A half diallel cross between eight common bread wheat varieties and/or lines (Triticum aestivum vulgare L.) was evaluated under three different nitrogen rates for six quantitative characters in RCBD with three replications. Nitrogen rates, genotypes, parents, hybrids, genotypes x nitrogen rates interaction and hybrids x nitrogen rates mean squares were significant for all traits under study. General and specific combing ability mean squares were found to be significant for all traits. High ratios which largely exceeded the unity were obtained for all studied traits except grain yield/plant in low and normal nitrogen fertilization rates indicating that large part of the total genetic variability associated with these traits was additive and additive by additive gene action. For the exceptional case GCA/SCA ratios was less than unity, therefore, it could be concluded that the large portion of the total genetic variability associated with this case is due to non-additive gene action. The largest heterotic magnitude express by the previous traits. May strength ened the conclusion about the 9 importance of non-additive gene effects in their inheritance. For grain yield/plant; eight, ten, eight and sit crosses expressed significant and positive ( ) effects in zero, low, normal nitrogen levels and the combined analysis, respectively. However, the most desirable ( ) effect were recorded for the crosses 3 x 6, 1 x 8, 2 x 4, 6 x 7 and 5 x 6 in the respective cases. Also, the cross 1 x 2 (8.40**) in zero nitrogen level, 2 x 4 (13.14**) in low nitrogen level and 1 x 8 (22.10**) in normal nitrogen level ranked the first best ( ) effects in the respective cases.
It could be concluded that the parental genotype P2 and P3 seemed to be the best general combiner for grain yield/plant and some of its components in the combined analysis of the three nitrogen rates. Also, P2 and P5 gave the best combiner for heading date. The most desirable ( ) effect were recorded by the crosses P3 x P6, P1 x P8, P2 x P4, P6 x P7 and P5 x P6 in the respective cases. Also, the cross 1 x 2 (8.40**) in zero nitrogen rate, P2 x P4 (13.14**) in low nitrogen rate and P1 x P8 (22.10**) in normal nitrogen rate ranked the first best ( ) effects.
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