Introduction

Mungbean [Vigna radiata (L.) Wilczek] being a self-pollinated crop the naturally existing genetic variability is not sufficient to achieve the desired improvement. Due to small size of flower, emasculation and pollination in Mungbean [Vigna radiata (L.) Wilczek] is very tedious and costly. Alternately, induced mutations with the application of mutagens are the best way to enlarge genetic variability in this species considerably within a short time. Mutation breeding has made significant contribution in increasing the production of different crops like rice, groundnut, castor, chickpea, pigeonpea, mungbean and urdbean in Indian subcontinent. Mutation is the source of variability in the existing varieties. For the development of better crop, mutation breeding is best method (Awan et al., 2004). The heritability of important traits like production, resistance to insect and pest, and quality can be analyzed through analysis of induced mutations (Fawad et al., 2015; Kanwal et al., 2015; Mumtaz et al., 2015; Ramzan et al., 2015; Naseer et al., 2015)

Mutations are the source  of variability in the existing varieties and early maturing lines  can  be  developed  which  are  helpful  in  the summer season. For the development of better crops, mutation  breeding  is the  best  method  (Awan et al., 2004).

The heritability of  the important traits such as production,  resistance against pest and diseases and quality can be  analyzed  through  analysis of  the induced mutation (Fawad  et al., 2015; Kanwal et al., 2015; Mumtaz et al., 2015;  Rizwan et  al., 2015; Naseer  et  al., 2015; Naseem et  al., 2015;  Masood et al., 2015a,b).

A number of chemical and physical mutagens are widely employed to induce genetic variability in plants. Ethyl Methane Sulphonate (EMS) is a well known potent mutagen, widely employed in induction of genetic variability; it is an alkylating agent and induces high frequency of base pair substitutions. Sodium azide (SA) is another most widely used chemical mutagen in crop improvement, generally used as respiratory inhibitor. Yield contributing traits (viz., Days to flowering, Days to maturity, plant height, primary branches plant^-1 , pods plant^-1, seeds pod^-1, 100 Seed weight and seed yield plant^-1 etc.) are the metric traits which are quantitatively inherited and directly or indirectly contribute to increase in yield of crop plants. Their inheritance is controlled by multiple (poly) genes. The selection of quantitative traits is more difficult than qualitative traits since they are characterized by the presence of whole spectrum of phenotypes (Naseem et al., 2015; Masood et al., 2015a,b; Javed et al., 2016).

The present study of frequency, spectrum, effectiveness, efficiency and effective concentration of these mutagens in inducing genetic variability in mungbean so that they can be assessed for their suitability for selection of elite genotypes and use in breeding programmes aimed at genetic improvement of mungbean. The experimental plant material used in the present investigation are two well adapted diverse variety of mungbean [Vigna radiata (L.) Wilczek.]–Pusa vishal and SML-668.

Materials and Methods

The present experiment was conducted during 2016–17 at Birsa Agricultural University, Kanke, Ranchi. The experimental plant materials used in the present investigation were two well adapted and genetically divergent, varieties of Mungbean [Vigna radiata (L.) Wilczek] cultivars–Pusa vishal and SML 668. These two varieties are widely cultivated in this area of the country. Seeds presoaked in water for 6 hours, were treated with different concentrations of EMS (0.2%, 0.4% and 0.6%) and Sodium azide (0.06%, 0.08% and 0.1%) for 7 hours at room temperature. Seeds not treated with the mutagens served as control. Some of the mutagen treated seeds (20) were grown in Petri plates with moist filter papers. These were used for germination studies and elongation of seedlings after 8 days of germination. About 200 seeds of each treatment were sown in the experimental field, along with controls, as treatment population to raise M₁ generation during kharif season of 2016. The individual seeds of M₁ plant progenies were sown in the field to raise M₂ progeny in subsequent season. Uniform cultivation methods and agricultural practices were followed raise all M₁ and M₂  generations.

