Abstract
Bambara groundnut, a future emerging legume, is usually known as a poor man’s crop or as “women’s crop” and newly noted as crop for new millennium. In this context, a field experiment was conducted during 2019/20 cropping season at Sonka testing site of Asosa zone in northwestern Ethiopia with objective to estimate the variability of Bambara groundnut genotypes with respect to important quantitative and qualitative traits. Treatments consisted in 20 accessions collected from five distracts of Benishangul Gumuz province in northwestern Ethiopia. The treatments were laid out in a randomized block design with three replications. All the qualitative characteristics recorded for genotypes showed different modalities and frequencies at vegetative stage with 70.12% of the fully expanded leaflet colour, 74.26% oval terminal leaflet shape, 76.24% petiole green colour, 78.22% stem green colour, 88.27% sparse hair on their stems, 70.41% spreading types, 81.33% rounded base with a point at the top, 90.11% yellowish brown pods at harvest, and 90.06% oval shaped seeds. With respect to quantitative traits, genotypes AMN/04/013, AMN/04/014, AMN/04/015, AMN/04/019, and AMN/04/020 gave relatively higher grain yield (>2000 kg/ha) with the highest grain yield recorded for genotype AMN/04/016. Thus, these genotypes showed relatively better adaptation at tested location. Higher correlation values were observed for days to maturity, plant height, number of stems per plant, pods per plant, seed per pod, and HSW, indicating that these traits are very important to be used in selection for the crop improvement. Agronomic and morphological evaluations have provided more accurate estimation of genetic diversity, the raw material of plant breeding. In general, this study enabled to determine how the influential traits direct to improve Bambara groundnut genotypes.
1. Introduction
Bambara groundnut (Vigna subterranea (L.) Verdc. L.) is a future emerging legume grown in Africa and Asia. It is usually known as a poor man’s crop or as “women’s crop” and newly noted as crop for new millennium. In drier parts of sub-Saharan Africa, Bambara groundnut is mainly grown by female on a small scale in pure culture without improved techniques [1]. Bambara groundnut is one of the grain legumes which belongs to family Papilionaceae and originates from northeastern Nigeria and northern Cameroon [2]. Bambara groundnut seeds are rich in many components such as protein and carbohydrate [3] and have high antioxidant activity. The word “Bambara” comes from a place name near Timbuktu in central Mali, West Africa. The word “groundnut” is because the pods setting occur under the ground soil; hence, jointly its common name is “Bambara groundnut” [4]. The highly nutritious content of Bambara groundnut and its high content of essential amino acids (methionine, leucine, isoleucine, lysine, phenylalanine, threonine, valine, and tryptophan) make it an important crop to consider for food security [5]. Bambara groundnut can possibly fulfill the regular demand of protein for the marginal users where the animal protein is not affordable with consumers with low incomes [6]. Seeds of Bambara groundnut contain considerable amount of minerals such as Ca, K, Fe, and Na. Moreover, the richness of K in seeds of Bambara groundnut has ability to lessen diabetes by prompting the insulin hormone [7]. Bambara groundnut is a medicinal plant used to treat diarrhea, anemia, abscesses, internal injuries, ulcers, infected wounds, epilepsy, cataracts, menorrhagia during pregnancy, nausea in pregnant women, kwashiorkor, and venereal diseases. Bambara groundnut also contains vitamin E, vitamin C, and vitamin A, indicating the crop has a potential for economic exploitation [8].
Bambara groundnut can well adapt to the tropical areas where cultivation of major crops such as rice, wheat, and maize are increasingly challenging due to drought and unpredictable rainfall patterns. In crop genetic variability, the morphological method is considered as the first step in description and classification of germplasm [9]. Characterization and evaluation of genetic resources aid to identify, differentiate, and distinguish traits’ agronomic importance for improvement of a crop in breeding scheme since they provide information on diversity within and between crop genotypes under consideration. Hence, this was initiated with objective to estimate the variability of Bambara groundnut genotypes with respect to important quantitative and qualitative traits.
2. Materials and Methods
2.1. Description of the Experimental Site
A field experiment was conducted during 2019/20 cropping season at Sonka testing site of Asosa zone in northwestern Ethiopia. The approximate geographical coordinates of the site are 9°45′44″ N latitude and 34°47′23″ E longitude having an altitude of 1477 meters above the sea level. The area is characterized by low and erratic rainfall with mean annual rainfall of 796 mm. The mean annual temperature is 21.2°C with a minimum of 14°C and maximum of 28.4°C.
