Combining Ability and Heterosis for Morpho-Physiological Characters on Bread Wheat (Triticum aestivum L.)

Wheat is the principal food crop in most areas of the world and also occupies prominent position in Indian agriculture after rice. It is nutritionally important cereal essential for the food security, poverty alleviation and for livelihoods. It is widely cultivated as staple food crop among the cereals and is contributing about 30% to the food basket of the country. India is the second largest producer of wheat in the world with the production around 75 million tonnes during the last decade and it is a major contributor to the food security system in India, occupying nearly 30.37 million hectares, producing 90.78 million tonnes and productivity 29.89q/ha and in Madhya Pradesh, grown in 5.56 million hectares with production of 13.37 million tonnes and productivity of 24.05q/ha (Anonymous 2014-2015). The substantial improvement in production is utmost necessary not only to meet ever increasing food requirement for domestic consumption, but also for export to earn foreign exchange. To feed the growing population, the country’s wheat requirement by 2030 has been estimated at 100 million metric tonnes and to achieve this target, wheat production has to be increased at the rate of <1per annum and this can be achieved through horizontal approach i.e. by increasing area under cultivation or through vertical approach i.e. varietal/hybrid improvement, which is one of the strongest tool to take a quantum jump in production and productivity under various agroclimatic conditions.


Introduction
Wheat is the principal food crop in most areas of the world and also occupies prominent position in Indian agriculture after rice. It is nutritionally important cereal essential for the food security, poverty alleviation and for livelihoods. It is widely cultivated as staple food crop among the cereals and is contributing about 30% to the food basket of the country. India is the second largest producer of wheat in the world with the production around 75 million tonnes during the last decade and it is a major contributor to the food security system in India, occupying nearly 30.37 million hectares, producing 90.78 million tonnes and productivity 29.89q/ha and in Madhya Pradesh, grown in 5.56 million hectares with production of 13.37 million tonnes and productivity of 24.05q/ha (Anonymous 2014(Anonymous -2015. The substantial improvement in production is utmost necessary not only to meet ever increasing food requirement for domestic consumption, but also for export to earn foreign exchange. To feed the growing population, the country's wheat requirement by 2030 has been estimated at 100 million metric tonnes and to achieve this target, wheat production has to be increased at the rate of <1per annum and this can be achieved through horizontal approach i.e. by increasing area under cultivation or through vertical approach i.e. varietal/hybrid improvement, which is one of the strongest tool to take a quantum jump in production and productivity under various agro-climatic conditions. Triticum aestivum (bread wheat) (2n=6x=42) is an allohexaploid produced from two separate naturally occurring hybridization events. The initial hybridization, that occurred between the two grass species Triticum urartu (2n=2x=14) (the A genome donor), and Triticum speltoides (2n=2x=14) (B genome donor). This new species would have been tetraploid wheat (2n=4x=28) viz., Triticum turgidum var. durum (Durum wheat). Hexaploid wheat arose as a result of a second hybridization between the new tetraploid and a third diploid species (2n=2x=14) viz., Triticum tauschii (D genome donor).

Results and Discussion
Line × tester analysis techniques has been extensively used in almost all the major field crops to estimates GCA and SCA variances and effect and to understand In the present research work, variances due to both lines and testers were significant for days to heading, plant height, tillers per plant, spike length, weight of spikes per plant, grains per spike, test weight, grain yield per plant, canopy temperature index, biological yield and harvest index. However, lines were significant for days to maturity, 1st inter-node length, weight of grains per spike. Variances due to Line × tester were significant for all the 14 characters [2,3] also reported similar results). In the present study both GCA and SCA variances were significant for most of the characters, thereby, indicating importance of both additive as well as non additive components of genetic variance in the control of these traits (Table 1). Singh et al. [2] & Kumar et al. [4] also reported similar results.

Genetic components
Genetic component of variances is assessed through the estimates of GCA and SCA variances. The additive genetic variance is equal to GCA variance and dominance variance is equal to SCA variance. In the present research, the genetic component's magnitudes showed that the additive components of variances were higher compared to dominance components of variances for all characters. The ratio of genetic components "σ2gca/ σ2sca" also showed more than one, indicating predominance of additive variances for almost all characters ( Table 2). Present finding are in confirmation with Kandil et al. [5], Khiabani et al.

Combining ability
Combining ability plays major role in the evaluation of inbred in terms of their breeding values; and this will help to decide efficient breeding method to be applied in segregating generation.

