Assessment of combining ability and heterosis for seed yield and related traits in linseed (Linum usitatissimum L.) through diallel analysis

Introduction

Linseed (Linum usitatissimum L.) is one of the earliest domesticated crops and holds significant economic value as an industrial non-edible oilseed. It has been cultivated for centuries for both its seeds and fiber [1] . While linseed is primarily grown for its oil, it also plays an important role in the food industry due to its nutritional properties. The seeds are rich in essential polyunsaturated fatty acids, such as alpha-linolenic acid, and contain high levels of soluble dietary fiber. Linseed oil, known for its abundant linolenic acid, is widely used as an industrial drying oil [2] . In our country, linseed occupies 3.84 lakh ha area with a production of 1.54 lakh tonne and contributes about 10.81% and 5.31%, respectively to the global area. The low yield can be attributed to the limited resources available to smallholder farmers and the absence of high-yielding varieties [4] . To improve competitiveness with other linseed-producing countries, the development of high-yielding varieties is crucial. Such varieties can be achieved through well-planned hybridization and selection programs aimed at identifying superior segregants [5] . Combining ability analysis is an effective tool for selecting ideal parent lines and understanding the genetic control of important traits [6] .

Materials and Methods

The experimental materials for this study included eight parental lines, which were crossed following a diallel cross design [7] during the Rabi season of 2010-11, resulting in 28 hybrid combinations (excluding reciprocal crosses). A total of 39 entries, consisting of the 8 parental lines, 28 F1 hybrids, and 3 check varieties, were sown on November 26, 2011, at the Field Experimentation Centre, Department of Genetics and Plant Breeding, SHIATS, Allahabad (U.P.).

The experimental layout followed a Randomized Block Design (RBD) with three replications. Data for days to flowering and days to maturity were recorded at the plot level. For other traits, such as plant height, number of primary branches per plant, number of capsules per plant, number of seeds per capsule, seed yield per plant (in grams), 1000-seed weight (in grams), and harvest index (%), measurements were taken from five randomly selected plants in each plot. The mean values for these traits were analyzed using analysis of variance (ANOVA) [9] . Further statistical analyses were carried out to estimate heterosis, heterobeltiosis, economic heterosis, and combining ability.

Results and Discussion

Flowering and Maturity: The analysis of variance (ANOVA) showed significant differences between both the parental lines and hybrids for days to 50% flowering and days to maturity, with notable variations observed between parents and hybrids for days to maturity (Table 1). The days to 50% flowering in the hybrids ranged from 82.05 days in the cross FRW-6(973) × GS-129(1018) to 98.71 days in the cross FRW-6(973) × POLF17 (1704) (Table 2). Significant mean squares for both general combining ability (GCA) and specific combining ability (SCA) underscored the involvement of both additive and non-additive genetic factors in the inheritance of these traits. However, the relatively low GCA/SCA ratios for days to 50% flowering (0.18) and days to maturity (0.14) (Table 3) suggested that nonadditive genetic effects played a dominant role in controlling these traits. Our findings are consistent with the observations of [8] and [9] , who also reported the dominance of non-additive genetic effects for days to flowering. However, our results differ from those of [10], who found a higher variance in general combining ability (GCA) compared to specific combining ability (SCA) for both days to flowering and maturity. Specifically, the parent GS-129(1018) exhibited the highest GCA effect for early flowering, with a reduction of -2.12 days, while FRW-6(973) showed the highest GCA effect for early maturity, with a decrease of -1.70 days (Table 4.1). These parents, therefore, hold potential for use in recombination breeding programs aimed at developing earlymaturing linseed varieties. The cross FRW-6(973) × POLF17(1704) exhibited the highest and significantly positive specific combining ability (SCA) effects for days to flowering, while the hybrid M-42(169) × FRW-6(973) showed the most significant positive SCA effects for days to maturity (Table 4.2). In contrast, the cross FRW-6(973) × GS-129(1018) demonstrated the highest negative and significant SCA effect for early flowering (-5.88 days), and M-42(169) × PbD2-42(2789) displayed the highest negative SCA effect for early maturity (-5.17 days) (Table 4.2). For early maturity, the hybrids M-42(169) × PbD2-42(2789), M-42(169) × POLF-19(1765), and FRW-6(973) × C91538 (456) showed significant heterobeltiosis, (Table 5), indicating superior performance compared to the best parent for this trait. Early flowering and maturity are important goals in flax breeding [10], and our results suggest that genetic improvement for these traits can be effectively achieved using the genotypes involved in this study.

Plant Height

The genotypes, including both the parents and their hybrids, showed significant variation for plant height (Table 1). Among the hybrids, the cross M-42(169) × GS-129(1018) produced the tallest plants, measuring 81.81 cm, while POLF-19(1765) × POLF-17(1704) had the shortest plants, at 58.70 cm. The analysis of variance (ANOVA) for combining ability revealed significant effects for both general combining ability (GCA) and specific combining ability (SCA) for plant height. However, the GCA/SCA ratio of 0.19 (Table 3) indicated that non-additive genetic effects played a more dominant role in the genetic variation observed for this trait. These results are in agreement with the findings of [8] , but contrast with those of [11] , who reported a greater influence of additive genetic effects. Both positive and negative specific combining ability (SCA) effects for plant height were observed in the hybrids (Table 4.2). The SCA effects varied from -10.37 cm for the cross GS-234(1703) × GS-129(1018) to +10.60 cm for the hybrid M-42(169) × POLF-17(1704) (Table 4.2).

