Corn (Zea mays L.) belongs to the grass family (Poaceae) and the tribe (Maydeae). It is a monoecious annual herbaceous plant. The tribe (Maydeae) comprises eight genera, the most important being the genus Zea, which includes the species Mays. Other genera include Tripsacum (Gamagrass) and Euchlaena (Teosinte) which are considered to be the closest wild species to cultivated corn (Al-Sahouki, 1990). Several attempts have been made to classify the species Mays into subspecies based on endosperm characteristics and grain components. These attempts have resulted in the following subspecies: dent corn, flint corn, waxy corn, popcorn and sweet corn. (Purseglove, 1972: Paliwal, 2000a; Darrah et al., 2003).
       
Yellow corn is one of the oldest and most productive cereal crops, with a global average yield of approximately 4 tons per hectare. (Paliwal, 2000b; Farnham et al., 2003).

Yellow corn is a versatile and important crop for human consumption. It is also used in animal feed, medicinal purposes and as a raw material in industry. For human consumption, it can be fried, roasted, or boiled. Corn flour is also mixed with wheat flour to produce bread, sweets and potato chips (Rooney and Saldivar, 2003).
       
Yellow corn constitutes 75% of the feed provided to poultry, as its grains are rich in beta-carotene precursors, which form vitamin A, important for poultry health. A deficiency of this vitamin causes paralysis in chicks, lethargy and feather loss and in severe cases, it leads to blindness (Jaber et al., 2008). Corn is also used for industrial purposes, as it is the main source of starch worldwide (White, 1994), which is used to produce alcohol, as well as to produce ethanol, which is considered a revolution in the field of biofuels, in addition to the manufacture of paper, insulation materials and paints. The oil and protein extracted from corn are also used in the food industry. (Paliwal, 2000a; Boyer and Hannah 1994; Hobbs, 2003).
       
Global demand for yellow corn has increased for both human consumption and animal feed. Global production is expected to rise by 50% to 79% in developed countries and this increase could reach as high as 93% in Asia and Africa by 2020 (CIMMYT, 2009). Finding ways to increase production has become an urgent and necessary objective. This can be achieved either by increasing the cultivated area or by raising the yield per unit area. In this context, vertical improvement of maize production may also be supported by intensive agronomic practices. High-density planting combined with adequate nutrient management has been reported as a practical approach to improve crop performance and maintain residual soil fertility in maize-based cropping systems (Priya et al., 2022). Given that horizontal expansion in Syria is very limited and sometimes even decreasing, the only way to increase production is through vertical expansion by raising the yield per unit area. This can be achieved by improving and developing various production resources, adopting modern technology in cultivation and crop management and developing superior varieties and hybrids that are better suited to local conditions and tolerant to varying environmental circumstances (Ghazal, 1989). Recent field evidence further indicates that improving maize productivity cannot be separated from the farming system and seasonal conditions. Intercropping maize with beans significantly increased maize yield under favorable seasonal rainfall, whereas rainfall variability reduced productivity in the less favorable season (Tumwesigye, et al., 2024). This supports the need for locally adapted maize genotypes able to maintain yield performance under variable production conditions.
       
The importance of maize is highlighted by its status as the first and most important crop studied for hybrid vigor, due to the natural separation of male and female inflorescences, which allows large-scale control of hybridization (Paliwal, 2000b).
       
Plant breeding is used to develop or modify crop traits such as plant height, number of ears, grain yield, maturity, grain characteristics and resistance to diseases and insects (Paliwal, 2000b; Sleper and Poehlman, 2006). Furthermore, plant breeding aims to increase the nutritional content of cultivated maize varieties (Zhu et al., 2007). Since maize breeding depends primarily on the availability and exploitation of genetic variability, the assessment of maize accessions through morphological traits and molecular markers provides valuable information for identifying favorable traits and selecting superior genetic materials for crop improvement (Kabululu et al., 2017).
       