Results and Discussion

3.1.  Mutagenic effectiveness

Effectiveness and efficiency of mutagens are quite important, as far as the plant breeding is concerned. Mutagenic effectiveness is a measure of the frequency of mutation induced by a unit dose of mutagen while mutagenic efficiency represents the proportion of mutation in relation to the associated undesirable biological effects, such as chromosomal aberration, lethality and sterility induced by mutagen in question (Konzak et al., 1965) opined that the mutagenic efficiency carries lot of theoretical and practical importance.

In the present investigation lower concentrations of EMS and Sodium azide showed higher effectiveness values. In other words the effectiveness of the mutagens decreased with increase in concentration of mutagens. The mutagen Sodium azide was found to be most effective than mutagen Ethyl methane sulphonate. It was also found that the lower concentrations of both mutagens were most effective in both the varieties (Table 1). Sharma et al. (2005) also reported higher mutagenic effectiveness at lower concentrations of EMS and Sodium azide in urdbean. The decrease in effectiveness with increasing concentrations of mutagen has been reported by several authors, Jaykumar et al. (2003), Sharma et al., (2006) in urdbean. Bashir et al. (2013) reported effectiveness decreases with increase in dose of Sodium Azide. Increasing

Table 1: Mutagenic effectiveness and efficiency of EMS and Sodium azide mutagens in mungbean cv. Pusavishal and SML 668

trend in lethality percentage was observed with increasing dose of sodium Azide in case of dry as well as soaked treatments in mungbean.

3.2.  Mutagenic efficiency

Mutagenic efficiency is the ratio of frequency of chlorophyll mutations induced in M₂ generation to various biological damages (such as seedling injury and lethality) observed in M₁ generation. Both the mutagens exhibited gradual decrease in mutagenic efficiency with the increasing concentration or doses with respect to seedling injury and lethality. From the data on mutagenic efficiency values, it could be observed that the 0.06% sodium Azide in both the variety Pusa Vishal (0.046) and SML 669 (0.32) were the most efficient in regard to lethality. Comparing the efficiency of both the mutagens it was at par with the variety Pusa Vishal, whereas in SML 668 Sodium Azide (SA) was more efficient than EMS (Table 1 and 2).

According to Konzak et al. (1965), higher efficiency at lower concentration of a mutagenic agent is due to the biological damage ( like seedling injury, lethality and sterility) which increases with increase in dose at faster rate than the mutations. Kumar  et al.(2017) reported the same in soyabean. Sharma et al. (2005) in urdbean reported that the lower doses of mutagens were more efficient than the higher doses. Sodium Azide are proved to be the most potent in inducing viable mutants (Rao et al. 1984).The responses of both the chemical mutagens are influenced by number of biological, environmental and chemical factors. Khan and Tyagi (2017) in soyabean observed more chlorophyll mutants at 0.2% EMS.

Bhosale et  al.(2013) reported that seed germination and survival of plants at maturity decreased with increasing concentration/dose of the mutagens, while seedling injury, pollen sterility and lethality were found increased with increasing concentration/dose of the mutagens in the present variety of Urdbean.

All the mutagens exhibited gradual decrease in mutagenic efficiency with the increasing concentrations or doses with respect to lethality. On the whole, when the mean values for mutagenic efficiency are compared, the higher values for lethality were found at higher doses of both Sodium Azide (SA) and EMS respectively. Similar result was reported by Khan and Wani (2006); Jain et al. (2013).

Table 2: Effect of EMS and sodium azide on seed germination, plant survival, seedling length and injury in M1 generation in mungbean

Conclusion

Mutagenic effectiveness and efficiency are two reliable parameters in plant breeding which are used to evaluate a mutagen. The mutagenic effectiveness was inversely proportional to the increasing concentrations or doses of mutagens in Sodium Azide. Low concentrations were found to be most effective. EMS was found to be most effective mutagen in mungbean as compared to sodium Azide (SA) (SA>EMS). The decrease in effectiveness with increasing concentrations attributed to the biological damage.

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