​Genetic Variability Studies in Short Statured Early Maturing M₃ Generation Cluster Bean [Cyamopsis tetragonoloba (L.) Taub.] Mutants

Cluster bean [Cyamopsis tetragonoloba (L.) Taub.] (Syn. C. psoraliodies D.C.) is a traditional underexploited vegetable crop grown mainly in the north western dry zones of India. Apart from being used as a vegetable cum fodder crop, it is getting industrial importance due to the galactomannan content. One of the major setbacks to the cluster bean production is the plant lodging due to adverse environmental conditions like downpour and strong winds. Hence, it is essential to develop short-statured crops with enhanced productivity.
       
The present germplasm of cluster bean lack the needed natural variability for developing superior short-statured high yielding lines. Generally, hybridization is the most common breeding method used to create the needed genetic variability in majority of the vegetable crops. The manual hybridization in cluster bean is a tedious process due to the tiny floral structure which makes it highly uneconomical to produce hybrid seeds. In such a scenario, induced mutagenesis can be efficiently used as an alternative source to create the desirable variability in physiological and morphological characters. Dahiya (1973) and Ram et al., (1982) stated theoretically some genetic gain would be expected from selection for quantitative traits, assuming that the variability observed in the treated population is largely genetic and most of the genetic variance is additive in nature from mutagenic treatments is given by means and components of variance.
       
Selection of mutants for synchronization with determinate type suitable for mechanical harvesting is a major concern. Since genetic variability is a prerequisite for any successful breeding, the creation and management of such induced variability becomes a central base for the improvement of any crop. Creation of genetic variability followed by screening and selection of the best mutants is important. Mutation is considered as one of the effective tools for crop improvement. Spontaneous mutation cannot be expected to serve the cause of crop improvement effectively. Therefore, the present investigation was undertaken to study the magnitude of heritable variability induced through induced mutation in M₃ generation of cluster bean variety MDU 1.

The experiment was conducted from March (2019) to May (2019) at the College Orchard, Department of Vegetable Science, Agricultural College and Research Institute, Tamil Nadu Agricultural University, Madurai. MDU 1 mutant was used in this work taking into consideration of its higher yielding potential (250 to 300 g/plant) and other desirable attributing characters. It is a variety developed from Agricultural College and Research Institute, Madurai (TNAU) by selection from IC 432117 in 2015.
       
The seeds of cluster bean cv. MDU 1 were mutagenized using gamma rays, electron beam and combination of gamma ray with EMS to raise M₁ and M₂ generation. In M₂ generation, the shortest mutant was observed in combination treatment of 100Gy+20mM of EMS and earliest flowering in combination treatment of 300 Gy+20mM EMS. The highest number of clusters, number of pods per plant and pod yield per plant was observed at 300 Gy of gamma ray treatment. So a total of ninety mutants were selected for terminal flowering with yield in M₂ generation and forwarded to M3 generation for the further variability studies. (Table 1). 
 

       
Observations were recorded on ten selected M3 plants from each family (Table 2) of treatments for six polygenically controlled traits viz., plant height, days to first flowering, number of clusters per plant, number of pods per plant, pod length and pod yield per plant. The same method was followed for recording observation on untreated parental materials also. Throughout the growth period, recommended cultivation practices and protection measures were followed.
 

 
The genetic variability parameters like per se performance, Genotypic Coefficient of Variation (GCV), Phenotypic Coefficient of Variation (PCV) were estimated according to Burton (1952), heritability (Lush, 1940); genetic advance (GA) and genetic advance as percentage of mean by Johnson et al., (1955). Correlations between the traits were estimated using SPSS software and skewness and kurtosis have been estimated as per the statistical procedure given by Panse and Sukhatme (1978).

