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Diallel Analysis of Wheat Grain Protein Content and Yield in F1 and F2 Generations under Contrasting Nitrogen Conditions

Grain yield and grain protein content (GPC) of bread wheat are crucial determinants of its profitability and product quality. In general, low levels of nitrogen fertilizer result in a lower protein content in wheat grains and lower grain yield. The main objective of this investigation was to get information on gene action, heritability and expected genetic advance from selection (GA) for GPC and grain yield/plant (GYPP) under contrasting soilnitrogen conditions. Diallel crosses in F1 and F2 generations among six contrasting wheat parents in GPC and GYPP were evaluated in field experiments across two seasons in a split plot design under two N levels, i.e. low-N (0kg N/fed) and high-N (75kg N/fed). In both F1's and F2's, the magnitude of dominance (H1) was much greater than that of additive (D) component for GPC under both high N and low N and GYPP under high-N, while the opposite was true for GYPP in F1's under low-N, where the additive was greater than dominance variance and GYPP in F2's under low-N, where the additive and dominance components were approximately equal. The average degree of dominance (H1/D) 1/2 in F1's was in the range of over-dominance, except for GYPP under lowN, which showed partial dominance, but in F2's was in the range of partial dominance for GYPP under both high and low N and GPC under high N, while GPC under low N showed no dominance. Narrow-sense heritability (h2n) in F1's and F2's was generally of low magnitude; it ranged in F1's from 6.18% (GPC under low-N) to 20.41% (GYPP under high-N); the exception was GYPP under low-N, where h2n was of high magnitude (67.14%) and ranged in F2's from 10.86% for GPC under low N to 46.90% for GYPP under low-N. The h 2 n was higher under low--N for GYPP and under high-N for GPC. In F1's, the values of GA were higher under low-N than under highN for GYPP, but were higher under high-N than low-N for GPC. In F2's, the values of GA for both traits were higher under low-N than under high-N, suggesting that it is better to practice selection in F2's for both traits under low-N conditions to obtain higher values of selection gain.


I. INTRODUCTION
Wheat (Triticum aestivum L.) is an important source of both carbohydrates and protein in human and livestock nutrition (Shewry, 2007). It is estimated to contribute as much protein as the total annual soybean crop, or 60 million tonnes of protein per year (Shewry, 2007). Grain yield potential and grain protein content (GPC) of a wheat crop are crucial determinants of its profitability and product quality.
Grain protein content is an important quality factor in wheat, as it helps to determine milling and baking quality (Bushuk 1977). Protein content of wheat is largely influenced by cultivar, available N, moisture and temperature conditions under which the crop is grown (Fowler et al. 1990 and Campbell et al. 1977). Changes in protein content with application of fertilizer N differ with cultivar (Clarke et al. 1989 andFowler et al. 1990). Semi dwarf wheat cultivars show a smaller increase in grain protein with small applications of N fertilizer than do cultivars of conventional height due to greater yield potential of semi dwarf wheats (Campbell et al. 1971 andClarke et al. 1989).
In general, high levels of nitrogen fertilizer result in a higher protein content in wheat grains and higher grain yield (Ortiz-Monasterio et al., 1997). Genotypic variability was found for wheat grain protein content and grain yield. Therefore a concomitant increase in grain yield and grain protein seems feasible (Bänziger et al., 1992). However, there are problems associated with N fertilizer use, because N can leach and cause eutrophication of water (Vitousek et al., 1997), and N fertilization increases emissions of the greenhouse gas nitrous oxide (N 2 O) from agricultural soils (Bouwman et al., 2002).

