الفهرس 
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Abstract
SUMMARY
Programs of maize breeding aim to develop new varieties characterized by superior economic traits as early tasseling and silking, high yield potentiality and flexibility in tolerance to unfavorable environmental conditions for growth in order to be more suitable with different environmental conditions and produce high yield under all conditions.
Therefore this research aimed to:
1. Study the genetic parameters for the correlated yield traits with drought tolerance.
2. Attempt to obtain genotypes of maize tolerant to drought stress.
3. Study the relationship between the ISSR-PCR and field results in an attempt to identify drought tolerant genotypes by using this technique.
The field experiments of this investigation were carried out at Giza Agricultural Research Station of the Agricultural Research Center (ARC) in the seasons from 2011 to 2013.
In the first growing season (2011), crossing was made among the four parents to produce the F1 hybrid grains of the two crosses; Gz-602 × Gz-628 (Cross I) and Sd-7 × Sd-63 (Cross П).
In the second growing season (2012), F1 hybrid’s of the two crosses and the four inbred lines were sown and F1 plants of each cross were backcrossed to their respective parents to produce BC1 and BC2 grains. At the same time, some of the F1 plants for each cross were selfed to produce the F2 grains and parents were also selfed to maintain parental purity. In addition, crossing was made between the parental inbred lines again to produce additional new F1 grains for each cross.
In 2013 season, grains of the six populations i.e. P1, P2, F1, F2, BC1 and BC2 of the two crosses were grown in one experiment under drought stress conditions (preventing the 4th and 5th irrigations during flowering stage) at Giza Agricultural Research Station of the ARC on 1st May in a
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randomized complete blocks design with 3 replicates. Each replicate comprised one row for each parent and F1 cross, four rows for each backcross and six rows for the F2 population for each cross. The row was 4 m long and 0.80 m wide with a distance of 0.25 m between hills (15 plants/row).
Observations and measurements were recorded on individual guarded plant basis where data were recorded on 30 plants for each of P1, P2 and F1, 180 plants for each of BC1 and BC2 and 270 plants for each of F2 population from the three replications for each cross.
The following readings and observations were recorded: tasseling date, silking date, plant height, ear height, number of rows per ear, number of kernels per row, 100-kernel weight, grain yield per plant, chlorophyll content and leaf proline content.
Inter-Simple Sequence Repeats (ISSR-PCR) was used to analyze the genetic polymorphisms of the two tolerant maize crosses (Gz-602 × Gz-628 and Sd-7 × Sd-63), the two sensitive maize parents (Gz-602 and Sd-7) and the two tolerant maize parents (Gz-628 and Sd-63), the most two sensitive F2 groups and the most two tolerant F2 groups were used to assess molecular markers associated with drought tolerant of maize plants.
The collected data were analyzed statistically and genetically to estimate the following parameters in each cross which included: means, variances, heterosis, inbreeding depression, degree of dominance, types of gene action, heritability in broad and narrow senses, genetic advance from selection ,phenotypic correlation coefficients and path coefficient analysis
The obtained results can be summarized as follows:
A. Analysis of variance and means performance:
1. Results of the analysis of variance showed significant differences among the six populations of each cross for the studied traits.
2. F2 populations followed by BC1 and BC2 populations gave the highest values for phenotypic variance (PV) and phenotypic coefficient of
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variation (PCV), while the parents and F1 populations gave the lowest values. The three traits i.e. tasseling date, silking date and chlorophyll content under drought stress conditions exhibited the lowest averages of PCV, while the three traits, no. of rows per ear, no. of kernels per row and grain yield per plant gave the highest averages. The latter traits displayed average more than 30% variation at the phenotypic level for segregating populations. Phenotypic coefficient of variation (PCV) values were slightly higher than GCV in cases of tasseling date, silking date, plant height, chlorophyll content and leaf proline content indicating the presence of little environmental influence on the expression of these traits. The remainder characters showed higher PCV than GCV.
