Induction of Genetic Variability Through Mutagenesis in Rabi Sorghum [Sorghum bicolor (L.) Moench]

Sorghum [Sorghum bicolor (L.) Moench] is the fifth most significant cereal in the world in terms of area and production. It is an often cross-pollinated, diploid (2n = 2x = 20) plant that is a member of the Poaceae family. The primary jowar-producing states in the nation include Maharashtra, Karnataka, Andhra Pradesh, Madhya Pradesh, Gujarat, Rajasthan, Uttar Pradesh (Bundelkhand region), and Tamil Nadu.

          Mutation breeding has played a beneficial role in sustainable agriculture, it is a complementary strategy for crop development that promotes unselected genetic variability for useful, practical breeding applications. Due to the extremely low frequency of natural mutation, induced mutations are employed to increase genetic variability for both quantitative and qualitative parameters. Among various physical mutagens such as x-rays, fast neutrons, thermal neutrons, ultraviolet radiation, beta radiation, and gamma rays, the electron beam is particularly well known for its effect on plant growth and development, inducing cytological, physiological, and morphological changes in cells (Thapa, 2004). Several reports are available where breeders use gamma radiations as a source for inducing genetic variability in quantitative characters to enhance yield and yield-contributing traits. Recently, high-power linear electron accelerators with energies ranging from 500 keV to 10 MeV have gained importance for various applications. The accelerators operate in a switch-on-off mechanism, similar to X-ray facilities. It produces an electron beam that can be used to irradiate materials in a high-throughput manner. Electron beam radiation is essential tool for inducing genetic variability, enhancing yield, and contributing to yield-related traits.

            Genetic variability for economic traits is the prerequisite for any successful breeding program, as the degree of response to selection depends on the amount of variability. In any crop, yield is a complex character influenced by many of its contributing characters, which are controlled by polygenes and environmental factors. Therefore, an understanding of the genetics of yield and its component traits is required.

 ‘TSV-32’ rabi sorghum genotype was chosen as base material. It was a selection from CSV-20 X Phule Anuradha. This is an early-maturing sorghum genotype (47-55 days to 50% flowering). The variability present in this segment is very low. Under these circumstances, creating variability through induced mutation will provide additional scope for improvement in Rabi sorghum. The primary focus of the current research is on inducing genetic variability in sorghum (TSV-32) through Electron beam.

The present investigation was conducted on sorghum (Sorghum bicolor (L.) Moench) at the Department of Agricultural Botany, Vasantrao Naik Marathwada Krishi Vidyapeeth, Parbhani. During Rabi 2022-23. which is situated at the latitude of 19^o 24' N, longitude of 76^o 78' E, and altitude of 458.38 m above mean sea level (msl), with the soil type of shallow to medium black.

Pure selfed seeds (1200) of TSV-32 with 10 per cent moisture content were exposed to 100, 200, 300, 400 and 500 Gy dose of electron beam radiation from electron accelerator at Nuclear Agriculture & Biotechnology Division, BARC Trombay, Mumbai and sown as M₁ generation along with the same number of untreated seeds of each variety served as control seeds during rabi 2021-22.

Dominant mutations observed in the M₁ generation were recorded, and individual plants were self-pollinated, harvested, and used to raise the M2 generation during the Rabi 2022-23 season. A total of thirty-six (36) selfed progeny selected from each treatment were grown in a head-to-row progeny design in the M2 generation in a randomized block design with seven treatments, including 2 controls and three replications. The observations were recorded on days to 50% flowering, days to maturity, plant height (cm), number of primaries per panicle, number of grains per primary, panicle length (cm), panicle width (g), 100 grain weight (g), grain yield per plant (g) and fodder yield per plant (g), Relative water content (%), Chlorophyll content (SPAD values), Leaf area (cm²), Flag leaf area (cm²). The standard statistical approach for the Random Block Design (RBD), described by Panse and Sukhatme (1954), was used to analyze the variance for differences between and within families. According to the method recommended by Burton (1952) the variability parameters genotypic coefficient of variance (GCV), phenotypic coefficient of variance (PCV), heritability, and genetic advance — were estimated.

The results of the analysis of variance for the M₂ generation of rabi sorghum are furnished in Table 1. Highly significant differences among the genotypes were observed for fourteen characters, indicating the presence of sufficient variability among genotypes for these fourteen characters.

For all fourteen of the traits under consideration, the mean sums of squares due to progenies within families were found to be highly significant, demonstrating that progenies within the family differ from one another. Anand and Kajjidoni (2014), Htun et al. (2015), Kham et al. (2015), Suthakar and Mullainathan (2015), and Takele et al. (2021) have all observed such variation yield and yield-attributing traits. The outcome of the variability parameters is shown in Table 2. For each of the fourteen characters in the M₂ generation of sorghum, the genetic components; genotypic variance, phenotypic variance, genotypic coefficient of variation (GCV), phenotypic coefficient of variation (PCV), heritability (in broad sense), genetic advance (GA) and genetic advance as the percent of mean (GAM) were calculated using the proper statistical formulas. The results are presented in Table 2.

