Exploring combining ability for the selection of promising parents to harness heterosis for yield and associated traits in Indian mustard (Brassica juncea (L.) Czern & Coss)

Introduction

Indian mustard (Brassica juncea (L.) Czern & Coss) is a predominant rabi season oilseed crop in India and occupies a premier position among oilseed species due to its high oil content (Singh et al., 2020) [24] . Belonging to the genus Brassica and family Brassicaceae (Cruciferae), B. juncea is an economically significant crop extensively cultivated across Asia and Europe (Warwick et al., 2006) [31] . Vavilov (1949) [30] identified Afghanistan and its adjoining regions as the primary centre of origin for B. juncea, with secondary centres of diversity reported in Asia Minor, central or western China, and eastern India. Historically, B. juncea is believed to have been introduced into India from China. Indian mustard thrives in a variety of environmental conditions, with annual rainfall ranging from 500 to 4,200 mm, temperatures between 6 and 27 °C, and soil pH levels from 4.3 to 8.3 (Karthik et al., 2024) [8] . In India, the rapeseed-mustard group comprises both traditionally cultivated indigenous species and a few non-traditional ones. [2] .

Non-traditional species include white mustard [ The primary goal of any crop improvement program is to increase the yield potential of the crop (Karthik et al., 2024) [8] . Despite its importance, the productivity of Indian mustard in India remains significantly lower than in countries like Germany, France, and the United Kingdom. Therefore, enhancing the seed yield is critical to achieving self-sufficiency in edible oil production (Yadav et al., 2020) [26] . Improvement in both seed yield and oil quality is necessary to meet the growing demand for edible oils, particularly in the context of a rapidly expanding population. One of the most effective approaches for yield enhancement is hybridization, which requires the strategic utilization of diverse and genetically rich germplasm. A key concept in hybrid breeding is combining ability, which refers to the capacity of a genotype to transmit superior performance to its offspring. This concept is foundational in identifying elite parental lines for hybrid development and capitalizing on heterosis (hybrid vigor). Combining ability is typically assessed through biometrical approaches such as diallel, partial diallel, and line × tester (L×T) analyses, which help partition the genetic variance into additive and non-additive components. General combining ability (GCA) represents the average performance of a line across multiple hybrid combinations and is primarily attributed to additive gene effects. In contrast, specific combining ability (SCA) captures the deviations of specific crosses from the expected performance based on parental GCA values, reflecting nonadditive gene action including dominance and epistasis. The concept of combining ability was formalized by Sprague and Tatum (1942) [27] during their work in maize, emphasizing its utility in plant breeding for selecting optimal parental combinations to achieve targeted genetic gains. In nutshell, understanding the genetic basis of combining ability is indispensable for hybrid breeding programmes, particularly in crops like Indian mustard where improving seed yield and oil content is a national priority. Through a well-structured combining ability analysis, breeders can identify superior parent lines and promising hybrid combinations that contribute to sustainable genetic improvement and yield enhancement. Hence, the objective of this study was to identify mutant lines possessing desirable agronomic traits and strong combining ability, with the aim of exploiting heterosis to enhance yield potential and contribute to sustainable agricultural development.

Materials and Methods

Location and Agro-climatic and weather condition of experimental site

The present investigation was conducted at the Agricultural Research Farm, Institute of Agricultural Sciences, Banaras Hindu University (BHU), Varanasi, located along the banks of the Ganga River in Uttar Pradesh. The research farm is situated in the southeastern part of Varanasi at 25°15' N latitude and 82°59' E longitude, at an altitude of 75.5 meters above mean sea level. Varanasi has a moist subtropical climate, marked by considerable temperature fluctuations between the summer and winter seasons.

Experimental material

In this experiment, 10 mutant lines (TM-143, TM-258, TM-130, TM-108, TPM-1, TM-217, TM-108-1, TM-117, TM-52, and TM-263-3) of Indian mustard obtained from Bhabha Atomic Research Centre, Trombay were crossed with three testers viz., Giriraj, RH-749 and HUJM-10-6 (obtained from germplasm collection maintained in department of Genetics and Plant Breeding, Institute of Agricultural Sciences, BHU.) during Rabi-2019-20 to develop a total of 30 crosses. These 30 F 1 crosses along with 13 parents and national checks (Kranti) were evaluated in randomized block design with three replications during Rabi 2020-21. The experimental material consisting of a total of 44 entries (30 F 1 crosses, 13 parents and 1 checks) were sown in randomized block design with 3 replications during rabi 2020-21. Each genotype was sown in a two rows of 5-meter length. The spacing between rows and plants were 30 cm and 10 cm, respectively.

