Variability, Heritability and Genetic Advance in Egyptian Sweet Melon (Cucumis melo var. aegyptiacus L.) Under Water Stress Conditions

INTRODUCTION

Sweet melon (Cucumis melo var. aegyptiacus L.) is a staple and refreshing fruit in Egypt. Fruits are consumed in the summer period and are popular because the pulp of the fruit is very refreshing, high nutritional and sweet with a pleasant aroma (Melo et al., 2000). A large number of local sweet melon genotypes are cultivated in Egypt but no serious attempts have been made to upgrade the productivity and acceptability of this crop within the country. The presence of genetic variation in the breeding material at hand determines the success or failure of any breeding or bio-engineering program. Therefore, the measurement of genetic variation and understanding of mode of inheritance of quantitative traits are essential steps in any crop improvement program.

The most of cultivation of sweet melon is based on local open pollinated varieties which are maintained by farmers, produced for self-consumption and sold on local markets. The commercially important improved cultivars of sweet melon are Kahera-6, Ananas El-Dokki and Shahd El-Dokki. So, developing local sweet melon, based on local genotypes, may result in very promising outputs, especially because the germplasm of sweet melon available in Egypt is having high genetic variability (El-Shimi and Ghoneim, 2006).

Egypt is expected to be seriously affected by the adverse impacts of climate change on water sources and availability and severe water stress conditions will affect crop productivity, particularly that of vegetables (McCarthy and IPCC, 2001; Boutraa, 2010; Xoconostle-Cazares et al., 2010; Ruzana Adibah and Ainuddin, 2011). So, the main goal of sweet melon breeding is improvement of stress tolerance that represents a major goal for the plant breeders and for the agriculture in the future. Agriculture researchers are trying to develop crops that use water more efficiently, i.e., with higher water use efficiency and can perform better for yield even in water limited conditions (Manickavelu et al., 2006; Kusvuran et al., 2010). There is dire need to make and select sweet melon genotypes that have no or only little effect of water stress.

The present study provides information on nature of genetic variability for yield and yield components under normal and drought condition in sweet melon genotypes. The information derived from the study will help in breeding sweet melon for drought tolerance.

MATERIALS AND METHODS

Thirteen sweet melon genotypes (10 local open-pollinated cultivars and 3 commercial cultivars) were used as genetic material for this study (Table 1). Local open-pollinated cultivars were collected from different places in Egypt and single plants from each cultivar were selfed for one generation during the growing season of 2008. The soil texture at the experimental site is clay-loam.

All genotypes were sown and evaluated under irrigated and drought conditions at Qaha vegetable research station, Qalubia governorate on 6 April of 2009 season. Each of two experiments was designed in a randomized complete blocks with three replicates. Each experimental unit area was consisted of four ridges each of 5 m length and 1.5 m in width and one plant per hill with 50 cm apart. The culture practices were done according to the general program of sweet melon cultivation. Drought conditions were started after first irrigation and created by reducing the frequency of irrigation watering by one half to that of irrigated crop, i.e., missing alternate irrigation.

At the harvesting time, a random sample of 12 plants was taken from each experimental unit and data were recorded for number of fruits per plant, fruit weight (g), total yield per plant (g), fruit length (cm), fruit width (cm) and flesh fruit thickness (cm).

Table 1:	The names and sources of sweet melon genotypes used in the study

Estimates of broad sense heritability for different traits were computed using the variance components method based on the combined analyses over both water conditions according to Snedecor and Cochran (1980). The components of variance were computed using the observed mean square values as outlined by Johnson et al. (1955) by using following formulae:

where, σ²g, σ²gw, σ²e and σ²ph are the variances due to genotypes, genotypesxwater levels (GxW) interaction, experimental error and phenotypes, respectively, MS_g, MS_gw and MS_e are the mean squares of genotypes, genotypesxwater levels (GxW) interaction and pooled error, respectively and w denotes the number of environments (i.e., water levels) and r the number of replicates.

Phenotypic (PCV) and genotypic (GCV) coefficient of variation were evaluated according to the methods of Johnson et al. (1955) and Hanson et al. (1956) as follows:

Image for - Variability, Heritability and Genetic Advance in Egyptian Sweet Melon (Cucumis melo var. aegyptiacus L.) Under Water Stress Conditions

where, σ²ph, σ²g and X are the phenotypic variances, genotypic variances and grand mean for each trait, respectively.

Broad sense heritability (h²B) was calculated according to Allard (1999) as the ratio of the genotypic variance (σ²g) to the phenotypic variance (σ²ph).

Expected genetic advance after one generation of selection (GA) and GA as percentage of the mean assuming selection of the superior 5% of the genotypes were estimated according to the formulae given by Johnson et al. (1955) as follows:

where, K is the selection differential (2.06 for selecting 5% of the genotypes).

RESULTS AND DISCUSSION

Analysis of variance: Combined analysis of variance for the studied traits is presented in Table 2. Water levels mean squares were significant for all the studied traits except fruit length. The mean squares for genotypes were highly significant for all the studied traits.

