Agronomical Evaluation of Aerial Yam / Dioscorea bulbifera / Accessions collected from South and Southwest Ethiopia

DOI: 10.15580/GJAS.2013.9.073113767 The experiment was conducted at Jimma Agricultural Research center. The objectives of the study were to estimate the level of diversity within collected accessions based on key agronomic trait(s) that can be used for selection of aerial yam accessions for high root yield and evaluate the accessions based on root yield and other related trait. A total of 47 accessions of aerial yam were tested in randomized complete block design with three replications. Both qualitative and quantitative data collected were subjected to multivariate analysis using principal component (PCA), cluster analysis and analysis of variance to determine the variability of accessions. The results of PCA based on qualitative traits indicated that traits have good contribution to the variability. The two-dimensional plot of the first two PCs showed a separation between accessions in big sized. Cluster analysis based on qualitative characters showed the creation of six distinct groups with different sizes and presence of variability, based on their above and below ground plant parts. The result of Shannon-Weaver diversity index (H’=0.18) showed low levels of diversity between aerial yam accessions. Analysis of variance indicated that tuber length, tuber diameter and vine length have highly significant (P>0.01) differences exists for most of the characters studied. Estimate of phenotypic and genotypic coefficients of variation also showed the presence of variability among the accessions for the a limited number of the characters namely, vine length, vine dry weight, tuber length and tuber diameter. Relatively high phenotypic (53.96, 44.99 and 26.07 %) and genotypic coefficients of variation (12.44, 22.81 and 11.18%) were observed for vine dry weight, tuber dry weight and vine fresh weight in the order of magnitudes. Heritability (39.69% and 25.70%) coupled with genetic advance as percent of mean (12.42% and 23.83%) were recorded for vine length and tuber dry weight per plot, respectively. Path coefficient analysis revealed that that vine dry weight (p =1.203) and leaf length (p =1.1949) was more important than other traits, hence can be used as a criterion for indirect selection. Therefore, selections based on vine dry weight and leaf length are vital to increase the yield and the genetic improvement of this crop. The overall results of this study showed that aerial yam accessions collected from south and southwestern Ethiopia have an enormous wealth of traits variation, indicating huge potential for its genetic improvements through selection and hybridization. Submitted: 31/07/2013 Accepted: 22/09/2013 Published: 29/09/2013


INTRODUCTION
Yam belongs to the genus Dioscorea in the family Dioscoreaceae.The family is believed to be among the earliest angiosperms and probably originated in Southeast Asia (Coursey, 1967;Jayasurya, 1984;Wilkin, 1998;Muluneh, 2006).There are eight economically most important species of yams that are cultivated as staples throughout the tropics (Coursey, 1967;Hahn, 1993;Muluneh, 2006).
Aerial yam (Dioscorea bulbifera) is one of the economically most important specie of yam; it is distinguished from all other species by having particular bulbils on the base of leaves petioles (Marthin, 1974;Tewodros, 2008).To such an extent that tuberization is solely aerial.In Ethiopia, aerial yam has been cultivated mainly and extensively in dense populated and high rainfall areas.In South, Southwest and Western parts of the country, a large pool of aerial yam germplasm is found in farmers' field and forest areas.However, the existence of different vernacular names for the same cultivar of the species, or vice versa has created problems to classify accessions while avoiding duplicates.Therefore, the need for collection, evaluation and characterization of the accessions is thus important for crop improvement and to reduce genetic erosion of this crop (Tewodros, 2012).Moreover, there is a need to broaden the knowledge base of the crop through studies on diversity and use of the available landraces.This includes detailed analysis of the extent, distribution and diversity of aerial yam, good understanding of farmers' perception and management of local accession.
In Ethiopia, some aerial yam collection work has been started.However, Dioscorea bulbifera is distributed across diverse agro-geographical areas of the country that have not been adequately represented (Muluneh, 2006).Moreover, the collected accessions have not been properly evaluated and their attribute remains unknown by breeders.
Consequently, detailed descriptions of accessions based on morphoagronomical characters have tremendous impact on the conservation and genetic improvement of the crop.Previous to this study, information on these aspects was very little to plan an effective breeding program.The present study, therefore, intended to assess the nature and extent of genetic diversity of aerial yam in Ethiopia.

