Study of genetic variability in F2 maize (Zea mays L.) populations

Introduction

Maize (Zea mays L.) is one of the most important cereal crops worldwide and belongs to the tribe Maydeae of the family Poaceae. It is a monoecious, diploid species with a chromosome number of 2n = 20 and is believed to have originated in the Americas. The crop has two closely related American relatives, Tripsacum (gamagrass) and teosinte (Euchlaena), the latter being considered its closest wild relative. The Asiatic members of the Maydeae include Chionachne, Sclerachne, Coix, Trilobachne and Polytoca. Maize is popularly known as the "Queen of Cereals" because of its high genetic yield potential, broad adaptability and diverse uses. As a C₄ plant, maize possesses high photosynthetic efficiency and can produce substantial biomass and grain yield across a wide range of agro-ecological environments. Maize is cultivated globally for food, feed and industrial purposes and plays a major role in supporting food security and agricultural economies. The crop is an important source of carbohydrates, proteins, oils and several industrially valuable products. Its grain is extensively used for human consumption, livestock and poultry feed, starch and oil production, biofuel, and a wide range of value-added industrial products.

With increasing population, expanding livestock and poultry industries, and growing demand forindustrial raw materials, the global demand for maize is expected to continue increasing. Consequently, enhancing maize productivity and stability under diverse production environments has become a major objective of contemporary maize improvement programmes. The increasing demand for maize must be addressed largely through improved productivity because opportunities for expanding cultivated land are limited by population growth, urbanization and environmental degradation. In many developing countries, maize production is constrained by a combination of abiotic and biotic stresses. Among these, drought is one of the most important environmental constraints afecting maize production, particularly in tropical and subtropical regions where a considerable proportion of the crop is grown under rainfed conditions. Drought stress may occur at any stage of crop development; however, its efects are particularly severe when it coincides with the flowering and grain-filling stages. Maize is highly sensitive to water deficit during the reproductive phase. Drought stress during flowering can cause delayed silk emergence, increased anthesis-silking interval (ASI), poor pollen-silk synchrony and reduced fertilization. These efects ultimately result in a reduction in kernel number, kernel weight and grain yield. Water stress during grain filling further restricts assimilate production and translocation, thereby reducing final grain accumulation. The severity of drought-induced yield losses depends on the timing, duration and intensity of stress, as well as on the genetic constitution of the genotype and the prevailing environmental conditions. The increasing frequency and unpredictability of drought events associated with climate variability further emphasize the need to develop maize genotypes with improved drought tolerance and stable yield performance. Drought tolerance in maize is a complex quantitative trait governed by several genes and influenced substantially by environmental conditions. Therefore, direct selection for grain yield under drought stress is often dificult because of strong genotype × environment interactions and reduced heritability under stressful conditions. The identification of secondary traits associated with drought adaptation can improve the eficiency of selection. Traits such as anthesissilking interval, relative water content, chlorophyll retention, plant and ear height, number of cobs per plant, kernel number and grain yield have been widely considered important indicators of drought response and productivity. In particular, a shorter anthesis-silking interval, greater maintenance of leaf water status and chlorophyll content, and superior grain yield under waterlimited conditions are generally associated with improved drought adaptation. Genetic variability is the fundamental prerequisite for the genetic improvement of any crop. The success of a breeding programme depends largely on the availability of adequate genetic variation and the efective identification of superior genotypes within segregating populations. Estimates of genotypic and phenotypic coeficients of variation provide valuable information regarding the magnitude