Field Evaluation of 6 Parents, 15 F1's And 15 F2's
In the seasons 2007/2008, 2008/2009, parents (6), F 1 's (15) and F 2 's (15) were sown on 17 th of November each season in the field of Noubarya Res. Stat., under two levels of nitrogen fertilizer; the low level was without fertilization (LN) and the high level was 75 kg Nitrogen/ feddan (HN); this is the recommended level of Ministry of Agriculture. This level of nitrogen fertilizer (168 kg Urea/fed) was added in two equal doses, the first dose was added just before the sowing irrigation and the second dose just before the second irrigation (21 days after irrigation). In this experiment, a split plot design in lattice (6x6) arrangement was used with three replications. The two levels of nitrogen were allotted to the main plots and the genotypes to the sup plots. Each parent or F 1 was sown in two rows and each F 2 was sown in four rows; each row was three meter long; spaces between rows were 30 cm and 10 cm between plants, and the plot size was 1.8 m 2 for parent or F 1 and 3.6 m 2 for F 2 . All other agricultural practices were done according to the recommendation of Ministry of Agriculture for growing wheat in Noubarya region.
Available soil nitrogen in 30 cm depth was analyzed immediately prior to sowing and N application at the laboratories of Water and Environment Unit, ARC, Egypt in the two seasons. Soil nitrogen was found to be 55  The soil analysis of the experimental soil at Noubarya Research Station, as an average of the two growing seasons, indicated that the soil is sandy loam (67.86% sand, 7.00% silt and 25.14% clay), the pH is 8.93, the EC is 0.55 dSm -1 , the soluble cations in meq l -1 are Ca 2+ (5.30), K + (0.70), Na + (0.31), Mg 2+ (2.60) and the soluble anions in meq l -1 are CO 3 2-(0.00), HCO 3 -(2.10), Cl -(5.30) and SO 3 2-(1.51).

Data Recorded
The following grain characters were measured on a random sample of each entry each replicate. 1. Grain protein content (GPC) measured as follows: GPC%= N g x 5.70 according to AACC (2000), where N g is grain nitrogen content. Grain samples were ground in powder and nitrogen of grains (N g ) was determined using

IV. BIOMETRICAL AND GENETIC ANALYSES
The analysis of variance (ANOVA) of the split plot design was performed on the basis of individual plot observation using the MIXED procedure of SAS ® (Littell et al., 1996). Combined analysis of variance across the two seasons was also performed if the homogeneity test was non-significant. Moreover, each environment (HN and LN) was analyzed separately across seasons as lattice design for the purpose of determining genetic parameters using GENSTAT 10 th addition windows software. Least significant differences (LSD) values were calculated to test the significance of differences between means according to Steel et al. (1997).

Estimating Combining Ability In F 1 's And F 2 's
Diallel crosses in F 1 and F 2 generations were analyzed to estimate general (GCA) and specific (SCA) combining ability variances and effects for studied traits according to Griffing (1956) Model I (fixed effect) Method 2.

Estimating Genetic Components And Ratios In F1's
Although Griffing's analysis was based on Model I (fixed effect) since parents of the diallels in this study were selected in purpose for the validity of diallel analysis, Hayman's approach (that assumes random model) was used to estimate genetic components and ratios The conclusions obtained from Hayman's analyses will not be generalized, but will help us to characterize our genetic material for its proper use in the future breeding programs. The genetic parameters and ratios were calculated according to methods developed by Jinks and Hayman (1953), Jinks (1954) and Hayman (1954a and b) and described by Sharma (2003). The variance and covariance statistics across replications were used to obtain estimates of the components of variation and their respective standard errors. The validity of the assumptions of diallel analysis was tested by the following formula (Sharma, 2003 Where: n = number of parents.The significance of (b) different from zero (t1) and from unity(=1) (t 2 ) can be tested by t-test as under: t 1 = (b-0)/SEb and t 2 = (1-b)/SEb The foregoing values were tested against the ‛‛t" tabulated value for (n-2) degrees of freedom according to (Jinks and Hayman, 1953). If all the assumptions were valid, the regression coefficient would be significantly different from zero but not from unity. Hayman (1954 a and b), derived the expectations for the statistics calculated from the  F1 diallel table and the  The following genetic parameters were also calculated: Average degree of dominance is estimated as (H 1 /D) 1/2 1. If the of this ratio is zero, there is no dominance. 2. If it is greater than zero, but less than one, there is partial dominance. 3. If it is equal to 1, there is complete dominance. 4. If it is greater than 1, it indicates over dominance.

