Because of essential economic and ecological concerns, there is increased interest worldwide in cereal cultivars that are more efficient in utilizing soil resources and better suited to water and nutrient limitations (El Bassam, 1998; Fageria & Baligar, 2005; Hirel et al., 2007; Sylvester & Kindred, 2009). Among cereals, bread wheat ( Triticum aestivum L.) is commonly identified as a species with higher requirements for nutrients, especially nitrogen. Thus, breeding wheat cultivars with improved adaptation to less favorable, but more optimized N fertilization regimes has gained importance.

Although numerous reports on genotypic variation in components of N efficiency already suggest potential applications of this genetic knowledge for wheat improvements (Ortiz-Monasterio et al., 1997; El Bassam, 1998; Le Gouis et al., 2000; Gorny et al., 2006; Kichey et al., 2007; Baresel et al., 2008; Barraclough et al., 2010), relatively fewer attempts have been made to breed wheat for these traits (Van Ginkel et al., 2001; Wolfe et al., 2008). Progress in breeding bread wheat better adapted to less favorable nitrogen fertilization is still restricted for several reasons. Wheat breeders are frequently skeptical not only because of the morpho-physiological complexity of the matter, but mainly due to limited data on both the variation among available wheat collections and the genetics of key traits involved. Hence, several important questions remain to be resolved, especially in regard to the most effective selection schemes, desirable plant ideotypes for low input ecosystems, appropriate selection criteria and features of the selection environment for such breeding programs ( e.g, Ceccarelli, 1996; Dawson et al., 2008; Wolfe et al., 2008). Furthermore, modern Egyptian wheat cultivars are phenotypically different but, in essence, represent a limited gene pool. The majority of them were developed under favorable or even luxurious fertilization regimes used at most breeding stations without or with scarce selection pressure for components of nutrient use efficiency. On the contrary, beneficial plant characteristics for low-input ecosystems may be different from those present in modern, high-yielding wheat (El Bassam, 1998; Murphy et al., 2007). The manner in which target traits are inherited has, of course, major consequences for the whole breeding strategy. However, our understanding of the inheritance of the components of N efficiency in wheat, as in other cereals, is still extremely limited. There is some information regarding various characteristics decisive for the uptake efficiency in juvenile wheat plants (Gamzikova, 1992; An et al., 2006; Gorny et al., 2006; Laperche et al., 2006), but the limited data may lead to uncertain conclusions on N efficiency over the whole growing season. However, the genetic control of whole-season N efficiency has rarely been examined in wheat (Gorny et al., 2011). In earlier investigations (Le Gouis et al., 2002), both additive and non-additive genetic effects were crucial for agronomic nitrogen use efficiency (NUE) components in F ₁ hybrids between modern French cultivars. In contrast, additive gene action was only important for NUE components among F ₂ progenies of wheat of Turkish origin (Yildirim et al., 2007). In Poland, Gorny et al. (2011) reported that under high N-fertilization, the efficiency components were inherited in a manner favorable for wheat selection (preponderance of additive effects). However, they found that 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. To the best of our knowledge, scarce information in wheat for the major physiological measures of N efficiency, i.e. nitrogen uptake efficiency (NUPE) and nitrogen utilization efficiency (NUTE) when whole-season indices and conventional genetic/breeding approaches are considered, and this may have an impact on current breeding methods and goals. Thus, the objectives of this study were (i) to assess the modes by which low-N tolerance adaptive traits are inherited (ii) to evaluate the effects of varied nitrogen levels on combining ability, heritability and expected genetic advance from selection for such traits and (iii) to identify the most promising genotypes to be included in breeding programs to improve tolerance to low rate of nitrogen fertilizer.

This study was carried out at Giza Research Station of the Agricultural Research Center (ARC), Giza, Egypt (30° 02’ N latitude and 31° 13’ E longitude with an altitude of 22.50 meters above sea level), in the 2005/2006 season and at Noubarya Research Station of the ARC, Noubarya, Egypt (30° 66’ N latitude and 30° 06’ E longitude with an altitude of 15.00 m asl), in 2005/2006, 2006/2007, 2007/2008 and 2008/2009 seasons.

Breeding wheat cultivars with improved adaptation to less favorable, but more optimized N fertilization regimes has gained importance in Egypt, like in other developing countries. Such breeding strategies are also justified by limited-nitrogen supply that supposes a major constraint for grain production. The NUE and plant adaptation to less favorable nutrition regimes is complex, involving various mechanisms (Sattelmacher et al., 1994; Coque & Gallais, 2007; Al-Naggar et al., 2008, 2009, 2010; Gorny et al., 2011). Different characteristics, associated with both the N-uptake capacity and efficiency of nitrogen utilization in grain mass formation (NUTE), appear to be critical components of NUE (Moll et al., 1982; Huggins & Pan, 2003).

In the present study, analysis of variance indicated that each of the two factors, genotype and N-level had a marked effect on all studied traits. In that context, the ranks of wheat genotype differ from one nitrogen level to another and from one year to another. Selection for improved performance under a specific soil nitrogen environment is possible as proposed by Al-Naggar et al. (2006, 2009, 2010 and 2015 a, b and c). The significant G×N interaction for grain yield was also a good evidence for varying responses of wheat genotypes at various N levels (Austin et al., 1980; Earl & Ausubel, 1983). The interactions G × Y and G × Y × N were also significant (p=0.01 or 0.05) for all studied traits, indicating that genotype ranks differ from one combination of Y × N to another.