Increasing crop yield is one of the most important goals that plant breeders aim for. However, direct selection for the high-yielding trait is ineffective, as this trait is considered as complex quantitative trait controlled by several genes. Some of these genes have a significant and clear influence and are called major genes, while others have a minor influence and are called minor genes (Hassan, 1991d). Therefore, selection for the components of yield is more effective than selection for the yield itself (Grafius, 1956).
       
Diallel crossing is widely used in genetic research to investigate the inheritance of important traits across genotypes and to estimate the combining ability of parental lines for use in hybrid development programs (Yan and Hunt, 2002). The basic principles of diallel analysis were established by Hayman, (1954), Jinks (1954). and Saleem et al., (2002). Diallel cross data are commonly analyzed according to the method of Griffing, (1956). which partitions the total variance into general combining ability (GCA) of the parents and specific combining ability (SCA) of the hybrids Yan and Hunt (2002).
       
Studies on heterosis of seven yellow maize varieties and their resulting hybrids using reciprocal crosses for grain yield. The results showed non-significant heterosis values of 6.4% and -0.8%, respectively, compared to the average of the parents and the best parent (El-Rouby and Galal, 1972).
       
On evaluating yellow maize varieties and their resulting reciprocal hybrids to study heterosis for grain yield it was found that the heterosis values were of 109.9% and 105.9%, respectively, compared to the average of the parents and the best parent (Galal et al., 1978).
       
A study conducted through a half-reciprocal cross between six inbreeding maize lines to estimate hybrid vigor relative to the average of the parents and the best parent in each of the following traits: yield and its components, plant and ear height and the number of days from planting to the emergence of 50% of the female inflorescences. The results showed significant values   for hybrid vigor, with the best values   being 70.89% and 59.89% for individual plant yield, 46.22% and 33.16% for plant height, -11.3% and -22.99% for ear height, -8.23% and -9.3% for the number of days from planting to the emergence of 50% of the female inflorescences, 108.9% and 95.83% for the number of rows per ear, 25.86% and 21.66% for the number of grains per row, 63.14% and 50.72% for ear length and 43.33% for the number of days from planting to the emergence of 50% of the female inflorescences. The weight of the ear of corn is 34.38%, compared to the average of the two parents and the best tester respectively (Nawar et al., 1980).
       
A study undertaken using ten single-cross hybrids resulting from half-alternating crosses between five inbreeding maize lines indicated significant values for all traits except for the number of rows per ear, which showed non-significant heterosis values. The best values were 58.25% and 27.50% for grain individual plant yield, 40.52% and 23.93% for number of grains per row, 24.70% and 17.08% for ear diameter, 47.36% and 26.09% for ear length, 17.82% and 17.82% for plant height, 18.65% and 7.44% for ear height and finally -14.24% and -3.58% for number of days (Nawar et al., 1981).
       
A study utilizing perfect reciprocal crosses between six inbreeding lines of yellow maize to estimate hybrid vigor relative to the average of the parents and the best parent indicated that values of hybrid vigor were 20.03 and 20% for plant height, 69.32 and 39.41% for ear height, 37.22 and 34.83% for ear length, 24.05 and 17.69% for ear diameter, 23.59 and 23.03% for the number of rows per ear, 42.63 and 34.47% for the number of grains per row, 66.08 and 47.60% for the trait of hundred grain weight and finally 83.32 and 58.21% for the trait of grain yield relative to the average of the parents and the best parent respectively (Shafey, 1998).
       
From a study utilizing fifteen hybrids resulting from half-diallel crosses between six inbreeding yellow maize lines, it was observed that the highest heterosis values   were 54.73% for ear length, 41.87% for ear diameter, 31.71% for number of rows per ear, 142.66% for number of kernels per row, 45.09% for 100-kernel weight and reached 266.48% for grain yield (Abd El-Sattar et al., 1999).
       
