Genetic Characterization of Soybean (Glycine max L.) Genotypes for Yield, Maturity and Rust Resistance

Soybean [Glycine max (L.) Merrill] is considered as “Yellow Jewel” and “Great Treasure” for its nutrition value. It contains about 40 % protein, 20 % oil and 80% soybean meal, as well as eight essential amino acids. It is high in lysine, as well as vitamins A, B and D. Because of its many uses, such as animal feed, biofuel, fertiliser, cleaning products, cosmetics, candle wax and a protein source through soymilk, tofu and other products, it is also acknowledged as the “Golden Bean” and “Miracle Crop” of the twenty-first century.
       
Globally, India holds the fourth position in soybean cultivation with an area of 13.2 million hectares and ranks fifth in production, yielding 11.87 million tonnes. The national average productivity stands at approximately 0.90 tonnes per hectare (Anonymous, 2023a). In Karnataka, for the year 2023-24, soybean was cultivated over 4.077 lakh hectares, producing 3.87 lakh tonnes with an average productivity of 0.95 tonnes per hectare. Among the districts, Bidar accounted for the largest share of the soybean area (48.70%), followed by Belagavi (26.10%) and Dharwad (10.47%) (Anonymous, 2023b). Limits in the area, production and productivity scenario of soybean was observed in recent years due to unavailability of biotic/abiotic stress resilient genotypes with high yield potential and early maturity. Soybean rust, caused by Phakopsora pachyrhizi, is a devastating disease that can lead to yield losses of up to 100%. The best solution for increasing area under soybean cultivation is the development of genotypes having combination of all these traits.
       
An inclusive spectrum of variation is expected to be shown by progenies derived from a set of diverse crosses, thereby providing a vast scope for isolating high yielding segregants. To rally the production potential of soybean, best solution is selection of superior lines in early segregating generations and advanced to next filial generations for the development of high yielding, early maturing and rust resistance varieties. This requires precise evidence on the nature and degree of genetic variation present in soybean and breeder has to formulate criteria for isolating superior genotypes from both segregating generations and advanced generations.
       
JS 335 is most promising variety which covers more than 80% of the soybean cultivation area in Karnataka. It is high yielding coupled with early maturing and agronomically superior variety but highly susceptible to rust disease. Though farmers suffer from heavy yield loss due to rust, they still cultivate JS 335 as it matures within 85-90 days. The improved genotypes of soybean are DSb 21 and DSb 23 which are high yielding and rust resistant varieties. However, these are late in maturity compared to JS 335 and hence farmers prefer JS 335 over these two improved varieties. In this direction, there is a scope to identify genotypes with high yield coupled with earliness and rust resistance, which will be boon to farmers.
       
Keeping the aforesaid views, an effort has been made with the following objectives
1   To study the extent of variability created in the early segregating generation to advanced generation for earliness and yield related traits.
2    To screen the segregating and advanced generation for rust resistance. 

With a purpose to identify high yielding, early maturing and rust resistance genotypes, DSb 23 was crossed with MACS 1575 and MACS 1460 during Kharif 2019 in AICRP on soybean, main agricultural research station, UAS, Dharwad. DSb 23 is highly resilient to rust with high yield potential, but it will take around 95-100 days to mature and it is developed from U A S, Dharwad. MACS 1575 and MACS 1460 are the genotypes from Agharkar Research Institute, Pune, Maharashtra, which mature in 80-85 days but are susceptible to rust and low yielders compared to DSb 23.
       
The first filial generation of both the crosses were grown in kharif 2020. Two hundred seventy-four seeds from cross 1 and four hundred eighty seeds in cross 2 were harvested in first generation and evaluated in an un-replicated trial during kharif 2021 under rainfed condition, which is second filial generation. Selection was progressed in both the crosses for early maturity, rust resistance and yield ascribing traits. Among 274 and 480 F₂ populations, 61 genotypes from cross 1 and 57 genotypes from cross 2 were selected based on our objective and advanced to third filial generation, which were evaluated in plant to progeny rows in augmented design during summer 2022.
       
Among F₃ generation, 22 genotypes from cross 1 and 14 genotypes from cross 2 were selected based on their superior performance compared to superior checks, JS 335 and DSb 21. Superior F₄ families of both the crosses selected from F₃ generation, were assessed in RCBD during kharif 2022 for earliness, rust resistance and yield assigning traits.
       
From the analysis of F₄ generation, 18 genotypes from cross 1 and 12 genotypes in cross 2 were selected based on their superiority and were further evaluated in summer 2023 under RCBD design for identification of high seed yielding genotypes combined with early maturity and rust resistance.