3. In general, the means of F1 followed by BC2 in the two crosses were the earliest in their tasseling and silking dates, and gave highest values for plant and ear heights, number of rows per ear, number of kernels per row, 100-kernel weight, grain yield per plant, chlorophyll content and leaf proline content.
B. Heterosis, inbreeding depression and potence ratio:
1. The two studied crosses exhibited significant positive heterosis percentages over the mid and better parent values for all of the traits under study, except tasseling and silking dates which exhibited significant negative (desirable) heterosis values in the two crosses. The percentages of significant positive heterosis relative to the mid parent values ranged from 11.07% for chlorophyll content in cross I to 286.83% for grain yield per plant in cross I. Whereas the percentages of significant positive heterosis relative to the better parent values ranged from 3.31% for chlorophyll content in cross I to 231.54% for grain yield per plant in cross I. On the other hand, percentages of significant negative heterosis relative to the mid-parent values ranged from -11.27% for Silking date in cross II to -8.69% for tasseling date in cross I, whereas percentages of significant negative heterosis
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relative to the better parent values ranged from -9.19% for tasseling date in cross II to -7.14% for tasseling date in cross I.
2. Significant positive inbreeding percentages ranged from 6.11% for grain yield per plant in cross I to 25.00% for number of rows per ear in cross I, whereas significant negative percentages were detected ranging from -2.02% for tasseling date in cross II to -1.21% for silking date in cross I. However, plant height and chlorophyll content showed insignificant inbreeding depression in crosses I and II.
3. Potence ratio indicated that over dominance were detected towards the earlier parent in tasseling and silking dates, the taller parent for plant and ear heights, the higher parent in number of rows per ear and number of kernels per row, the higher parent in 100-kernel weight, the higher parent for grain yield per plant, the higher parent in chlorophyll content and leaf proline content in the two crosses.
C. Scaling test (epistasis test), F2-deviation (E1), backcrosses deviation (E2) and types of gene action:
1. The results of scaling test (A, B, C and D) indicated the presence of non-allelic interaction e.g., the presence of epistasis type of gene action in addition to the additive and dominance gene effects in the inheritance of the traits under drought stress condition.
2. The results showed significant deviation of F2 populations than the average of F1 performance and mean parental performance (E1), as well as significant deviation for backcross performance than the average of F1 and recurrent parent performance (E2), suggesting contribution of epistatic gene action in the inheritance of the studied traits in the two crosses.
3. The results of the nature of gene action controlling the inheritance of different traits under study using the method of generation mean analysis (six parameters model) revealed that the F2 mean effect parameter (m) was found to be highly significant for all studied traits
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in the two crosses under investigation, indicating that all traits were quantitavely inherited.
4. The results showed also that dominance gene effect (d), followed by the additive gene effect (a) and the additive × dominance (ad) type of epistatic gene action contributed with the large part of genetic component controlling the inheritance of the studied traits compared to the dominance × dominance (dd) additive × additive (aa) and type of epistatic gene effects.
As previously shown, the studied traits were significantly affected by the six parameters of gene action with different degrees of effects. Meantime, the positive and significant estimates for various types of gene action demonstrated that the dominance gene effect which represented 16 cases from a total of 20 cases (80.00%) followed by the additive gene effect (65.00%) and the additive x dominance type of epistatic gene action (50.00%) contributed by the large part of the genetic components controlling the inheritance of the studied traits. Presence of additive gene action in a trait suggested that early selection is effective for obtaining further improvement for this trait. On the other hand, the additive x additive and dominance x dominance types of epistatic gene effects appeared to be relatively of less importance than the three above mentioned types of gene effects in the inheritance of these traits.
5. The results of generation’s variance analysis indicate the important role of the dominance variance compared to the additive variance in the inheritance of the studied traits. This finding is coincidence with the result obtained from the generation mean analysis.
D. Heritability estimates and expected genetic advance from selection:
1. The results showed moderate to high heritability estimates in the broad sense for most of the studied traits with values ranging from 51.58% for chlorophyll content in cross II to 95.12% for ear height in cross II.
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Low to moderate heritability estimates in the narrow sense were detected in most of the studied traits with values ranging from 14.56% for plant height in cross I to 63.39% for leaf proline content in cross II.