Description of the Study Area
The experiment was conducted at Jimma Agricultural Research Center located at 366 km south west of Addis Ababa and located at 1753 meters above sea level at 7 o 46' N latitude and 36 o E' longitude.It receives average rainfall of 1432 mm annually with average maximum temperature of 29.2 0 C, and minimum of 8.90 0 C. The soil type is Eutric Nitosole with a pH of 5.3.

Experimental Materials and Design
Forty-seven accessions of aerial yam were considered in this study.The accessions were collected from south and southwestern parts of Ethiopia.All accessions were grown randomized complete block design with three replications.Single row plots, with each row 6 meter long were used in the experiment.The spacing between plants and rows was 1.5m and 1m respectively.The middle four plants of the row were used for data collection and for harvesting.

Morphological Data collection
Description of yam (Dioscorea Spp.) developed by International Plant Genetic Resource Institute (IPGRI, 1997) was followed by data collection.There were two sets of data (qualitative and quantitative) collected for assessing the diversity and to find key morphological traits in yam cultivars.In regard to the qualitative traits, only the first replication of the experiment was considered, whereas, for quantitative characters the entire replications were considered.Among the descriptor developed by IPGRI (1997) to characterize yam cultivars, 17 qualitative and 15 quantitative traits were used, most of which were distinguished as highly heritable traits and were used for analysis.Both foliar and subterranean data were considered.Traits from above ground plant parts were scored at seven months after planting, while underground traits were evaluated at harvest (9 months).Most of the data (qualitative and quantitative traits) were recorded on individual plant basis using sample averages of four plants selected at random from the row.

Statistical Analysis
Data of quantitative characters were subjected to analysis of variance (ANOVA) as suggested by Gomez and Gomez (1984).The multivariate analysis (principal component and cluster analysis) was performed Crossa, 1990 andMahalanobis's D2 statistics.Afifi (Malanobis 1936).
The analysis of variance for all traits were carried out using SAS version 8.2 (SAS, 2008) statistical computer packages to examine the presence of statistically significant differences among genotypes for these characters.Least Significant Difference (LSD) was employed to identify genotypes that are significantly different from each other.The analysis of Variance (ANOVA) (Table 2) was made using model for randomized complete block design.
Where, µ=the overall mean, τi=the effect of the i th treatment, βj=the effect of the j th block, and εij=the error effects.The genotypes are considered as random effect.
Phenotypic and genotypic coefficients of variation were computed by Burton and Dewane (1953) considering genotypes as random effects using SAS statistical packages (SAS 2008).Genotypic variance component σ 2 g = MS g -MSe )/r.Where MS g is genotypic mean square, MSe is error mean square and r is replication Phenotypic variance component was calculated using the formula: σ 2 p = σ 2 g +σ 2 e Where σ 2 p = Phenotypic variance; σ 2 g = Genotypic variance and σ 2 e = Environmental variance component (On genotypic mean basis σ 2 e = MSe/r Genotypic and phenotypic coefficients of variation were calculated according to the method suggested by Burton and Dewane and Lu (1959) as: Genotypic coefficients of variation (GCV) GCV= √σ 2 g * 100 X - Phenotypic coefficients of variation (PCV) Where X -is the grand mean value of the trait Broad sense heritability ( h 2 ) in percents were estimated in each character using variance components as described by Allard (1960).
The expected gain or genetic advance with one cycle of selection, assuming the selection intensity of 5%, was predicted as suggested by Johnson et al., (1955a).
Genetic advance in percent of the mean (GAM) was calculated to compare the extent of predicted genetic advance of different traits under selection, using the following formula: Covariance analysis was carried out in the same way in that of analysis of variance, and the mean cross produce was equated with the expected mean square product.
Covariance components were used to compute genotypic and phenotypic correlation coefficients.
Genotypic covariance of traits σ 2 g xy = MSCPgxy -MSCPexy r Where, MSCPgxy is genotypic mean cross product of traits x and y.MSCPexy, is error mean cross product of traits x and y.
Phenotypic covariance σ 2 p xy = σ 2 g xy + σ 2 gexy r Genotypic and phenotypic correlation coefficients of fresh tuber yield and its components were estimated calculating the variance and covariance at phenotypic and genotypic level by using the formula suggested by Singh and Chaudhury (1985).
Phenotypic correlation, the observable correlation between two variables, which includes both genotype and environmental components between two variables, was estimated using the formula suggested by Miller et al., (1958).
Genotypic correlation between traits x and y was computed as Where, σ 2 gx and σ 2 px are genotypic and phenotypic variance components of trait x.The coefficient correlation at phenotypic level was tested for their significance using the t-test as: t= rpxy√g-2 /√ (1-r 2 pxy) The calculated 't' value was compared with tabulated 't' at n-2 degree of freedom, where n is the number of characters.The correlation coefficients at genotypic level were tested with the following formula suggested by Robertson (1959).