Ratio of dominant and recessive genes in the parents (K D /K R )
is estimated as follows: .0, it means nearly equal proportion of dominance and recessive alleles in parents, i.e. symmetrical distribution; p = q = 0.5. Any deviation from 1.0 indicates asymmetry of distribution (p # q). Thus: Ratio > 1 refers to excess of dominant alleles and minority of recessive alleles (p > q). Ratio < 1 means minority of dominant alleles and excess of recessive alleles (p < q). The ratio of dominant genes with positive or negative effects in parents (H 2 /4H 1 ) was determined. The maximum theoretical value of 0.25 for this ratio arises when, p = q = 0.5 at all loci. A deviation from 0.25 would stem when p ≠ q Thus: if this ratio ≈ 0.25, it means symmetrical distribution of positive and negative dominant genes in parents, while if this ratio ≠ 0.25, it means asymmetry of distribution. Narrow-sense heritability (h 2 n) was estimated using the following equation: h 2 n = [1/4D / (1/4D + 1/4H 1 -1/4F + Ê]. Expected genetic advance from selection (GA). The expected genetic advance (GA) from direct selection as a percentage of the mean (x) was calculated according to Singh and Narayanan (2000) based on 1% selection intensity as follows: GA = 100[(k.h 2 n δ ph )/x] Where: k = 2.64 (selection differential for 1% selection intensity), and δ ph = square root of the dominator of the narrow sense heritability.

Estimating Variance Components And Ratios In F2's
Hayman's approach was performed for estimating variance components and ratios in F 2 generation of the same diallel crosses, based on the same assumption proposed in F 1 , but with the following modifications in F 2 for some genetic parameters (according to Sharma, 2003)

Analysis of Variance
Combined analysis of variance of a lattice design for studied traits under each environment (high N and low N) across two seasons is presented in highly significant for both traits under the two levels of nitrogen, indicating the presence of significant heterosis inbreeding effects for the two studied grain traits. Mean squares due to the interaction G x Y were highly significant for both studied traits under the two levels of nitrogen, except for GPC under low N, which were not significant. Mean squares due to the interaction P x Y under high level of nitrogen were significant or highly significant for GYPP and non significant for GPC. Mean squares due to the interactions F 1 's x Y and F 2 's x Y under high-N were significant or highly significant for both studied traits. Mean squares due to the interactions F 2 ' s x Y were significant or highly significant for both studied traits under low N. Mean squares due to the interactions P's vs F 1 's x Y and F 1 's vs F 2 's x Y under the two levels of nitrogen were significant and highly significant for both studied traits, except GPC for F 1 's vs. F 2 's x Y.

Effects of low N on grain characters
A comparative summary of means of the two studied traits across all 36 genotypes (6 parents, 15 F 1 's and 15 F 2 's) subjected to two levels of nitrogen conditions and across two years is presented in Table (2). In general, low N caused a significant reduction in both studied traits, namely GYPP and GPC). Mean grain yield/plant (GYPP) was significantly decreased due to low-N by an average of 18.96, 21.17, and 15.40% for parents, F 1 's and F 2 's, respectively. Reduction in grain yield of wheat due to low soil nitrogen was reported by several investigators. A positive relationship between N application levels and the grain yield has already been shown in many studies (Austin et al., 1980 andDesai andBahatia, 1978). In general, high levels of nitrogen fertilizer result in a higher protein content in wheat grains (Ortiz-Monasterio et al., 1997).  Moreover , low nitrogen caused a significant reduction in grain protein content (GPC) by 21.3, 14.71 and 13.12% for parents, F 1 's and F 2 's, respectively. Reduction in grain protein content due to low soil-N is predicted due to the important role of N in protein synthesis. This conclusion was previously reported by previous investigators (e.g Fowler et al. 1990).