2.2. Treatment and Experimental Design
The plant materials consisted of 20 accessions collected from five districts of Benishangul Gumuz province in northwestern Ethiopia. Description of the genotypes used in the trial is given in Table 1. The treatments were laid out in a randomized block design with three replications. Seeds were hand planted by placing two seeds per hill at row spacing of 50 cm and plant spacing of 20 cm. Thinning was after emergence to maintain the proposed plant density per plot. Crop management activities such as hoeing and weeding were carried out during crop growing season. Diseases and pest events were visually monitored in the field.
2.3. Data Collection and Measurements
Agronomic traits recorded were days to flowering, days to maturity, plant height, number of branches, leaf number, terminal leaf length, number of nods, number stems, pods per plant, number of seeds per pod, hundred seed weight (HSW), and grain yield. Similarly, morphological traits recorded were terminal leaflet shape, petiole colour, stem colour, stem hairiness, growth habit, pod shape, pod colour, seed shape, and pod hardness. The estimation of genetic parameters was done to identify and ascertain the genetic variability among the varieties and to determine the extents of the environmental effect on various characters. Variance components due to phenotype , genotype , and the environment were calculated by adopting the following formula suggested by Burton and De vence [10].where MSg is the mean squares due to genotypes, MSe is the mean squares due to error, and r is the number of replications.
According to Singh [11], the phenotypic genotypic coefficients of variances were expressed aswhere PCV is the phenotypic coefficient of variation.where GCV is the genotypic coefficient of variation and x is the the grand mean of a character.
Heritability in broad sense was calculated for each trait by using the formula [12] aswhere H is the heritability in broad sense, is the genotypic variance, and is the phenotypic variance.
Genetic advance (GA) under selection, assuming the selection intensity of 5%, was calculated as proposed by Johnson et al. [13] aswhere GA is the expected genetic advance and K is the selection differential (K = 2.056 at 5% selection intensity).
Genetic advance as percent of mean was calculated to compare the extent of predicted advances of different traits under selection, using the formula given by Falconer and Mackey [14] aswhere GAM is the genetic advance as percent of mean, GA is the genetic advance under selection, and x is the mean value.
3. Results and Discussion
3.1. Variation of Qualitative Traits
The data for frequencies of qualitative characters of Bambara groundnut genotypes/accessions are given in Table 1. All the qualitative characteristics observed as well as the different modalities and their frequencies showed that at the vegetative stage 70.12% of the fully expanded leaflet colour of Bambara groundnut exhibited green colour. About 74.26% of the genotypes had oval terminal leaflet shape. Information recorded on petiole and stem colour showed that 76.24% and 78.22% of genotypes exhibited green colour, respectively. With respect to stem hairiness, about 88.27% of genotypes showed sparse hair on their stems. Regarding the growth habit, genotypes with spreading growth habits dominated with 70.41%, followed by bunch type growth habit with proportion of 20.68%. Likewise, genotypes with rounded base with a point at the top dominated with amount to 81.33% and rounded base without a point at the top of 16.56%. After harvest, observation of the accessions showed mostly yellowish brown pods with proportion of 90.11% and brown pods 6.92%. Seed shape was dominated by oval with proportion of 90.06% and pod hardness was nearly in equal proportion with 55.56% hard and 43.46% moderately hard.
3.2. Phenological Traits
Analysis of variance showed that genotypes were significantly differed for days to maturity and did not significantly differ on days to flowering (Table 2). Thus, all the genotypes (entries) were characterized by their relative homogeneity in flowering. Ouedraogo et al. [15] reported that all the entries characterized exhibited their relative homogeneity in flowering. In general, days to maturity for Bambara groundnut genotypes ranged from 120.33 to 132.53. The longest days to maturity (132.53) was recorded for genotype AMN/04/016. The shortest days to maturity (120.33) was seen for genotype AMN/04/005. As this investigation indicated, genotypes AMN/04/001, AMN/04/002, AMN/04/003, AMN/04/005, AMN/04/006, AMN/04/008, AMN/04/011, AMN/04/012, and AMN/04/017 took relatively shorter days to maturity. Conversely, genotypes AMN/04/004, AMN/04/007, AMN/04/009, AMN/04/010, AMN/04/013, AMN/04/014, AMN/04/015, AMN/04/016, AMN/04/018, AMN/04/019, and AMN/04/020 exhibited relatively longer days to maturity. The difference of 12.20 days was observed between the longest and shortest days to maturity. This is an indication that there was relatively a wide range of variability among genotypes for days to maturity. Judicaëlle et al. [16] reported that days to maturity was significant for genotypes with nonsignificance of days to flowering for genotypes studied in Burkina Faso.