Half-diallel crosses generated using seven single-cross hybrids to calculate heterosis relative to the average of the parents and the better parent for yield and its components, revealed that the highest values   for heterosis relative to the average of the parents and the better parent were 2.11% and 4.75% for the number of days from planting to the emergence of 50% of the female inflorescences; 8.80% and 14.58% for plant height; 31.4% and 31.38% for ear height; 7.9% and 7.48% for ear length; 11.99% and 14.20% for ear diameter; 6.49% and 3.31% for the number of rows per ear respectively; and 5.54% and 5.40% for the number of grains per row. It was 19.9 and 52.93% for the 100-grain weight trait and finally, it was 2.91 and -0.6% for the grain yield trait (Abd El-Aty and Katta, 2002).
               
A semi-reciprocal cross study initiated using ten open-pollinated maize varieties to estimate heterosis revealed that heterosis values were significant for both the yield trait and its components, ranging from 2.5% to 64.7% for the yield trait and from 1.3% for grain depth to 8.2% for 100-grain weight for the yield component (Soengas et al., 2003).

This study aimed to determine the inheritance mechanism of selected quantitative traits by estimating the general combining ability (GCA) and specific combining ability (SCA) of the studied hybrids. It also aimed to estimate heterosis over the mid-parent and the better parent.
       
Six highly purified (95%) inbred lines of yellow maize named (IL.155-22) P1, (IL.130-22) P2, (IL.262-22) P3, (IL.257-22) P4, (IL.422-22) P5 and (IL.424-22) P6 were utilized. These lines were genetically distinct and obtained from the gene bank at the Maize Research Department, General Commission for Agricultural Scientific Research, Syria. The control line, Ghouta 82, was also used during the 2023 and 2024 growing seasons. The seeds of the different varieties were sown on May 7, 2023. During the flowering stage, hybridization was carried out between the lines in all combinations except the reciprocals to obtain hybrid seeds for fifteen single hybrids. These F1 seeds, along with seeds from the six parent varieties, were sown in the 2024 season following a randomized complete block design (RCBD) with three replications. Each entry was planted in four rows, each 6 meters long, with a spacing of 70 cm between rows and 25 cm between plants within a row. All agricultural operations, including weeding, fertilization and thinning, were performed according to the recommendations of the Ministry of Agriculture and Agrarian Reform regarding the agricultural practices required for maize production. Field readings were taken on ten plants enclosed in a row, including the number of seeds per row, the weight of 100 seeds, individual plant yield and the total grain yield. The data for all readings were collected and tabulated using Excel. The general ability to combine (GCA) and the specific ability to combine (SCA) were calculated, along with the effects of each. Additionally, the components of variance were calculated using Method 4, Model 2 (Griffing, 1956). The values   of heterosis were calculated as a measure of the average of the parents and the best parent using Excel software and the significance of heterosis was estimated using the T-Test (Wynne et al., 1970).

Analysis of variance and comparison of means
 
The highly significant variance among the lines (Table 1) confirmed their genetic divergence for the number of kernels per row. This result is consistent with the findings of (El Absawy, 2002; Malik et al., 2004; Muraya et al., 2006).


       
The mean number of kernels per row for the parental lines ranged from 17.2 kernels (P1) to 43.0 kernels (P2), with an overall mean of 32.65 kernels (Table 2).


       
The hybrids exhibited highly significant variation (Table 1), confirming the genetic divergence between the parental lines used in the hybridization process. This result is consistent with the findings of (El-Hosary, 1988a; El-Hosary, 1994a; Hassan, 1999).
       
The averages of the hybrids for the number of grains per row (Table 3) ranged from 35.5 grains (P3 × P6) to 46.4 grains (P2 × P4 and P2 × P6), with an overall average of 41.29 grains. Comparison of the averages showed that four hybrids outperformed the control Ghouta 82 with positive significant differences.


 
Hybrid vigor
 
Table (4) showed highly significant positive heterosis values relative to the mid-parent and better parent for the number of grains per row. These values ranged from 25.89% (P3 × P6) to 86.67% (P1 × P4) and from 18.91% (P1 × P2) to 53.85% (P1 × P4), respectively. This result is supported by the findings of (Shafey, 1998; Abd El-Sattar, 1999; Abd El-Aty and Katta (2002).