Path Coefficient Analysis
The direct and indirect effect of yield related traits on yield per plot were worked out through path coefficient analysis.The analysis was made following the method suggested by Dewey and Lu (1959).The formula was as follows.
Where: -rij = Mutual association between the independent character (i) and dependent Character (j) as measured by the correlation coefficient.Pij = Component of direct effects of the independent character (i) on dependent character (j) as measured by the path coefficient and, ∑rikpkj = Summation of components on indirect effect of a given independent character (i) on the given dependent character (j) via all other independent character k).
Residual effect will be estimated by the formula

RESULT AND DISCUSSION
Variability of aerial yam based on qualitative characters using cluster and principal components analysis.Cluster analysis based on qualitative traits classified accessions using distance between cluster and UPGMA clustering which gave six major clusters, with distance between cluster ranging from 0.0-2.0.The number of accessions belonging to each cluster varied from one in clusters V and VI to 26 in cluster I (Figure 1).Cluster I was the largest and consisted of 26 accessions (53.31%), 13 from Jimma, five from Gamo-Gofa, four from Kefa, two from Dawro and one accession each from Illubabor and Bench-maji collections (Figure 1).Accessions grouped under this cluster have predominantly dark green leaf, green vein on upper and lower surface, green petiole and petiole wing, dark brown bulbils skin, round bulbils, outer purple and inner yellowish tuber flesh, dark brown tuber skin, oval-oblong tuber, cordate leaf, green leaf margin, yellow green vine, a long acumen leaf apex, light green petiole wing, smooth bulbils surface and medium maturity.Likewise, Cluster II which comprised 15 accessions (31.91%), eight from Jimma, two each from Bench-maji and Kefa and one each from Dawro, Wolaita and Gamo-Gofa collections.Accessions falling into this cluster were characterized by pale green leaf, green vein upper and lower surface, brown bulbils skin, round and oval bulbils, outer purple and inner white tuber flesh, cordate leaf, yellow green vine, dark brown tuber skin, oval oblong tuber, a long acumen leaf apex, green petiole and petiole wing, green leaf margin, light green tip, smooth bulbils surface and with medium maturity.In addition, two accessions (4.25%) were grouped under cluster III, one each from Gamo-Gofa and Illubabor collections.They typically possess yellowish and dark green leaf, yellowish and green leaf vein on upper surface, light brown bulbils skin, light yellowish leaf vein on lower surface, oval and elongated bulbils, purple with white tuber flesh, light and dark brown tuber skin, oval-oblong tuber, smooth and rough bulbils surface and are early and late maturing group.
Two accessions (4.25%) categorized under cluster IV one from Gamo-Gofa and the other Jimma collections have predominantly yellowish leaf, yellow leaf vein on upper surface, irregular bulbils, dark brown bulbils skin, a long acumen leaf apex, cordate leaf, purple with white tuber flesh, light and dark brown tuber skin, oval-oblong tuber, green petiole, petiole wing and tip, smooth and rough bulbils surface, yellow vine, light yellowish leaf vein on lower surface, and are in early and late maturing group.Finally, two accessions were (4.25%) grouped each under cluster V and VI, all from south Ethiopia, and typically possess pale green, yellowish and dark green leaves, green and yellow green vein on upper and lower surface, light brown bulbils skin, round and oval, oval and elongated bulbils, outer purple inner white and purple with white tuber flesh, cordate leaf, light and dark brown tuber skin, green petiole and leaf margin, smooth bulbils surface, oval-oblong tuber and are medium, early and late maturing groups.Accessions used for this study were collected from diverse agro-ecological areas of south and south western part of the country.Nevertheless, accessions collected from the same or different areas fall into different clusters, showing that the accessions collected from the same or different areas of the country might have different genetic make-up.The results of principal component analysis based on 17 qualitative traits also revealed the existence of diversity among the aerial yam accessions used in this study.
Ordination among accessions showed that the first five principal components (PCs) had Eigen values greater than one and cumulatively accounted for 80.6% of variation (Table 1).The first component alone explained 37.8% of the total variation and was mainly associated with characters such as leaf color (LC), Leaf margin color (LMC), Bulbils shape (BS) and Bulbils skin color (BSC).The second principal components (PCs) explained 16.8% of variation and were associated with petiole color (PC), Tuber skin color (TSC) and Vine color (VC).The third principal components (PCS) explained 9.8% of the variation and were associated with leaf vein upper surface (LVUS), leaf vein lower surface (LVLS) and maturity (Ma).The fourth component was mainly associated with bulbils surface texture (BST) and tuber shape (TS) and had 9.3% of the variation.The other principal components (PCs) covered 26.3% of the variation.
To assess the score of individual accessions PC1 and PC2 were plated in (Figure 3).The total 34 (72.34 %) of accessions are occupied on the bottom part of the first quadrant of the plot indicating that most of accession scores were above their mean performance for both PC1 and PC2.Most of accessions in this study fall in the bottom of first quadrant of the plot and have high loadings on the first and second principal components (Table 1).On the other hand, 13 (27.66%) of accessions occupied the top and lower position of the plot indicating relatively lower score for PC1 and relatively higher score for PC2.In this study there was no clear grouping of the accessions according to geographical regions of collection.Accessions of one region were grouped into different clusters; although some of them also belonged to the same cluster.Therefore, it may be of considerable importance to enlarge the genetic base of aerial yam by sustainable and continual collection of accessions throughout the growing areas of the country, conduct genetic improvement work to exploit desirable traits of the accessions.

