Research Article
Utilization of Diverse Germplasm for Soybean Yield Improvement
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Muhammad Ashraf
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Afsari Sharif Qureshi
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Abdul Ghafoor
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The profitable yield can be obtained through genetic improvement for high yield potential. The examination of genetic diversity is important for plant breeder in general and particularly in a new introduced crop like soybean, which is not grown commercially in Pakistan. Introduced genotypes are an important source to help us meet our national food/oil demand. In Pakistan, soybean is one of the non-conventional oilseed crops which can be successfully grown in the country during both spring as well as the autumn seasons. Nutritional value of soybean lies in its protein (40-42%) and oil contents (18-22%) and is free from cholesterol, so it is highly desirable in the human diet.
Introgression of diverse germplasm into the current soybean genetic base may increase genetic variability and lead to greater gains from selection (Thompson and Nelson, 1998). Annual wild soybeans (Glycine soja), the ancestors of cultivated soybeans (G. max), are important sources of major genes for resistance to pests, diseases and environmental stresses. The study of their genetic diversity is invaluable for efficient utilization, conservation and management of germplasm collections (Dong et al., 2001). Khanghah and Sohani (1999), Muhammad and Shah (2003) showed significant differences among varieties in terms of traits like pods/plant, seeds/plant, plant height, days to flowering, days to pod initiation, 100 seed weight, grain yield/plant and seeds/pod, indicating the existence of genetic variation among varieties. Choudhry et al. (1999) also revealed that all cultivars varied significantly in yield components. Jagdish et al. (2000) and Jain and Ramgiri (2000) reported that seed yield per plant, biological yield, pods per plant and plant height showed high heritability with high genetic advance as a percentage of mean. Rajanna et al. (2000) showed significant and positive correlation of number of pods per plant, number of clusters per plant and 100 seed weight with seed yield. Similar results were found by Amanullah and Hatam (2001) who observed significant variations in all the parameters studied and showed significantly positive correlation of plant height, branches per plant, pods per plant and dry matter with grain yield. The correlation between grain yield and 100 seed weigh was, however, significantly negative. Iqbal et al. (2003) showed that pods per plant had maximum positive direct effect on yield per plant followed by 100 seed weight and seeds per pod.
Genetic improvements could be accelerated if physiological attributes were used as selection criteria. Present study is one of such efforts in which soybean germplasm was used to study the genetic variability and to evaluate the performance of different genotypes. Moreover, The study of direct and indirect effects of yield and its components provides the basis for increased bean yield and selection for closely associated traits. This information would ultimately lead to the determination of suitable plant types available for further studies in soybean.
The study was carried out during summer season (July to Oct. 2002) at experimental farm of Oilseed Programme, NARC, Islamabad. Experimental material consisting of seventeen soybean genotypes (Table 1) was obtained from the Institute of Agro-biotechnology and Genetic Resources (IABGR) of National Agricultural Research Center (NARC), Islamabad. The experimental design was randomized completely block design with 3 replications; each plot consisted of a single row of 5 m length with a row to row distance of 60 cm maintaining 20-24 plants m-1 in length. The crop was grown under normal conditions. Sowing was done with the help of hand drill. Five selected plants were used to take the data from each plot of each replication. Data were recorded for leaf area, chlorophyll content, 1st pod height, days to 50% flowering, days to flowering completion, days to pod initiation, days to 50% maturity, plant height, number of pods per plant, number of branches per plant, number of unfilled pods, number of shattered pods, 100 seed weight, grain yield, oil content and protein content.
The mean squares of various traits (Table 1) indicated that there were significant differences among genotypes for all the characters under study. High range of variation was observed in leaf area, plant height, 50% flowering, days to 50% maturity, plant height, pods/plant, branches/plant, 100 seed weight and grain yield/plant. This high variation indicated that improvement in soybean could be successful by simple selection for these characters. The other characters with low variations needed to be improved by acquiring germplasm from other sources or using breeding techniques. Evaluation and classification of broad base germplasm needs to be built up by making extensive local collection and obtaining germplasm from abroad to develop a sound breeding program (Ghafoor et al., 1992).