Effect of genotypes
Ranges of means of the two studied traits of 6 wheat parents and their 15 F 1 and 15 F 2 diallel crosses across studied N levels and across two seasons are presented in Table (3). In general, parents varied in both studied traits, especially in grain protein content (GPC), indicating their usefulness as parents of diallel crosses for studying inheritance of these traits. The parental line L26 showed the highest means for GPC and GYPP (16.45% and 29.16 g, respectively ). On the contrary, the parent Gemmeiza 9 showed the lowest mean for GYPP (21.83g) and GPC (96%). In general, the first three parents L25, L26 and L27 show significantly higher means than the second three parents Gem7, Gem 9 and Gz168 for GYPP, and GPC traits. Such significant differences among wheat parents in this study are prerequisite for the validity of using them as parents of diallel crosses to study the inheritance of both traits. The studied diallel crosses varied greatly in both studied traits either in F 1 or F 2 generation ( Table 3). The cross Gem 7 x Gem 9 exhibited the lowest means in F 1 for GYPP and in F 2 for GPC. The cross L25 x Gem 9 exhibited the lowest GYPP in F 2 . On the contrary, the highest means were exhibited by the cross L26 x L27 for GYPP and GPC in the F 1 , L2 x Gz 168 for GYPP, in F 2 and L1 x Gz 168 for GPC in F 2 . It is interesting to mention that the rank of crosses for GYPP and GPC differed from F 1 to F 2 generation.

Genotypes
In general, F 1 crosses showed higher means for GPC and GYPP than their parents (Table 3), indicating that heterozygotes exhibit better (more favorable) values for these two traits than homozygotes, which is logic and may be attributed to heterosis phenomenon.
On the contrary, F 2 crosses exhibited lower means than their corresponding F 1 crosses for GYPP, indicating the role of inbreeding depression in this trait, while for GPC trait, the F 2 's were higher than both F 1 's and parents, indicating appearance of transgressive segregants for grain protein content trait. Thus, selection in F 2 could improve grain protein content in wheat.

Genotype x nitrogen interaction
Means of each parent, F 1 cross and F 2 cross for studied traits under two nitrogen levels (0 and 75 kg N /Fed) across two seasons are presented in Table ( 4). In general means of GYPP and GPC of the three parents L25 , L26 and L27 were higher in magnitude than those of the three other parents Gem 7, Gem 9 and Giza 168 under both high-N and low-N levels. Reduction in GYPP and GPC, due to low-N stress was lower in the first three parents than that in the latter parents. The first three parents (L25, L26 and L27) were therefore considered as low-N tolerant (N-efficient) genotypes and the latter ones (Gem 7, Gem 9 and Giza 168) as low-N sensitive (N-inefficient ) parents. The rank of crosses in F 1 and F 2 generation for most studied traits was changed from one environment (N-level) to another. The highest mean of GYPP under low-N was obtained from L26 x L27 followed by L25 x L26 and L25 x L27 in F 1 and L25 x L27 followed by L25 x L26 and L26 x Gz 168 in F 2 generation. These crosses also showed the lowest reduction due to low-N stress and the highest NUE means, and therefore were considered tolerant (Nefficient) to low-N stress. The highest mean of GPC under low-N was shown in L26 x L27 (16.83%) and L27 x Gem9 (15.62%) in F 1 and L26 x Gz168 (17.49%) followed by L26 x Gem9 (16.94%) in F 2 generation. It is observed that the cross L26 x L27 in the F 1 showed the highest GYPP and the highest GPC. This means that it is possible to improve both traits (GYPP and GPC) simultaneously. Several authors described the negative relationship between the percentage of grain protein and grain yield (Cox et al., 1985a andLöffler andBusch, 1982). Implications to overcome the negative correlation between the percentage of grain protein and grain yield were reviewed by Feil (1997). Increased efficiency of N utilization is realized when the nitrogen concentration in the kernels increases and grain yield remains stable (Kramer, 1979). Genotypic variability was found for N accumulation and/or N remobilization. Therefore a concomitant increase in grain yield and grain protein seems feasible (Bänziger et al., 1992).
On the contrary, the three crosses Gem 7 x Gem 9, Gem 7 x Gz168 and L27 x Gem 9 in F 1 and F 2 generations showed the lowest GYPP under low-N, and high reduction due to low-N and therefore were considered sensitive (Ninefficient) to low-N stress. Changes in protein content with application of fertilizer N differ with cultivar (Clarke et al. 1989 andFowler et al. 1990). Semidwarf wheat cultivars show a smaller increase in grain protein with small applications of N fertilizer than do cultivars of conventional height due to greater yield potential of semidwarf wheats (Campbell et al. 1971 andClarke et al. 1989).
The rank of parents for GYPP and GPC was similar in the two N-environments, indicating less effect of interaction between parent and nitrogen level on these traits. The three tolerant parents showed the highest GYPP under high-N and therefore were considered responsive parents. Moreover, L26x L27 and L25 x L27 in F 1 and L26 x Gz 168 in F 2 generation had the highest GYPP under high-N and are therefore considered responsive crosses.