Frequency distribution of major qualitative traits
Vegetative characters: Variation among aerial yam accessions based on vegetative characters was observed.In this study, the predominant colors are dark green (40.43%), yellowish (31.91%) and pale green (27.66%).The color of upper surface leaf vein was yellowish (14.89%) and green (85.11%).The color of leaf vein lower surface was varied from yellow (12.76%), green (76.59%), and yellow green (10.65%).Leaves of the accessions are simple with alternate arrangement and long petioles.The margins are not serrated; they are cordate in shape (Table 2). .

Tuber characteristics:
The tuber skin color of aerial yam varied from light brown (36.17%) to dark brown (63.83%).The flesh tuber color varied from accession to accession as purple with white (27.66%), white with purple (12.77%) and outer purple with inner white (59.77%).The shape of the tuber was also varied as round (10.64%), oval (8.51%) and oval-oblong (80.85%) shape.Similarly, it is in agreement with the works of Muluneh et al., (2006) who reported that there is a wide range of variability of tubers among Dioscorea species in Ethiopia.Furthermore, similar result was reported by Asfaw (2006)

Analysis of Variance
The analysis of variance of quantitative characters showed significant difference (P < 0.01) among the accessions for three of the 15 characters examined indicating the existence of considerable amount of variability for the characters (Table 3).The characters that manifested significant difference are vine length, tuber length and diameter.Non-significant difference was observed among the accessions for leaf length, leaf width, vine fresh weight, vine dry weight, number of bulbils/plant, bulbils fresh and dry weight, bulbils length and diameter, petiole length, tuber fresh and dry weight revealed that the contribution of these characters to the variability was low.

Genetic variability, estimation of heritability and genetic advance
In this study, the highest phenotypic and genotypic variances were observed for number of bulbils/plot, leaf length and width (cm).The higher fraction of phenotypic variance on these characters was contributed from the genotypic and environmental variances; so, detailed descriptions of accessions based on genetic variance have tremendous impact on the genetic improvement of the crop.Moreover, the value of phenotypic and genotypic variance can be used for comparing degrees of variability while different traits have different means.For this reason genotypic and phenotypic coefficient of variations were used for this study.

