Induced variability for yield and its attributing traits in cluster bean [Cyamopsis tetragonoloba (L. ) Taub] through gamma irradiation

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Cluster bean (Cyamopsis tetragonoloba L. Taub) is an underutilized vegetable from the family Fabaceae. But its nutritional value, hardiness and ease in growing makes it a suitable crop for future. Presently, it is being grown widely in north-western states of India for guar gum production which has a high industrial value. Whereas, in south India, it is grown mainly for vegetable purpose. In Karnataka, it is grown year- round in northern districts like Dharwad, Belagavi, Vijayapura and Haveri. Despite its nutritional importance, the area under vegetable cluster bean cultivation is very small due to non-availability of suitable variety. For developing an improved variety, genetic variability for desired traits is a pre-requisite. Cluster bean is a self-pollinated crop with a very narrow genetic base and creation of variation through manual hybridization is further difficult and not economical, owing to small flowers. Looking into this limitation, efforts were made to create variability in cluster bean using induced mutations and selecting the elite genotypes.Mutations are the heritable changes which alter the phenotype of an organism. It is created by changes in the sequence of base pairs in the genes or changes in chromosome structure and number. The process of mutagenesis alters the biological organization of an individual. It is desirable to select and screen the mutants in M. generation as recessive mutation will express at this stage. The use of mutagens is a valuable supplemental strategy to plant breeding in cluster bean (Yadav et al., 2014; Akhtar et al., 2015). It creates variability both in terms of qualitative and quantitative traits which are needed for selection. Mutations are very helpful particularly when it is aimed to improve one or two easily identifiable characters in a well-adapted variety. The present investigation was carried out to generate variability in one of the widely cultivated commercial variety ‘Pusa Navbahar’ which has reached a plateau in terms of yield and attributing traits.

The present study was conducted using cluster bean variety Pusa Navbahar which was treated with 12 different doses of gamma rays from 50 Gy to 600 Gy with 50Gy interval at the Gamma irradiation facility available at ICAR- Indian Institute of Horticultural Research, Bengaluru. The M. seeds were planted in a replicated trial in field and all the survived M. plants (605 lines) were harvested separately to form the M. generation seeds. These M. seeds were sown in plant to progeny rows along with parent Pusa Navbahar and 3 other check varieties (MDU 1, Swarna and Sonali) in an augmented block design. The whole plot was divided into 5 blocks and in each block 122 lines were sown except for block 1 where 121 lines were sown. Seeds were sown on raised beds of length 5 m and width 1 m with a spacing 30 x 15cm between rows and plants (December 2018 to April 2019). All the recommended package of practices were followed and from each treatment, 10 plants were randomly selected for recording the data on different quantitative characters in M. generation. Similarly, 10 plants were picked up from the checks for comparative assessment. Observations were recorded on days to 50 per cent flowering, number of clusters per plant, number of pods per cluster, total number of pods per plant, pod length (cm), pod width (cm), 10 pod weight (g), pulp to seed ratio, yield per plant (g) and plant height (cm). Based on these observations the parameters of variability like GCV, PCV, heritability (h2) (Allard, 1960) and genetic advance (Johnson et al.,1955) were calculated to identify the traits based on which selections can be made. OPSTAT software was used for the estimation of above parameters.

The analysis of variance (Table 1) revealed significant variation for most of the traits studied. Mutants exhibited significant variation for plant height, days to 50 per cent flowering, pod length, pod width and pulp to seed ratio while it was non-significant for number of clusters per plant, number of pods per cluster, number of pods per plant, pod yield and pulp to seed ratio. The variance between the control and the mutants was found to be significant for plant height, number of pods per cluster, number of pods per plant, pod length, pod width, pulp to seed ratio and for only these selected traits, the estimates of variability such as genotypic coefficient of variation (GCV), phenotypic coefficient of variation (PCV), heritability in broad sense (h2), genetic advance (GA) was calculated along with mean, range, standard error of mean, critical difference and coefficient of variation (Table 2). Graphical representation of GCV, PCV, h2 and GA is presented in Fig 1 and 2 respectively.

Table 1
Analysis of variance for yield and its attributing traits in M2 generation

* denotes significant at P005NS denotes Non-significant at P 0.05

Fig. 2
Heritability and genetic advance for yield attributing traits in M2 generation

Table 2
Parameters of variability for yield and its significantly attributing traits in M2 generation

For all the traits, PCV showed higher values compared to GCV but small difference between these values indicated less environmental influence. PCV ranged from 31.99 per cent (pulp to seed ratio) to 14.28 per cent (pod width), whereas GCV ranged from 24.16 per cent (pulp to seed ratio) to 10.10 per cent (pod width). Traits like plant height and pulp to seed ratio showed moderate PCV and GCV. According to Khan and Wani (2006) moderate to high phenotypic and genotypic variance in the quantitative traits in all the mutagenic treatments indicates better chances for selection to be successful. However, coefficient of variation alone is not an effective indicator, to determine the amount of variation and it also depends on the heritability and genetic advance of the trait. The heritability in broad sense ranged from 50 per cent (pod width) to 79.99 per cent (Plant height). Genetic advance expressed as percentage over mean ranged from 2.06 per cent (pod width) to 222.44 per cent (plant height). According to Singh (2001), if heritability of a character is very high (>50 %), selection for such traits could be fairly easy. Mishra et al., 2020 has also reported in cluster bean that characters like number of pods per plant, pod length, pod weight, plant height, number of clusters per plant, pods per cluster showed high coefficient of variations and high heritability with high genetic gain indicating their suitability for effective selection.This is because there would be a close correspondence between the genotype and the phenotype due to the relatively small contribution of the environment to the phenotype. Johnson et al. (1955) suggested that high heritability combined with high genetic advance as per cent mean is indicative of additive gene action and selection based on these parameters would be more reliable. In this study, plant height (Fig. 3) showed moderate GCV but high PCV and high heritability along with very high genetic advance indicating the additive gene. action, thus making it responsive to selection. Likewise, pulp to seed ratio showed moderate GCV but high PCV, moderate heritability but high genetic advance indicating the dominance of additive gene action favouring selection for improvement of this trait.

Fig. 3
Variation in plant height obtained in M2 generation (dwarf mutants)

Whereas, traits like number of clusters per plant, number of pods per plant, ten pod weight and yield per plant revealed non-significant differences among mutants indicating selection for this trait from the mutant population will be ineffective because of predominance of environmental effect. For days to 50 per cent flowering, variation was found significant between mutants but non- significant over checks that means mutants are on par with the checks or they are of late maturing types which is not the objective of the present study. The variation created through mutation for pod length (Fig. 4) was found significant between the mutants and also over checks. It showed low GCV and PCV but high heritability coupled with moderate genetic advance. Selection would be effective for this trait because of higher heritability and moderate genetic advance due to additive gene action in its background. However, pod width of mutants exhibited low GCV, PCV with moderate heritability along with very low genetic advance. This may be due to higher environmental influence and non-additive gene action. Therefore, selection followed by few generations of selfing would help in breaking the non-additive gene action. It can be concluded here that sufficient variability could be created for traits like plant height, pod length, pod width and pulp to seed ratio through induced mutation in cluster bean. Further improvement of these traits through selection is possibly result in a superior mutant variety/line in cluster bean.

Fig. 4
Variation in pod length obtained through mutation in M2 generation

The authors express their gratitude to Director, ICAR- IIHR for extending all necessary facilities by granting permission to carry out the research work at ICAR- IIHR, Bangalore and providing the gamma chamber facility for the mutation study.