Abstract
Field evaluation of 33 Bambara groundnut lines were carried out to estimate genetic variability, heritability and genetic advance. One-way analysis of variance (ANOVA) result showed significant differences for 14 of the 17 agronomic traits studied. The results on the variance components revealed that phenotypic variance had values (vigour index 2.30, pod length 10.09, seed length 1.64) that were slightly higher than the respective genotypic variance (vigour index 1.68, pod length 9.88, seed length 0.26). Similarly, the values (number of branches 41.91, number of nodes 68.72, internode length 59.02) of phenotypic coefficient of variation (PCV) were slightly higher than the corresponding genotypic coefficient of variation (GCV) (number of branches 40.11, number of nodes 66.98, internode length 57.31), suggesting a substantial genetic variability that can serve as a base for Bambara groundnut improvement. High estimates of heritability were observed for most characters like number of branches (95.70%), number of nodes (97.46%), internode length (97.10%), pod length (97.91%), and seed length (93.79%). Likewise, genetic advance values for most traits were high, pod length (201), number of nodes (200), internode length (200), number of branches (197) and seed yield (195), implying that improvement of seed yield in Bambara groundnut can be achieved through direct selection.
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Key words: Bambara groundnut, Genetic variability, Heritability, Agronomic traits
INTRODUCTION
Genetic diversity indicates differences in gene composition and frequencies within and between species. In both wild and cultivated plant species certain factors determine the extent and distribution of genetic diversity which include evolution, ecological and geographical factors, human factors like breeding programmes. Fundamentally, genetic diversity is important in the continuity of plant species; their existence, adaptability and performance in the ever-changing environment. It also forms the basis for selection to be effective in the improvement of traits in crops.
Assessment of genetic variability is very useful in crop improvement programmes as it broadens and exposes the gene pool of crops, while estimates of heritability provides information on the extent of genetic control over the expression of a character, hence the effectiveness of selection (
Chopra 2000). This can be a useful guide to breeders to the extent that they can rely on phenotypic variability in their selection programme for crop improvement. In fact, to improve agronomic characters through selection, it is important that total variability be partitioned into heritable and non-heritable components through the estimation of variance components (like genotypic and phenotypic variances, genotypic and phenotypic coefficients of variation, and heritability), since heritable variations are sometimes masked by non-heritable (Ariyo 1987). In addition, it has been reported that the best result in a crop improvement programme is achieved when heritability estimates are considered together with genetic advance (
Shukla et al. 2006;
Asfaw et al. 2017). Usually, selection of traits that showed high heritability values alongside with high values of genetic advance generally lead to improved yield.
Bambara groundnut is a very important but neglected and underutilized pulse. It is relatively free from pest and disease attack and tolerates extreme weather conditions. This crop grows on soils with varied nutrient fertility and can produce reasonable yields where other crops failed. The seed of Bambara groundnut has been described as a complete food, and its gross energy value is greater than that of other pulses like cowpea, lentil and pigeon pea (
FAO 1982;
Amarteifio et al. 2002;
Lacroix et al. 2003). However, Bambara groundnut is cultivated as landrace till date. There are no improved varieties as genetic improvement through conventional breeding has not been achieved. Currently the available improvement strategy is to apply selection on the existing genotypes. However, the progress that can be made through direct selection is determined by available heritable variability. Further heritability influences the choice of selection method that would be most useful to improve a trait. It also helps to predict gain from selection and to determine the relative importance of genetic effects (
Umar et al. 2014). Therefore, the aim of this study was to measure the genetic variability in agronomic traits of Bambara groundnut with the specific objective of determining estimates of heritability and genetic advance, as a basis for selecting cultivars with high yielding potentials.
MATERIALS AND METHODS
This research was carried out at the Teaching and Research Farm of School of Agriculture and Agricultural Technology, Federal University of Technology, Owerri located at longitudes 70° 00E-07° 07E and latitudes 05° 20N-05° 27N. It is in the humid tropical agroecological zone. The mean annual rainfall ranged from 2250 mm to 2500 mm, and mean daily temperature varied between 27℃ and 28℃ (Owerri meteorological station).
