Genetic Enhancement of Variability Through Induced Mutagenesis in Two Genotypes of Brassica napus L.

The present study deals with the extent of variability for yield, yield components and oil content in the 24 gamma-ray induced mutants (M 7 generation) of two genotypes of Brassica napus L. A highly desirable shift in mean values in the mutants as compared to national check, GSL-1 was observed for almost all the characters. The highest range of variation was recorded in grain yield (3.58-9.46 g) followed by number of primary branches (5.2-12.3), 1000-grain weight (1.76-3.93 g) and no. of siliquae per plant (113.10-236.1). The maximum value of PCV was obtained for number of grains per siliqua followed by grain yield per plant and 1000-grain weight. The characters, such as, days to 50 per cent flowering, days to maturity, length of siliqua, plant height, number of primary branches, grain yield per plant and 1000-grain weight showed high heritability (more than 80%). Among component traits, plant height, length of siliqua, no. of grains per siliqua, no. of siliquae per plant and 1000 – grain weight showed strong positive correlation with seed yield per plant. Mutants, such as, GSM-1, GSM-2 and GSM-14 showed enhanced level of oil content, test weight and grain yield. The isolation of certain promising mutants signified the role of mutation breeding in enhancing the genetic variability in Brassica napus L

Existing varieties as well as germplasm pool of B. napus have at present very limited genetic variability with respect to yield and yield components. So, it is highly imperative to induce and enhance genetic variability in the existing cultivars through mutagenesis. Hence an experiment on induction of mutants in two B. napus lines (GSL-1 and HNS-*Corresponding author email: hriday_bhu@rediffmail.com 9601) was initiated at this laboratory during 2004-05 using gamma rays; the study led to the isolation of large number of mutants of economic significance.

Materials and Methods
The experimental materials for the present study consisted of 24 gamma-ray induced mutants of two genotypes of B. napus, namely, GSL-1(national check) and HNS-9601. The promising mutants were maintained at Institute of Agricultural Sciences, Banaras Hindu University, Varanasi by Dr. H. Kumar. The present experiment was carried out during Rabi 2009-10 at the Experimental Research Farm of the Institute. The mutants along with check, GSL-1 were evaluated in Randomized Block Design (RBD) with three replications. Each genotype was grown in single row of 3 m length, row to row and plant to plant distance was maintained at 45 and 10 cm, respectively. All the recommended agronomic practices were adopted to raise a good crop.
Ten plants were randomly selected and tagged before flowering from each genotype in each replication for data recording on ten quantitative characters, namely, days to 50 per cent flowering, plant height, number of primary branches per plant, length of siliqua, number of siliquae per plant, number of grains per siliqua, grain yield per plant, 1000-grain weight and oil content. The oil content in grains was estimated following the Soxhlet's method. A few seeds from sample plants were bulked and two composite samples (each of 2-3g) were drawn from each of the three replications.
The oil content was estimated using the following relationship Oil content (%) = (W2 -W3) x 100/ (W2 -W1) Where, W1 = Weight of filter paper (container) W2= weight of filter paper + weight of initial dried seeds (crushed) W3= Weight of filter paper + weight of dried seeds (crushed) after oil extraction.
The statistical data of sampled plants were averaged to get mean values. The character means for each replication were then utilized for various statistical analyses; usual statistical procedures were adopted to calculate mean, range, co-efficient of variation and critical difference. The mean value of characters was subjected to analysis of variance (ANOVA) following Panse and Sukhatme (1967). The correlation coefficient was estimated following Hayes et al.(1955). The heritability and genetic advance were estimated following Johnson et al. (1955).