Trait assessment and data analysis

Data were recorded on 16 different traits namely plant height (PH), days to 50% flowering (DF), days to maturity (DM), number of primary branches (NPB), number of secondary branches (NSB), length of main raceme (LMR), number of siliquae on main raceme (NSMR), number of siliquae per plant (NSPP), siliqua length (SL), seeds per siliqua (SPS), seed yield per plant (SYPP), biological yield per plant (BYPP), harvest index (HI), test weight (TW), canopy temperature deficit (CTD), chlorophyll content (CC). Five competitive plants were tagged randomly from each genotype in each replication for recording field observations for all the traits except for days to 50% flowering and days to maturity, which were observed on plot basis during both the years. Harvest index was calculated by dividing seed yield per plant by biological yield per plant. The data were first subjected to the usual analysis followed for a randomized block design for individual environment as suggested by Panse and Sukhatme (1967). The analysis of variance for combining ability (gca and sca) and estimation of variance components for various characters were carried out using the procedure suggested by Kempthorne (1957) [11] for line × tester analysis.

Results and Discussion

Analysis of variance for combining ability

The details of the analysis of variance are presented in Table 1. The analysis, conducted on 30 hybrid combinations, partitioned the total variance into three components: line effect, tester effect, and line × tester interaction effect. The mean squares due to crosses were significant for all the traits studied, indicating substantial genetic variability. The line effects were significant for all traits except canopy temperature deficit, chlorophyll content, number of secondary branches, number of siliquae per plant, seeds per siliqua, biological yield, and harvest index. The tester effects showed significance for days to 50% flowering, canopy temperature deficit, seeds per siliqua, test weight, seed yield per plant, and seed yield per hectare. Additionally, the line × tester interaction effects were significant for all traits except the number of primary branches. Similarly, significant effect of line, tester and line × tester component for the various yield component traits were reported in Indian mustard (Patel et [18,19,13,3,12] .

Estimation of general combing ability (gca) and specific combing ability (sca) effects