(PCV) was found to be higher than the genotypic coefficient of variation (GCV) for all the characters with extremely minor differences, indicating a substantial genetic influence. Anand and Kajiidoni (2014), Kham et al. (2015), Shivaprashad et al. (2019), Htun et al. (2015), and Thange et al. (2021) agreed with these findings. GCV and PCV values for grain yield per plant were moderate (15.97% and 18.19%, respectively), indicating moderate variability across progenies for this character and a greater possibility for improving this character by selection in the appropriate direction. Additionally, the progeny exhibited moderate GCV and PCV values for 100-seed weight (8.36% and 13.25%) and flag leaf area (6.08% and 10.38%). The other variables such as days to 50% flowering (1.32% and 1.99%), plant height (3.24% and 3.86%), days to maturity (0.47% and 1.11%), number of primaries per panicle (2.15% and 3.23%), number of grains per primary (2.00% and 2.49%), panicle length (cm) (1.66% and 2.63%), panicle width (g) (2.62% and 3.96%), fodder yield per plant (g) (6.15% and 9.17%), Relative water content (%) (2.53% and 4.96%), Chlorophyll content (SPAD values) (7.40% and 9.11%) and Leaf area (cm²) (3.60% and 5.13%). These characters had low GCV and PCV levels. Low PCV values, accompanied by marginally low GCV values in these traits, suggested a limited level of variability in the materials and a low likelihood of genetic improvement. This suggests that the level of variability and the odds of genetic improvement are moderate. Similar results were found for grain yield per plant by Patel et al., (1980b), Kumar and Singh (1986), Cheralu and Rao (1989) for days to flowering and days to maturity by Nimbalkar et al. (1988), Narkhede et al, (2001); for plant height by Patel et al. (1980a), Patel et al., (1980b), Singh et al., (1980), Amrithadevarathinam et al. (1994) for panicle length and chlorophyll content (spad values) by Narkhede et al, (2001); for 100 grain weight by Negash et al., (2005); for relative water content by Pawar (2007).

High heritability was observed for grain yield per plant (77.05%), plant height (70.53%), number of grains per primary (64.49%), and chlorophyll content (SPAD values) (66.00%). Additionally, the progeny showed moderate heritability for days to 50% flowering (44.12%), number of primaries per panicle (44.38%), panicle length (39.67%), panicle weight (43.62%), 100 grain weight (39.84%), fodder yield per plant (44.98%), leaf area (49.15%) and flag leaf area (34.28%). This suggests that the genotypic effect of phenotypes may be well measured and that selection may be effectively used to enhance these traits. These results are consistent with the results reported by Khaing Wah Htum et al., (2015). Anand and Kajidoni (2014).

The characters viz., plant height (9.99%), leaf area (28.83%), flag leaf area (21.08%), and grain yield per plant (15.73%), all showed moderate genetic advance. Supporting results were observed by Khaing Wah Htum et al., (2015). Anand and Kajidoni (2014). The traits with low estimates of genetic advance were days to 50% flowering (1.16%), days to maturity (0.48%), number of primaries per panicle (1.54%), number of grains per primary (3.47%), panicle length (cm) (0.48%), panicle width (g) (0.26%), 100 grain weight (g) (0.25%), fodder yield per plant (g) (3.92%), Relative water content (%) (1.26%) and Chlorophyll content (SPAD values) (6.28%). Shivaprashad et al. (2019) discovered similar results for these traits.

In predicting selection response, heritability combined with genetic advance is more valuable than heritability alone. Plant height, grain yield per plant, number of grains per primary and leaf area all these traits showed high heritability and moderate genetic advance, which indicates the role of additive gene effects and least the effects of environmental factors on the expression of the traits. Thus, simple phenotypic selection offers a greater potential for improving these traits. Muduli and Misra (2008), Anand and Kajjidoni (2014), Shivaprashad et al. (2019), all found comparable results for these traits.

High heritability coupled with low genetic advance was observed for the number of grains per primary and chlorophyll content (SPAD values), indicating non-additive gene action. So, these traits cannot be improved through simple selection. Similar results were also observed by Shivaprashad et al. (2019).

Table 1: Analysis of variance for yield and yield contributing characters in rabi sorghum

                                                                                * and ** Significant at 5 % and 1% level of significance, respectively.

DFF= Days to 50% Flowering, PH= Plant Hight, DM= Days to Maturity, NPPP= Number of Primaries Per Panicle, NGPP= Number of Grains Per Primary, PL= Panicle Length PW= Panicle Width, 100 SW= 100 Seed Weight, FYPP= Fodder Yield Per Plant, RWC= Relative Water Content, CC= Chlorophyll Content, LA= Leaf Area, FLA= Flag Leaf Area, and GYPP= Grain Yield Per Plant.

Table 2: Genetic variability parameters for yield and yield contributing characters in M₂ generation of rabi sorghum.

GV = Genotypic variance (σ² g) (%), GCV = Genotypic coefficient of variation (%) PV = phenotypic variance (σ² p) (%), PCV = Phenotypic coefficient of variation (%) h² _bs = Heritability (Broad sense) (%), GAM = Genetic Advance as per cent of mean (%)

The present investigation clearly demonstrated that electron beam irradiation is an effective physical mutagen for inducing genetic variability in rabi sorghum genotype TSV-32. Significant differences among M₂ progenies for key agronomic traits such as grain yield per plant, plant height, chlorophyll content, and leaf area indicate successful generation of useful variability. Moderate to high estimates of GCV, PCV, heritability, and genetic advance in these traits suggest that additive gene effects are at play, and phenotypic selection can be effectively applied to enhance these traits. Particularly, grain yield per plant showed promising improvement potential due to its high heritability (77.05%) and moderate genetic advance (15.73%). In contrast, traits like days to maturity and 100 seed weight showed low genetic variability, indicating limited scope for improvement through selection alone. Overall, the use of electron beam mutagenesis has opened new avenues for enriching the genetic base of sorghum, and further selection in subsequent generations could lead to the development of superior genotypes suited for rabi season cultivation.