of variability present for important traits. However, the presence of variation alone does not necessarily indicate the efectiveness of selection. Heritability estimates, together with genetic advance, provide a better understanding of the relative contribution of genetic and environmental factors to the expression of quantitative traits. High heritability coupled with high genetic advance as a percentage of the mean generally indicates the predominance of additive genetic efects and suggests that efective improvement may be achieved through phenotypic selection. The development and evaluation of inbred lines through recurrent selection and selfing of superior plants represents an important strategy for generating improved genetic material in maize. Early-generation testing of segregating progenies under contrasting moisture regimes enables breeders to identify genotypes that combine desirable agronomic performance with drought adaptation. Evaluation under both stress and non-stress conditions is particularly important because genotypes that perform well under optimum moisture may not necessarily maintain their productivity under drought. Comparative assessment of genetic variability, heritability and genetic advance under contrasting moisture environments can therefore provide valuable information for designing efective selection strategies. In view of the increasing importance of maize and the substantial yield losses associated with drought stress, the identification of genetically superior and drought-adapted breeding material is essential. Therefore, the present investigation was undertaken to assess the genetic variability, heritability and genetic advance for important morphological, physiological and yield-related traits in maize inbred populations evaluated under drought-stress and non-stress conditions. The study was aimed at identifying traits with a strong genetic basis and high selection potential for improving grain yield and drought tolerance in maize breeding programmes. Water availability is one of the most important factors determining crop productivity per unit area. Although maize is relatively tolerant to water deficit during the early vegetative stages, water stress during the reproductive and grain-filling stages can severely reduce yield. The magnitude of yield reduction depends on the timing, duration and intensity of water stress, as well as on the genetic constitution of the genotype. Quaranta et al. (1998) [1] emphasized the importance of water availability for maximizing crop productivity. Khan et al. (2001) reported that increasing water stress adversely afected flowering time and grain yield in maize. Similarly, Tahir and Mehdi (2001) [2] observed significant reductions in 100-grain weight and grain yield per plant under waterlimited conditions. Ramadoss et al. (2004) identified water stress and high temperature as major constraints to maize production and reported that drought reduced seed-set percentage, ultimately resulting in lower grain yield. Monneveux et al. (2005) [3] demonstrated significant genetic diferences in yield performance under drought conditions, highlighting the potential for selection of drought-adapted genotypes. Zhang et al. (2008) [4] reported significant reductions in grain yield of maize inbred lines under drought stress, accompanied by delayed 50% silking. Hussain et al, while evaluating maize inbred lines under normal and drought conditions, observed significant diferences among genotypes for all the traits studied, indicating the presence of substantial genetic variability for drought response. The availability of diverse and genetically variable source populations is fundamental to the success of any crop improvement programme. Such populations provide valuable breeding material for generating and exploiting genetic variability for desirable traits. The assessment of genetic variability, heritability and genetic advance in source populations helps determine the relative contribution of genetic and environmental factors to trait expression and provides an indication of the potential efectiveness of selection. Conventional maize breeding programmes largely rely on the development of inbred lines through repeated selfing of genetically diverse material, followed by their evaluation using appropriate biometrical and quantitative genetic approaches. The identification of superior lines at an early stage of breeding is particularly important for improving breeding eficiency and reducing the time and resources required for the development of drought-tolerant cultivars.

