الفهرس 
REVIEW OF LITERATUREOnly 14 pages are availabe for public view
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Abstract
The present study aimed to evaluate mean performance of six bread wheat genotypes and their respective 15 F1 crosses for grain yield and its contributing traits at two nitrogen fertilization levels, to determine suitable selection criteria for selecting genotypes tolerant to nitrogen stress conditions and to estimate some genetic parameters for the studied traits as basis to improve bread wheat under nitrogen stressed conditions.
The field experimental work was conducted during three successive growing seasons of 2016/17, 2017/18 and 2018/19. The four local cultivars; Giza 168 (P2), Sakha 93 (P4), Sids 12 (P5) and Misr 1 (P6) as well as the introduced Syrian cultivar Cham 4 (P1) and promising line 36 (P3) were chosen on the basis of their genetic diversity in some agronomic traits and nitrogen stress tolerance to achieve this investigation. In 2016/17 growing season, a half diallel set of crosses involving the six parents excluding reciprocals was done to obtain the seeds of 15 F1’s. In 2017/18 season, the half diallel cross was again made to increase quantity of F1 seeds. In 2018/19 season, the six parental genotypes along with their respective 15 F1’s (21 entries) were evaluated in the Agric. Res. Stat. of the Fac. of Agric., Ain Shams Univ., at Shalakan, Kalubia Governorate, Egypt, in two separate and adjacent field experiments representing two different nitrogen fertilization levels i.e., 80 (recommended level) and 40 (low level) kg N/fed. Each experiment was designed in a randomized complete block with three replications. The experimental plot consisted of two rows for each genotype. The row was 2 m in length and rows spacing and distance between plants within row were 25 and 10 cm, respectively. Plants were thinned at one plant per hill before first irrigation.
Heading and maturity dates were recorded. Also, flag leaf area and chlorophyll content were measured at anthesis stage. At harvest, ten guarded plants randomly chosen from each replicate and data were recorded for the following traits; plant height, number of spikes/plant, spike length, number of spikelets/spike, number of kernels/spike, 100-kernel weight, grain yield/plant, straw yield/plant, harvest index, biological yield/ plant and grain protein content. The obtained data for all studied traits from each experiment were subjected to the ordinary analysis of variance. Combined analysis over the two nitrogen treatments was done when the homogeneity test of error variances was insignificant. Fifteen low nitrogen tolerance indices were computed and rank correlation coefficients among different N tolerance indices as well as yield potential (Yp) and stress yield (Ys) were calculated to detect the most desirable nitrogen stress tolerance criteria. Heterosis and narrow sense heritability were estimated for the studied traits. Combining ability variances and effects were estimated by applying Griffing’s (1956) method 2 and model 1.
The obtained results can be summarized as follows:
Analysis of variance
The results showed that nitrogen fertilization levels mean squares were highly significant for all studied traits, indicating that these traits are influenced by different rates of nitrogen fertilization. Also, mean squares due to genotypes, parents and crosses were significant for all studied traits at both nitrogen levels and their combined analysis. Significant mean squares for interactions of genotypes, parents and crosses with nitrogen treatments were observed for most studied traits, showing that the relative ranking of the genotypes for these traits are inconsistent under different nitrogen fertilization levels. Parents vs. crosses mean squares as an indication to average heterosis over all crosses at the two nitrogen levels and their combined analysis were significant with few exceptions, revealing that average heterosis overall crosses was pronounced for most crosses under such conditions. Moreover, mean squares of parents vs. crosses x nitrogen fertilization levels interactions were significant for all studied traits except for heading date, flag leaf area and biological yield/plant.
