Bulb Fresh Weight Mode of Inheritance in Onion ( Allium cepa L . )

One of the most important traits of onion bulb is its bulb fresh weight. Also, this trait is in a group of morphological markers that, together with RAPD, represent the parameter of the most precise identification of onion genotype. For the purpose of this study, the chosen genotypes were: Makoi bronzi, Piroska, AC 101, Jasenicki crveni, Bukino beo. Also, they were of a different geographical origin. Method of full diallel without reciprocals was applied in order to obtain F1 and F2 generation. Field trial with parents and hybrids F1 and F2 generation was set in a random block system with five replications at the Institute for Vegetable Crops, Smederevska Palanka, Serbia. Considering all crossing combinations, superdomination and domination are the modes of inheritance for bulb fresh weight. The best general combiner was the line Makoi bronzi. The highest level of SCA in F1 and F2 generation was found in hybrid combination Makoi bronzi x Piroska. Also, this hybrid combination had the highest values of heterosis.


Introduction
Onion bulb is the most important part of the onion, from the market point of view, so the breeders of this crop have to be focused on its high quality.One of the most important traits of onion bulb is its weight.Also, this trait is in a group of morphological markers that, together with RAPD, represent the parameter of the most precise identification of onion genotype (Pavlović et al. 2012).
Weight of onion bulb implicates the size of the bulb.This trait along with the realized set per unit area, directly influences total yield.The size of the bulb is a variety trait influenced by the way of growth, edaphic and ecologic conditions (Pavlović et al. 2002).
Determining the mode of inheritance is of extreme importance for breeders (Ortiz et al. 2001).By determining the combining abilities the choice of parental lines for obtaining good commercial onion varieties and hybrids is narrowed.Evaluation of parental lines is the starting point for its further application in selection.The success of the selection depends on the gene determination of traits chosen as the aim of selection.Breeders can expect the positive answer in breeding with traits with high heterosis, which is important for the success in creating new varieties (Haydar et al. 2007, Zdravković et al. 2010, Singh et al. 2010, Pavlović et al. 2011).

Material and Methods
For the purposes of this study the parental lines were chosen from the Institute for Vegetable Crops onion germplasm collection.Genotypes were divergent, which was determined in previous studies (Pavlović 1999).Method of diallel cross was applied.Genotypes Makoi bronzi, Piroska, AC101, Jasenicki crveni, Bukino beo all of a different geographical origin were the subject of this study.
Field trial with parents and F1 and F2 hybrids was set in a random block system with five replications with 30 plants per replica.Sowing was done in containers ( 103 The mode of inheritance of the bulb fresh weight was evaluated by testing significance of mean values of F1 hybrids and F2 generation comparing to original scientific paper/originalni naučni članak parental average according to Borojević (1986).Segregation of the genetic variance was performed by applying the method of Hayman's (1954) and Mather and Jinks (1971), and the analysis of combining abilities was performed according to Griffing (1956), method 2, mathematic model 1, which besides parental lines includes F1 and F2 generation.