In F 1 's
Variances estimates for general (GCA) and specific (SCA) combining ability of the F 1 diallel crosses of wheat for combined data across two years under high and low levels of nitrogen are presented in Table ( 5). Mean squares due to GCA and SCA were highly significant, for the two studied traits, indicating that both additive and non-additive gene effects play an important role in the inheritance of both studied traits under different N application rates. In the present study, the magnitude of GCA mean squares was higher than that of SCA, since the ratio of GCA/ SCA mean squares was higher than unity for both studied traits under the two levels of N. The mean squares due to GCA x year was higher than those due to SCA x year for both studied traits, except for GPC under high-N, suggesting that GCA (non-additive) variance is more affected by year than SCA (additive) variance in most studied cases.

In F2's
Analysis of variance of general (GCA) and specific (SCA) combining ability of F 2 crosses of wheat for combined data across two years under high and low levels of nitrogen are presented in Table (6). Results of F 2 crosses also show highly significant estimates of GCA and SCA mean squares under both high-N and low-N for both studied traits. The ratio GCA/SCA mean squares was greater than unity for both studied traits of F 2 crosses under both high-N and low-N conditions. These observations are in partial conflict with data reported by Le Gouis et al. (2002) who in Nlimited diallel F 1 hybrids between modern French cultivars found markedly higher GCA/SCA ratios for grain yield, grain N yield and total above ground N than in those grown under high-N nutrition. More recently, a similar preponderance of GCA effects for N uptake and NUTE was identified in F 2  Results indicate that mean squares due to GCA x year and SCA x year interactions in F 2 's were significant or highly significant in the two levels of N, except for GPC under low-N, indicating that the additive and non-additive gene effects in most cases were affected by years. The mean squares due to GCA x year were higher in magnitude than those due to SCA x year for GPC of F 2 crosses, under both high and low N and, suggesting that GCA (additive variance) is more affected by year than SCA for GPC of F 2 crosses. The opposite was true for GYPP of F 2 's.

GCA Effects In F 1 's
Estimates of general combining ability (GCA) effects of parents for studied traits under the two levels of nitrogen across two years are presented in Table 7. In general, the best general combiners in F 1 's for GYPP and GPC attributes were L27 followed by L26 parents under both high-N and low-N, L25 under low-N and Gem.7 under high-N. On the contrary, the worst general combiners in F 1 's were Giza 168, Gem9 and Gem7 and for the two studied traits under both high-N and low-N environments. It is worthy to note that the best general combiners in this study (L25, L26 and L27) showed also high per se performance for the two studied traits under both high and low-N environments.