B. Mean performance for genotypes under two nitrogen treatments
Results revealed that soil nitrogen deficit significantly reduced all the studied traits with different degrees of reduction. Number of spikes/plant had the highest reduction (35.46%), while maturity date exhibited the lowest reduction (1.34%). The genotypes registered the lowest means reduction under N-stress conditions were line 36 for heading date and number of spikes/plant, cross P4 x P5 for plant height and harvest index, cross P2 x P3 for maturity date, cross P3 x P5 for flag leaf area, cross P1 x P6 for chlorophyll content, cross P1 x P4 for spike length, cross P3 x P6 for number of spikelets/spike, Sakha 93 for number of kernels/spike and straw yield/plant, cross P4 x P6 for 100-kernel weight, cross P5 x P6 for grain yield/plant, cross P2 x P4 for biological yield/plant and cross P2 x P6 for grain protein content. For heading date, mean values for genotypes ranged from 103.67 d for each of Giza 168, P2 x P3 and P3 x P4 to 107.67 d for each of Sakha 93, P2 x P6 and P3 x P6 at recommended N level. Meanwhile, the mean values varied from 99.67 d for Giza 168 to 104.67 d for each of line 36, P2 x P6 and P3 x P6 under N-stress conditions. With regarding to maturity date, the mean values ranged from 152.67 d for Sakha 93 to 156.67 d for Misr 1 at recommended N level, while mean values varied from 151.33 d for Giza 168 and Sakha 93 to 154.67 d for Misr 1 and cross P2 x P3 under N-stress conditions. With respect to plant height, mean values ranged from 112.47 cm for Cham 4 to 117.25 cm for Sids 12 at recommended N level, while mean values varied from 104.07 cm for Cham 4 to 107.42 cm for P1 x P2 under N-stress conditions. Regarding flag leaf area, mean values ranged from 35.03 cm2 for Giza 168 to 62.31 cm2 for P2 x P5 at recommended N level, while mean values varied from 30.80 cm2 for Misr 1 to 54.99 cm2 for P3 x P5 under N-stress conditions. With respect to chlorophyll content, mean values of genotypes ranged from 46.94 Spad for line 36 to 54.43 Spad for cross P2 x P5 at recommended N level and from 37.49 Spad for cross P2 x P6 to 47.78 Spad for cross P1 x P2 under N-stress conditions. Concerning number of spikes/ plant, mean values for genotypes ranged from 12.33 spikes for Cham 4 to 17.74 spikes for cross P1 x P5 at recommended N level, while mean values varied from 7.81 spikes for Sids 12 to 12.11 spikes for cross P1 x P3 under N-stress conditions. Regarding spike length, mean values of genotypes varied from 12.77 cm for Misr 1 to 16.67 cm for cross P3 x P5 at recommended N level and from 11.64 cm for Misr 1 and cross P2 x P4 to 13.83 cm for cross P1 x P4 under N-stress conditions. For number of spikelets/spike, mean values for genotypes ranged from 23.40 spikelets for cross P3 x P6 at recommended N treatment and from 22.12 spikelets for Misr 1 to 24.08 spikelets for cross P1 x P5 under N-stress conditions. Concerning number of kernels/spike, mean values for genotypes varied from 50.90 kernels for Sakha 93 to 79.09 kernels for cross P1 x P6 at recommended N treatment and from 38.37 kernels for Sids 12 to 53.75 kernels for cross P1 x P2 under N-stress conditions. For 100-kernel weight, mean values for genotypes ranged from 4.26 g for cross P3 x P6 to 4.74 g for Sakha 93 at recommended N treatment and from 3.34 g for Sids 12 to 3.80 g for cross P1 x P2 under N-stress conditions. Regarding grain yield/plant, mean values for genotypes ranged from 21.25 g for cross P5 x P6 to 27.02 g for the two crosses; P3 x P4 and P3 x P6 at recommended N treatment and from 13.48 g for Sids 12 to 18.77 g for cross P1 x P2 under N-stress conditions. Concerning biological yield/plant, mean values for genotypes ranged from 57.92 g for cross P2 x P4 to 72.72 g for cross P4 x P5 at recommended N treatment and from 39.61 g for Sids 12 to 56.77 g for cross P1 x P2 under N-stress conditions. With respect to straw yield/plant, mean values for genotypes varied from 33.11 g for line 36 to 54.42 g for cross P3 x P5 at recommended N treatment and from 25.26 g for cross P3 x P4 to 37.99 g for cross P1 x P2 under N-stress conditions. For harvest index, mean values of genotypes ranged from 35.41% for cross P3 x P5 to 44.39% for cross P2 x P4 at recommended N treatment and from 30.24% for Giza 168 to 39.45% for cross P3 x P4 under N-stress conditions. Regarding grain protein content, mean values of genotypes ranged from 13.41% for Giza 168 to 14.61% for the two crosses; P3 x P4 and P4 x P6 at recommended N treatment and from 11.35% for cross P4 x P5 to 13.57% for cross P4 x P6 under N-stress conditions.
