Research Article | Open Access
Amos Afolarin Olajide, Christopher Olumuyiwa Ilori, "Effects of Drought on Morphological Traits in Some Cowpea Genotypes by Evaluating Their Combining Abilities", Advances in Agriculture, vol. 2017, Article ID 7265726, 10 pages, 2017. https://doi.org/10.1155/2017/7265726
Effects of Drought on Morphological Traits in Some Cowpea Genotypes by Evaluating Their Combining Abilities
An evaluation was conducted to understand the genetic effects of combining ability for four different morphological traits, on 42 hybrids in randomized complete block design with three replications in water-stressed and well-watered environments. The significance of the additive variance (D) and dominance variance (H1) indicated the presence of both additive and nonadditive gene actions in both environments. Among the parents, there was asymmetrical distribution of positive and negative dominant genes and the preponderance of overdominance gene action for all the traits in both environments. This study also indicated a minimum of ten genes for plant height in water-stressed environment and minimum of three and eight genes for terminal leaflet area and number of leaves per plant in both environments, respectively. Estimates of narrow-sense heritability ranged from 13.0% for number of branches per plant in water-stressed to 95.0% in well-watered environment for terminal leaflet area. The study revealed that Danilla, IT93K-432-1, and IT97K-499-35 were the best general combiners for all traits, Danilla × IT97K-499-35, and Danilla × IT93K-432-1 were found to be the best specific combiners for all traits in water-stressed environment. Genetic interactions, additive × additive and additive × dominance, were more pronounced in the inheritance of the traits. This indicated that the selection for these traits should be delayed till advanced generations.
Cowpea (Vigna unguiculata (L.) Walp) is a legume of African origin (Baghizadeh et al. ). It is a legume annual crop that thrives in warm conditions. It is being cultivated in the tropics and in subtropics during the warm season (Hall et al. , Valenzuela and Smith , Noubissié et al. , and Abadassi ). Poor people in less developed countries of the tropics derived their protein, animal feed, and cash income from the production of the crop (Diouf ). FAOSTAT  reported that most cowpeas are grown in the African continent, particularly in Nigeria and Niger which account for 66% of world cowpea production. FAOSTAT  earlier estimated the worldwide production at 10 million hectares, while Nigeria and Niger produced 3.04 and 0.69 million tonnes seed of the crop, respectively (Quinn and Myers ). These two countries alone account for more than 81% of the world crop (FAOSTAT ). Cowpea as a species is considered to be moderately tolerant to water stress, particularly the spreading cultivars (Shimelis and Shiringani  and Hayatu et al. .) There is considerable intercultivar variation in the tolerance to water stress (Summerfield et al. ). Precipitation of around 50 mm per month is considered to be sufficient for crop growth in soil with good water holding capacity. However, International Institute of Tropical Agriculture  and Ishiyaku and Aliyu  reported that drought is a major constraint in semiarid tropics due to erratic rainfall in the beginning and towards the end of rainy season. The crop is often subjected to drought stress in both seedling and terminal growth stages which cause the substantial reduction in grain yield as well as biomass production. Hence the necessity of effective breeding program would support high yielding and well-adapted varieties for water deficit conditions. Simultaneous selections in water-stressed and nonwater-stressed environments for nonwater-stressed yield and water-stressed stability will achieve the desired goals of evolving high yielding drought-tolerant genotypes (Ishiyaku and Aliyu ). Understanding the genetic mechanism involved in the inheritance of a particular trait will help the plant breeder in effective selection and selecting for the best traits that would contribute to better yield. Combining ability study provides information on the genetic mechanisms controlling quantitative traits and enables breeder to select suitable parents for further improvement (Hira Lal et al.  and Kadam et al. ). General combining ability is a good measure of additive gene action, whereas specific combining ability is a measure of nonadditive gene action (Rojas and Sprague ). The problem of water stress can be solved by developing drought-tolerant genotypes. Erratic rainfall caused by climate change has become a threatening factor to crop production in Nigeria. The areas that are supposed to be rainforest zones are gradually becoming savannah belts and the desert is advancing at fast rate towards the southwest part of Nigeria from the North. This is resulting in a real problem in the farming community creating joblessness. However, breeding along with better crop management has a major role to play in meeting the changes in the global climate change. Therefore the objective of this study was to evaluate the general and specific combining ability of some morphological characteristics in cowpea aimed to develop cowpea genotypes with increased drought tolerance.
2. Materials and Methods
This research was conducted at the Teaching and Research Farm of the University of Ibadan. University of Ibadan is located at latitude 070 341 and 030 541E at an altitude of 220 m. Soil type is Alfisols of Egbeda and Gambari soil series. The ten cowpea genotypes of diverse drought tolerance used were collections maintained by Genetic Resources Unit, International Institute of Tropical Agriculture, Ibadan, Nigeria. The cowpea genotypes planted were as follows: Danilla, IT93K-452, IT97K-499-35, IT89KD-288, IT98K-205-8, TVU7778, TVU12349, IT92KD-357-2, IT98K-491-4, and IT99K-573-2-1. From screening experiment, crosses in a diallel design involving the ten parental lines, with diverse drought tolerance, were selected for diallel analysis. Hybridization method was carried out to obtain a total of 42 hybrid combinations following partial diallel mating design. Parental lines and 21F1 and 21RF1 hybrids were selected for evaluation in a randomized complete block design with three replications at Teaching and Research Farm, University of Ibadan, during the dry season, November to February 2011/2012 and 2012/2013. Each entry was grown in a single row plot 3 m long with row spacing 75 cm apart and plant spacing 20 cm within a row. Two seeds were sown per hole which was later thinned to one seed per hole.
