Genetic Variability for Yield and Other Agronomic Traits in Sweet Potato
Engida Tsegaye ,
E.V. Devakara Sastry
Understanding the nature and magnitude of variability among sweet potato (Ipomoea batatas (L) Lam.) genotypes for traits of economic importance is vital to plan effective breeding programs. A replicated field experiment was carried out using thirty sweet potato genotypes selected at random from the germplasm collection of diverse sources. The specific purpose was to estimate the nature and magnitude of variability among yield and yield related traits in the crop plant. Observations were recorded on sixteen characters. The analysis of variance revealed that there were highly significant (p<0.01) differences among the genotypes for all the characters. Wide ranges of genotypic variability were observed among the genotypes for most of the characters. In vine traits, 32.4 to 82.5% of the observable variability was due to the genetic variation while in root traits this value ranged from 43.04 to 76.81% indicating the existence of immense inherent variability among the genotypes. The contributions of genetic variance to phenotypic variance were only 32.4 and 43.04%, for above ground dry matter content and storage root dry yield per plant, respectively suggesting the important role of environment in the expression of these traits. High genotypic coefficients of variation along with high heritability and expected genetic advances were recorded for vine length, vine internode length, leaf area, above ground fresh and dry weights, number of storage root per plant, individual storage root weight, storage root fresh yield per plant. Thus, future improvement of the crop plant should exploit the genetic variability available in the germplasm collection.
Sweet potato (Ipomoea batatas (L.) Lam.) is a dicotyledonous plant belonging to the family Convolvulaceae. This family includes about 55 genera and more than 1000 species (Watson and Dallwitz, 2000). However, only Ipomoea batatas is of economic importance as a source of food (Onwueme and Charles, 1994).
The amount of variability that exists in the germplasm collections of any crop is of utmost importance towards breeding for better varieties. Particularly, genetic variability for a given character is a basic prerequisite for its improvement by systematic breeding.
Sweet potato is a highly heterozygous and cross-pollinated crop in which many of the traits show continuous variation. Since it is highly heterozygous, there is extensive variability within the species, which is available for exploitation by plant breeders (Jones et al., 1986). Hence, consideration of quantitative approaches for exploitation of the extensive genetic variability available in sweet potato is of paramount importance, which in turn is dependent on good estimates of the genetic parameters. Estimates of genetic parameters serve as a base for selection and hybridization as the degree of variability for a given character is a basic prerequisite for its improvement. Although several sweet potato germplasm accessions have been introduced to Ethiopia from International Potato Center (CIP), International Institute for Tropical Agriculture (IITA), Asian Vegetable Research and Development Center (AVRDC) and a considerable amount have been collected from farmers field for evaluation and utilization, information on the nature and extent of variability among these collections for traits of economic importance is lacking. Thus, this study was undertaken to estimate the nature and magnitude of variability for yield and yield related characters with the help of genetic parameters as such as phenotypic as well as genotypic coefficients of variation, heritability and genetic advance.
MATERIALS AND METHODS
Experimental site: The study was carried out at Awassa Agricultural
Research Center, Ethiopia, during 2003/04 growing season under rainfed condition.
Awassa lies at 7° 04' N and 38° 31' E, at an elevation of 1700 masl.
The average annual rainfall of Awassa is 1033.6 mm with a minimum/maximum mean
air temperature of 13.2/27.4°C. The soil is volcanic in origin and is classified
as Vitric Andosol. The textural class is a well-drained sandy loam with a pH
of 7 and CEC 22.6 meq per 100 g.
