

The dominant worldview is that the cowpea originated in West Africa, probably in the sub humid savanna grasslands of Nigeria, the area of its greatest diversity (Faris, 1965, Rawal, 1975, Lush and Evans, 1981 Ngand Marechal, 1985). It is, however agreed that the cowpea was domesticated about 4000 years ago from the wild progenitors Vigna unguiculata ssp. Dekindtiana var, dekintiana (in sub Sahelian West Africa) and mensensis (in the humid and sub humid zones) and distributed thereafter throughout sub Saharan Africa, reaching Middle East by 2000 B.C. and India by 1500 B.C. (Lush and Evans, 1981 and Steele, 1976). The Indian cultigroups of V. unguiculata ssp. unguiculata namely, Biflora (the catjang bean) and Sesquipedalis (the yard long or asparagus bean) arose probably by selection from the early cowpea domesticates (Faris, 1965). According to Bressani (1985), the mature legume contains 23-25% protein and 50-67% carbohydrate, 1.9% fats, 6.35% fibre and small percentage of the B-vitamins such as folic acid, thiamine, riboflavin and niacin as well as some micronutrients such as iron and zinc. The richness in protein makes cowpea a source of cheap plant protein (Anderson, 1985) to people who hardly can afford animal protein derived from meat, fish, milk and eggs. According to Yadav and Yadav (1986), cowpea fix 563 kg of atmospheric nitrogen ha-1.

Cowpea is avery diverse, usually glabrous, annual herb which is twinning to sub-erect and rarely erect. The first pair of leaves is simple and opposite. According to Fery (1985), the inflorescence is axillary and formed of a peduncle 10 to 30 cm long, at the end of which there is a rachis with each node bearing a pair of flowers. The pod is green at early stage and when maturing it becomes usually yellow, light brown, pink or purple. The pod length may vary from less than 11 cm to more than 100 cm (Rachie, 1976).

Cowpea can withstand considerable drought and a moderate amount of shade. Cowpeas are short-day, warm-weather plants, sensitive to cold and damaged by frost (Duke and James, 1990). According to Duke and James (1990), cowpeas can thrive on highly acid to neutral soils but they are less well adapted to alkaline soils. The crop is more tolerant of low fertility, due to its high rates of nitrogen fixation (Elawad and Hall, 1987) and effective symbiosis with mycorrhizae (Kwapata and Hall, 1985). Singh (1987) reported that the best cowpea yields are obtained in well-drained sandy loam to clay loam soils between pH 6 to 7.

The success of artificial pollination has been reported to below ranging from 0.5 to 50% (Rachie et al., 1975) and varies with genetic and physiological factors as well as the care taken in handling floral parts during the process of emasculation. In autogamous crop like cowpea, the objective of recombination breeding is to develop high yielding pure line varieties. Recombination breeding involves hybridization, the long process of selection and critical evaluation. Early generation testing and selection have gained momentum in self-pollinated crops, as additive genetic variance is more important. Genetic analysis in early generation helps the selection of desirable transgressive segregants and thereby reducing the population size in letter generations. The basic objective of any breeding method is to increase yield per unit area, to meet the demand food for increasing populations. A distinct knowledge about type of gene effect, its magnitude and composition of genetic variancei.e., additive, dominance and epistatic help in formulating an effective and sound breeding programme.

The precise knowledge of nature and magnitude of gene action for characters related to productivity is helpful in the choice of effective breeding methods to accelerate the pace of genetic improvement of seed yield and other economically important characters. However, epistasis is important in the inheritance of quantitative traits besides additive and non-additive effects. Therefore, the present study was to study the nature and magnitude of heterosis and gene action for yield and yield components.