The combining ability plays an important role in the selection of suitable parents for hybridization. Parents having high mean performance may not necessarily transmit their superior performance to their offspring (Sprague and Tatum, 1942) [27] . Similarly, parents with good GCA effects may not always produce hybrids with good SCA effects (Griffing, 1956) [6] . GCA variance represents additive gene action, where simple selection is effective for the improvement of such traits, while SCA variance indicates the preponderance of non-additive gene action, where heterosis breeding may be more rewarding (Falconer and Mackay, 1996) [4] . For several traits, gca variance may be equal indicates that both additive and non-additive genes are equally important in that expression of that character (Fasahat et al., 2016) [5] . Out of the thirteen parental lines evaluated, four-TPM-1, TM-143, HUJM-10-6, and TM-52-were identified as good general combiners for days to 50% flowering, days to maturity, and plant height (with the exception of HUJM-10-6 for plant height), corroborating the findings of Kaur et al. (2019) [9] . For chlorophyll content, TM-130, TM-108-1, TM-258, and RH-749 exhibited strong general combining ability. TM-130 and TPM-1 were effective general combiners for the number of primary branches per plant, while TM-263-3 and TPM-1 showed good GCA effects for the number of secondary branches per plant, consistent with earlier reports by Kumar et al. (2017) [13] and Singh et al. (2016) [25] . Five parents-TM-217, TM-258, TM-130, TM-117, and HUJM-10-6-demonstrated significant GCA effects for main raceme length. TM-130 was the sole parent with desirable GCA for the number of siliquae on the main raceme, whereas TPM-1 and TM-263-3 showed good combining ability for the total number of siliquae per plant. TM-143 was found to be a good general combiner for siliqua length, and Giriraj for the number of seeds per siliqua, aligning with the reports of Kumar et al. (2017) [13] , Kumar et al. (2018) [14] . Among the lines, TM-143, TM-130, and TM-217, and among testers, HUJM-10-6, exhibited significantly positive GCA effects for test weight. Additionally, four lines-TM-130, TM-108-1, TM-263-3, and TM-117-and one tester, RH-749, were identified as superior general combiners for biological yield. The list of lines and testers exhibiting significant general combining ability (GCA) effects for various traits, along with a summary of GCA estimates, is provided in Table 4. For traits such as plant height, days to 50% flowering, canopy temperature deficit, and days to maturity, significant negative GCA effects are considered desirable. Accordingly, lines and testers demonstrating positive and significant GCA effects for the remaining traits are regarded as good general combiners. In this context, TM-108 and TM-52 were identified as superior general combiners for harvest index. Furthermore, the parents TM-130, TM-117, TM-263-3, and RH-749 exhibited significant positive GCA effects for both seed yield per plant and seed yield per hectare. These genotypes represent valuable genetic resources for use in future breeding programs aimed at enhancing specific yieldrelated traits in Indian mustard. These observations are consistent with the results reported by Kumar et al. (2017) [13] and Tirkey et al. (2020) [29] and Choudhary et al. (2020) [3] . The list of crosses exhibiting significant specific combining ability (SCA) effects for various traits is presented in Table 5. The cross Giriraj × TM-143 demonstrated a significantly negative SCA effect for days to 50% flowering, indicating its potential for early maturity. Among the thirty crosses evaluated, none exhibited a significant negative SCA effect for days to maturity. However, the hybrid HUJM-10-6 × TM-258 showed a significantly negative SCA effect for plant height, which is desirable in terms of plant architecture and lodging resistance. These findings align with the results of Tirkey et al. (2020) [29] and Choudhary et al. (2020) [3] . Several hybrids-Giriraj × TM-52, RH-749 × TM-217, RH-749 × TM-143, RH-749 × TM-117, HUJM-10-6 × TPM-1, and HUJM-10-6 × TM-263-3-exhibited significant positive SCA effects for seed yield per plant, biological yield (excluding RH-749 × TM-217), and seed yield per hectare, in line with the findings of Saikia et al. (2019) [21] and Meena et al. (2017) [16] . Moreover, the crosses RH-749 × TM-130, RH-749 × TM-117, Giriraj × TM-143, Giriraj × TM-108, Giriraj × TM-52, and HUJM-10-6 × TM-263-3 showed significantly positive SCA effects for chlorophyll content. Only three hybrids-RH-749 × TM-117, Giriraj × TPM-1, and HUJM-10-6 × TM-258-displayed significant positive SCA effects for canopy temperature deficit, suggesting improved drought resilience. Significant positive SCA effects for the number of secondary branches per plant were recorded in HUJM-10-6 × TPM-1, HUJM-10-6 × TM-263-3, RH-749 × TM-143, RH-749 × TM-217, and Giriraj × TM-108. However, none of the crosses showed significant positive SCA effects for the number of primary branches per plant or number of seeds per siliqua, consistent with the findings of Meena et al. (2017) [16] , Singh et al. (2019) [23] , and Tirkey et al. (2020) [29] . Among all hybrids, only HUJM-10-6 × TM-108 exhibited significantly positive SCA effects for the number of siliquae per plant. The cross HUJM-10-6 × TM-258 showed positive and significant SCA effects for both thousand seed weight and harvest index, corroborating the observations of Singh et al. (2019) [23] and Choudhary et al. (2020) [3] . Furthermore, the crosses RH-749 × TM-143, RH-749 × TM-258, Giriraj × TM-52, and HUJM-10-6 × TM-108 demonstrated significant positive SCA effects for the length of the main raceme. Notably, no hybrid exhibited significant positive SCA effects for siliqua length. These specific cross combinations can be strategically employed in breeding programs to generate superior transgressive segregants. The parental lines involved in these hybrids may also be considered for introgression into the backgrounds of welladapted cytoplasmic male sterile (CMS) or restorer lines for future hybrid development. These conclusions are in agreement with the reports of Kaur et al. (2019) [9] and Rashmi et al. (2018) [20] . In the present investigation, a higher magnitude of SCA variance was observed compared to GCA variance for the majority of the traits studied, indicating a predominance of non-additive gene action. Specifically, the condition σ²sca > σ²gca was evident for all traits except days to 50% flowering, days to maturity, and number of primary branches per plant, where additive gene action was predominant (σ²gca > σ²sca). These findings are consistent with those reported by Meena et al. (2015) [17] and Synrem et al. (2015) [28] , who also observed a preponderance of SCA variance in their studies. High SCA effects observed in crosses involving two good general combiners (good GCA × good GCA), such as RH-749 × TM-117 ( Kranti', demonstrating strong potential for future breeding efforts. These findings underscore the utility of these hybrids and their respective parents for exploitation in heterosis breeding and the development of high-yielding mustard cultivars. To ensure broad adaptability and stable performance, these hybrids must be further evaluated through multilocation trials.

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