Combining Ability for Grain Yield and Some Agronomic Traits in Diallel Crosses of Bread Wheat under Nitrogen Stressed Conditions

Six bread wheat genotypes and their 15 F1 crosses were evaluated under two nitrogen fertilization levels (40 and 80 kg N/fed) to estimate combining ability for days to heading, plant height, spike length, number of spikes/plant, kernels number/spike, weight of 100-kernels and grain yield/plant during 2018/19 season. Results revealed that significant nitrogen levels (N) mean squares were detected for all studied characters. Mean squares due to genotypes (G), parents and crosses for the studied characters were significant at both nitrogen levels and their combined analyses. Also, G x N interaction was highly significant for the studied characters. The general and specific combining ability variances were found to be significant for all studied characters at both-N-levels and their combined analyses. GCA/SCA ratios were less than unity for all studied characters at both N-levels and their combined data. The behavior of gene action varied from nitrogen level to another where the interactions of GCA and SCA with nitrogen levels were significant for most studied characters. Sakha 93 was identified as the most useful source for increasing favorable alleles for grain yield/plant, kernels number/spike and weight of 100-kernels under N-stress conditions, while Giza 168 was the best combiner for earliness at the two N levels. Cross combinations; Sakha 93 x Sids 12 and Sakha 93 x Misr 1 gave the best SCA effects for grain yield/plant under N-stress conditions. Significant positive correlations were detected between means of parents and F1 crosses with their GCA and SCA effects for most studied characters under the two nitrogen levels, respectively