Keywords

General Combining Ability (GCA); Specific Combining Ability (SCA); Heterosis

Introduction

Maize (Zea mays L.) is a versatile crop with wider genetic variability and able to grow successfully throughout the world covering tropical, subtropical and temperate agro-climatic conditions. Maize acreage and production have an increasing tendency with the introduction of hybrids due to its high yield potential. Efforts are, therefore, required to be made to develop hybrids with high yield potential in order to increase production of maize. Most efficient use of such materials would be possible only when adequate information on the amount and type of genetic variation and combining ability effects in the materials is available. Heterosis and combining ability is prerequisite for developing a good economically viable hybrid maize variety. Combining ability analysis is useful to assess the potential inbred lines and also helps in identifying the nature of gene action involved in various quantitative characters. Combining ability is dissected into two parts General Combining Ability (GCA) and Specific Combining Ability (SCA). Both GCA and SCA variances have been determined and related to the possible types of gene action involved. GCA is a good estimate of additive gene action, whereas SCA is a measure of non-additive gene action [1]. This information is helpful to plant breeders for formulating hybrid breeding programmes. A wide array of biometrical tools is available to breeders for characterizing genetic control of economically important traits as a guide to decide upon an appropriate breeding methodology to involve in hybrid breeding. Line × tester mating design developed by Kempthorne, which provides reliable information on the general and specific combining ability effects of parents and their hybrid combinations was used to generate the information. The design has been widely used in maize by several workers like, Joshi and Sharma continues to be applied in quantitative genetic studies. The line × tester analysis provides information on GCA of parents and Specific Combining Ability (SCA) of hybrids which helps to identify good quality in breeds and hybrids, respectively [2,3]. The present investigation was carried out to determine the nature and magnitude of gene action for yield and other important traits in maize.

Materials and Methods

Experimental site

The experiment was carried out during rabi season, 2018-2019 at the experimental field of Plant breeding division of Bangladesh Agricultural Research Institute (BARI), Gazipur. The institute is located at 23º59’ N latitude and 90º25’ E longitude. The climate of the area is characterized as tropical with mean monthly maximum and minimum temperature of 28.9ºC and 18.8ºC, respectively. The soil of the experimental field of BARI, Gazipur is characterized by sandy loam with 62.72%sand, 21.95% silt and 15.33% clay. The soil of the field is slightly acidic to neutral and thus pH varied from 6.1 to 6.9. The organic matter content of the soil is also low which was only 1.34% available Phosphorous (P) content is 14.60 ppm.

Experimental materials

Experimental materials comprised a total of 47 entries including 44 test crosses produced by crossing twenty two elite inbred lines with two testers (CML 429 and CML 425) and three standard checks (BHM 9, 981, Elite). The lines were obtained from PBD, BARI, but some are originally introduced from CIMMYT breeding program.

Experimental design and management

The experiment was laid out in alpha lattice design with two replications having plot consisted of two row of 4 meter lengths with row to row distance of 60 cm and plant to plant of 25 cm. Two seeds were planted per hill on 26^th November 2018 and later thinned out to one plant per hill after seedlings established well. Fertilizers were applied @ 250, 55, 110, 40, 5 and 1.5 kg/ha of N, P, K, S, Zn and B respectively. Other standard agronomic practices like weeding and pest management has been done manually throughout the entire growing season as required.

Data collection

Days to Tasseling (DT): Number of days from planting to when 50% of the plants in a plot shed pollen. Days to Silking (DS): Number of days from planting to when 50% of the plants in a plot produced 2-3 cm long silk. Plant Height (PH): The average height of five randomly selected plants measured in cm from base of the plant to the first tassel branch. Ear Height (EH): The average height of five randomly selected plants measured in cm from base of the plant to the node bearing the upper most ear ofthe same plants used to measure plant height. Anthesis Silking Interval (ASI): Number of days interval between Days to anthesis or tasseling (DT) and Days to Silking (DS). Grain Yield (GY): The total grain yield in kg per plot and adjusted to 12.5% moisture level and converted to t/ha.

Data analysis

Combining ability analysis was carried as per procedure given by Kempthorne [3]. The mean performances of all characters were analyzed using Crop Stat software. Data were analyzed for variance for all the characters studied. Using the mean data of all the single cross hybrid and check variety, the standard heterosis (against the BHM 9, standard check hybrid variety) Percent heterosis was calculated by using the following formula:

Standard heterosis (%) = [(F1-CV)/CV] × 100

Where, F1 and CV represent the mean performance of hybrid and standard check variety .The significance test for heterosis wasdone by using standard error of the value of check variety (Table 1).

Table 1: List of lines, testers and check varieties used in the experiment.

Results and Discussion

The analysis of variance for different characters is presented in which indicated that there were highly significant differences among the genotypes for all the characters. ANOVA partitioned the variance into cross/hybrid variance, line variance, tester variance and line × tester variance. All the variance revealed that there were significant differences in all the characters.