GCA Effects Of Parents In F2 Crosses
Estimates of general combining ability (GCA) effects calculated from the analysis of F 2 diallel crosses under the two levels of N are presented in Table ( 8). Data in Table (8) indicates that under low-N and high-N, the best general combiners based on F 2 diallel analysis for both traits were L27 followed by L26 and then L25 for GYPP under low-N and Gem7 for GPC under high-N. The best combiners identified from both F 1 and F 2 diallel analyses under high-N and low-N are more or less similar in most cases under low-N conditions. L25, L26 and L27 are generally the best combiners for grain protein content and grain yield traits based on diallel analyses of both F 1 and F 2 crosses. These parents are expected to have more additive genes for the studied grain quantity and quality characters. Estimates of specific combining ability (SCA) effects of the F 1 crosses for the studied traits under the two levels of N are presented in Table (9). The rank of F 1 crosses for SCA effects was changed from high-N to low-N conditions. The best cross for SCA effects ofr GPC was the F 1 cross L25 x L26, L27 x Gem7 and Gem7 x Gem9 under high-N and L25 x L26, L26 x Gz168 and Gem7 x Gem9 under low-N. For GYPP, the best combiner was L27 x Gem7 under high-N and L25 x Gz168 under low-N. These F 1 's include at least one parent of high GCA effects.

In F2's
Specific combining ability (SCA) effects of the F 2 crosses under two levels of N are presented in Table (10). The best F 2 cross in SCA effects for GPC was L25 x L25, L27 x Gem7, L27 x Gem9, L27 x Gz168 and Gem7 x Gz168 under high-N and L25 x L26, L27 x Gem9 and L27 x Gz168 under low-N. For GYPP, the best F 2 cross was L27 x Gem9 and L27 x Gz168 under high-N and L25 x Gem7 under low-N.

In F 1 's
Estimates of genetic components and ratios for studied traits in F 1 's under high-N and low-N environments across two years are presented in Table ( 11). The dominance genetic component of variation (H 1 ) was highly significant for both studied traits under both high and low-N environments, indicating that the dominance gene effects in F 1 's of this experiment are important for the inheritance of GPC and GYPP traits under low-N and high-N. The additive component of variation (D) was also significant (P≤ 0.01or 0.05) for both traits in F 1 's under both high-and low-N, indicating that selection may be efficient for improving most studied traits.

