Six bread wheat varieties representing different traits were crossed in a half-diallel in 2017/2018 season. The 6 parents and its 15 F1 crosses were evaluated under normal and N-stress conditions in 2018/2019 season, to study the mean performance and the combining ability for earliness and morpho-physiological traits and biomass yield of wheat under normal and N-stress conditions. Mean squares of genotypes, parents, crosses, and parents versus crosses were significant or highly significant for most of the studied traits and biological yield under both conditions, reflecting a sort of heterosis for these characters. Mean squares of GCA and SCA were significant or highly significant for most of the studied traits under both conditions, indicating the presence of both additive and non-additive types of genes in the genetic system controlling these traits. The best general combiners were P1 (Giza 168) and P2 (Sakha 94) at both conditions, P3 (Shandweel 1) at N-stress, and P4 (Gemmeiza 11) at normal condition for earliness and P6 (Misr 1) at both conditions and P5 (Sids 12) at N-stress and P3 (Shandweel 1) at normal condition for biomass yield plant-1. The best cross combinations were crosses No.3 (P1xP4), No.5 (P1xP6), No.6 (P2xP3), No.7 (P1xP2), No.8 (P2xP5) and No.11 (P3xP5) under N-stress for earliness; and crosses No.3 (P1xP4), No.7 (P2xP4), No.8 (P2xP5), No.12 (P3xP6), No.14 (P4xP6) and No.15 (P5xP6) at both conditions, and cross No.2 (P1xP3) under N-stress for biomass yield plant-1.

Six wheat varieties were single crossed at 2017/18. Six parents and its 15 F1 single-crosses were cultivated under N-normal and N-stress at 2018/19, to study heterosis and the genetic parameters for yield and its components under normal and N-stress. Mean squares of wheat-gentypes, parent, F1-crosses and parent versus cross were highly significant or significant for most of traits under both conditions. The highest desirable heterosis and heterobeltiosis were detected by crosses No. 7 and No.8 for earliness; No.14, No.8 and No.3 for 100-Grain weight; crosses No.8 and No.13 for number of spikelets/spike; crosses No.9, No.7 and No.13 for number of grains per spike; cross No.4 for number of spikes/plant; crosses No.3, No.12 and No.10 for grain yield per plant, therefore, it might be decided that these crosses may be beneficial for enhancing wheat grain-yield programs under low or high N conditions. The median grade of dominance was greater than one for all characters under both conditions, suggesting the importance of over dominance gene effects in the genetics of these traits. Heritability in broad sense (hb.s.) had high values for all traits under both conditions. Lower heritability estimates in narrow sense (h 2 n.s.) were detected for all the studied traits under both conditions, except each of spikes number /plant, weight of 100-grain and protein % at both conditions which had moderate values of narrow sense heritability, reflecting the part of environmental factors and dominance genes in heirloom scheme of these characters.

In order to determined of 6 bread wheat varieties representing different traits were crossed in a half-diallel in 2013/2014 season. The 6 parents and its15 F 1 crosses were evaluated under normal watering and water stress conditions in 2014/2015 season the objective of this investigation was aimed to study the combining ability and mode of gene action for some wheat traits under normal and stress conditions. Mean squares of genotypes were highly significant for all the studied traits. Both GCA and SCA variances were found to be highly significant for most traits under investigation at both conditions, indicating the importance of additive and non-additive effect in determining the performance of these characters. The ratio of GCA/SCA at the two conditions were more than unity for all studied traits, except heading date and flag leaf area under normal condition, and biomass yield per plant at water stress condition. This indicates that these traits are predominantly controlled by additive gene action, and it could be concluded that selection based on the accumulation of additive effects would be more effective in the early generations. P5 (Sids 14) at normal, P6 (Misr 2) under stress and combined data showed the highest biomass yield /plant. Among crosses, cross No. 12 (P3 x P6) under normal and combined data and cross No. 14 (P4 x P6) under stress showed the highest biomass yield /plant. P1 (Giza 168) and P2 (Sakha 93), at both normal and stress conditions were the best combiners for days to heading (earliness). P6 (Misr 2) was the best combiners for biomass yield per plant under both conditions. The best cross combinations for heading date (earliness) were crosses No. 1 (P1xP2), No. 6 (P2xP3) No. 10 (P3xP4) and No. 15 (P5xP6) at normal condition, crosses No. 5 (P1xP6) and No. 9 (P2XP6) under stress condition, and cross No. 2 (P1xP3) under both conditions. Also, the best cross combinations for biomass yield per plant were crosses No. 3 (P1xP4), No.8 (P2xP5) and No.15 (P5xP6) at both conditions, No. 10 (P3xP4) at normal watering condition, and No. 4 (P1xP5) and No. 14 (P4xP6) at water stress condition.