Table 1: | Means of 16 characters studied in 17 lines of soybean grown during July-Oct. 2001 |
X1 = Leaf Area (cm2), X2 = Chlorophyll content, X3 = 1st Pod height (cm), X4 = Days to 50% flowering, X5 = Days to flowering completion, X6 = Days to pod initiation, X7 = Days to maturity, X8 = Plant height (cm), X9 = No. of pods/plant, X10 = No. of branches/plant, X11 = No. of unfilled pods/plant, X12 = No. of shattered pods/plant, X13 = 100 seed weight (g), X14 = Grain yield/plant (g), X15 = Oil content (%), X16 = Protein content (%) |
Table 2: | Genotypic, phenotypic and environmental correlation coefficients among 16 characters studied in 17 genotypes of soybean grown during July-Oct. 2001 |
X1 = Leaf area (cm2), X2 = Chlorophyll content, X3 = 1st Pod height (cm), X4 = Days to 50% flowering, X5 = Days to flowering completion, X6 = Days to pod initiation, X7 = Days to maturity, X8 = Plant height (cm), X9 = No. of pods/plant, X10 = No. of branches/plant, X11 = No. of unfilled pods/plant, X12 = No. of shattered pods/plant, X13 = 100 Seed weight (g), X14 = Grain yield/plant (g), X15 = Oil content (%), X16 = Protein content (%) |
Table 3: | Direct (Bold) and indirect effects of 15 characters (Independent variables) on srain yield (Dependent Variable) in 17 lines of soybean |
X1 = Leaf area (cm2), X2 = Chlorophyll content, X3 = 1st Pod height (cm), X4 = Days to 50% flowering, X5 = Days to flowering completion, X6 = Days to pod initiation, X7 = Days to maturity, X8 = Plant height (cm), X9 = No. of pods/plant, X10 = No. of branches/plant, X11 = No. of unfilled pods/plant, X12 = No. of shattered pods/plant, X13 = 100 Seed weight (g), X14 = Oil content (%), X15 = Protein content (%) |
The partitioning of variance (Table 1) revealed that high heritability was recorded in 100 grain weight, days to maturity, days to flowering completion, days to pod initiation, leaf area, days to 50% flowering, shattered pods/plant, grain yield/plant, oil contents and protein contents respectively indicating the additive type of gene action. Moderate heritability was noted for plant height, pods/plant, branches/plant, unfilled pods/plant, On the basis of heritability, selection for above traits would provide the greatest improvement in soybean.
The results regarding genotypic, phenotypic and environmental coefficients of correlation given in Table 2 showed that the grain yield was positively and significantly correlated with leaf area, pods per plant and oil contents. Positive association of yield with these characters illustrated that selecting for above traits can increase the yield.
Path co-efficient analysis presented in Table 3 revealed that the maximum direct effect was observed for days to pod initiation. Days to pod initiation suggested important selection criteria. Leaf area, chlorophyll content, days to flowering completion, days to pod initiation, number of branches per plant, number of pods per plant and oil content and protein content also exerted positive direct effects. However, first pod height, days to 50% flowering, days to maturity, plant height, 100 seed weight had negative direct effects on yield. This suggested that selection on the basis of these traits might lead to the loss in terms of bean yield.
Germplasm evaluation must be considered the first step in any plant breeding programme and it is commonly based on a simultaneous examination of a large number of populations for several characters of both agronomic and physiological traits. It was observed that some genotypes possessed desirable genes for more than one character and hence these could be utilized directly or included in a hybridization programme for varietal development. Ghafoor et al. (2001) has suggested selection on the basis of best performance. The identification of plants with suitable combinations of characters from a population with genetic variability is dependent upon the knowledge of breeder on that population. This knowledge is utilized to decide a selection criterion which is expected to prove effective for yield improvement together with improvement in other traits.