In the context of increasing water scarcity and the growing frequency of drought episodes, the development of maize genotypes capable of maintaining stable productivity under water-limited conditions has become a major breeding objective. Therefore, the present investigation was undertaken to assess the extent of genetic variability and the heritability of important agronomic, physiological and yieldrelated traits in maize populations evaluated under droughtstress and non-stress conditions. The study was specifically aimed at identifying superior breeding lines and traits with high selection potential that could be efectively utilized in breeding programmes for the development of high-yielding and drought-tolerant maize genotypes.

Material and Methods Experimental Site and Weather Conditions

The investigation was conducted at the experimental fields of the Botanical Garden, Main Agricultural Research Station (MARS), University of Agricultural Sciences, Dharwad, Karnataka, India. The first experiment was conducted during the post-rainy season of 2012-2013. During the crop growth period, the minimum and maximum temperatures ranged from 14.8 to 29.5 °C, and a total rainfall of 201.1 mm was received. A second experiment was conducted at the same location during the summer season of 2013-2014. During this crop growth period, the minimum and maximum temperatures ranged from 14.5 to 36.9 °C, with a total rainfall of 382 mm.

Evaluation of S₁ Generation under Stress and Non-Stress Conditions

A total of 200 S₁ • -generation selfed seeds, together with nine parental genotypes and 16 checks, were evaluated as ear-to-row progenies during the post-rainy season of 2012-2013. The experiment was laid out in a 15×15 lattice square design with two replications under both managed droughtstress and non-stress conditions. Under non-stress conditions, the genotypes received recommended agronomic practices and regular furrow irrigation at 10-12-day intervals throughout the crop growth period to prevent the development of moisture stress. Under managed drought-stress conditions, recommended agronomic practices were followed, but irrigation was provided only up to 20 days after sowing. No further irrigation was applied until harvest, thereby exposing the genotypes to moisture stress during the flowering and grainfilling stages. The stress and non-stress experiments were separated by a 4-m-wide bufer zone planted with maize to minimize the movement of irrigation water between the two experimental areas.

Each selfed seed was grown as an ear-to-row progeny in a single 4-m-long row. Under stress conditions, a spacing of 75 × 20 cm (inter-row × intra-row) was maintained, whereas under non stress conditions, the spacing was 60 × 30 cm. Two seeds were sown per hill and subsequently thinned to maintain one healthy plant per hill. Approximately 20 plants were maintained in each progeny row. Five competitive plants were randomly selected, tagged and used for recording observations. The mean values of the five plants were calculated for each trait and used for statistical analysis.

Evaluation of S₂ Generation under Stress and Non-Stress Conditions

A total of 80 S₂generation selfed seeds, along with 12 inbred lines, three composite populations and five checks, were evaluated as ear-to-row progenies during the summer season of 2013-2014. The experiment was conducted in a 10 × 10 lattice square design with two replications under managed drought-stress and non-stress conditions. Under non-stress conditions, the genotypes were maintained using recommended agronomic practices and regular furrow irrigation at 10-12-day intervals to ensure adequate moisture throughout the crop growth period. Under managed drought-stress conditions, recommended agronomic practices were followed, but irrigation was provided only up to 30 days after sowing. Thereafter, irrigation was withheld until harvest to impose moisture stress during the flowering and grain-filling stages. The stress and non-stress experiments were separated by a 4-mwide bufer zone planted with maize. Each progeny was grown in a single 4-m-long row. Interrow and intra-row spacing of 75 × 20 cm was maintained under stress conditions, whereas 60 × 30 cm spacing was used under non stress conditions. Two seeds were dibbled per hill and subsequently thinned to retain one healthy plant per hill. Approximately 20 plants were maintained in each progeny row, from which five competitive plants were randomly selected and tagged for recording observations. The mean of the five plants was calculated for each trait and used in subsequent analyses.

Observations Recorded

Observations were recorded for the following morphological, physiological and yield-related traits: days to 50% anthesis, days to 50% silking, anthesis-silking interval (ASI), plant height (cm), ear height (cm), days to maturity, cob length (cm), cob girth (cm), number of cobs per plant, number of kernel rows per cob, number of kernels per row, shelling percentage, 100-seed weight (g), grain yield per plant (g), relative water content (RWC) and chlorophyll content.

Statistical Analysis

The data obtained for the evaluated traits were subjected to appropriate statistical analyses to estimate the magnitude of genetic variability and other genetic parameters under drought-stress and non-stress conditions. Statistical analyses were performed using WINDOSTAT version 8.0.