Primary Branches per Plant

The analysis of variance (ANOVA) showed significant differences among the entries for the number of primary branches per plant (Table 1). The hybrid crosses displayed considerable variation in branch number, with M-42(169) × GS-234(1703) having 3.71 branches per plant, and POLF-17(1704) × GS-234(1703) producing 8.93 branches per plant, indicating substantial diversity among the hybrids. The significant mean squares for both general combining ability (GCA) and specific combining ability (SCA) highlighted the influence of both additive and non-additive genetic effects on this trait. However, the GCA/SCA ratio of 0.23(Table 3) further indicated that nonadditive genetic effects played a dominant role in the inheritance of primary branches per plant. These results align with the findings of [8], but differ from those reported by [12] . Estimates of combining ability revealed positive and significant general combining ability (GCA) effects of 0.87 for the parent C91538(456) and 0.36 for POLF-17(1704) (Table 4.1). The specific combining ability (SCA) effects for primary branches per plant ranged from -1.18 to +2.94, observed in the hybrids C91538(456) × POLF-17(1704) and POLF-17(1704) × GS-234(1703), respectively (Table 4.2). Additionally, notable and significant heterosis over the midparent was recorded, with values varying from -35.67% to +72.26% across the different crosses. The parents C91538(456), POLF-17(1704), and POLF-19(1765) (Table 4.1) exhibited higher general combining ability (GCA) effects for branching and also performed well in terms of seed yield. These parents could therefore be beneficial in recombination breeding programs. Incorporating these parents into breeding programs could help gather genes linked to better branching, which may improve the performance of future generations of recombinant inbred lines.

Seed Yield and Its Components

There was significant variation in seed yield and its component traits, such as the number of capsules per plant, number of seeds per capsule, 1000-seed weight, and seed yield per plant, among the parental lines and their F1 hybrids (Table 1). The analysis of variance (ANOVA) for combining ability showed that both general combining ability (GCA) and specific combining ability (SCA) had a significant impact on these traits (Table 3). This indicates that both additive and non-additive genetic effects contribute to the observed variation, with non-additive effects having a more dominant influence, which aligns with the results of [8] .

Significant differences were observed among the hybrids for the number of capsules per plant, with counts ranging from 265.80 in the cross FRW-6 (973) × C91538 (456) to 52.50 in FRW-6(973) × GS-129(1018) (Table 2). The specific combining ability (SCA) effects for this trait showed a wide range, from -96.44 to +79.64, with highly significant effects noted in the hybrids FRW-6(973) × GS-129(1018) and C91538(456) × GS-234(1703) (Table 4.2). In several crosses, significant heterosis was observed over both the mid-parent and better-parent, with the highest heterobeltiosis of 161.11% recorded in the hybrid POLF-19(1765) × PbD2-42(2789) (Table 5). For the number of seeds per capsule, the general combining ability (GCA) effects were positive and significant for the parents M-42(169), GS-129(1018), and POLF-17(1704), while negative GCA effects were recorded for FRW-6(973) (Table 4.1). The highest specific combining ability (SCA) effect of 1.17 seeds per capsule (Table 4.2)was found in the hybrid POLF-19(1765) × GS-234(1703), whereas the lowest SCA effect of -2.71 seeds per capsule was observed in the cross FRW-6(973) × C91538(456). Several hybrid combinations showed significant and positive heterosis over both the mid-parent and betterparent, with the highest heterobeltiosis of 4.98% seen in POLF-19(1765) × GS-234(1703) (Table 5). There was substantial variation in 1000-seed weight among the hybrids, with the highest average of 8.85 g recorded in the cross FRW-6(973) × POLF17 (1704), and the lowest of 5.01 g in M-42(169) × POLF-17(1704). The parents POLF-17(1704), FRW-6(973), and GS-234(1703) showed positive and significant general combining ability (GCA) effects for 1000-seed weight (Table 4.1). Several hybrids also exhibited significant positive specific combining ability (SCA) effects, with the highest SCA value of 1.40 g in FRW-6(973) × POLF17(1704) and the lowest SCA value of -1.58 g in M-42(169) × POLF-17(1704) (Table 4.2). Some hybrids demonstrated significant heterosis over both mid-parent and better-parent, with the highest heterobeltiosis of 22.45% observed in FRW-6(973) × POLF17 (1704) (Table 5). Seed yield per plant, an important economic trait, showed highly significant variation among the F1 hybrids, with yields ranging from 2.95 g in M-42(169) × PbD2-42(2789) to 7.95 g in M-42(169) × POLF-19(1765) (Table 2). According to the general combining ability (GCA) estimates, FRW-6(973) and POLF-19(1765) were identified as strong combiners for seed yield, suggesting their potential for use in breeding programs. The specific combining ability (SCA) effects for seed yield per plant varied considerably among the hybrids, with the highest SCA values of 3.76 g and 3.03 g seen in PbD2-42(2789) × GS-129(1018) and M-42(169) × POLF-19(1765), respectively (Table 4.2). Significant and favorable heterosis was observed in most of the crosses, consistent with the findings of [13 ]. The highest heterobeltiosis of 154.03% was recorded in the cross PbD2-42(2789) × GS-129(1018) (Table 5). Seed yield, a key economic trait, exhibited substantial variation among the parental lines and their F1 hybrids. The GCA/SCA ratio for seed yield (0.14) (Table 3).Further supported the dominance of non-additive genetic effects in controlling this trait's inheritance.

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