The plant materials used for this study comprised 33 lines of
Vigna subterranean obtained from the germplasm collection of the International Institute of Tropical Agriculture, Ibadan. A portion of land measuring 40 m × 20 m, was cleared, ploughed and marked out into 3 blocks. The space between two blocks was 1 meter. Planting was done in August under wet cropping season. Two seeds were sown and later thinned to one on 6 m length ridges spaced 100 cm apart. The intra ridge spacing (planting distance) was 20 cm. Poultry manure was applied at 15 t/ha as reported by
Duruigbo (2004). Standard cultural practices like supplying, thinning, and weeding at 3 and 7 weeks after planting (WAP), and earthing up at 9 WAP were adopted to ensure optimum crop growth and development (
Ntundu et al. 2006).
The design of this experiment was a randomized complete block design (RCBD) with three replications. Data were collected on 17 agronomic characters (
Table 1) of Bambara groundnut as listed by the International Bambara Groundnut Network (BAMNET), (
IPGRI, IITA, BAMNET. 2000). The data collected were subjected to Analysis of Variance (ANOVA) using GENSTAT 5.0 Release 4.23DE, Discovery Edition 3. Treatment means were separated by least significant difference (LSD) at 5% probability. Using the expected mean square for ANOVA, the variance components were calculated as suggested by
Uguru (2005).
Using the expected mean square for ANOVA, the variance components was calculated as
Heritability estimates
Broad Sense Heritability (BSH) gives an estimate of the presence of the proportion of the total variance (phenotypic) that is due to genetic effect and was calculated as:
Relative difference
Genetic advance
Genetic Advance was estimated using
RESULTS
The results of the mean values which were the actual values of the 17 agronomic characters measured and the one-way analysis of variance of these parameters are showed in
Table 2. There were significant differences for almost all the characters used to discriminate among the 33 Bambara groundnut accessions. Only three parameters, plant height, number of leaves per plant and seed length did not differ significantly.
The estimates of genotypic variance, phenotypic variance, genotypic coefficient of variation, phenotypic coefficient of variation, relative difference, broad sense heritability and genetic advance are presented in
Table 3. Apparently, the result showed that genotypic variance (s
2g) varied from 0.26 for seed length to 40325.01 for seed yield. Similarly, pod weight had the least phenotypic variance value of 1.47 while seed yield recorded the highest (42994). The values for Genotypic coefficient of variation (GCV) ranged from 1.96% for seed length to 13947.49% for seed yield, respectively. Phenotypic coefficient of variation (PCV) had values between 10.84% for pod width and 14870.64% for seed yield. Relative difference (RD) which is an estimate of GCV in relation to respective PCV, ranged from 6.20% for seed yield to 84.13% for seed length. The broad sense heritability estimates were generally high for most of the traits. Apparently, characters like pod length, internode length and number of nodes had values that were above 97%. On the other hand, heritability estimates for traits like seed length (15.85%), plant height (20.91%), canopy width (33.72%) and terminal leaflet length (38.82%) were relatively low. Genetic advance for most traits were high, pod length (201), number of nodes (200), number of branches (197), seed yield (195) etc., similar to the result on heritability. Only very few traits had relatively low genetic advance values, and these were plant height (43.08%), canopy width (69.48%), terminal leaflet length (79.97%) and number of leaves (98.71%).
DISCUSSION
The improvement of agronomic characters through selection depends on the magnitude of genetic variation present within a population. In addition, it also depends on degree of transmissibility of a particular trait which is a measure of estimates of heritability. Again, to determine the expected gain from selection, it is expedient that heritability estimates be considered alongside with values of genetic advance. Assessment of these factors is an important preliminary step that must be taken before commencing any breeding programme. In this study, the choice of selection method specified for the improvement of yield in Bambara groundnut was based on the measure of the amount of variation that exists in the gene pool of the crop, estimates of heritability and genetic advance. This is in line with the report of previous studies on heritability, which observed that the selection made for the improvement of a character is not only dependent on available genetic variation but also on the extent of heritability of such variations (
Umar et al. 2014;
Langat et al. 2019). Further, the estimates of heritability together with genetic advance provide profound advantage over the use of heritability alone (
Shukla et al. 2006;
Asfaw et al. 2017).