Results and Discussion

Performance and variability study of S inbred lines derived from the seven diferent population under stress and non stress conditions Evaluation of 200 S1 inbredlines was carried out under both stress (drought) and non stress (irrigated) conditions to isolate inbred lines for drought tolerance. Variability is very important for improvement of any trait. The nature and magnitude of variation for diferent traits apart from mean performance decides the worth of the cross for exploitation after hybridization. Adopting the procedures described in the material and methods phenotypic and genotypic variances were computed for diferent traits. In order to know the nature and magnitude of variances for divergent traits, genetic parameters viz., phenotypic and genotypic coeficients of variation, heritability in broad sense and genetic advance over mean were computed and presented in Table (2). Analysis of variance under stress the analysis of variance indicated highly significant diferences for 18 characters except relative water content at 60 days and relative water content at 90 days (Table 1).whereas under non stress condition analysis of variance indicated highly significant diferences for 17 characters and 1character showed significant diference (chlorophyll content content at 90 days) except chlorophyll content content at 90 days and relative water content at 90 days (Table 1). There is a considerable variation in the mean values for all the traits as indicated by highly significant mean sum of squares. Genetic variability parameters of S1 progenies like mean, range and the components of genetic variability viz., phenotypic coeficient of variation (PCV), genotypic coeficient of variation (GCV), broad sense heritability and genetic advance as per cent mean (GAM) were worked out for the 20 quantitative characters under both stress and non stress conditions. Results obtained in the present investigation for each character is presented in Table (2). Days to 50 per cent anthesis ranging from 54.0 to 79.0 with an average of 65.8 days (S₁ progenies) under stress condition. The variability parameters, like GCV (6.58%) and PCV (6.09%) exhibited low and S₁ progenies showed high heritability (85.5%) coupled with moderate genetic advance as per cent of mean (11.59%) under stress condition. it ranging from 59.0 to 80.0 with an average of 68.43 days (S₁) under non stress condition, The genetic variability components, νiz., GCV (5.69%) and PCV (5.94%) exhibited low values and S₁ progenies showed high heritability (91.7%) coupled with moderate genetic advance as per cent of mean (11.23%) under non stress condition. Days to 50 per cent silking ranging from 56.0 to 82.0 with an average of 68.2 days (S₁) under stress condition. The genetic variability components, viz., GCV (6.25%) and PCV (6.78%) exhibited low values and S₁ progenies showed high heritability (84.9%) coupled with moderate genetic advance as per cent of mean (11.86%) under stress condition. ranging from 61.0 to 84.0 with an average of 70.62 days (S₁) under non stress condition. The variability parameters, like GCV (5.63%) and PCV (5.92%) exhibited low values and S₁ progenies showed high heritability (90.5%) coupled with moderate genetic advance as per cent of mean (11.04%) under non stress condition. Days to anthesis to silk interval ranging from 1.0 to 6.0 with an average of 2.4 days (S₁) under stress condition. The genetic variability components, viz., GCV (30.53%) and PCV (37.73%) exhibited high values and S₁ progenies showed high heritability (65.6%) coupled with high genetic advance as per cent of mean (50.93%) under stress condition. ranging from 1.0 to 4.0 with an average of 2.19 days (S₁) under non stress condition, The genetic variability components, viz., GCV (21.48%) and PCV (30.38%) exhibited high values and S₁ progenies showed high and moderate heritability (50.0%) coupled with high genetic advance as per cent of mean (31.28%) non stress condition. At 60 days after sowing chlorophyll content ranging from 29.8 to 56.8% with an average of 39.9 % (S₁) under stress condition. Genetic components of both GCV (5.19%) and PCV (8.07%) were low, and S₁ progenies showed moderate heritability (41.3%) coupled with low genetic advance as per cent of mean (6.87%) under stress. ranging from 28.6 to 49.8% with an average of 42.6 % (S₁) under non stress condition. The variability parameters, like GCV (2.94%) and PCV (6.27%) exhibited low values, S₁ progenies showed moderate heritability (21.9%) coupled with low genetic advance as per cent of mean (2.83%) non stress conditions. At 90 days after sowing chlorophyll content ranging from 23.0 to 53.5% with an average of 38.5 %(S1) under stress condition. The genetic variability components, νiz.., GCV (8.06%) and PCV (11.65%) exhibited low and moderate values, S₁ progenies showed moderate heritability (47.8%) coupled with moderate genetic advance as per cent of mean (11.48%) under stress, ranging from 26.7 to 45.8% with an average of 36.5% (S₁) under non stress condition. Genetic components of both GCV (3.04%) and PCV (7.69%) were low and S₁ progenies showed low heritability (15.7%) coupled with low genetic advance as per cent of mean (2.48%) non stress condition. Relative water content at 60 days ranging from 0.49 to 0.99% with an average of 0.70 % (∇ S₁) under stress condition. The genetic variability components, viz., GCV (2.83%) and PCV (9.62%) exhibited low values and S₁ progenies showed low heritability (8.7%) coupled with low genetic advance as per cent of mean (1.71%) under stress. ranging from 0.42 to 0.94% with an average of 0.73% (S₁) under non stress condition, varaiability components of both GCV (7.87%) and PCV (8.97%) were low and S₁ progenies showed high heritability ( 77.0%) coupled with moderate genetic advance as per cent of mean (14.22%) non stress condition.