Experimental materials, design and management: Thirty sweet potato genotypes randomly taken from the germplasm collection of materials introduced from CIP, IITA and AVRDC over different years and maintained at Awassa Agricultural Research Center were used for this study. The experiment was arranged in a randomized complete block design with four replications. Each genotype was planted on 3 m long and 2.4 m wide plot consisting of four rows, which accommodated ten plants per row and thus forty plants per plot. A distance of 1m was maintained between the plots. Vine cuttings from the top portion of 3-4 months old mother plants were taken for planting. The vine cuttings were then cut into 30 cm length and thereafter planting was done on 29th July 2003 with a spacing of 60 cm between rows and 30 cm between plants. Earthing up was done twice, 45 and 75 days after planting. Fertilizers were not applied during the course of the experiment. During the course of this experiment, no serious disease or insect pest infestations were noticed and thus crop protection measures were not employed.
Data collection: For each character under study, data were recorded on five randomly taken plants from the middle two rows of each plot and expressed on per plant basis. The mean of five plants was used for statistical analyses. Total storage root yield per plot was recorded as weight of storage roots harvested from the middle two rows of each experimental plot. The following sixteen characters were measured during the course of this study: Vine length (cm), vine internode length (cm), vine internode diameter (mm), leaf area (cm2), above ground fresh weight per plant (g), above ground dry matter content (%), above ground dry weight per plant (g), storage root number per plant, storage root length (cm), storage root diameter (cm), individual storage root weight (g), harvest index per plant (on dry weight basis), storage root fresh yield per plant (g) storage root dry matter content (%), storage root dry yield per plant (g) and storage root fresh yield per plot (kg).
Statistical analysis: The mean values of each character under the study
were computed and subjected to analysis of variance following the procedures
described by Gomez and Gomez (1984) using MSTAT computer software. Phenotypic
and genotypic variances were calculated by the methods suggested by Burton and
σp2 = σ2g
+ σ2e, σ2g = MSg-MSe/r
||Environmental variance (error mean square).
The genotypic (GCV) and phenotypic (PCV) coefficients of variation were estimated
according to the procedure outlined by Johnson et al. (1955):
where, = grand mean . Broad
sense heritability and the genetic advance expected under selection, assuming
the selection intensity of 5% were calculated as suggested by Allard (1960):
||Heritability in broad sense (in percentage).
||Expected genetic advance,
||Selection differential (2.06 at 5% selection intensity),
||Phenotypic standard deviation.
Genetic advance as percent of mean (GAM) was calculated using the following
RESULTS AND DISCUSSION
The analysis of variance showed that there were highly significant (p<0.01) differences among the genotypes for all the characters (Table 1). This may be attributed to the existence of large variability among genotypes maintained at Awassa Agricultural Research Center, the germplasm collected from diverse collections at CIP, IITA and AVRDC.
The estimates of genotypic and phenotypic variances along with the phenotypic
(PCV), genotypic coefficients of variation (GCV), broad sense heritability,
genetic advance and genetic advance as percent of the mean for sixteen characters
in thirty sweet potato genotypes are presented in Table 2.
|| Analysis of variances for 16 characters in 30 sweet potato
genotypes grown at Awassa, 2003
|Msr = replication mean square MSg= mean
square due to genotypes, Mse = Error mean square, ¥Figures
in parenthesis indicate the degree of freedom, ** * Significant at
1% and 5% probability levels, respectively
||Estimates of components of variance, broad sense heritability,
GA and GA as percent of the mean for 16 characters in sweet potato genotypes
evaluated at Awassa, 2003
|VL = Vine Length (cm), VIL = Vine Internode Length (cm), VID
= Vine Internode Diameter (mm), LA = Leaf Area (cm2) AGFW = Aboveground
Fresh Weight Plant-1 (g), AGDW = Aboveground Dry Weight Plant-1
(g), AGDMC = Aboveground Dry Matter Content (%), SRN = Storage Root Number
Plant-1, SRL = Storage Root Length (cm), SRD = Storage Root Diameter
(cm), ISRW = Individual Storage Root Weight (g), HI = Harvest Index Plant-1
(%), SRFY = Storage Root Fresh Yield Plant-1 (g), SRDMC = Storage
Root Dry Mater Content (%) SRDY = Storage Root Dry Yield Plant-1
(g), SRFYP = Storage Root Fresh Yield Plot-1 (kg) σ2g
= genotypic variance, σp2 = phenotypic
variance, GCV = Genotypic Coefficient of Variation, PCV = Phenotypic Coefficient
of Variation and GA = genetic advance
The results revealed considerable phenotypic and genotypic variances among
the genotypes for the traits under consideration. In all the traits a large
portion of the phenotypic variance was accounted for by the genetic component
except for above ground dry mater content and storage root dry yield per plant
in which the contributions of genetic variance to phenotypic variance were only
32.4 and 43.04%, respectively. Generally, in vine traits 32.4 to 82.5% of the
observable variability was due to the genetic variation while in root traits
this value ranged from 43.04% for storage root dry yield per plant to 76.81%
for individual storage root weight. This is an indication for the existence
of immense inherent variability that remains unaltered by environmental conditions
among the genotypes, which in turn is more useful for exploitation in selection
and hybridization programs. These results are in conformity with the results
obtained by Jones et al. (1969) who investigated considerable phenotypic
variance among ten storage root traits in sweet potato and in every case a large
part of this variance was accounted for by the genotypic variance.