 
Combining ability
 
General combining ability showed highly significant variance, while specific combining ability variance was not significant for the number of grains per row (Table 1). This indicates the dominance of additive gene action in the inheritance of this trait. This is consistent with the additive genetic variance (19.50), dominance variance (1.67) and degree of dominance (0.29). The results of (Kassem et al., 1979; Barakat, 2001) also support this finding.
       
The effects of general combining ability (Table 5) ranged from -3.44 (P3) to 4.72 (P2). These effects indicated that both lines (P2) and (P4) exhibited good general combining ability for the number of grains per row trait.


       
The effects of specific combining ability (Table 6) ranged from -2.0366 (P1 × P2) to 2.488 (P2 × P6). These effects indicated that the hybrid P2 × P6 possessed a non-significantly favorable specific combining ability for the number of grains per row.


 
100-grain weight
 
Analysis of variance and comparison of means
 
Table (1) shows a highly significant variance among the lines for the 100-grain weight trait, indicating genetic divergence between them. This is in confirmation with the findings of (Shafey, 1998; Saeed et al., 2000).
       
The mean weight of the 100 grains (Table 2) of the lines ranged from 24.6 g (P3) to 29.7 g (P2), with an overall mean of 28.03 g.
       
The hybrids showed highly significant variation (Table 1), confirming the genetic divergence between the parental lines, consistent with the findings of (Shafey, 1998; Saeed et al., 2000). The mean weight of the hybrids (Table 3) ranged from 31.6 g (P6 × P2) to 40.6 g (P5 × P2), with an overall mean of 35.21 g. The results indicated that twelve hybrids showed significant positive differences compared to the control Ghouta 82.
 
Heterosis
 
All hybrids exhibited positive and highly significant heterosis for the 100-grain weight trait (Table 4), with values ranging from 11.86% (P6 × P2) to 37.41% (P5 × P3) and from 6.40% (P6 × P2) to 36.70% (P5 × P2), compared to the average of the parents and the better parent, respectively. These results are consistent with the results of (Shafey, 1998; Abd El-Aty and Katta, 2002).
 
Combining ability
 
The results of the analysis of variance for combining ability (Table 1) indicated highly significant variance for both general (GCA) and specific (SCA) combination abilities. This suggests that both additive and non-additive genetic actions contribute to the inheritance of this trait. The σ2GCA/σ2SCA ratio, which was less than one, demonstrated the dominance of the non-additive genetic action in the inheritance of the hundred-grain weight trait. A dominance degree greater than one (1.095) further confirmed this inheritance behavior. The variance of the additive genetic action (6) was smaller than the variance of the dominant genetic action (7.20), which supports the findings of (Shafey, 1998; Saeed et al., 2000).
       
The effects of general combining ability (Table 5) ranged from -2.45 (P6) to 2.88 (P5), with lines P1, P4 and P5 exhibiting good general combining ability for this trait.

The effects of specific combining ability (Table 6) ranged from -3.4754 (P5 × P1) to 3.94971 (P6 × P1), indicating that the hybrids P6 × P1, P5 × P2, P4 × P1, P5 × P3 and P4 × P3 exhibited good specific combining ability for the 100-grain weight trait.
 
Single-plant yield
 
Analysis of variance and comparison of means
 
The lines exhibited highly significant variation (Table 1), demonstrating genetic divergence among the parental lines for the trait of individual plant yield. This result is consistent with the findings of (Shafey, 1998; Saeed et al., 2000;  Saleem et al., 2002).
       
The average values of the parental lines for single-plant yield (Table 2) ranged from 71.8 g (P1) to 190.9 g (P5), with an overall mean of 139.50 g.
       
The hybrids showed highly significant variation (Table 1), confirming the genetic divergence among the parental lines used in the hybridization process. This result is consistent with the findings of (El-Hosary, 1988a; El-Hosary et al., 1990b; Ibrahim, 2003). The averages of the hybrids for single plant yield (Table 3) ranged from 196 g (P4 × P3) to 251.3 g (P2 × P1), with an overall average of 226.38 g. The results of comparing the averages showed that all hybrids outperformed the control Ghouta 82 by significant positive differences.
 