Results and Discussion
Gobhi sarson (Brassica napus L.) is a comparatively recent crop introduction as compared to other Brassicas and it has shown excellent performance in regions with cooler climatic conditions. However, to facilitate the spread of Gobhi sarson in the country, the breeding focus is now shifted to development of varieties keeping in view the location specific problems associated with B. napus. At present, there is very limited genetic variability available in this crop; only seven promising varieties (5) /hybrids (2) have been released in India and that too suffer from narrow adaptability apart from other inherent drawbacks. The scope of improving B.napus through intervarietal hybridization is limited owing to lack of genetic variability. Mutation breeding appears to be the one of the alternatives to improve this crop through enhancing genetic variability of traits of economic importance. The treatment variance of each character (except for number of grains per siliquae) was found to be significant indicating the presence of adequate variability among the mutants.
Mutagenesis has been found to cause both morphological abnormalities (Grover and Tejpaul, 1979) and genetic changes in yield and yield contributing traits (Muhammad et al, 2007 andKumar et al., 2011). A marked reduction in mean value coupled with positive shift in means of primary branched per plant, number of siliquae per plant, no. of seeds per siliqua, 1000-grain weight and grain yield per plant in Indian mustard was reported by Khan et al. (2008). High amount of variability were also reported by various workers, such as, Khatri et al. (2005) and Muhammad et al. (2007) in B. juncea, Khan et al. (2008) in B. napus.

Genotypic and Phenotypic Co-efficients of Variation
The values of genotypic and phenotypic coefficients of variation along with heritability and genetic advance are presented in Table 2. In general, as expected, phenotypic coefficient of variation (PCV) was higher than genotypic coefficient of variation (GCV). The magnitude of both GCV and PCV varied with the traits. Maximum value of PCV was obtained for number of seeds per siliqua followed by grain yield per plant and grain weight. Days to 50 per cent flowering, days to maturity, length of siliqua, plant height, number of primary branches, grain yield per plant and 1000-seed weight showed high heritability (more than 80%). Plant height and number of  High genetic advance as percent of mean was obtained for seed yield and number of primary branches. Whereas moderate values were obtained for days to 50 per cent flowering, length of siliqua, oil content and grain weight. Very low values of genetic advance were observed for days to maturity and plant height. The GCV provides a measure for comparison of the genetic variability and sometimes give an idea regarding validity of the traits under selection. However, it does not provide a clear picture of the extent of genetic gain to be expected from selection based on phenotypic basis of traits unless heritable fraction of variation is not known (Burton, 1952). A character exhibiting high broad sense heritability might not necessarily give high genetic advance . Gandhi et al. (1964) reported that high heritability accompanied by high genetic advance was found to be more reliable in deriving any valid conclusion. Therefore, selection should not be based solely on heritability (broad sense) but due consideration should be given to genetic advance as well.
Increase in heritability values in mutagenic population were also reported by Labana et al. (1980) in B. juncea. They found high estimates of heritability and genetic advance for plant height, number of grain per siliqua, grain yield per plant. Mahla et al. (2003) reported very high heritability for seed yield per plant, test weight, number of siliquae per plant. Rai et al. (2005) found high heritability for most of the traits except days to maturity, days to 50 per cent flowering and oil content. Khan et al. (2008) reported high heritability and genetic advance as percent of mean for number of siliquae per plant, test weight and grain yield per plant in B. napus.

Association between yield and yield contributing traits
In the 24 mutants of B. napus, plant height, length of siliqua, number of seeds per siliqua, number of siliquae per plant and test weight showed significantly positive correlation with grain yield per plant (Table 3). Among component traits, plant height showed significant and positive correlation with length of siliqua and test weight. Days to maturity showed a highly significant positive correlation with number of primary branches but a highly significant negative correlation with oil content and test weight.
Number of primary branches showed significant and positive correlation with days to 50 per cent flowering, days to maturity. Length of siliqua showed significant positive correlation with number of grains per siliqua, no. of siliquae per plant and grain weight, while it was negatively correlated with number of primary branches. Oil content did not show significant positive correlation with any of the yield contributing traits  Length of siliqua, number of siliquae per plant, number of grains per siliqua and plant height were found to exhibit significant positive correlation with seed yield. Earlier workers have reported similar relationship (Mitra et al., 2006 andMarjanovic et al., 2008). Number of primary branches per plant and siliquae per plant exhibited significant positive correlation with yield per plant (Rai et al., 2005 andMitra et al., 2006). Earlier workers reported both positive (Mitra et al., 2006) as well as negative correlation (Muhammad et al., 2007) of grains per siliqua with seed yield per plant. Test weight showed highly significant and positive correlation with grain yield per plant. Both positive (Mitra et al., 2006) and negative correlation (Kumar and Srivastava, 2004) between grain weight and seed yield per plant were reported.