3.3. Growth Traits
The data of growth traits for varieties are given in Table 2. Analysis of variance indicated that genotypes were significantly different for plant height, terminal leaf length, number of stems per plant, and nodes per stem (Table 3). Plant height for genotypes ranged from shortest (171.53 cm) for AMN/04/001 and the tallest height (191.33 cm) for AMN/04/016. Similarly, terminal leaf length ranged from 50.82 to 64.27 cm with the longest for genotype AMN/04/016 and the least for AMN/04/005. This result showed that plant height and terminal leaf length measured were significantly different due to effect of genotypes which might be attributed to their inherent differences. Number of stems per plant is one of the growth traits that varied from 5.83 to 12.49 with the greatest number of stems per plant (12.49) for AMN/04/016 and the lowest (5.83) for AMN/04/001 (Table 2). In line with this, the number of nodes per stem varied from 2.72 to 3.87 with greatest number of nodes per stem (3.87) observed for genotype AMN/04/016 and least number of nodes per stem (2.72) seen for genotype AMN/04/003. This result showed that plant height, terminal leaf length, number of stems per plant, and nodes per stem measured were significantly different for genotypes which might be attributed to their inherent variability. On the other hand, the number of branches per plant and leaf number were not significantly different for genotypes (Table 3).
3.4. Yield Components and Yield
Analysis of variance revealed that genotypes of Bambara groundnut were significantly different for yield components and yield (Table 4). The number of pods per plant varied from 20.00 to 41.87 with the highest number of pods per plant (41.87) recorded for genotype AMN/04/016. The lowest number of pods per plant (20.00) was seen for genotype AMN/04/003. Genotypes had variable HSW which ranged from 16.00 to 22.00 g, where the highest HSW (22.00 g) was achieved from genotype AMN/04/016 and the lowest HSW (16.00 g) was obtained from AMN/04/005 (Table 4). Grain yield for genotypes varied from 1107 to 3163 kg/ha. The highest grain yield (3163 kg/ha) was obtained from genotype AMN/04/016 and the lowest (1107 kg/ha) from genotype AMN/04/003.
3.5. Variance Components
3.5.1. Phenotypic and Genotypic Variations
Genotypic and phenotypic coefficients of variation are used to measure the variability that exists in a given population under consideration [10]. Phenotypic variance varied from 0.01 for number of seeds per pod to 91.02 for grain yield (Table 5). In line with this, relatively higher magnitude of difference between genotypic and environmental variance was observed for the characters number of pods per plant and grain yield. This implies greater influence of environmental factors for the phenotypic expression of these characters [10]. In contrast, relatively lower phenotypic variance (˂50) was recorded for other traits. Genotypic variance ranged from 0.01 to 36.64 with relatively higher genotypic variance for the number of pods per plant and grain yield. In general, phenotypic coefficient of variation (PCV) varied from 0.52 for grain yield to 26.25 for number of pods per plant (Table 5). According to [10] Sivasubramanian and Madhavamenon (1973), PCV was categorized as high if PCV >20%, moderate if PCV is 10–20%, and low if PCV is below 10%. Based on this categorizing, traits number of branches, number of stems, and number of pods per plant had higher PCV. Conversely, traits leaf number and number of nodes per stem exhibited moderate PCV, whereas days to flowering, days to maturity, plant height, terminal leaf length, number of seeds per pod, HSW, and grain yield showed lower PCV with PCV value below 10%. This reflected the pronounced influence of environmental factors for the expression of these characters. In line with this, genotypic coefficient variance (GCV) varied from 0.33 to 17.82% (Table 5). The lowest GCV (0.33%) was recorded for grain yield, while the highest GCV (17.82%) was recorded for the number of pods per plant. This investigation indicated that moderate GCV (10–20%) was recorded for traits number of branches, number of stems, and number of pods per plant. On the other hand, lower GCV (<10%) was seen for days to maturity, days to maturity, plant height, leaf number, terminal leaf length, number of stems per plant, number of seeds per pod, HSW, and grain yield.