Hybrid vigor
 
The results for hybrid vigor (Table 4) indicated highly significant positive values  compared to the average of the parents and the best parent for this trait. Hybrid vigor values   ranged from 85.83% (P6 × P5) to 235.26% (P4 × P1) and from 92.37% (P6 × P5) to 167.18% (P4 × P1), compared to the average of the parents and the best parent, respectively. This result supports the findings of research by (Nawar et al., 1981; AL-Ahmad, 2001; Shafey et al., 2003).
 
Combining ability
 
The general combination ability (GCA) and specific combination ability (SCA) resulted in highly significant variance for this trait (Table 1), indicating the contributions of both additive and non-additive genetic actions to the inheritance of this trait. The σ2GCA/σ2SCA ratio, which was less than one (0.22), showed the dominance of the non-additive genetic action in the inheritance of the trait. This result was confirmed by a degree of dominance greater than one (1.502), where the variance of the additive genetic action (124.65) was approximately half that of the dominant genetic action (281.39). This finding was supported by research conducted by (Kassem et al., 1979; Nawar, 1980;  AL-Ahmad, 2001; Abou-Deif (2007).
       
The effects of general combining ability (Table 5) ranged from -19.22 (P3) to 11.40 (P2). These effects indicated that both lines (P1) and (P2) possessed good general combining ability.
       
The effects of specific combining ability (Table 6) ranged from -19.317 (P5 × P6) to 27.5863 (P3 × P5). These effects indicated that the hybrids P3 × P5 and P1 × P6 exhibited good specific combining ability for this trait.
 
Grain yield per plot (tons/hectare)
 
Analysis of variance and comparison of means
 
The lines showed highly significant variance (Table 1), indicating genetic divergence among them for grain yield. This result is consistent with the findings of (Yasien, 2000;  AL-Ahmad, 2001; Al-Kaddoussi et al., 2004). The average grain yield of the lines (Table 2) ranged from 4.009 t/ha (P1) to 7.448 t/ha (P2), with an overall mean of 6.222 t/ha.

The hybrids exhibited highly significant variation in grain yield (Table 1), confirming the genetic divergence between the parent lines used in the hybridization process. This result is consistent with the findings of (Soliman and Sadek, 1998; Malik et al., 2004; Ojo et al., 2007).
       
The average grain yield of the hybrids (Table 3) ranged from 11.215 t/ha (P1 × P3) to 16.083 t/ha (P1 × P2), with an overall average of 13.42 t/ha. Yield formation in maize is also closely associated with dry matter accumulation and its partitioning among vegetative and reproductive organs. Under adequate irrigation, dry matter allocation to cobs increased progressively up to harvest, confirming the importance of efficient assimilate transfer to reproductive organs in determining final yield components (Meena et al., 2015).
       
The results showed that thirteen hybrids outperformed the control Ghouta 82 with significant positive differences.
 
Hybrid vigor
 
The hybrid vigor results showed highly significant positive values, compared to the average of the parents and the best parent for the grain yield trait (Table 4). Hybrid vigor values   ranged from 72.30% (P6 × P5) to 162.43% (P2 × P1) and from 63.28% (P6 × P3) to 104.93% (P2 × P1), respectively. These results are consistent with those of (Galal et al., 1978; Soengas et al., 2003; AL-Ahmad, 2001), Unay et al., 2004; Ojo et al., 2007).
 
Combining ability
 
Both additive and non-additive gene action contributed to the inheritance of grain yield, as evidenced by the highly significant variance of GCA and SCA (Table 1). The σ2GCA/σ2SCA ratio, which was less than one (0.41), indicated the dominance of the non-additive gene action in the inheritance of grain yield. This result was confirmed by the degree of dominance, which was also greater than one (1.103), as the variance of the additive gene action (1.11) was lesser than the variance of the dominant gene action (1.35). This result was consistent with the findings of Galal et al., (1989); Sedhom, (1994c); AL-Ahmad, (2001) and Unay et al., (2004). On the other hand, this result contradicted what was found by (El-Rouby and Galal, 1972; El-Sherbieny et al., 1996; Betrán et al., 2003).
       
