The objective of the studies was to identify potential parental genotypes or crosses that can be used in future breeding programs for developing promising genotypes. Gene action, combining ability and the extent of heterosis was estimated by analyzing diallel cross data between eight bread wheat parents, Pak81, LU26S, Inqliab91, Rohtas90, Barani83 Rawal87 and 5039. The data obtaind from the diallel progenies were analyzed following the methods given by Maher and Jinks (1982) and Hayman and Jinks (1954). Combining ability studies were conducted as described by Griffing (1956) and heterosis was estimated following Matzingar et al. (1962). The characters studied included plant height, flag leaf area, flag lead weight, specific flag lead area, specific flag lead weight, flag leaf venation, stomatal frequency, stomatal size, epidermal cell size, number of tillers per plant, spike length, number of spikelets per spike, spike density, number of grains per spike, 1000-grain weight, biomass per plant, grain yield per plant, protein content and harvest index.
Scaling tests were used to test the adequacy of the data for analysis using additive-dominance model. The test showed that except number of grains per spike, data was adequate or partially adequate for the characters under study. Analysis of variance of diallel tables displayed that additive (a) effects were significant for all the characters studied. However, dominant (b) effects were significant for most of the characters except flag leaf weight, flag lead venation, stomatal size, number of tillers pr plant and biomass per plant.
The estimate of average degree of dominance displayed partial dominance for stomatal frequency, spike length, number of spikelets per spike and spike density while over dominance was operative for plant height, and biomass per plant while over dominance was operative for plant height, 1000-grain weight, grain yield per plant, protein content and harvest index. Graphical representation of the data also supported the results obtained by the average degree of dominance.
Additive effects appeared to be more important than the dominant effects for flag leaf area, flag leaf weight, flag leaf venation, stomatal size, number of tillers per plant and biomass per plant, suggesting the importance of additive variation in the inheritance of these characters.
Mean squares due to GCA ere significant for all the characters and mean squares due to SCA were significant for most of the characters while non-significant SCA was recorded for flag leaf weight, flag lead venation, number of tillers per plant and biomass per plant. The relative magnitude of variation due to GCA and SCA indicated the importance of additive effects for all the characters except plant height, specific flag leaf area, specific flag leaf weight, spike length, number of spikelets per spike, spike density, 1000-grain weight and grain yield per plant where importance of dominant genetic effects was indicated.
Generally greater SCA effects were obtained in the crosses involving both parents with high GCA (high x high), which showed the possibility of genetic improvement of wheat plant through pedigree selection. Similarly cross showing high SCA and involving both parents as low general combiner (low x low) for a character indicated the involvement of epistasis or non-allelic interaction at the heterozygous loci. This situation was indicated in the cross Rawal87 x 5039 for epidermal cell size and Pak 81 x Rohtas 90 for biomass per plant. Therefore, utilization of these crosses through single plant selections in the later generations is suggested.
Crosses presenting high SCA and involving one parent with high GCA (low x high) indicated the involvement of additive x dominance gene interaction for the expression of the particular trait. This situation was indicated in the cross Rohtas90 x 5039 for 1000 grain weight, grain yield per plant and harvest index, and Barani83 x 5039 for plant height, and Chakwal86 x Rohtas90 for flag leaf area, and Pak 81 x Inqilab91 for spike length, and LU26S x Barni83 for spike density.
Heterosis was revealed in some of the crosses but its magnitude was generally low. The hybrids Rawal87 x 5039 showed positive heterosis over mid parent for flag leaf weight and epidermal cell size and Inqilab91 x Rawal87 showed positive heterosis over mid parent for 1000 grain weight, grain yield per plant and harvest index. The cross Inqilab91 x Rawal87 showed positive heterobeltiosis for 1000-grain weight, grain yield per plant and harvest index. As the cross exhibited heterobeltiosis for the most economical characters, therefore it is suggested that this cross (Inqilab91 x Rawal87) may be useful in hybrid breeding programme. It may be concluded from the present diallelic studies in spring wheat, that the genotypes, LU26S, Chakwal86 and 5039 showing additive gene effects and better estimates of GCA, SCA and heterosis would be useful in the development of promising breeding material.
Furthermore, it is suggested that wheat breeding and genetic studies maybe supplemented with the use of modern tools and technique of genetic engineering and biotechnology. The information thus derived would be more helpful in breeding programmes aiming at the evolution of modern wheat cultivars.