Significant differences among studied parents of diallel crosses in all studied traits are a pre-requisite for performing the diallel analysis for estimating the inheritance of studied traits under different N-application rates. Significance (p=0.01) of mean squares due to parents vs F ₁’s and F1’s vs F ₂’s for most studied traits under the two levels of nitrogen, indicated the presence of significant heterosis and inbreeding effects, respectively. The significance of the interactions P’s vs F ₁’s × Y and F ₁’s vs F ₂’s × Y indicates that heterosis and inbreeding effects differ from season to season in all studied traits due to interactions with the environment.

It is worthy to note that the magnitude of N-induced alterations due to low-N stress in the majority of the N-efficiency components and other studied traits was distinctly dependent upon the genotype, as evident by the significant genotype × environment interactions. These results are consistent with observations previously reported in wheat (El Bassam, 1998; Le Gouis et al., 2000, 2002; Al-Naggar & Shehab-El-Deen, 2012), barley (Ceccarelli, 1996; Gorny & Sodkiewicz, 2001) and maize (Di Fonzo et al., 1982; Medici et al., 2004; Al-Naggar et al., 2010, 2014, 2015a,b,c), corroborating that an evaluation of breeding materials under diverse fertilization regimes is necessary for choice of the most efficient parental forms and/or cross combinations, as suggested in wheat by Brancourt-Hulmel et al. (2005), Laperche et al. (2006), Dawson et al. (2008), Wolfe et al. (2008) and in maize by Al-Naggar et al. (2010, 2014, 2015a,b,c).

It is worthy to note that the best general combiners under low-N in this study (L25, L26 and L27) showed also high per se performance for the most studied grain yield and NUE traits. Based on diallel analyses in both F ₁ and F ₂, these parents are expected to have more additive genes for the respective traits. The crosses L25 × L26, L26 × L27 and L25 × Gem7 were superior in both F ₁ and F ₂ for SCA effects under low-N conditions. These crosses showing high SCA effects under low-N and including at least one parent of high GCA effects are expected to release more transgressive segregants if additive gene effects existed in the high general combiner parent and epistasis acts in the cross in the same direction for decreasing the undesirable traits and increasing the desirable traits. These crosses besides their high per se performance, are the best in specific combining ability effects for the respective traits and are therefore good genetic material in their segregating generations for practicing selection for improving adaptive traits to low-N conditions.

Hayman’s analysis of the present experiment indicated that both additive and dominance gene effects in F ₁’s and F ₂’s of this experiment are important for the inheritance of all studied traits under both low-N and high-N, indicating that selection in segregating generations as well as heterosis breeding in wheat could be efficient for improving all studied traits. Results indicated that the dominance gene effects are more important than additive and plays the major role in the inheritance of all studied traits under both low-N and high-N. In this context, 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 for N uptake and NUTE was identified in F ₂ and F ₃ progenies of factorial hybrids between modern and exotic cultivars of barley grown under reduced N fertilization (Gorny & Ratajezak, 2008). On the other hand, results of Gorny et al. (2011) on wheat appear to be in accord with similar N-shortage- induced increases in the importance of non-additive effects for grain yield and components of NUE previously reported in maize (Al-Naggar et al., 2015a,b,c) and those for NUTE in barley (Gorny & Sodkiewicz, 2001). Gorny et al. (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 agreed with the conclusion of Gorny et al. (2011).The preponderance of dominance (non-additive) effects in this study suggests that it is preferable to postpone the selection 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 grain yield and nitrogen use efficiency traits.

Broad-sense heritability (h ² _b) in F ₁’s and F ₂’s for all studied traits in this experiment was of medium to high magnitude, indicating that environment had a small effect on the phenotype of F ₁’s and F ₂’s for most studied traits. Narrow-sense heritability (h ² _n) in F ₁’s was generally of low to medium magnitude, but in F ₂’s was generally of medium to high magnitude. The big difference between broad- and narrow- sense heritability estimated from F ₁’s and 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 ₁’s and F ₂’s of the present study was generally higher in magnitude under low-N than under high-N for most studied traits. Results of this study are in agreement with one group of researchers, e.g. Shabana et al. (1980), Blum (1988a,b), Hefny (2007), Al-Naggar (2007, 2009, 2010, 2015a,b,c), who support 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 & Hamblin, 1981; Atlin & Frey, 1990; Banziger & Laffite, 1997; Banziger et al., 1997; Worku, 2005). Our results were in agreement with the first group of researchers. Moreover, the values of expected genetic advance from selection (GA) of this study were higher under low-N than under high-N for most studied cases. It is therefore expected that to improve grain yield and nitrogen use efficiency components in the present germplasm, it is better to practice selection for these traits under low-N conditions to obtain higher values of selection gain.

In this study, for nitrogen use efficiency and grain yield traits under low-N conditions, the best general combiners were L25 and L26 and L27, the best F ₁ cross for SCA effects wasL25 × Gz168 and the best F ₂ cross was L25 × Gem 7. These genotypes could be used in the future breeding programs for improving low-N tolerance adaptive traits. The preponderance of dominance (non-additive) effects of this study suggests that selection methods that eliminate masking non-additive effects and take advantage of the additive variance should be employed to improve nitrogen efficiency traits under low-N environment. Therefore, selection should be postponed to later segregating generations under low-N conditions to develop N-efficient genotypes of wheat. The results expected that to improve nitrogen use efficiency components in the present germplasm, it is better to practice selection for studied nitrogen efficiency traits under low-N conditions to obtain higher values of selection gain.