The result on the variance components (genotypic and phenotypic variances) showed that phenotypic variances were slightly higher than the corresponding genotypic variances in all the phenotypic descriptors. This indicates environmental influence in the expression of the characters. Therefore, selection for the improvement of any of these characters should be delayed until genetic influence appreciates. The result of this study is in line with the reports of other workers (
Ashok et al. 2000;
Uguru 2000;
Adebola et al. 2001). Further, the genotypic coefficient and phenotypic coefficient of variation results were analyzed based on the report of the works of
Deshmukh et al. (1992). They proposed that values of genotypic coefficient of variation (GCV) and phenotypic coefficient of variation (PCV) that were greater than 20% indicated high variation, while GCV and PCV values that were less than 10% showed low variation. Also, they classified values between 10 to 20% as moderate variability. Based on this benchmark, genetic component analysis result showed that both genotypic coefficient of variation (GCV) and phenotypic coefficient of variation (PVC) were high for almost all the traits. Invariably the agronomic traits evaluated showed a wide range of variability except for seed length and pod length that had low values of 1.96% and 4.72% respectively. Typically, characters with reasonable variation offers a wide range of opportunity for selection for their improvement (
Fakuta et al. 2014), On the other hand, characters that recorded low GCV and PCV values showed low variability among the Bambara groundnut lines. They cannot be effectively used to discriminate among the collections, and again offer little or no opportunity for selection for crop improvement. The relative difference (RD) estimates was high for some characters like plant height (78.08%), canopy width (66.27%), terminal leaflet length (61.17%) and seed length (84.13%). This implies that the observed variations in these characters were mostly due to environmental effect, hence their improvement cannot be achieved by direct selection (
Bello et al. 2012;
Umar et al. 2014). On the contrary, the characters that had low RD were number of branches (4.29%), number of nodes (2.53%), internode length (2.90%), pod length (2.09%) and seed yield (6.20%). This result indicates that the variation that exist in these characters are due to genetic effect. Invariably the genetic improvement of these characters can be achieved through direct selection. This result is in agreement with the works of
Fakuta et al. (2014).
Estimates of heritability and genetic advance play a very important role in predicting the dependability of phenotypic value as a guide to breeding value.
Johnson et al. (1955) classified heritability estimates as low for 0 to 30%, moderate for values from 30 to 60%, and high for values above 60%. Apparently, almost all the agronomic characters evaluated in this study had high heritability alongside with high genetic advance values. Apparently, traits like number of branches, number of nodes, internode length, and seed yield had heritability estimates of 95.70%, 97.46%, 97.46%, and 93.46% in that order. Similarly, these traits (number of branches, number of nodes, internode length, and seed yield) also recorded high values (197.16%, 200.17%, 200.03%, and 195.21%) respectively for genetic advance. High heritability values alongside with high values of genetic advance suggest greater additive gene effect which offers opportunity for selection, unvaryingly direct selection for these traits could be effective. Previous studies have reported that direct selection can be imposed on traits that recorded high estimates of heritability together with high values of genetic advance (Fakuta
et al. 2013;
Olaniyi et al. 2014;
Langat et al. 2019). Canopy width, number of leaves, and terminal leaflet length had moderate heritability estimates of 33.72%, 47.915%, 38.82%, respectively. Moderate heritability values would tend to implicate considerable influence of environmental effects (
Cornelius 1994) with some implications on selection. Low heritability estimates observed for characters like plant height (20.91%) and seed length (15.85%), alongside with observed relatively low values of genetic advances of 43.08% and 32.65% respectively indicated non-additive genetic (possibly, dominance and or epistasis) and environmental effects or a combination of both factors. It has been reported that selection of character with low and moderate heritability estimates alongside with low genetic advances values be postponed until their genetic effects appreciate well over environmental effects (
Subranmayan 1986).
It is evident from the present study that the improvement of yield and other yield related characters of V. subterranean can be achieved through selection by the estimates of heritability and genetic advance. In addition, the magnitude of variations observed for almost all the agronomic characters were substantial and therefore can be exploited to an advantage in improvement of agronomic characters of Bambara groundnut by plant breeders.
Table 1Morphological characters, their abbreviations and units of measurement for Bambara groundnut traits evaluated.