After sowing, all the plants were watered until the emergence of the trifoliolate leaves after which watering was suspended on water-stressed plots. The water level was devised by irrigating the plots with supplemental water whenever required, while on the other plot it was completely held throughout the growing season. The soil moisture content of the plots was also estimated on regular basis. Crop management was uniform following recommended production package. Data were collected on plant height, terminal leaflet area, the number of leaves per plant, and number of branches per plant at six weeks after planting. Ten plants for each parent, F1, and RF1, were sampled per replication. Data collected on parental lines and the hybrids for all the traits instudy were averaged and utilized for statistical analysis. Combining ability analysis was carried out following model II of Method 1 modified for partial diallel cross (parents and F1 and RF1) for combining ability analysis of Griffing . The various genetic components were calculated based on the formulae of Hayman  as demonstrated by Aksel and Johnson .
3.1. Combining Ability Analysis
Table 1 showed the descriptions of parental cowpea genotypes used. Analysis of variance for combining ability (Table 2) showed highly significant differences in both general combining ability (GCA) and specific combining ability (SCA) of the cowpea lines for the entire traits indicating the importance of both additive and nonadditive gene effects in their inheritance in both environments.
|: significant; ns: not significant; PH6: plant height at six weeks; TLA: terminal leaflet area; NL: number of leaves per plant; and NB: number of branches per plant.|
3.2. General Combining Ability Effect
The estimates of GCA effect of the parents are furnished in Table 3. All the parental lines except IT99K-513-21 exhibited significant GCA for plant height at six weeks in well-watered environment, while IT98K-205-8 and IT99K-513-21 only were not significant in water-stressed environment. However, IT97K-499-35 had high positive significant GCA effect and thus good general combiners for plant height in both environments. On the contrary, TVU12349, IT89KD-288, and IT98K-491-4 had high negative and significant GCA effect and were, therefore, poor combiners for plant height in both environments. Danilla had intermediate GCA effect in both environments, while IT99K-513-21 was also poor combiners in both environments.
|: significant; ns: not significant.|
With respect to terminal leaflet area, the estimates of GCA effect indicated that IT97K-499-35, Danilla, and IT92KD-357-3 had high positive significant GCA effect and thus were good combiners in well-watered environment, while TVU12349, IT89KD-288, and IT98K-491-4 had high negative GCA and therefore poor combining ability for plant leaflet area in nonwater-stressed environment. However, IT92KD-357-3 and IT97K-499-35 had high positive GCA effect and thus good combiners in water-stressed environment, while TVU12349, IT89KD-288, and IT98K-491-4 had negative significant GCA effect and, therefore, were poor combiners for plant leaflet area in water-stressed environment.
With respect to the number of leaves per plant, IT93K-432-1 and IT97K-499-35 had positive significant GCA effect and therefore were good combiners for number of leaves in well-watered environment, while TVU12349, IT89KD-288, and IT98K-491-4 had negative GCA effect and therefore were poor combiners. TVU7778, IT98K-205-8, and IT99K-513-21 were not significant and therefore were poor combiners in well-watered environment. IT97K-499-35 had high positive significant GCA effect and therefore was good combiners in water-stressed environment, while TVU12349 and IT89KD-288 had negative significant GCA effect and thus were poor combiners. IT98K-205-8 and IT99K-513-21 were not significant in water-stressed environment and therefore were poor combiners.
With respect to number of branches per plant, IT97K-499-35 had relatively high significant GCA effect and, therefore, was good combiners in both environments.
3.3. Specific Combining Ability
In Tables 4 and 5, among 42 hybrids observed, Danilla × IT93K-452-1, Danilla × IT97K-499-35, IT92KD-357-3 × IT98K-205-8, IT92KD-357-3 × IT99K-513-21, Tvu7778 × IT93K-452-1, Tvu7778 × IT89KD-288, Tvu12349 × IT97K-499-35, IT93K-452-1 × IT99K-513-21, IT97K-499-35 × IT98K-205-8, and IT98K-205-8 × IT99K-513-21 were found to be best specific combiners for plant height as evident from their high significant positive SCA effect in well-watered environment. However, Danilla × IT93K-452-1, IT92KD-357-3 × IT93K-452-1, IT92KD-357-3 × IT99K-513-21, Tvu7778 × IT89KD-288, Tvu12349 × IT97K-499-35, IT98K-205-8 × IT99K-513-21, Danilla × IT97K-499-35, and IT93K-452-1 × IT99K-513-21 were best specific combiners in water-stressed environment for plant height at six weeks. Reciprocal combining ability (RCA) effects indicated the crosses IT93K-452-1 × Danilla, IT97K-499-35 × Danilla, IT98K-491-4 × Danilla, IT93K-452-1 × Tvu7778, IT97K-499-35 × Tvu7778, IT89KD-288 × Tvu7778, IT97K-499-35 × Tvu12349, and IT98K-205-8 × IT97K-499-35 to be highly significant in well-watered environment, while crosses IT93K-452-1 × Tvu7778 and IT97K-499-35 × Tvu7778 were only significant in water-stressed environment.
|: significant; ns: not significant.|