2. The expected genetic advance of the F2 mean (Δg%) for the studied traits was found to be low or moderate with values ranged from 2.24% for silking date in cross I to 17.89% for number of rows per ear in cross II, suggesting that selection for improving the studied traits would be effective in the segregating generations.
3. Moderate genetic advance was found to be associated with high narrow sense heritability values for number of rows per ear, number of kernels per row, 100-kernel weight and grain yield per plant in the two crosses, therefore, phenotypic selection for these characters would likely to be effective and satisfactory for successful breeding purposes. On the other hand, low or moderate genetic advance was found to be associated with moderate narrow sense heritability values for tasseling and silking dates, plant and ear heights, chlorophyll content and leaf proline content, indicating slow improvement of this trait in subsequent generations.
E. Correlation and path coefficient analysis:
1. The results of the phenotypic correlations coefficients under drought stress conditions for each cross between all possible pairs of characters including grain yield except chlorophyll content and leaf proline content, showed that grain yield showed positive significant and highly significant correlations with each of number of rows per ear, number of kernels per row and 100-kernel weight in the two crosses. Such positive significant associations between grain yield and these attributes means the important contributions of these components towards grain yield in maize populations by which selection for these traits would be effective in improving plant grain yield.
2. from the results of path coefficient analysis, it was found that number of rows per ear, number of kernels per row and 100-kernel weight in
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the two crosses proved to be the major grain yield contributors. Thus, the breeder should take into his consideration these traits as selection criteria for maize yield improvement especially those positively and significantly correlated with grain yield per plant and had moderate to high narrow sense heritability estimates.
F. Molecular marker results:
Five primers (14-A, 44-B, HB-08, HB-13 and HB-15) showed association with drought tolerance as follows:
1. In the first cross (Gz-602 × Gz-628):
Primer 14-A, One universal band at MW 540 bp was characterized the tolerant groups (the tolerant parent Gz-628, the most tolerant F2 group and the tolerant F1) while, was absent in the sensitive groups (the sensitive parent Gz-602 and the most sensitive F2 group). Primer 44-B, One universal band at MW 450 bp was characterized the tolerant groups while, was absent in the sensitive groups. Primer HB-08, Two universal bands at MW 445 and 250 bp were characterized the tolerant groups while, were absent in the sensitive groups. Primer HB-13, One universal band at MW 395 bp was characterized the tolerant groups while, was absent in the sensitive groups. Primer HB-15, Two universal band at MW 350 and 160 bp were characterized the tolerant groups while, were absent in the sensitive groups.
2. In the second cross (Sd-7 × Sd-63):
Primer 14-A, one universal band at MW 540 bp was characterized the tolerant groups while, were absent in the sensitive groups. Primer 44-B, One universal band at MW 450 bp was characterized the tolerant groups while, was absent in the sensitive groups. Primer HB-08, One universal band at MW 340 bp was characterized the tolerant groups while, was absent in the sensitive groups. In the same time one universal band at MW 160 bp was characterized the sensitive groups while, was absent in the tolerant groups. Primer HB-13, One universal band at MW 540 bp was characterized the tolerant groups while, was absent in the sensitive
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groups. In the same time one universal band at MW 395 bp was characterized the sensitive groups while, was absent in the tolerant groups. Primer HB-15, Two universal bands at MW 365 and 160 bp were characterized the tolerant groups while, were absent in the sensitive groups.
According to these results, the primers 14-A, 44-B, HB-08, HB-13 and HB-15 could be used as markers assistant selection for drought tolerance maize genotypes which showed close relationship for drought tolerance.
from the results of this study it is clear that the two crosses; Gz-602 × Gz-628 ( Cross I ) and Sd-7 × Sd-63 ( Cross II) have good performance in all the studied traits especially grain yield per plant and earliness in tasseling and silking dates. Therefore these two crosses may be utilized in the maize breeding programs for producing single crosses and/or developing lines characterized by earliness in tasseling and silking with high grain yield per plant under drought stress condition.