Relative water content at 90 days ranging from 0.33 to 0.77% with an average of 0.57 % ( S₁) under stress condition. Genetic components of both GCV (1.94%) and PCV (8.04%) were low, S₁ progenies showed moderate heritability (47.0%) coupled with low genetic advance as per cent of mean (7.54%) under stress. ranging from 0.30 to 0.76% with an average of 0.47% (S₁) under non stress condition. The genetic variability components, viz., GCV (4.92%) and PCV (12.16) [5] , Om Prakash et al. (2006) [6] Prashanth M. (2008) [7] , Alake et al.

(2008), Kuchanur, P. H., (2010) and Salman S., (2011) [8] . [6] , Prashanth M. (2008) [7] , Alake et al. (2008), Kuchanur, P. H., (2010) and Salman S., (2011) [8] . Under stress conditon cob girth and chlorophyll content at 90 days the GCV was low and PCV was moderate. [9] Prashanth M. (2008) [7] , Alake et al. (2008), Kuchanur, P. H., (2010) and Salman S., (2011) [8] . [6] , Kumar et al. (2006), Prashanth M. (2008) [7] , Alake et al. (2008), Kuchanur, P. H., (2010) and Salman S., (2011) [8] . Whereas under non stress condition grain yield per plot and bulk cob weight were indicating higher variances in the parents as compared to S₁ progenies. Hence, the genetic parameters were non estimable in these two characters, Evaluation of 80 S2 inbred lines was carried out during rabi season of 2013-14 under stress (drought) and under irrigated conditions to isolate inbred lines for drought tolerance.

Variability is very important for improvement of any trait. The nature and magnitude of variation for diferent traits apart from mean performance decides the worth of the cross for exploitation after hybridization. Adopting the procedures described in the material and methods phenotypic and genotypic variances were computed for diferent traits. In order to know the nature and magnitude of variances for divergent traits, genetic parameters viz., phenotypic and genotypic coeficients of variation, heritability in broad sense and genetic advance over mean were computed and presented in Table (4).

The analysis of variance under stress condition indicated highly significant diferences for 17 characters except relative water content at 60 days, relative water content at 90 days and number of kernel rows per cob (Table 3). There is a considerable variation in the mean sum of square values for all the 17 traits. Under non stress condition the analysis of variance indicated highly significant diferences for 7 characters and 4 character showed significant diference except chlorophyll content at 60 days, chlorophyll content at 90 days, relative water content at 60 days, days to maturity, plant height, ear height, cob girth, number of kernel rows per cob and number of kernels per row (Table 3). There is a considerable variation in the mean values for 11 traits as indicated by significant mean sum of squares. Genetic variability parameters of S₂ progenies like mean, range and the components of genetic variability viz., phenotypic coeficient of variation (PCV), genotypic coeficient of variation (GCV), broad sense heritability and genetic advance as per cent mean (GAM) were worked out for the 20 [6] , Prashanth M., (2008) [7] and [6] Prashanth M., (2008) [7] and Kuchanur, P. H., [6] Prashanth M., (2008) [7] and [6] Prashanth M., (2008) [7] and Kuchanur, P. H., (2010).

Conclusion

The present investigation demonstrated substantial genetic variability for important agronomic, physiological and yield-related traits in maize under both drought-stress and non-stress conditions. Under drought stress, high heritability coupled with high genetic advance as a percentage of the mean was observed for bulk cob weight, grain yield per plot, number of cobs per plant, anthesis-silking interval and number of kernels per row. Under non-stress conditions, the same combination of high heritability and high genetic advance was recorded for bulk cob weight, grain yield per plot, cob length, number of kernels per row, number of cobs per plant, 100-seed weight and plant height. The association of high heritability with high genetic advance suggests that these traits are predominantly governed by additive genetic efects and can therefore be efectively improved through selection. Accordingly, these traits may serve as useful selection criteria in maize breeding programmes aimed at improving grain yield and drought adaptation. The superior inbred lines identified in the present study may provide valuable genetic resources for the development of high-yielding, drought-tolerant maize

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