Mean performance of the parents and checks for different characters in soybean is given in Table 1. The values of mean, minimum and maximum of quantitative traits of cross 1 (DSb 23 × MACS 1575) and cross 2 (DSb 23 × MACS 1460) from segregating generation to advanced generation are given in Table 2 and Table 3. The phenotypic coefficient of variation, genotypic coefficient of variation, heritability and genetic advance over mean from segregating generation to advanced generation are presented in Table 4 and Table 5 respectively.



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Days to 50% flowering
 
In second filial generation, the overall mean of days to 50% flowering in cross 1 (DSb 23 × MACS 1575) was 41 days, whereas overall mean number of days taken for flowering in plants of cross 2 (DSb 23 × MACS 1460) was 40 days. Whereas, in F₅ generation, mean of 50% flowering took around 36 days in cross 1 and 37 days in cross 2. Plant number 7-1, 178-2 and 38-2 of cross 1 were earliest to flower (35 days). Plant number 60-2 and 72-1 of cross 2 flowered at 36 days followed by parent MACS 1575 (38 days).
       
The phenotypic and genotypic coefficient of variation in both the crosses were moderate in F₂ generation. High heritability entailed with moderate genetic advance over mean was registered for 50% flowering in both the crosses from segregating generation to advanced generation indicates this trait is governed by both additive and non-additive gene action. Similar result was obtained by Osekita and Olorunfemi, (2014).
 
Days to maturity
 
The overall mean days taken to attain maturity in cross 1 (DSb 23 × MACS 1575) and cross 2 (DSb 23 × MACS 1460) was in the range of 80 to 112 days in cross 1 and 70 to 110 days in cross 2. In advanced generation, cross 1 and cross 2 plants took around 89 days and 93 days to attain physiological maturity. In cross 1, plant number 178-2 and 38-2 of cross 1 were earliest to mature (87 days) followed by 7-1 (88 days). Plant number 60-2 and 72-1 of cross 2 matured at 93 days respectively.
       
Low to moderate PCV and GCV values were registered from F₂ to F₅ generation for days to maturity in both the crosses. However, high heritability associated with low genetic advance over mean was observed in both the crosses from F₂ to F₅ generation indicates this character is governed by non-additive gene action. Similar consequences were obtained by Baraskar et al., (2013); Chandrawat et al., (2017); Malek et al., (2014).
 
Plant height (cm)
 
Cross 1 (DSb 23 × MACS 1575) chronicled the overall mean of 43.85 cm for plant height, whereas in cross 2 (DSb 23 × MACS 1460), it was around 41.28 cm respectively in second filial generation. However, in fifth filial generation, cross 1 and cross 2 genotypes recorded 48.05 cm and 49.88 cm.
       
The lines of cross 1 recorded high PCV and GCV values for plant height throughout the generation. But in cross 2, PCV and GCV values were moderate in segregating generation and becomes low in advanced generation. High heritability coupled with high genetic advance over mean was recorded in both the crosses from segregating to advanced generation. Hence, this result was also confirmed by Jain et al., (2018) and Bairagi et al., (2023).
 
Productive pods per plant
 
The data revealed high variability among the lines of both the crosses for this trait. The average productive pods/plant was 66.00 in cross 1 with a range 6 to 160, whereas in the cross 2 range values were 6 to 175 with a mean value of 74. However, in advanced generation, cross 1 and cross 2 registered a maximum of 131.00 and 139.00 average productive pods/plant. Plant number 21-7 of cross 2 recorded highest number of pods (181.00 pods) followed by plant number 92-1 and 58-1 (176.00 pods) of cross 2 in F₂ generation.
       
The values of PCV, GCV and heritability coupled with genetic advance over mean were high in both the crosses from F₂ to F₅ generation for productive pods/plant. Similar out-turn was reported by Akram et al., (2016), Jain et al., (2018), Bairagi et al., (2023) and Spoorthi et al., (2024).
 
Test weight (g)
 
In F₂ generation, the mean of test weight in cross 1 was 11.43 g with a wide range of 6.5 g to 15.5 g and in cross 2 the mean value was 10.34 g with a range of 6.5 g to 14.5 g. In F₅ generation, genotypes of cross 1 recorded mean value of 14.09 g and genotypes of cross 2 showed mean value of 14.57 g respectively.
       
The genotypes of both the crosses from segregating generation to advanced generation registered high values of phenotypic coefficient of variation, genotypic coefficient of variation, heritability coupled with genetic advance over mean. Similar outcomes were registered by Jain et al., (2018) and Jain et al., (2015).
 
