Abstract

Introduction

Further advancement in the yield of wheat requires certain information regarding the nature of combining ability of parents available for use in the hybridization program, and also the nature of gene action involved in expression of quantitative and qualitative traits of economic importance. General and specific combining ability effects are very important in constructing the next phase of a breeding programme. Diallel analysis provides a unique opportunity to test a number of parental lines in all possible combinations. Thus, the main objective of the present study was to identify the best combiners and their crosses on the basis of their general and specific combining ability for yield and its component traits.

Materials And Methods

Ten varieties of hexaploid wheat (Triticum aestivum L. Thell), namely, Raj 3765, HD 2285, UP 2425, UP 2338, PBW 373, HD 2329, WH 542, Raj 1482, PBW 343 and Raj 3077 were crossed in all possible combinations excluding reciprocals. The 10 parents and their resulting 45 F₁’s and 45 F₂’s were grown in a randomized block design with three replications at Agricultural Research Station, Durgapura, Jaipur, Rajasthan, India. Plots of parents and F₁s consisted of two rows of 3 m length while each plot of F₂ consisted of four rows with the spacing of 30 cm between rows and 10 cm between plants. Ten competitive plants in parents and F₁’s and 30 plants in F₂ progenies were sampled randomly. Combining ability analysis was carried out using Method II, Model I as suggested by Griffing (1956).

Results and Discussion

Analysis of variance for combining ability revealed GCA and SCA were highly significant for all traits studied in both F₁ and F₂ generations (Table 1). Thus, additive and non-additive gene effects were important in controlling inheritance of all characters studied. However, the GCA:SCA ratio normally favoured SCA in all the traits except for days to heading in both F₁ and F₂ generations indicating the preponderance of non-additive gene effects in the genetic control of traits. Other studies also indicated a preponderance of non-additive effects (Menon and Sharma 1997).

Estimates of GCA effects revealed that parents PBW 373 and UP 2425 were the best general combiners for grain yield and good to average combiners for most of the yield component characters (Table 2). However, the rest of the parents were poor combiners for grain yield and average to poor general combiners for most of the yield contributing traits. Parents HD 2329, WH 542, Raj 3077 and UP 2338 were the best general combiners for harvest index, grain yield per spike, early heading and dwarfness, respectively. Similar parental trends for GCA effects were observed in both F₁ and F₂ generations. Parent PBW 373 was a good combiner for early heading, tillers per plant and 1000-grain weight whereas UP 2425 was a good general combiner for early heading, tillers per plant, flag leaf area and 1000-grain weight. Therefore, there is still further scope for improving combining ability for component traits, as none of high combiners for grain yield was a high combiner or at least an average combiner for the desirable traits. Raj 3765 was a good combiner for early heading, shorter grain filling period, spikes number and harvest index; HD 2285 for early heading, dwarfness and harvest index; and Raj 1482 for dwarfness and harvest index. The parent PBW 343 was inconsistent in combining ability in both generations. However, it was a good combiner for shorter grain-filling period, flag leaf area, grain yield per plant in the F₁ and dwarfness in the F₂ generations. Similarly, combining ability was inconsistent for Raj 3077. However, it was a good combiner for tillers per plant, grain yield per plant in the F₁, and flag leaf area and harvest index in the F₂ generation. The parents PBW 373 and UP 2425 could be utilized extensively in the hybridization programme to accelerate the pace of genetic improvement of grain yield in hexaploid wheat.

Table 1. Analysis of variance showing mean squares for combining ability for different characters in hexaploid wheat

*Significant at p=0.05 and ** Significant at p=0.01

Table 2. Estimates of general and specific combining ability effects for important characters in hexaploid wheat.

*Significant at p=0.05 and ** Significant at p=0.01; P₁ = Raj 3765, P₂ HD 2285, P₃ = UP 2425, P₄ = UP 2338, P₅ = PBW 373, P₆ = HD 2329, P₇ = WH 542, P₈ = Raj 1482, P₉ = PBW 343 and P₁₀ = Raj 3077

The estimates of SCA revealed that out of 45 crosses, 19 crosses in the F₁ and 15 crosses in the F₂ were found to be good specific combiners for grain yield. It is noteworthy that 14 crosses showed positive and significant SCA effects for grain yield in both F₁ and F₂ generations. The generation effects were also noticed in the SCA effects of the crosses. The highest positive significant SCA effect was exhibited by the cross Raj 3765 × HD 2285 in both generations. Other good combinations, which showed significant SCA effects for grain yield in both the generations were HD 2285 × PBW 343, Raj 3765 × UP 2338, PBW 343 × Raj 3077, Raj 3765 × Raj 1482, PBW 373 × HD 2329, PBW 373 × Raj 1482, UP 2425 × WH 542, UP 2425 × UP 2338 and UP 2425 × PBW 373 in both F₁ and F₂ generations. These crosses were higher yielding, and because in most crosses contained at least one good combiner parent, indicating that such combinations are expected to throw desirable transgressive segregants. All the best crosses for grain yield also showed average to high SCA effects for most of the yield components. It is, therefore, recommended that new materials should be used in future breeding efforts for recombining the desirable traits in the envisaged elite genotypes.

The study demonstrates that both additive (fixable) and non-additive (non-fixable) components of genetic variances were involved in governing inheritance although additive genetic variance was predominant. Therefore, biparental mating (Joshi and Dhawan 1966) and/or diallel selective mating (Jensen 1970) which allows intermating of selections in different cycles to exploit both additive and non-additive gene effect, could be useful in genetic improvement of characters of hexaploid wheat. Restricted recurrent selection by way of intermating the most desirable segregates followed by selection (Joshi 1979) may also prove to be an effective alternative approach to improving grain yield in this important species. Inclusion of F₁ hybrids showing high SCA and having parents with good GCA, into multiple crosses, could also prove a worthwhile approach for tangible advancement of grain yield in hexaploid wheat

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