Similarly, parent and parent’s crosses were significant for all the characters except ASI indicating greater diversity in the parental lines of the traits. The present observations are in agreement with the earlier report [4]. A comparison of the magnitude of variance components due to GCA and SCA confirms the gene action in controlling the expression of traits. The ratio of GCA and SCA variance for all the traits were less than one, which indicates that all these characters were predominantly governed by non-additive gene effects. Similar findings were reported by [5]. For grain yield, cob length, plant height, ear height, 100 grain weight, grain rows per cob, days to 50% tassel and days to 50% silk for number of grain rows per cob and 100- grain weight in maize in their study (Table 2).

Table 2: Mean squares and estimates of variance for grain yield and yield components in maize evaluated at Gazipur duringrabi 2018-19.

The proportional contributions of lines (female), testers (male) and their interactions (crosses) to total variance for different traits revealed that female lines (maternal) contributed much higher compared to male lines (paternal) in all studied traits. Results showed that maternal parents play the most important role for those traits. Similar conclusion was reported by Bhavana et al. who observed the greater effect of female lines for grain yield and other traits [6]. (Table 3).

Table 3: Proportional contribution of lines, testers and their interactions to total variance in maize.

General combining ability (GCA) effects

The General Combining Ability (GCA) effects of lines (females) and testers (males) are presented. Among the parents, three line (viz., BMZ 55, BMZ 53, BMZ 4) showed significant and negative GCA effect for both days to 50% tasseling and silking, indicating good general combiners for earliness [7]. Also reported the additive gene action for days to 50% tassel and silk. Lines BMZ 15, BMZ 55, BMZ 53 and BMZ 68 showed significant and negative GCA effects for both plant and ear height. The lines (BMZ 55, BMZ 53) also recorded negative GCA effect for days to tasseling, and silking indicated that these parents were suitable for earliness and/or short stature breeding. Similar observations in maize were reported by [8]. Three parental lines (BIL 79, Pinacle 17 and BIL 182) exhibited desirable significant positive GCA for grain yield. These lines could be desirable parents for hybrids as well as for inclusion in breeding program, since they may contribute favourable alleles in the synthesis of new varieties. The parents exhibited significant and positive GCA for yield, were good general combiner and those could be used for exploiting more positive alleles for yield. Significant GCA effect for yield in maize was also reported [9]. As GCA is generally associated with additive gene action in inheritance of characters, the lines with high GCA may be utilized in hybridization program to improve a particular trait through transgressive segregation.

Specific combining ability (SCA) effects

In respect of days to tassel and days to silk, no cross combination recorded significant and negative SCA effects. In case of maize, significant and negative value is expected for plant and ear height to develop short stature plant. The lowest days for 50% tasseling and silking was found in the cross BMZ 53 × CML 425. Lowest plant height and ear height was observed in cross BMZ 15 × CML 425. Positive SCA effect is expected for yield and yield components. In case of grain yield, only one cross (CML 451 × CML 429) exhibited significant positive SCA effects. Among the cross combination highest yield (13.3 t/ha) was produced by BIL 79 × CML 429 followed by BIL 182 × CML 429 (12.6 t/ha).

Heterosis

The degree of heterosis in F1 hybrids varied from character to character or from cross to cross. Days to pollen shedding and silking determine the maturity of the hybrid. For heterosis Days to tasseling and silking ranged from - 10.0 to 7.6 % and -9.3 to 3.8% respectively. Negative heterosis is desirable for these two characters. Considering commercial hybrid BHM9 as a check four crosses BMZ 15 × CML 425, BMZ 55 × CML 425, BMZ 68 × CML 425, BMZ 53 × CML 425 showed significant l and negative heterosis for days to pollen shedding. Maximum negative heterosis was observed in the cross BMZ 68 × CML 425for this trait. For days to silking three crosses BMZ 15 × CML 425, BMZ 68 × CML 425, BMZ 53 × CML 425 exhibited significantly and negative heterosis and highest negative heterosis was observed in the cross of BMZ 53 × CML 425.

Negative heterosis is desirable for plant height and ear height which helps for developing short statured plant leading tolerant to lodging. Heterosis for different crosses ranged from -36.2 to -6.7% and -43.9 to 2.2%, respectively, for plant and ear height [10].In case of grain yield, the percent of standard heterosis varied from -52.6 to 0.6%. Most of the crosses showed significant and negative heterosis except BIL 79 × CML 429 which showed positive value.

Conclusion

Good general combining ability effects for yield and important yield contributing characters were noticed in the lines viz. BMZ 55, BMZ 53, BMZ 4 (earliness), BMZ 68, BMZ 15, BMZ 53, BMZ 55 (dwarf character) and, BIL 79, BIL 182 and Pinnacle 17 (higher yield). These parents could result in the production of superior single crosses. Four crosses BMZ 15 × CML 425, BMZ 55 × CML 425, BMZ 68 × CML 425, BMZ 53 x CML 425 showed significant and negative heterosis for days to pollen shedding. For days to silking three crosses BMZ 15 × CML 425, BMZ 68 × CML 425, BMZ 53 x CML 425 exhibited significant and negative heterosis. In case of grain yield most of the crosses showed significant and negative heterosis except BIL 79 × CML 429 which showed positive value. Hybrid CML 451 x CML 429, BIL 182 x CML 429 and BIL 79 x CML 429 could be advanced for commercial hybrid development after verifying the performance over locations.

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