Table11. Estimates of genetic parameters and ratios for studied traits under high and low-N in F 1 's across two
seasons. The overall dominance effects of heterozygous loci in Hayman's model (h 2 ) controlling both studied traits of F 1 's under both high-and low-N environments, were significant (P< 0.01 or 0.05); that could be due to the presence of a considerable amount of dominant genes for GPC and GYPP in the parental genotypes.
The average degree of dominance (H 1 /D) 1/2 in F 1 's was in the range of over-dominance (greater than unity) for both studied traits under the two levels of nitrogen, except for GYPP trait under low-N, which showed partial dominance.
Lower ratio of (H 2 /4H 1 ) than 0.25 (Table 11) indicated symmetrical distribution of positive dominant genes in parents for both studied traits of F 1 's under both high-and low-N environments.
The ratio (K D /K R ) in F 1 's was less than unity for both traits under the two levels of nitrogen, indicating minority of dominant alleles and the excess of recessive alleles (p<q).
Number of genes or groups of genes controlling the inheritance of a given trait (h 2 /H 2 ) was one group of genes for both studied traits (GPC and GYPP) of F 1 's under both high-N and low-N environments (Table 11).
Broad-sense heritability (h 2 b ) in F 1 's for both studied traits in this experiment was of high magnitude and ranged from 77.26% (GPC under low-N) to 91.27% (GYPP under high-N) ( Table 11), indicating that environment had a small effect on the phenotype of F 1 's for both studied traits. Grain protein content (GPC) trait showed smaller h 2 b value under both high and low N environments than GYPP, indicating a larger effect of environment on this trait (GPC).
Narrow-sense heritability (h 2 n ) in F 1 's ( Table 11) was generally of low magnitude and ranged from 6.18% (GPC under low-N) to 20.41% (GYPP under high-N). The exception was GYPP under low-N, where h 2 n was of high magnitude (67.14%). The big difference between broad-and narrow-sense heritability estimated from F 1 's in this experiment could be attributed to the high estimates of dominance as compared to additive component, except GYPP under low-N. It is observed that narrow-sense heritability (h 2 n ) in F 1 's of the present study was generally higher in magnitude under low-N than under high-N for both studied traits. This increase in h 2 n under low-N compared to high-N was more pronounced in GYPP.
Our results are in agreement with some researchers, e.g. Blum (1988 a and b), Hefny (2007), Al-Naggar et al. (2008, 2009, 2011a,b, 2014 and 2015), who support the idea that heritability is higher under stressed than nonstressed environment. On the contrary, other investigators reported that heritability is higher under good (nonstressed) environment ( Rosielle and Hamblin, 1981, Atlin and Frey, 1990, Banziger and Laffite, 1997, Banziger et al., 1997and Worku, 2005. Expected genetic advance (GA) from selection (based on 1% selection intensity) across two years ranged from 5.11% for GYPP under high-N to 19.70% for GYPP under low-N. In general, the values of GA were higher under low-N than under high-N for GYPP, but were higher under high-N than low-N for GPC. These results indicated that to improve grain yield in the present germplasm, it is better to practice selection under low-N conditions to obtain higher values of selection gain.

