الفهرس 
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Abstract
The research presented in this study was designed to: 1) - Study the effect of nitrogen level on different maize traits and to identify the most associated characters with yield under low and high nitrogen levels. 2) - Test the genotypic differences among many maize genotypes in stress (low nitrogen level and to identify the best ones. 3) - Evaluate the role of general and specific combining ability of inbred lines in crosses performance under stress nitrogen level. 4) - Estimate the heritability and heterosis under both low and high nitrogen levels.
Genetical traits: General and specific combining ability, Expected gain from direct and indirect selection, Phenotypic (rph) of simple correlation coefficients and heritability.
Statistical analysis showed that nitrogen level had significantly affected all studied flowering, vegetative, yield and yield components. Normal nitrogen level for parents, resulted a significant decrease in all studied flowering and vegetative traits, except ear leaf area. On other hand, for crosses Decreasing nitrogen level decreased all flowering and vegetative traits, except Anthesis Silking Interval( ASI). Decreasing nitrogen level for parents and crosses, resulted a significant decrease in all studied yield and yield components traits, except shelling percentage.
Results showed that the (84) inbred line (P1) showed significant positive GCA effects (best general combiner) for ear height, plant height and ear diameter at normal nitrogen level and for plant height, ear height, 100-kernel weight and row number/ear at stress nitrogen level. (95) inbred line (P2) showed significant or highly significant positive GCA effects (best general combiner) for ear diameter at both nitrogen levels, kernels no./row at normal nitrogen level. (144) inbred line (P3) showed significant positive GCA effects (best general combiner) for ear leaf area at stress nitrogen level, ears number per plant, grain yield per plant and row number per ear at both nitrogen levels. (204) inbred line (P4) showed significant positive GCA effects (best general combiner) for plant height, ear height, ear leaf area, ears number per plant and grain yield per plant at both nitrogen levels, kernels number per row at normal nitrogen level. (213) inbred line (P5) showed significant positive GCA effects (best general combiner) for ear length and row number per ear at both nitrogen levels.
Results given in Table (21) revealed that the cross combinations viz., P1xP3,P1XP4 and P3XP4 at both nitrogen levels, show the highest positive significant heterosis over mid and better-parent for grain yield/plant,For, 100-kernel weight the highest positive significant heterosis over mid and better-parent were recorded by P2xP3 and P2xP4 at both nitrogen levels. For ear diameter, the highest positive significant heterosis over mid and better-parent were recorded by P1xP5 at both nitrogen levels. For kernels number/row, the highest positive significant heterosis over mid and better-parent were recorded by P1xP3 and P1xP4 at both nitrogen levels. For ear length the highest positive significant heterosis over mid and better-parent were recorded by P2xP3 and P2xP4 at both nitrogen levels.