Both phenotypic and genotypic coefficients of variation were highest for above
ground dry weight per plant followed by above ground fresh weight per plant
and vine length. In addition to these three traits, six other traits had PCV
and GCV higher than 20% (Table 2) and thereby indicating high
variability and great scope for improvement. These findings are in agreement
with the works of Alam et al. (1998) and Hossain et al. (2000)
who investigated high GCV and PCV for vine length, number of storage root per
plant, individual root weight and storage root fresh yield per plant. The GCV
was relatively moderate for storage root dry yield per plant, harvest index,
storage root dry matter content, vine internode diameter, storage root length,
storage root diameter and low for above ground dry mater content. While the
PCV was moderate for vine internode diameter, storage root length, storage root
dry mater content, harvest index and low for above ground dry mater content.
For all characters under the study PCV values were greater than GCV values. However, in most cases the two values differed in narrow indicating that the variability due to the genetic constitution of the genotypes was more than the variability exerted by environmental factors. Hence, selection for desirable traits on these highly variable characters would be effective in sweet potato improvement programs. The moderate to high value of genotypic coefficients of variation for all characters except above ground dry matter content indicated that most of these traits might be further improved through clonal selection.
Estimates of heritability in broad sense: Although the genotypic coefficient of variation revealed the extent of genetic variability present in the genotypes for various traits, it does not provide full scope to assess the variation that is heritable. As stated by Burton (1952) the genotypic coefficient of variation along with heritability estimates provide reliable estimate of the amount of genetic advance to be expected through phenotypic selection. The results of the present study indicated that except for above ground dry matter content, moderate to high heritability values were recorded for all characters (Table 1). The heritability estimates were higher for vine length, above ground dry weight, individual storage root weight, above ground fresh weight, harvest index, storage root fresh yield per plant, storage root number, vine internode length, leaf area, vine internode diameter, storage root dry matter content and storage root fresh yield per plot indicating the lesser influence of environment on them. These results were in agreement with the findings of Jones (1969) who reported high heritability for vine length, vine internode length and vine internode diameter; Mok et al. (1997) for number of storage roots per plant, storage root weight per plant, storage root dry matter content, root size and harvest index; Kamalam (1977) for foliage weight and individual storage root weight. Hence, satisfactory, progress can be achieved by selection on these characters. Moderate heritability estimates were also observed in storage root length, storage root diameter and storage root dry yield. The heritability of above ground dry matter content was the least suggesting that this trait was greatly influenced by environmental factors. As yield is a complex character liable to have more environmental influences, compared to all other traits except above ground dry matter content, the heritability of storage root dry yield per plant (43.04%) was lower. Nevertheless, Jones et al. (1986) suggested that in sweet potato, a heritability estimates above 60% are quite adequate for good selection advance and estimates as low as 40% by variance-covariance analysis could be considered favorable provided that the selection techniques have enough precision. Thus, although the heritability value recorded on storage root dry yield per se was the second from the least, it can be considered as favorable to achieve satisfactory progress by selection.