Results and Discussion
The results in Table 1 show that the average bulb fresh weight of parental lines was from 50.80 g (Jasenicki crveni) to 88.76 g (AC 101).Analysis of the average weight of onion in F1 generation showed that the lowest bulb fresh weight was from 48.60 g (hybrid derived from Makoi bronzi x Jasenicki crveni crossing combination) to 165.30 g (Makoi bronzi x Piroska).In F2 generation, the lowest and the highest bulb fresh weight were in the same crossing combinations as in F1 generation.
Analysis of variation effect proves significant variation of this quantitative trait (Table 1).In parental lines, the line with lowest variability was Piroski (9.49%) while AC101 (16.74%) had the highest variability.
In F1 generation the variability, expressed as a coefficient of variability (Cv) had great interval of variation: from 1.66% (Makoi bronzi x Bunkino beo) to 16.88% (AC101 x Jasenicki crveni).Parameters of variability in F2 generation were higher: from 3.22% (AC101 x Jasenicki crveni) to 17.89% (Jasenicki crveni x Bunkino beo).High variability of traits studied in our study was also found in Agič (1996) and Pavlović et al. (2002) who emphasized that the main reason of high variability was the impact of the outdoor conditions.
In study of the mode of inheritance of the bulb fresh weight in F1 hybrid, the following types were established: super-dominance, partial-dominance, dominance and intermediary.Super-dominance of better parent was the most common mode of inheritance and it was found in three hybrid combinations and then the dominance of better and worse parent (Table 1).In F2 generation different results were obtained (Table 1).
Extremely high heterosis in both generations was found in hybrid combination Makoi bronzi x Piroska.This hybrid had the highest bulb fresh weight.The significant heterosis was found in five other hybrids in both studied generations (Table 1 Additive component of genetic variance (D) for the studied trait was lower than dominant (H1 and H2) in both generation levels, which proves that the heredity of bulb fresh weight was influenced by dominant genes.
Negative F value (interaction of additive x dominant gene effect) proves that the heredity of this trait was more influenced by recessive alleles.Also, this proves that the ratio of total number of dominant and recessive alleles (Kd/Kr = 0.95 and 0.98) was less than one.Dominant and recessive alleles were not equally positioned in parental lines (H2/4H1 = 0.22, Table 2).
The average level of domination ( = 3.25 and 3.08) was higher than one, which proves the super-dominance as the mode of inheritance of bulb weight, regardless to the crossing combination.Heritability in narrow sense in F1 generation was 0.31, and in broader 0.99.Similar values were found in F2 generation.High values of heritability for the studied trait prove higher level of genetic factors in inheritance of the average bulb fresh weight.High level of heritability was also confirmed by Singh et al. (1995), Neykov et al. (1997), Pavlović et al. (2002).
The analysis of variance of combining abilities in our study proved significant values of GCA and SCA in both generations (Table 3).The GCA values were a little higher than SCA, which proves the dominant influence of additive genes in inheritance of bulb fresh weight.However, the influence of non-additive genes was extremely high.
The GCA values (Table 4) prove that the studied lines had statistically significant differences in both researched generations.The best GCA was found in line Makoi bronzi both in F1 and F2 generation, followed by Piroska and AC101, while lines Jasenicki crveni and Bunkino beo had statistically significant negative values of GCA.
The highest level of SCA in F1 generation was found in hybrid derived from the crossing combination Makoi bronzi х Piroska.Four other combinations had positive significant values of SCA.Negative significant values were calculated in three hybrid combinations (Table 5).Almost identical values were found in F2 generation.These SCA values could be explained by equal participation of additive and non-additive genetic variance in inheritance onion bulb fresh weight.
Regression coefficient (b) was significantly different from 1 both in F1 and F2 generation, which proves the presence of inter-allele interaction of genes in inheritance of onion bulb weight.
Figures 1 and 2 show that the regression line was distant from the limiting parabola, which proves that the influence of non-additive genes was higher.The intersection of the expected regression line with Wr axis was above the coordinate start which proves partial domination as heredity mode of this trait (Figures 1 and 2).
Distribution of points at the diagram along the expected line of regression proves that genotypes chosen for the hybridization i.e. parental genotypes were genetically divergent.
Parental line with the greatest number of dominant genes in both generations was Bunkino beo, while the bearer of the largest number of recessive genes for the bulb fresh weight in both generations was Makoi bronzi.

Conclusions
The obtained results of our study show that both generations of hybrid combinations Makoi bronzi x Piroska had the highest bulb fresh weight.Considering all crossing combinations, super-dominance and dominance are the mode of inheritance for bulb fresh weight.The best general combiner was the genotype Makoi bronzi.Hybrid Makoi bronzi x Piroska had the highest level of SCA in F1 and F2 generation.Also, this hybrid combination had the highest values of heterosis.
holes) on 15 February 2006 in greenhouse.Planting date was 4 April 2006 in the open field.

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Figure 1.VrWr regression for the average onion bulb fresh weight in F1 generation

Table 1 .
). Mean value ( ), standard error (Sх), variation coefficient (Cv), mode of inheritance, absolute and relative heterosis (Hа and Hr) for the average bulb fresh weight of researched lines, F1 and F2 hybrids of onion i intermediary inheritance, Pd partial dominance, d dominance, Sd superdominance

Table 2 .
Components of genetic variance for content of the onion bulb fresh weight

Table 3 .
Analysis of variance of combining abilities for the average onion bulb fresh weight

Table 4 .
Value of GCA parental lines for the average onion bulb fresh weight