Table 3. Analysis of variance for 15 quantitative characters of Dioscorea bulbifera accessions grown at Jimma
The estimation of phenotypic coefficients of variation (PCV) ranged from 7.78% for bulbils length to 53.96% for vine dry weight, whereas genotypic coefficients of variation (GCV) ranged from 4.08% for leaf width to 22.81% for tuber dry weight (Table 4).This indicates that bulbils length and leaf width had the least PCV and GCV; and vine dry weight and tuber dry weight had the highest PCV and GCV and suggesting that environmental factors have high contribution to the observed variation among aerial yam accessions.High GCV values on tuber dry weight indicates the existence of genetic variation for such characters.Therefore, selection based on vine dry weight and tuber yield is effective.Similar results were observed by Baye et al. (2005), Asfaw et al. (2006) on potato and taro in respect to number of tubers and corm per hill.In general, all accessions that are used in this study were phenotypically and genotypically diverse.This indicates the existence of large diversity in aerial yam for quantitative characters.Heritability estimates ranged from 10.36 for bulbils fresh weight to 53.14 for vine dry weight (Table 4).Maximum heritability was obtained from vine dry weight per plot followed by tuber diameter and tuber length.On the other hand, bulbils fresh weight, vine fresh weight and number of bulbils per plot have relatively low heritability estimates (Table 4).Moderate heritability estimates were observed for leaf width and tuber dry weight.Heritability indicates the ease with which a trait can be improved through selection and could vary with materials studied and the environment.The values of genetic advance for different characters of aerial yam accessions were different.These values are also expressed as percentage of the accession mean for each character so that comparisons could be made among various characters, which had different units of measurement.The result indicated that the progress that could be expected from selection of accessions ranged from 5.20% for bulbils length to 23.83% for tuber dry weight (Table 4).Moderately higher heritability coupled with high genetic advance and genetic advance in percentage of means was exhibited for vine length, tuber length and diameter.These three traits could, therefore, be improved more easily than the rest of the characters and will result in the selection and improvement of aerial yam.

Path coefficient analysis
The result of path coefficient analysis showed that vine dry weight and leaf length had maximum direct positive effect on bulbils fresh weight.Vine fresh weight also had positive indirect effect on bulbils fresh weight through most of the traits where the indirect effects of vine fresh weight are negative (Table 5).The negative direct effect of vine fresh weight on bulbils fresh weight may be explained by the fact that high vine fresh weight may contain a large percentage of water.Tuber yield

CONCLUSION
This study was initiated with the objectives of studying the genetic diversity and identifying key morphological traits to characterize aerial yam accessions grown in Jimma, southwest Ethiopia with the ultimate goal of bringing the crop to the attention of researchers and policy makers through broadening the knowledge base of the crop.Towards this effort, multivariate analysis (principal component and cluster analysis) was used based on 17 qualitative traits and analysis of variance (ANOVA) using the 15 quantitative traits.
The analysis of variance for quantitative traits showed highly significance (p>0.01)differences among accessions for the number of characters indicating the presence of genetic variability among aerial yam accessions.Heritability estimates ranged from 10.36 for bulbils fresh weight to 53.14 for vine dry weight.Relatively higher heritability coupled with high genetic advance and genetic advance in percentage of means was exhibited for tuber dry weight, vine length and tuber diameter reflecting the chance that these traits are the basis for selection and further improvement work in aerial yam.Path coefficient analysis at genotypic level showed that vine dry weight had maximum direct positive effect on bulbils fresh weight (p = 1.2023) followed by leaf length (p = 1.1949).This seems to suggest that these characters are good contribution to the fresh bulbils yield of aerial yam.The residual effect (h=0.28) indicated that the characters considered in this analysis have successfully explained the variation existing in aerial yam accessions.Therefore, the characters identified in the present analysis are the most important for breeders, so that selection based on these characters are efficient to maximize the fresh bulbils yield of aerial yam.

Figure 1 .
Figure 1.Dendrogram showing relationship of 47 aerial yam accessions based on cluster distance and UPGMA clustering using 17 qualitative traits Figure 3. Groups formed from the first (PCI) principal components for 47 aerial yam accessions collected from south and southwest Ethiopia by using 17 qualitative traits

Table 5 . Genotypic direct (bold and underlined) and indirect effects of some characters on bulbils fresh weight of aerial yam h=0
Rubaihayo et al. (2001) competitive effect with bulbils fresh weight at path coefficient analysis level.With the same analogy,Pandy et al. (2005)andRubaihayo et al. (2001)also reported similar direct positive effect on number of tubers/roots per plant on potato and cassava respectively.The residual effect (0.280) is relatively low showing that the characters considered in this analysis fruitfully explained variation in aerial yam accession.Consequently, selection based on these traits is efficient to improve tuber fresh yield and further breeding program of this crop.