Pod yield per plant or per plot
 
Pod yield per plant (g) varied greatly from 0.5 g to 42.5 g with a mean value of 16.50 g in cross 1 whereas in cross 2 mean value was 18.71 g with a range of 1 g to 45 g in F₂ generation. In F5 generation, lines of cross 1 and cross 2 registered mean pod yield value of 1.46 kg/plot and 1.95 kg/plot respectively. High magnitude of GCV observed for grain yield indicates the presence of wide variation to be allowed for further improvement by selection (Jandong et al., 2020). These results are in contract with the findings by Hakim et al., (2014), Mahbub et al., (2015).
       
The values of PCV, GCV, heritability coupled with genetic advance over mean were high in both the crosses from early to advanced generation for pod yield. Amit et al., (2014); Savita and Koti, (2016) and Spoorthi et al., (2024). confer the same results.
       
The differences in the magnitude of PCV and GCV observed in both the generations of two crosses was very meagre for all the characters studied and similar findings were registered by Malek et al., (2014). Akram et al., (2016) stated that selection based on those characters, which have low influence of the environmental factors would be effective. Similar findings of small differences between the PCV and GCV values were reported by Mahbub et al., 2015 and Jain et al., (2018).
       
From the F₂ to F₅ generations, there was a consistent decrease in the number of days to 50% flowering and days to maturity in both crosses. This trend aligns with our breeding objective of selecting early-maturing lines that also exhibit favourable traits contributing to higher seed yield. As a result, the values of independent traits associated with seed yield showed a progressive increase across generations.
 
Screening the F₂ generation and F₄ generation for rust resistance
 
The two generations were screened for rust resistance during kharif 2021 and 2022. The rust reaction of three parents revealed that, parent DSb 23 showed resistant reaction, whereas other two parents viz., MACS 1575 and MACS 1460 were highly susceptible to rust. Rust reaction of parents and checks are given in Table 6. Patil et al., (2004); Basavaraja et al., (2012); Kurundkar et al., (2011), Inayati and Yusnawan, (2016) and Immadi et al., (2022) characterized soybean genotypes for rust reaction under natural and green house conditions and classified them as resistant and susceptible lines.

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The progenies derived from two crosses i.e., 274 plants from cross 1 and 480 progenies from cross 2 were screened for rust disease under natural epiphytotic condition during kharif 2021. 66 plants out of 274 plants showed highly resistant reaction (rust score=1) to rust and 127 plants exhibited moderate resistance (rust score=3) to rust in cross 1. Whereas in cross 2, 133 plants out of 480 plants exhibited highly resistant reaction (rust score=1) and 208 plants exhibited moderate resistant (rust score=3) reaction under natural epiphytotic condition. Rust reaction of F₂ plants under natural epiphytotic condition in cross 1 and cross 2 of soybean is given in Table 7.

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In F₂ generation, 61 plants from cross 1 and 57 plants from cross 2 were selected based on earliness, rust resistance and yield attributing traits. These 118 genotypes were raised in summer 2022. Due to lower disease incidence, we could not able to screen the plants of both the crosses for rust disease. In F₄ generation, 22 lines from cross 1 and 14 lines from cross 2 were selected based on the yield, earliness and screened for rust disease under natural epiphytotic condition in kharif 2022. Among the 22 genotypes tested, none of the genotypes recorded highly resistant reaction to rust and fourteen genotypes (129-12, 7-1, 9-4, 178-2 and 176-1) exhibited moderately resistant, seven genotypes showed moderately susceptible and one genotype registered susceptible reaction to rust as depicted in Table 8.

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In cross 2, nine genotypes (479-1, 58-8, 23-15, 58-1, 439-4, 92-1, 21-7, 377-10 and 92-6) out of fourteen genotypes screened exhibited moderately resistant reaction and four genotypes recorded (60-2, 72-1, 205-4, 129-4 and 217-7) moderately susceptible reaction to rust disease as quoted in Table 9. 

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The investigation from crossing the contrasting parents to F₅ generation was carried out to identify lines or genotypes which are early flowering with high yield attributing traits coupled with resistant to rust disease. Variability study revealed highly significant variation among the genotypes in both the crosses for all the characters studied. Considerable range of variation was observed for all the yield traits under study indicating enough scope for bringing about improvement in the desired direction. High heritability coupled with high genetic advance were observed for productive pods per plant, pod yield per plant or per plot, plant height and 100 seed weight in both the crosses. Hence, we can rely upon these characters for selection of promising lines. The superior genotypes identified in F₅ generation based on days to maturity, rust resistance and pod yield can be evaluated in multilocation trial to assess their stability performance before releasing as elite genotype and can also be further used as parents in breeding programme for developing early maturing, high yielding and rust resistant genotypes.

The authors say that there is no conflict of interest.