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GENETICS AND INHERITANCE PATTERN OF TIME OF FLOWERING AND QUANTITATIVE TRAITS IN COWPEA [Vigna unguiculata (L.) Walp].

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INTRODUCTION

 

Cowpea [Vigna unguiculata (L.) Walp.] is a tropical grain legume which plays an important nutritional role in developing countries (Singh et al., 1997). Due to its high protein content (20-25%), cowpea has been referred to as “poor man’s meat” (Fall et al., 2003) and its young leaves and pods contain vitamins and minerals. Recent survey however, showed that the crop is now food for the rich, the informed and those that can afford it (Nnanyelugo et al., 1997; Othman et al., 2006). Cowpea is cultivated in all tropical areas on at least 12.5 million hectares, with an annual production of over 3 million tons world-wide (Feleke et al., 2006).  Nigeria, Brazil and Niger are among major producers and account for over 70% of world production; and Nigeria alone produces 900,000 tons annually (Jackai and Daoust, 1986). Cowpea farmers in the dry savanna areas of sub-Saharan Africa obtain low yields, estimated at about 350 kg per hectare; and in Nigeria yield can be as low as 220 kg ha-1 (IITA, 1998). The reasons for the poor yield of cowpea range from low yield potential of traditional cultivars to poor crop husbandry and insect damage (Jackai and Daoust, 1986; Omoigui, et al., 2006). Genetic information is needed to develop efficient breeding procedures which could lead to the development of improved and high yielding cowpea varieties suitable for different ecological zones and cropping systems.

Flowering is an important physiological process in crop survival and assurance for continuity. Time of flowering is particularly of great importance in annual crops, including cowpea (Vigna unguiculata [L.] Walp), as it is a component of the adaptation of a variety to a particular environment and it also determines pod set and crop yield (Ishiyaku et al. 2005). Information on the inheritance of days to flowering will help breeders develop strategies for improvement of seed yields and their adaptation to various agro-climatological zones (Adeyanju et al., 2007). Plant growth and development, especially flowering, is dependent on the interaction of many complex processes, which are influenced by both genetic and environmental factors (Uarrota, 2010). Craufurd et al. (1996) and Mukhtar and Singh (2006) reported that in West and Central Africa, photoperiod is the most important environmental variable affecting time of flowering and that most cowpea varieties under cultivation are unimproved, local types which are photoperiod sensitive. Photoperiod has been reported to influence plant growth characteristics, including flowering. Cha-um and Chalermpol (2007) reported that plant height, leaf length, leaf area and flag leaf area as well as flowering were highly regulated by short-day photoperiod in rice.

Earliness in cowpea (Vigna unguiculata [L.] Walp) is an important agronomic trait since it has been reported to yield the dividends of opening the possibility of successful sole cropping in areas with short rainy season, double/triple cropping in rice and/or wheat based systems.  It is also beneficial in relay cropping in areas with relatively longer rainfall after millet, sorghum, or maize as well as parallel multiple cropping with cassava, yam and cotton (Adeyanju and Ishiyaku, 2007). If the genetic basis of early and continuous flowering is understood, it can be exploited in the development of cowpea varieties that can flower and pod continually thereby ensuring all year round availability of cowpea for the teaming population in sub Saharan Africa.

Responses to selection may be affected by cytoplasmic and maternal effects. For many characters, the genotype of the mother via maternal effects ractunt for a considerable portion of the genetically based variation in the expression of the phenotype of the progeny. Therefore, selection based on direct genetic effect is insufficient and could be misleading as it may lead to omission of potentially important source of genetic variance contributed by the cytoplasm of the maternal strain (Wolf et al., 2002). It is therefore important to consider parental influence, especially maternal effect, when genetic studies of reproductive and quantitative characters are assessed.

Although maternal effects and its evolutionary consequence on improvement in the fitness of progeny, are well known in plants (Mousseau and Fox 1998), there is almost no information about their expression in nature (Galloway, 2001), especially on an important crop like cowpea. This is because most studies on maternal effects have been conducted on temperate crops and these studies were carried out under controlled environments. While these works have partly led to an understanding of maternal influence on phenotypic expression in offsprings, little is known of their contribution to phenotypic variation in nature. For instance, a controlled-environment study found that both maternal and paternal environments influenced seed and germination characters (Galloway 2001), but it is not known whether these parental effects are expressed under field conditions. In some other cases however, the parents used for hybridization by workers (Ishiyaku et al., 2005; Sene, 1967), belonged to either photoperiod-sensitive or photoperiod-insensitive group. This has generated some confusion as to the actual inheritance pattern of time to flowering of the two main photoperiodic groups under natural field conditions. This work was therefore initiated to address this obvious lapse, as the parents used in this study belonged to short day and day neutral groups. To date, there is no information on the inheritance of time to flowering and quantitative characters in cowpea using short day and day neutral lines.

The objectives of this study are therefore to:

  1. investigate the inheritance pattern of time to flowering and quantitative characters in crosses between short day and day neutral cowpea accessions and
  2. establish the level of crossibility between the two photoperiodic groups of cowpea.

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