In F2's
Genetic parameters and ratios for studied traits in F 2 's under high-and low-N environments across two years are presented in Table ( 12). Additive genetic variance (D) was highly significant for both studied traits under both high and low-N environments, indicating that the additive gene effects in F 2 's of this experiment are important for the inheritance of both studied traits under low-N and high-N, and that selection may be practiced in this segregating generation of the studied cross populations of wheat for improving GPC and GYPP traits. It is observed that the magnitude of additive variance was higher under low-N than under high-N conditions in both studied traits.
The dominance genetic component of variation (H 1 ) in F 2 's was highly significant for both studied traits under both high and low-N environments, indicating that the dominance gene effects in F 2 's of this experiment are important for the inheritance of GPC and GYPP traits under low-N and high-N. It is also observed that the magnitude of dominance variance was much higher under low-N than under high-N conditions in GPC trait.
The magnitude of dominance (H 1 ) in F 2 's was much greater than that of additive (D) variance for GPC under both high N and low N and GYPP under high-N, while for GYPP under low-N, the additive and dominance components were approximately equal. This indicates that the dominance gene effects are generally more important than additive in F 2 's and play the major role in the inheritance of GPC and GYPP under the two levels of nitrogen. The preponderance of dominance (non-additive) effects in F 2 generation of this study suggest that it is preferable to postpone the selection for high grain protein and grain yield/plant, either under high or low N environments, to later segregating generations, until completing the homozygosity process and eliminating masking non-additive influences in order to take advantage of the additive variance to improve these traits.
In this respect, Le Gouis et al. (2002) reported that in N-limited, diallel F 1 hybrids between modern French cultivars found markedly preponderance of additive variance for grain yield, grain N yield and total above ground N than in those grown under high-N nutrition. More recently, a similar preponderance of additive effects was identified in (2011) reported that under high N-fertilization, the efficiency components were incanted in a manner favorable for wheat selection (preponderance of additive effects) however the enhanced contribution of non-additive gene effects and increased dominance under N-limited conditions could impede wheat selection to improve the N efficiency and adaptation to less luxurious fertilization regimes. They concluded that selection methods that eliminate masking non-additive effects and take advantage of the additive variance should be employed to improve those traits. Results of the present study assure the conclusion of Gorny et al. (2011).
The overall dominance effects of heterozygous loci in Hayman's model (h 2 ) controlling all studied traits of F 2 's under both high-and low-N environments were significant (P< 0.01 or 0.05), that could be due to the presence of a considerable amount of dominant genes for most studied traits in the parental genotypes.
The average degree of dominance (H 1 /D) 1/2 in F 2 's was in the range of partial dominance (smaller than unity) for GYPP under both high and low N and GPC under high N, while GPC under low N showed no dominance, i.e. additive (Table 12).
Lower ratio (H 2 /4H 1 ) than 0.25 (Table 13) indicated symmetrical distribution of positive dominant genes in parents for both studied traits of F 2 's under both high-and low-N environments.
The ratio (K D /K R ) in F 2 's was greater than unity, indicating excess of dominant alleles and minority of recessive alleles (p>q) for GYPP under low-N. However, for GPC under low and high N and GYPP under high N in F 2 's, the ratio (K D /K R ) was less than unity, indicating minority of dominant alleles and the excess of recessive alleles (p<q).
Number of genes or groups of genes controlling the inheritance of a given trait (h 2 /H) was one group of genes for both studied traits of F 2 's under both high-N and low-N environments (Table 12). Broad-sense heritability (h 2 b ) in F 2 's for both studied traits under both environments in this experiment was of high magnitude and ranged from 75.82% for GPC under high N to 98.44% for GYPP under low N (Table 12), indicating that environment had a small effect on the phenotype of F 2 's for both studied traits.
Narrow-sense heritability (h 2 n ) in F 2 's ( Table 12) was generally of low to medium magnitude and ranged from 10.86% for GPC under low N to 46.90% for GYPP under low-N. It is observed that GPC trait recorded lower h 2 n than GYPP under both high-and low-N and that h 2 n for GYPP was higher under low-N than under high-N. The big difference between broad-and narrow-sense heritability estimated from F 2 's in this experiment could be attributed to the high estimates of dominance as compared to additive component. It is observed that narrow-sense heritability (h 2 n ) in F 2 's of the present study was generally higher in magnitude under low-N than under high-N for GYPP trait, while the opposite was true for GPC. In this aspect, Blum (1988 a and b), Hefny (2007) , Al-Naggar et al. (2008, 2009, 2014 and 2015), supported the idea that heritability is higher under stressed than non-stressed environment. On the contrary, other group of investigators reported that heritability is higher under non-stressed environments ( Rosielle and Hamblin, 1981, Atlin and Frey, 1990, Banziger and Laffite, 1997, Banziger et al., 1997and Worku, 2005. Expected genetic advance (GA) from selection in F 2 's (based on 1% selection intensity) across two years ranged from 7.70% for GYPP under high-N to 23.14% under low-N. In general, the values of GA were higher under low-N than under high-N. It is therefore expected that to improve grain yield and grain protein content in the present germplasm, it is better to practice selection in F 2 's for both traits under low-N conditions to obtain higher values of selection gain.

XI. CONCLUSIONS
This investigation concluded that under low-N, there were great differences among genotypes in grain protein content (GPC) and grain yield/plant (GYPP) that permit for successful selection for high values of these traits. The parent L26, the F 1 (L26 x L27) and the F 2 (L26 x Gz168) showed the highest means of both GPC and GYPP under low-N, indicating that it is possible to improve both traits simultaneously under limited N environment and these genotypes could be useful in future breeding programs. In both F 1 's and F 2 's, the magnitude of dominance (H 1 ) was much greater than that of additive (D) component for GPC under both high N and low N and GYPP under high-N, while the opposite was true for GYPP in F 1 's under low-N, where the additive was greater than dominance variance and GYPP in F 2 's under low-N, where the additive and dominance components were approximately equal. The results of this study concluded that selection in early segregating generations would be effective in improving grain yield/plant under low-N, while for grain protein content, selection would be more effective if it is postponed to the later segregating generations, when additive variance is released due to complete homozygosity by more selfing generations.