GENETICS AND INHERITANCE PATTERN OF TIME OF FLOWERING AND QUANTITATIVE TRAITS IN COWPEA [VIGNA UNGUICULATA (L.) WALP].

ABSTRACT

Field experiments were carried out in 2006 and 2007 cropping seasons at the Teaching and Research Farm of the Department  of  Crop  Science,  University  of  Nigeria,  Nsukka,  to study the inheritance pattern of days to flowering and quantitative traits in cowpea. The experimental materials comprised 10 cowpea accessions sourced from  Plateau  and  Enugu States of  Nigeria. Reciprocal crosses were generated and the parents, F1, F2 and BC populations were evaluated for days to flowering under long day length   (averaged   13.4   hd-1)   and   short   day   length   (10hd-1) conditions. Characters assessed included days to 50% emergence, number of leaves plant-1, number of primary branches plant-1, vine length, number of peduncles plant-1, number of flowers plant-1, number of pods plant-1  and grain yield. The study established the accessions from Plateau state as short day (SD) plants and those from Enugu state as day neutral (DN) plants. The parental lines differed significantly (P<0.05) in days to flowering, pod and grain yields. The mean number of days to first and 50% flowering for SD accessions were delayed significantly (P<0.05) when compared to DN accessions. The result of the principal component (PC) analysis revealed that the most effective characters for distinguishing the parents and progenies included days to first and 50% flowering, number of peduncles plant-1, number of flowers plant-1, number of pods plant-1 and grain yield. The scatter plot and cluster means of the first two principal axes grouped the cowpea accessions into two main clusters, based on  their response  to day-length.  Cluster I comprised day neutral (DN) cowpea accessions (Akidi-ani, Akidi- enu1, Akidi-enu2 and Akidi-enu3) that flowered early (<45 days) with poor yield components and cluster II comprised the short day (SD) accessions (Bwa-Tal, Bwa-Chip, Gag, Gazum and Du’ut) that flowered late (>45 days), and were prolific in the yield traits assessed. However, the accession, Jalbang alienated itself from the clusters and was genetically independent. The F1 and F2 populations also  differed  significantly  in  days  to  first  and  50% flowering, pod and grain yields.   The distribution of the progenies with respect to days to flowering showed combined effects of both maternal and nuclear influence on the inheritance of flowering time in cowpea. Photoperiod sensitive short day accessions used as maternal parents produced F1 progenies that were short day, while day neutral mothers produced F1 hybrids that flowered irrespective of day length. Pearson correlation coefficients revealed significant positive relationship between grain yield and number of peduncles plant-1(0.796**), number of flowers plant-1(0.774**), number of pods

plant-1  (0.758**) and pod length (0.728**). However, inverse relationships   were   obtained   when   days   to   flowering   were associated with grain yield. Grain yield of the SD accessions were significantly higher (>1000 Kgha-1) than those of the DN accessions

(<1000 Kgha-1). The accession, Gag was distinct in grain yield as it produced over 1600Kg ha-1.

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.]

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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:

i.    investigate the inheritance pattern of time to flowering and quantitative characters  in  crosses  between  short  day  and  day  neutral  cowpea accessions and

ii.     establish the level of crossibility between the two photoperiodic groups of cowpea.

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