Estimates of expected genetic advance: Even though heritability estimates provide the basis for selection on the phenotypic performance, the estimates of heritability and genetic advance should always be considered simultaneously as high heritability will not always be associated with high genetic advance (Johnson et al., 1955). The estimates of genetic advance help in understanding the type of gene action involved in the expression of various polygenic characters. High values of genetic advance are indicative of additive gene action whereas low values are indicative of non-additive gene action (Singh and Narayanan, 1993). Thus the heritability estimates will be reliable if accompanied by a high genetic advance. The expected genetic advance values for 16 characters of sweet potato genotypes evaluated is presented in Table 2. These values are also expressed as percentage of the genotypes mean for each character so that comparison could be made among various characters, which had different units of measurement. Progress that could be expected from selecting the top 5% of the genotypes ranged from 6.21% for above ground dry matter content to 76.98% for above ground dry weight (Table 2). For all characters except for above ground dry matter content the genetic advance values were considered moderate to high. Comparatively, the highest genetic advance as per cent of the mean was recorded for above ground dry weight followed by above ground fresh weight and vine length. This indicates that selecting the top 5% of the genotypes could make an advance of 76.98% in above ground dry weight, 73.06% in above ground fresh weight and 62.63% in vine length (Table 2).
High heritability along with high genetic advance is an important factor for
predicting the resultant effect for selecting the best individuals. In the present
investigation, high heritability along with high genetic advance as per cent
of the mean were obtained for above ground fresh as well as dry weight, vine
length, individual storage root weight, storage root fresh yield per plot, storage
root number, vine internode length, leaf area and storage root fresh yield per
plant. This findings is in agreement with that of Hossain et al. (2000)
and Choudhary et al. (1999) who reported high estimates of both heritability
and genetic advance for number of roots per plant and storage root fresh yield
per plant. Kamalam (1977) and Hossain et al. (2000) also observed high
heritability coupled with high genetic advance for individual storage root weight
whereas Alam et al. (1998) obtained similar result for vine length. As
stated by Panes (1957), high heritability associated with equally high genetic
advance is chiefly due to additive gene effect but if the heritability is mainly
due to dominance and epistasis, the genetic gain would be low. Hence, selection
for these characters would prove quite effective since the characters seemed
to be governed by additive genes action.
On the other hand, characters such as vine internode diameter, storage root dry matter content and harvest index exhibited relatively moderate genetic advance. However, their heritability estimates were high. The presence of high heritability and moderate genetic advance is the effects of equal contribution of additive and non-additive gene action (Shelby, 2000). Storage root length, storage root diameter, storage root dry yield per plant also showed relatively moderate genetic advance along with moderate heritability.
High GCV along with high heritability and genetic advance will provide better information than a single parameter alone (Sahao et al., 1990). Hence, in this study above ground fresh as well as dry weight, vine length, individual storage root weight, storage root fresh yield per plot, vine internode length, storage root fresh yield per plant, leaf area and storage root number exhibited high genotypic coefficient of variation, high heritability together with high genetic advance indicating that these characters would be very useful as a base for selection in sweet potato improvement programs. Alam et al. (1998) and Hossain et al. (2000) also found high genotypic coefficients of variation coupled with high heritability and genetic advance for the traits vine length, number of roots per plant and individual root weight. Thus, from the present investigations it is concluded that future improvement of the crop plant should exploit the genetic variability available in the germplasm collection at Awassa Agricultural Research Center from which the test genotypes were sampled at random.
This study was financially supported by the Agricultural Research and Training Project of Ethiopian Agricultural Research organization and South Agricultural Research Institute. The first author is grateful to Alemaya University for facilitating his M.Sc. study at the School of Graduate studies. This manuscript is part of the thesis submitted to Alemaya University in partial fulfillment of the requirements for M.Sc. degree in Horticulture.
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