Table 1
|
Abbreviation |
Character |
Unit |
|
VGI |
Vigour index |
(-) |
|
PTL |
Petiole length |
(mm) |
|
PTS |
Plant spread (canopy width) |
(cm) |
|
PHT |
Plant height |
(cm) |
|
NLP |
Number of leaves per plant |
(-) |
|
NSP |
Number of stems per plant |
(-) |
|
POL |
Pod length |
(mm) |
|
PDW |
Pod width |
(mm) |
|
NBP |
Number of branches per plant |
(-) |
|
SWT |
100-seed weight |
(g) |
|
SDL |
Seed length |
(mm) |
|
SDW |
Seed width |
(mm) |
|
TLL |
Terminal leaflet length |
(mm) |
|
TLW |
Terminal leaflet width |
(mm) |
|
INL |
Internode length |
(mm) |
|
NNS |
Number of nodes per stem |
(-) |
|
SYD |
Seed yield |
(g) |
Table 2One-way analysis of variance (ANOVA) of the quantitative morphological characters evaluated.
Table 2
|
Trait |
Mean |
Probability (P) |
|
Vigour index |
7.32 |
0.000**
|
|
Plant height |
21.84 |
0.158 |
|
Canopy width |
52.63 |
0.001*
|
|
No of leaves per plant |
266.27 |
0.824 |
|
Terminal leaflet length |
56.81 |
0.019*
|
|
Terminal leaflet width |
24.81 |
0.000**
|
|
Petiole length |
136.81 |
0.000**
|
|
No of stem per plant |
8.47 |
0.000**
|
|
No of branches per plant |
8.9 |
0.000**
|
|
No of nodes per stem |
8.6 |
0.000**
|
|
Internodes length |
16.96 |
0.000**
|
|
Pod length |
22.95 |
0.000**
|
|
Pod width |
13.56 |
0.004*
|
|
Seed length |
13.28 |
0.066 |
|
Seed width |
11.32 |
0.000**
|
|
Seed yield |
289.12 |
0.000**
|
|
Seed weight |
83.25 |
0.000**
|
Table 3Estimates of variance components, relative difference, heritability and genetic advance of the evaluated 17 agronomic characters of the Bambara groundnut.
Table 3
|
Character |
δ2g |
δ2p |
GCV (%) |
PCV (%) |
RD (%) |
h2 (%) |
GA (%) |
|
Vigour index |
1.68 |
2.30 |
22.95 |
31.42 |
26.95 |
73.04 |
150.46 |
|
Plant height |
2.93 |
14.01 |
13.42 |
64.15 |
78.08 |
20.91 |
43.08 |
|
Canopy width |
24.73 |
73.32 |
46.98 |
139.31 |
66.27 |
33.72 |
69.48 |
|
Number of leaves |
1113.33 |
2323.33 |
418.12 |
872.54 |
52.08 |
47.91 |
98.71 |
|
Terminal leaflet length |
20.77 |
53.50 |
36.56 |
94.17 |
61.17 |
38.82 |
79.97 |
|
Terminal leaflet width |
9.43 |
11.93 |
38.00 |
48.08 |
20.98 |
79.04 |
162.83 |
|
Petiole length |
431.04 |
484.97 |
315.06 |
354.48 |
11.12 |
88.87 |
183.09 |
|
Number of stems |
1.52 |
1.80 |
17.95 |
21.25 |
15.53 |
84.84 |
173.95 |
|
Number of branches |
3.57 |
3.73 |
40.11 |
41.91 |
4.29 |
95.7 |
197.16 |
|
Number of nodes |
5.76 |
5.91 |
66.98 |
68.72 |
2.53 |
97.46 |
200.77 |
|
Internode length |
9.72 |
10.01 |
57.31 |
59.02 |
2.90 |
97.10 |
200.03 |
|
Pod length |
9.88 |
10.09 |
43.05 |
43.97 |
2.09 |
97.91 |
201.71 |
|
Pod width |
0.64 |
1.47 |
4.72 |
10.84 |
56.48 |
43.43 |
89.68 |
|
Seed length |
0.26 |
1.64 |
1.96 |
12.35 |
84.13 |
15.85 |
32.65 |
|
Seed width |
8.62 |
14.20 |
76.15 |
125.44 |
39.29 |
60.70 |
125.05 |
|
Seed yield |
40325.01 |
42994.03 |
13947.49 |
14870.64 |
6.20 |
93.79 |
195.21 |
|
Seed weight |
226.62 |
318.67 |
272.21 |
382.78 |
28.88 |
71.11 |
146.49 |
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