General combining ability effects
It is primarily a function of additive genetic variance; it helps in the selection of suitable good general combining parents for hybridization. Data presented in Table 3 revealed that 5 lines viz., 33rd ESWYT150, PBW 658, KB 2013-03, KB 2013 and VW921 reported significant and positive GCA effect for grain yield, test weight, weight of spike/plant, biological yield and harvest index, thereby, suggesting good general combiner for these traits. Tester 'GW273' was good general combiner for grain yield, test weight and biological yield, whereas, tester SUJATA registered significant and positive GCA effect for grain yield, days to heading, spike length and harvest index, thereby, suggested good general combiner for these traits. Lines 33rd ESWYT150 was also reported significant and positive GCA effect for tillers/plant and weight of grains/ plant. Line KB2013-03 also exhibited significant positive GCA effect for plant height, weight of grain/spike and grains/spike. Significant positive GCA effect was reported in KB2013 and VW921 for canopy temperature index. These genotypes can be used in the development of high yielding varieties through the pedigree selection and progeny selection or mass selection in later generations in promising segregating generations in wheat Present finding are in confirmation with Kandil et al. [5], Samier and Ismail (2015)

Specific combining ability Effects
It is mainly a function of dominance variances; it helps in the identification of superior cross combination for commercial exploitation of heterosis. In self-pollinated crops like wheat, SCA effects are not much important as they are mostly related to non-additive gene effects excluding those of arising from complementary gene action or linkage effects they cannot be fixed in pure lines. Further superiority of the hybrids might not indicate their ability to yield transgressive segregates; rather SCA would provide satisfactory criteria. However, if a cross combination exhibiting high SCA as well as high per se performance having at least one parent as good general combiner for a specific trait, it is expected to throw desirable transgressive segregants in later generations. Data presented in Table 4 revealed that 19 crosses showed significant and positive SCA effect, thereby, indicating good specific combinations for grain yield and other attributing traits. Crosses viz., KB2013-03×SUJATA', KB2013×GW273', PBW658×SUJATA' and 'VW921×SUJATA' registered best good specific combiner for grain/yield because these crosses were the results of good x good general combiners and reported

Heterosis
The aim of estimation of heterosis in the present study has to spot out the best combination of parents giving high degree of useful heterosis and characterization of parents for their genotypic worth for future use in breeding programme. All characters had shown considerable amount of heterosis over better parent (Heterobeltiosis) and mid parent (relative heterosis). Present finding are in confirmation with Devi et al. [10]. The degree of heterosis however differed for different characters. Data presented in Table 5 revealed that 4 cross viz., 'KB2013×RVW4106', '33rd ESWYT150×RVW4106', 'KB2013-03×SUJATA' and 'KB2013-03×RVW4106' recorded significant heterobeltiosis and relative heterosis in positive direction for grain yield, while another 3 cross '33rd ESWYT150×GW366', '45th IBWSN1021×RVW4106' and 'PBW658×RVW4106' recorded significant positive relative heterosis for grain yield. Devi et al. [10] also reported similar result [11][12][13][14][15]. Cross 'KB2013×RVW4106' was also reported significant positive heterobeltiosis and relative heterosis for weight of spike/plant and test weight. Cross '33rd ESWYT150×RVW4106' also showed significant heterobeltiosis and relative heterosis in positive direction for tillers/plant, weight of spike/plant, test weight and biological yield. Cross 'KB2013-03×RVW4106' was also reported significant positive heterobeltiosis and relative heterosis for days to maturity, test weight and biological yield [16][17][18]. Cross 'KB2013-03×SUJATA' showed significant positive relative heterosis for 1st inter-node length, weight of spike/ plant, test weight, biological yield and harvest index, while significant positive heterobeltiosis was reported for test weight and canopy temperature index. Cross '33rd ESWYT150×GW366' was also reported significant positive relative heterosis for weight of spike/plant, test weight, and biological yield. Cross '45th IBWSN1021×RVW4106' was also reported significant positive heterobeltiosis and relative heterosis for biological yield. Cross 'PBW658×RVW4106' also showed significant positive relative heterosis for biological yield. Further it was noticed that both significant heterobeltiosis or/and relative heterosis were recorded for attributing traits without showing heterosis for grain yield [19][20][21][22][23].
Additive variance played the major role in determining all of the characters. Some of the cross recorded significant heterosis for grain yield, thereby, [31,32] indicating further scope of hybrid development in near future. The crosses KB2013-03XSUJATA was desirable for selecting higher yielding and short duration plant, while cross KB2013XRVW4106 was desirable for the selection of high yielding dwarf plant over the better parent. The crosses KB2013-03XRVW4106, KB2013XSUJATA and KB2013-03XSUJATA were suitable for selection of high yielding and short duration plant over the mid parent. The cross ESWYT 150 X RVW4106 was desirable for selecting higher yielding and short duration plant over both better and mid parent [33].

Suggetions for Further Work
The following relevant suggestions could be drawn for further scope of improvement from the present study: Good general combiners like 33 rd ESWYT150, PBW658, KB2013-03, KB2013 and VW921, GW273 and SUJATA could be used in the development of high yielding varieties through the simple/recurrent selection from promising segregating cross generations in wheat. New recombinant promising single plants may be selected in F2 generations of good cross 33 rd ESWYT150XRVW4106, KB2013-03XSUJATA and 33 rd ESWYT150XGW 366 involving one of the good specific combining parents. The cross 33 rd ESWYT 150XRVW4106 was found higher yielding and short duration plant over both better and mid parent. Thus it was very desirable for further breeding programme while selecting the genotype [34,35].