Table 2:	Mean squares from the combined analysis of variance for the studied traits of 13 sweet melon genotypes over two tested water conditions

***Significant at 0.05 and 0.01 probability, respectively, df: Degrees of freedom

Table 3:	Estimates of components of variance, phenotypic coefficient of variation (PCV) and genetic coefficient of variation (GCV), broad sense heritability (h²) and genetic advance (GA) for the studied traits of 13 Egyptian sweet melon genotypes tested in two environments

*σ²ph, σ²g and σ² are phenotypic, genotypic and error variances of genotype means, respectively

This indicates the existence of a high degree of genetic variability in the material to be exploited in breeding program and that also reflected the broad ranges observed for each trait. The water levelsxgenotypes interactions were found to be significant for all the studied traits except fruit length. These results have partial agreement with Kohpayegani and Behbahani (2008) and Naroui Rad et al. (2010).

Genetic variation: The values of genotypic, phenotypic and error variance, genotypic (GCV) and phenotypic (PCV) coefficients of variation, heritability and genetic advance are presented in Table 3. For all the studied traits, the genotypic and phenotypic estimated variance appeared large, in comparison with the estimated values of error variance, such a result seemed to indicate that the number of replicates used in the evaluation experiment of these genotypes were adequate to give a better estimation for the error variance.

High coefficients of phenotypic (PCV) and genotypic (GCV) variation were observed for several characters, the highest being for fruit weight followed by yield per plant and fruit length. The estimates of Phenotypic Coefficient of Variation (PCV) in general, were higher than the estimates of Genotypic Coefficient of Variation (GCV) for all the characters, which suggested that the apparent variation is not only due to the genotypes but also due to the influence of environment. The characters with high phenotypic coefficient of variation indicated more influence of environmental factors. Therefore, caution has to be exercised during the selection program because the environmental variations are unpredictable in nature and may mislead the results. Similar, findings were reported by Rahman et al. (2002) on snake gourd, Rakhi and Rajamony (2005), Taha et al. (2007) and Naroui Rad et al. (2010) on melon, Torkadi et al. (2007) and Tomar et al. (2008) on muskmelon, Pandit et al. (2009) on bottle gourd and Yadav et al. (2009) on cucumber, Dar and Sharma (2011) on tomato, Abd El-Kareem and El-Saidy (2011) on wheat and Degewione et al. (2011) on shallot.

Broad sense heritability estimates among all the traits studied are very high (>90%). High heritability estimates indicate the presence of large number of fixable additive factors and hence these traits may be improved by selection. However, selection should be made very carefully as heritability is measured in broad sense, which may be influent. High heritability does not mean a high genetic advance for a particular quantitative character. Johnson et al. (1955) reported that effectiveness of selection depends not only on heritability but also on genetic advance. Therefore, genetic advance was also computed as percentage of mean. In the present investigation, high heritability associated with high genetic advance was found in the characters like fruit weight and yield per plant. This indicated that these two characters were mostly governed by additive gene action.

Table 4:	Ranges, means, percentage decrease under water stress (D%), standard deviation (SD) and coefficient of variation (CV) for 6 characters for 13 sweet melon genotypes at the two tests water conditions

Breeding methods based on progeny testing and mass selection could be useful in improving these traits (Panse, 1957). High heritability accompanied by low genetic advance for number of fruits per plant is indicative of non-additive gene actions' predominance which could be exploited through heterosis breeding. These results were in harmony with those obtained by Rahman et al. (2002) on snake gourd, Rakhi and Rajamony (2005) and Taha et al. (2007) on melon, Torkadi et al. (2007) and Tomar et al. (2008) on muskmelon, Pandit et al. (2009) on bottle gourd, Yadav et al. (2009) on cucumber and Shiri et al. (2010) on maize.

Ranges, means, standard deviation and coefficient of variation among sweet melon genotypes revealed the presence of genetic variation for all studied traits in this experiment at both studied water conditions (Table 4). Moreover, magnitude of mean performance for all the traits decreased in water stress environment. Mean value of fruit weight, number of fruits per plant, total yield per plant, fruit length, fruit width and flesh fruit thickness decreases 12.9, 12, 22.8, 8.7, 9.4 and 11.1%, respectively. Fruit length was very less affected by water stress, but fruit weight was very high affected by water stress. Possible reason for decrease of mean value of traits can be deficiency of water that slowed the physiological processes. Similar observations were made by Cabello et al. (2009) on melon, who reported that under severe deficit irrigation, the yield was reduced by 22% mainly due to decrease fruit weight.

CONCLUSION

The data in this study showed the possibility of improving Egyptian sweet melon by selection for fruit weight and yield per plant under water stress conditions in Egypt. Based on the high heritability and high genetic advance shown by these characters, it could conclude that the determinant genetic effects of the phenotypic expression of these characters are fundamentally of the additive type. For this reason, a high response should be achievable after several selection cycles.