Present investigations revealed that the genotypes under study varied significantly for all the characters under study. These results are confirmed with the findings of Ghatge and Kadu (1993) who showed significant differences for days to maturity, plant height and seed yield. Maestri et al. (1998) reported significant differences in oil content and protein content. Muhammad and Shah (2003) showed significant differences among varieties in terms of traits like pods/plant, seeds/plant, plant height, days to flowering, days to pod initiation, 100 seed weight, grain yield/plant and seeds/pod, indicating the existence of genetic variation among varieties. In order to maintain, evaluate and utilize germplasm efficiently, it is important to investigate the extent of genetic diversity it contains (Ghafoor et al. 2003). Considerable amount of variation was found among genotypes for days to 50% flowering. Results are contradictory to the findings of Dadson (1976) who evaluated different cultivars of soybean and revealed that most cultivars flowered at the optimum date, between 30-35 days after sowing. The difference in the results might be due to the difference in genetic constitution of breeding material. Some genotypes produced smaller seeds with relatively lighter weight. These results are somewhat contradictory to the findings of Maestri et al. (1998) who conducted trials to compare different genotypes of soybean and reported that seed size varied between 13.9-21.0 g/100 seeds. The results are different because of the type of genotypes which produced lighter seeds. These lines of low 100 seed weight may be used as fodder purpose or in breeding program to improve this character.
High estimates of heritability were recorded in 100 grain weight, days to maturity, days to flowering completion, days to pod initiation, leaf area, days to 50% flowering, shattered pods/plant, grain yield/plant, oil contents and protein contents respectively indicating the additive type of gene action. On the basis of heritability, selection for above traits would provide the greatest improvement in soybean.
These results are comparable to the results of and Jain and Ramgiry (2000) who reported that plant height, pods per plant and 100 seed weight had high estimates of heritability. Ghatge and Kadu (1993) and Rasaily et al. (1986) also showed that heritability was high for leaf area, days to 50% flowering, days to maturity, plant height and seed yield. Zhu (1992) revealed that heritability for 100 seed weight and plant height was high.
The results regarding genotypic, phenotypic and environmental coefficients of correlation showed that the genotypic correlations were higher than the phenotypic and environmental ones for most of the characters exhibiting high degrees of genetic association among traits under consideration. The environmental correlation coefficients were not very important in most of the cases indicating low environmental influence in the experiment. Chand (1999) performed experiments on different varieties of soybean and revealed that the genotypic correlation coefficients for all characters studied were higher than the phenotypic and environmental correlation coefficients. Number of pods per plant and leaf area are an important characteristic which finally affects yield of soybean. It is clear that more number of pods per plant and higher leaf area will produce higher yield per plant. The potential of genotypes may be utilized as the oil content is positively and significantly correlated with yield and both characters could be combined in the same genotype. The results are in line with those of Rajanna et al. (2000). Positive association of yield with leaf area, pods per plant and oil contents illustrated that selection on the basis of these traits can increase the yield. The results are in line with those of Manzoor and Kaleri (1971), Khurana and Sandhu (1972), Amaranath et al. (1990), Khanghah and Sohani (1999), Chand (1999) and Rajanna et al. (2000).
Path co-efficient analysis revealed that days to pod initiation exerted highest positive direct effect on grain yield followed by leaf area and days to flowering completion. Therefore it is an important character contributing maximum to yield and emphasis may be given during selection. Harer and Deshmukh (1992) presented similar results and suggested greater emphasis on longer duration during selection. However, the results are contradictory to the findings of Srinives et al. (1986) who carried out experiments on soybean and reported that direct effect of leaflet index on yield was small and negative. The contradiction to the results might be due to the difference in the genetic material.
It is concluded that the germplasm exhibited considerable amount of variability for characters like leaf area, plant height, 50% flowering, days to 50% maturity, plant height, pods/plant, branches/plant, 100 seed weight and grain yield/plant. However, there is a need to collect more germplasm to increase the diversity in terms of chlorophyll contents, oil content and protein content. On the basis of correlation and path analysis, it is clear that selection of genotypes on the basis of leaf area, pods per plant, days to flowering completion and oil content can be helpful to increase the yield.