The effects of general combining ability (Table 5) ranged from -1.16 (P3) to 1.65 (P2), with line (P2) exhibiting the best general combining ability for grain yield.

The effects of specific combining ability (Table 6) ranged from -1.5417 (P2 × P4) to 1.648 (P3 × P4).
       
These effects indicated that the hybrids P3 × P4 and P2 × P6 had the highest specific combining ability for grain yield.
 
Contribution of parental lines based on pedigree background and combining ability
 
The contribution of each parental line was interpreted according to its pedigree identity as an inbred line maintained in the GCSAR maize germplasm collection and according to its GCA and SCA behavior. IL.155-22 (P1) contributed favorably to single-plant yield and 100-grain weight, particularly through its crosses with P2, P4 and P6. IL.130-22 (P2) was the most important general combiner for grain yield and related traits, as it showed favorable effects for number of grains per row, single-plant yield and grain yield and participated in the highest-yielding hybrid P1 × P2. IL.262-22 (P3) showed lower general effects for some traits, but its favorable contribution appeared through specific combinations, especially with P4 for grain yield and with P5 for single-plant yield, indicating complementary non-additive effects. IL.257-22 (P4) contributed positively to number of grains per row and 100-grain weight and formed a superior specific combination with P3 for grain yield. IL.422-22 (P5) contributed mainly to 100-grain weight and single-plant yield, as reflected by its favorable GCA and its specific combination with P3. IL.424-22 (P6) showed its major value through specific combinations, especially with P2 for grain yield and with P1 for single-plant yield. Therefore, the parental contribution was not uniform across traits; P2 was the best broad contributor for yield improvement, whereas P3, P4, P5 and P6 expressed their value mainly through complementary specific crosses.

Use of the lines IL.155-22 and IL.130-22 in maize yield-improvement programs is recommended, as they demonstrated good general combining ability for grain yield-related traits.
       
The hybrids IL.155-22 × IL.130-22 and IL.130-22 × IL.424-22 are recommended for advanced productivity trials, as they outperformed the check variety Ghouta 82.
       
The number of grains per row can be used as a selection indicator in breeding programs aimed at indirectly increasing yellow maize productivity in early segregating generations. This is due to the dominance of additive genetic action in its inheritance, which allows the development of lines with desirable traits that can produce high-yielding hybrids when crossed.

The results of this study can be summarized as follows:

The heterogeneity of the hybrids and lines was highly significant for all the studied traits indicating genetic and geographical divergence among the parental lines involved in the hybridization process.
       
The variation in general and specific combining abilities was significant for most of the studied traits, indicating an equal contribution of additive and non-additive genetic action to the inheritance of these traits. However, the variation in specific combining ability was not significant for the number of grains per row trait, demonstrating the dominance of the additive genetic action.
       
The σ2GCA/σ2SCA ratio, which was greater than one for the number of grains per row, indicates the dominance of additive gene action in the inheritance of this trait. In contrast, the non-additive gene action dominated the inheritance of the remaining traits, as the σ2GCA/σ2SCA ratio was less than one.
       
Comparison with the check Ghouta 82 showed that thirteen hybrids achieved higher grain yield, exceeding the check by 1.816 to 5.520 t/ha. All hybrids demonstrated desirable heterosis relative to the mid-parent and better parent.
       
The lines IL.155-22 and IL.130-22 showed good general combining ability for both plot yield and single-plant yield.
       
The hybrids IL.262-22 × IL.257-22 and IL.130-22 × IL.424-22 exhibited the highest specific combining ability for grain yield.

The authors gratefully acknowledge the General Commission for Agricultural Scientific Research (GCSAR), Crop Research Department, Damascus, Syria, for providing the field facilities and genetic materials required for this study.
 
Disclaimer
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institution. The authors are responsible for the accuracy and completeness of the information provided.
 
Ethical approval / Informed consent
 
Not applicable. This study was conducted on maize plant materials and did not involve human participants or experimental animals.

The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.