3.5.2. Broad Sense Heritability and Genetic Advance
Different characters have different levels of heritability that can contribute for yield improvement in breeding programs. In general, heritability in broad sense (H2) ranged from 2.25% for days to flowering which was the lowest to 73.93% for HSW which was the highest value. Johnson et al. [13] classified heritability estimates as low (<30%), moderate (30–60%), and high (>60%). Based on this classification, only HSW exhibited high H2 estimate. This result revealed that environment had low influence for the expression of the trait which suggests direct selection using this character as a major contributor of yield components to improve yield of the study area. Thus, selection could be effective in genotypes for this trait and the possibility of improving Bambara groundnut grain yield through direct selection for grain yield related trait. Relatively moderate H2 was recorded for traits days to maturity, plant height, terminal leaf length, number of stems, number of pods per plant, and grain yield which may be occurred due to influence of the environment on the polygenic nature of these traits. It was observed that heritability (H2) was low for traits number of branches, leaf number, number of nodes per stem, and number of seeds per pod. Low heritability that occurred for these traits limits the possibility of including the traits in order to select desirable genotypes. This may be due to the higher influence of environment for the expression of phenotypic variation than genotypic variation. Genetic advance as a percent mean ranged from 1.95% for leaf number to 26.95% for days to flowering (Table 5). This result indicated that selecting the top 5% of the genotypes could result in an advance of 2.23–74.52% over the respective population mean.
As suggested by Sivasubramanian and Madhavamenon (1973), genetic advance as percent of mean was classified as low (<10%), moderate (10–20%), and high (>20%). Based on this classification, traits days to flowering and number of pods per plant exhibited high genetic advance. Traits number of nodes per stem and HSW attained moderate genetic advance. In contrast, traits number of branches per plant, leaf number, terminal leaf length, number of nodes per stem, number of seeds per pod, and grain yield had low genetic advance. Trait such as HSW exhibited high heritability coupled moderate genetic advance, whereas pods per plant had moderate heritability coupled with high genetic advance. Moreover, the trait number of stems per plant showed moderate heritability coupled with moderate genetic advance. Hence, these traits should be given top priority during selection breeding in Bambara groundnut because they are the major portions of genetic variation attributable to additive gene action and selection may be effective in early generations for these traits. Moderate heritability accompanied with moderate genetic advance as percent of mean was an indication that additive and nonadditive gene actions involved in the expression of the traits. The correlation of selected agronomic traits with grain yield is given in Table 6. In general, correlation coefficient (r) of agronomic traits with grain yield ranged from 0.42 to 0.92. With exception of days to flowering, all other traits were significantly and positively correlated with grain yield (Table 5). Higher correlation (r) values were observed for days to maturity, plant height, number of stems per plant, pods per plant, seed per pod, and HSW.
4. Conclusion
Bambara groundnut genotypes exhibited differently for qualitative and quantitative traits recorded and measured due to their genetic variability. Genotypes AMN/04/013, AMN/04/014, AMN/04/015, AMN/04/019, and AMN/04/020 gave relatively higher grain yield (>2000 kg/ha) with the highest grain yield recorded for genotype AMN/04/016. Thus, these genotypes showed relatively better adaptation at tested location. Higher correlation values were observed for days to maturity, plant height, number of stems per plant, pods per plant, seed per pod, and HSW indicating, that these traits are very important to be used in selection for the crop improvement. Agronomic and morphological evaluations have provided more accurate estimation of genetic diversity, the raw material of plant breeding. In general this study enabled to determine how the influential traits direct to improve Bambara groundnut genotypes.
Data Availability
The data used to support the findings of this study are available from self sponser of authors.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Authors’ Contributions
Mesay Paulos, Gobeze Loha, and Mesfin Kassa contributed equally to this study.
Acknowledgments
The authors acknowledge the staff members of Department of Plant Science, Wolaita Sodo University and Biodiversity Institute of Ethiopia, Asosa Branch Office, for providing them with the necessary support to conduct this study. This work was supported by Ethiopian Ministry of Education and Wolaita Sodo University and Biodiversity Institute of Ethiopia, Asosa Branch Office, Asosa, Ethiopia.