Estimation of genotypic variability in irradiated population would reveal the heritable portion of total variation created. The GCV provided a mean to study the genetic variability generated in quantitative characters Johnson et al., (1955). The GCV alone is not sufficient to account the heritable variability. In addition, assessment of heritability and genetic advance as percentage of mean is also required to assess the heritable portion of total variation and extent of genetic gain anticipated for effective selection. As heritability in broad sense includes both additive and epistatic gene effects, it will be reliable, if accompanied by high genetic advance Burton (1952). Johnson et al., (1955) suggested that heritability estimates coupled with genetic advance are more helpful than the heritability values alone. This is because the heritability estimates are subjected to genotype–environment interactions Kaul and Kumar (1983). Moreover, genetic advance gives the extent of stability and genetic progress for a particular trait under a suitable selection system.
       
Here, six polygenically controlled traits viz., plant height, days to first flowering, number of clusters per plant, number of pods per plant, pod length and plant yield per plant were recorded in the progenies raised in M₃ generation. (Table 3).
 

 

 

 

 

 

       
Studies on the variability parameters in M₃ generation indicated that the phenotypic variance, genotypic variance, heritability and genetic advance were higher in combination treatment for all the traits. For plant height, the highest PCV (30.72%), GCV (30.17%) and  GAM (61.04%) were recorded in combination treatment (300Gy of gamma ray with 20 mM  EMS) and the  highest h2 (98.24 %) was recorded in 200 Gy of electron beam. For pod yield per plant, the highest PCV (42.11%), GCV (41.78%), h2 (98.42%) and GA as percentage of mean (85.38%) were observed in combination treatment of 300Gy of gamma ray with 20 mM EMS.The high value of heritability accompanied with high value of genetic advance indicated that heritability is most likely due to additive gene effect and selection would be effective method for forwarding these traits to next generation. The interrelation of various yield components showed that the plant height exhibited high positive and significant association with number of clusters per plant and number of pods per plant. Similar results were reported by Jukanti et al., (2015) in cluster bean, Usharani and Kumar (2015a) in black gram, Baisakh et al., (2014) in blackgram and Lavanya et al., (2014) in green gram.
 
Mean performance for quantitative characters for selected dwarf mutants in M₃ generation
 
Observations were recorded based on mean performance and fifteen best performing mutants were selected for terminal flowering with high yield viz.  3M1, 3M2, 3M3, 3M4, 3M5, 3M6, 3M7, 3M8, 3M9, 3M10, 3M11, 3M12, 3M13, 3M14 and 3M15. The tallest mutants were 3M1 (77.5 cm followed by 3M4 (76 cm) and the shortest mutants were 3M11 (53.9 cm) followed by 3M14 (55.7 cm) and 3M12 (56.5 cm). The earliest flowering was observed 3M11 and 3M12 at 21^st day and highest number of clusters was in 3M3 (15) and 3M1 had 14 clusters. The highest number of pods per cluster was recorded in 3M1 and 3M2 with 6.4 pods. The highest number of pods per plant was observed in 3M1 (74) and 3M2 (71). The longest pods were observed in 3M4 (13.20 cm) and 3M1 had 13.10 cm. The maximum number of harvest was observed in 3M3 (13) followed by 3M1,3M2 and 3M4 had 12 harvest. The highest pod yield was obtained in 3M1 (174.64 g) followed by 3M2 (167.56g) and 3M4 (162.84 g). (Table 4). The findings for morphological characters are similar to findings of Jagtap and More (2016) in lablab, Sultan et al., (2012) in cluster bean and Shinde et al., (2017) in cluster bean.
 

       
The shortest mutant 3M11 and earliest flowering mutant 3M11 were found at combination of 100Gy of gamma ray+ 20mM EMS. Whereas mutant 3M3 with the highest number of clusters was observed at combination of 300 Gy gamma ray with 20 mM EMS. Also the highest number of pods per plant in 3M1 and 3M2 and highest pod yield in 3M1 were observed at 400 Gy of gamma ray treatment.  The selected mutants have to be tested for stability in different environments.

The macro mutants viz., tall, dwarf, early maturity, late maturity, synchronized maturity, determinate type, terminal flowering and high yielding mutants were isolated from the M3 generation of cluster bean. The determinate mutants with synchronized maturity were selected based on suitability for mechanical harvesting and were forwarded to next generation for testing the stability in different environments.