C. Low nitrogen tolerance indices
Correlation analysis revealed that yield potential (YP) and stress yield (YS) had significant positive correlation coefficients with N-stress tolerance indices; Mean productivity (MP), Geometric mean productivity (GMP), Harmonic mean (HM) and Stress tolerance index (STI) as well as between yield potential (YP) and Modified stress tolerance index (K1STI). Moreover, significant positive correlations were observed among different stress tolerance indices used in this study. However, Modified stress tolerance index (K1STI) was not correlated with Ys. Results indicated that Mean productivity (MP), Geometric mean productivity (GMP) and Stress tolerance index (STI) indices seemed to be employed in breeding programs for wheat to identify most low-N tolerant genotypes and are much more effective to recognize genotypes, which have high yield in both environments, since these indices exhibited significant positive correlations with grain yield under recommended and N-stress environments. Meantime, in consideration to all indices, average rank (R) and ranks under N stress conditions, the crosses; P1 x P2, P2 x P3, P4 x P5 and P4 x P6 were identified as the most low-N tolerant genotypes, while the cvs.; Cham 4, Giza 168, Sids 12 and Misr 1 were considered as the most sensitive therefore, these genotypes are recommended for studying genetic analysis of low-N tolerance and developing desirable genotypes in breeding programs.
Estimates of genetic parameters
1. Heterosis
The results indicated that the best crosses over mid-parents at recommended N level were P3 x P4 for heading date, P1 x P6 for maturity date and number of kernels/spike, P2 x P6 for plant height and spike length, P2 x P5 for flag leaf area, P2 x P4 for chlorophyll content and grain yield/plant, P1 x P3 for number of spikes/plant, P1 x P4 for straw yield/plant, P3 x P6 for biological yield/plant, P1 x P5 for harvest index and P1 x P2 for grain protein content. On the other hand, the best crosses under N-stress conditions were P1 x P4 for heading date, P5 x P6 for maturity date and number of spikes/plant, P2 x P6 for flag leaf area and number of spikelets/spike, P1 x P6 for chlorophyll content and spike length, P1 x P2 for plant height, number of kernels/spike, 100-kernel weight and biological yield/plant, P5 x P6 for grain yield/plant, P3 x P5 for straw yield/plant, P3 x P4 for harvest index and P4 x P6 for grain protein content. Meantime, the best crosses over the better parent at recommended N level were P1 x P6 for heading date, maturity date, spike length, number of spikelets/spike and number of kernels/spike, P2 x P6 for plant height, P1 x P2 for flag leaf area and grain protein content, P2 x P4 for chlorophyll content, P1 x P3 for number of spikes/plant, P3 x P4 for grain yield/plant, P1 x P4 for straw yield/plant and biological yield/plant. Meanwhile, the best crosses under N-stress conditions were P1 x P4 for heading date, P1 x P2 for plant height, number of kernels/spike, 100-kernel weight and biological yield/plant, P2 x P6 for flag leaf area and number of spikelets/spike, P1 x P6 for chlorophyll content and spike length, P2 x P5 for number of spikes/plant, P5 x P6 for grain yield/plant, P3 x P5 for straw yield/plant, P3 x P4 for harvest index and P4 x P6 for grain protein content.
2. Combining ability analysis
The combining ability analysis for the studied traits indicated significant mean squares due to general (GCA) and specific (SCA) combining abilities at both N levels and their combined analysis with few exceptions. The ratios of GCA/SCA variances were found to be less than unity for most studied traits, suggesting preponderance of non-additive gene action in the inheritance of these traits and therefore, selection procedure will be important to improve these traits in late or advanced generations. In addition, mean squares for interactions of both GCA and SCA with nitrogen treatments were significant, except GCA x N for grain yield/plant. The best general combiners for grain yield/plant and one or more of its attributes were obtained in Sakha 93 and line 36 under N-stress conditions, while Sids 12 and line 36 were the best general combiners for flag leaf area under both environments. Moreover, the best general combiners for chlorophyll content, number of spikes/plant, biological yield/plant and straw yield/plant were Sids 12 at recommended nitrogen level, Sakha 93 for number of kernels/spike and 100-kernel weight and line 36 for harvest index under N-stress conditions. The best F1- cross combinations at recommended nitrogen fertilization treatment were shown by cross P1 x P2 for heading date, spike length, 100-kernel weight and number of spikelets/spike, cross P2 x P5 for flag leaf area and chlorophyll content, cross P1 x P5 for number of spikes/plant and grain yield/plant, cross P1 x P6 for number of kernels/spike, cross P3 x P5 for biological yield/plant and straw yield/plant, cross P2 x P4 for harvest index and cross P4 x P6 for grain protein content. The best crosses under N-stress conditions were P5 x P6 for maturity date and spike length, P4 x P5 for plant height, number of kernels/spike and grain yield/plant, P3 x P5 for flag leaf area, P1 x P6 for chlorophyll content, P4 x P5 for number of spikes/plant, P4 x P6 for 100-kernel weight, P1 x P2 for biological yield/plant, straw yield/plant and grain protein content and P3 x P4 for harvest index.
3. Correlations between X ̅p and GCA effects and between X ̅F1 and SCA effects
Significant correlations between X ̅p and GCA effects existed for all studied traits under both recommended and low-N environments, except for maturity date, flag leaf area, number of spikelets/spike and straw yield/plant under low-N conditions. In general, the magnitude of correlation coefficient between X ̅p and GCA effects was higher at low-N than recommended N conditions for most studied traits. The highest correlation coefficients under low-N between X ̅p and GCA effects were observed for 100-kernel weight followed by plant height and harvest index. On the contrary, the lowest correlation coefficients between X ̅p and GCA effects were shown under recommended N conditions for plant height. These results indicate that the best performing parents for grain yield and its components are also the best general combiners and vice versa, and therefore, the mean performance of a given parent under low-N and recommended N conditions is an indication of its general combining ability. The correlation coefficients between mean performance of F1’s (X ̅F1) and SCA effects for all studied traits, except number of spikelets/spike under recommended N conditions were significant positive; with little higher estimate under low-N than recommended N conditions in most studied traits. This indicates that mean performance of a given cross could be used as an indication of its specific combining ability effects for all studied traits, either under recommended or under low-N conditions.
4. Heritability
Narrow sense heritability values ranged from 3.98% for grain yield/ plant to 58.96% for 100-kernel weight at recommended N level and from 2.65% for number of spikelets/spike to 53.85% for plant height under N-stress conditions. Low narrow sense heritability values for most studied traits indicated that these traits are mainly genetically controlled by non-additive type of gene action and selection for improving these traits should be delayed to later generations. Moreover, heritability estimates at recommended nitrogen fertilization were lower than those under N-stress conditions for heading and maturity dates, plant height, number of spikes/ plant, number of kernels/ spike, grain yield/ plant, straw yield/ plant and harvest index.