Combining ability analysis for evaluation of maize hybrids under drought stress

The present study was conducted to evaluate the genetic basis of yield related traits under drought conditions. A high heritability and genetic advance was found for plant height, 100-grain weight, grain rows per cob and grain yield per plant, suggesting that the selection of high yielding maize genotypes is possible through this approach. The high specific combining ability of W64SP, A495, A509 and A50-2 suggested that the pre-screening of inbred lines may be an efficient approach to develop higher yielding maize hybrids through heterosis breeding under drought.


INTRODUCTION
Maize (Zea mays) is an important cereal crop worldwide and is ranked third after wheat and rice for its nutritional quality and uses (Cassamon, 1999;Ali et al., 2014a;b).It is a monoecious and highly cross pollinated crop mostly used as food, feed, forage, green fuel (ethanol), vegetable oil and starch and is the backbone of the poultry feed industry.Maize grain constitutes about 9.74 % grain protein, 4.85 % grain oil, 9.44 % grain crude fibre, 71.97 % grain starch, and 11.77 % embryo, while fodder contains 22.98 % acid detergent fibre, 51.69 % neutral detergent fibre, 28.797 % fodder cellulose, 40.18 % fodder dry matter, 26.85 % fodder crude fibre, 10.35 % fodder crude protein and 9.09 % fodder moisture (Ali et al., 2014 b;c; Saif-ul-Malook et al., 2014a;b;c).The Punjab region contributes to about 39 % of the total area under maize cultivation with 30 % of the total produce in Pakistan.The major share belongs to Sindh and KPK with 56 % area and 63 % production, respectively.The average production of maize in Pakistan is 3672 kg/ha, which is very low compared to other countries (Anonymous, 2012(Anonymous, -2013)).
Maize is affected by many biotic and abiotic factors.Drought badly affects plant growth from seedling to maturity (Areous et al., 2005) and maize is more susceptible to drought compared to the other cereals except barley (Banziger & Araus, 2007).Drought causes reduction in leaf area, stem extension, root proliferation, low water use efficiency, metabolism, enzyme activity, ionic balance and solute accumulation (Khan et al., 1995;Farooq et al., 2002).It reduces the chlorophyll content, resulting in low photosynthesis and ultimately reductions in crop yield (Athar & Ashraf, 2005).Water stress affects silking and extends the anthesis-silking-interval (ASI), which ultimately leads to lower crop yield (Edmeades et al., 1992).Grain yield is a quantitative trait, which depends on many factors such as plant height, plant vigour, efficient water availability, optimum nutrient availability, enhanced solar radiation interception and conversion of solar to chemical energy.The selection of a genotype for water stress is complex due to genotype interaction with the environment (Messmer, 2006;Naseem et al., 2015a;b).The present study was conducted to evaluate the genetic basis of yield related traits under drought conditions.
technique (Steel et al., 1997) to evaluate the differences in performance among the genotypes.Line × tester analysis (Kempthorn, 1957) was used to compute the combining ability effects of parents and crosses.

Genetic components
The heritability substantially increased and genetic advance was also high for plant height, leaf area, grain rows per cob, grain yield per plant, 100-grain weight and cob length under normal irrigated conditions (Table 1).These responses were similar under drought conditions, for which all the variables except leaf area displayed a high heritability, and leaves per plant, leaf area, grain rows per cob, cobs per plant, and 100-grain weight showed substantial genetic advance (Table 2).

METHODOLOGY
The parents (A-495, A-509, W-64SP, W-10, A-545, A427-2, A50-2, A-239) and F 1 hybrids were grown in a research field managed by the Department of Plant Breeding and Genetics, University of Agriculture Faisalabad, Pakistan.The seeds were sown at a 2.5 cm depth using a randomised complete block design with three replicates.The plot size was 4 m 2 .The plant-to-plant and row-to-row distances were 25 and 75 cm, respectively.Data were recorded for the following traits from 10 guarded plants of each genotype: leaves per plant, leaf area, plant height, cobs per plant, grains rows per cob,100-grain weight, cob girth, grain yield per plant and cob length.

Statistical analysis
The data were analysed by using analysis of variance

Cobs per plant
Genotype A-502 (0.076) (Table 3) showed the best general combining ability for cobs per plant, while A-509 (-0.21) showed the lowest value for general combining ability under normal irrigation conditions.Inbred lines W-10 (0.07) and A50-2 (0.07) exhibited the highest general combining ability for cobs per plant, while A-239 (-0.17) showed the lowest general combining ability under drought conditions (Table 4).

100-grain weight (g)
highest general combining ability effects were reported for the 100-grain weight of A-495 ( under both normal and drought conditions, respectively (Tables 3 and 4).These high values of general combining ability suggested the additive effects of genes.

Grain yield per plant (g)
Maximum general combining ability among the parents were reported for A-509 (28.  3 and 4).

June 2016
Journal of the National Science Foundation of Sri Lanka 44(2)

Grain yield per plant (g)
Tables 5 and 6 shows that A-495 × A-239 (

DISCUSSION
High heritability values were reported for various traits in this trial, which suggests that the selection of high yielding maize hybrids under drought conditions may be helpful to increase the maize grain yield (Tables 1  and 2).Higher genetic advance indicates that selections can be made to develop synthetic maize cultivars under drought conditions.Eagles (1982) reported that the elite lines of endosperm and embryo were of great importance as compared to the female parents in determining the differences of germination period and relative growth of maize seedlings (Nass et al., 2000).Khidse et al. (1983) have reported that the non-additive genetic effects contribute to grain size and seedling vigour traits of sorghum, viz., seedling volume, plumule length, radicle length and root/shoot fresh and dry weights of maize seedlings.Higher shoot length suggests that higher crop biomass may be produced due to more water content, and an inbred line with a higher shoot length may be selected for fodder breeding as well as for quantitative traits.Similar results have been reported by Mehdi et al. (2001).Several crosses in this trial led to substantial genetic advance, particularly under drought conditions (Tables 1 and 2).Ali and Ahsan (2015) have reported that heterosis can be used for the maintenance of germplasm and pedigree similarities among maize hybrids.Ahsan et al. (2010).It was suggested that increased fresh shoot length, fresh root weight and decreased stomata frequency and epidermal cell size may be useful criteria for selection under drought conditions.Ali et al. (2011a) had conducted an experiment on 40 maize genotypes at seedling stage and concluded that root length, root dry weight, leaf temperature, root density and shoot dry weight were correlated at genotypic and phenotypic levels and hence may be used as selection criteria for higher yielding maize genotypes.Ali et al. (2011b) also estimated the genetic variability and the association among different seedling traits of 40 maize genotypes.It was observed that selection may be made on the basis of shoot length and shoot weight.Higher values of general combining ability suggested that the inbred lines may be used for the development of synthetic cultivars for improving grain yield under drought conditions.The higher specific combining ability suggests that the breeding programme for the development of hybrid seed production in maize may be preceded in next generations.The selection of maize genotypes on the basis of 100-grain weight, grain rows per cob, cob girth, cob length and grain yield per plant may be effective to develop synthetic and hybrids against drought (Ali et al., 2013;2014b;d;Masood et al., 2015a;b).

CONCLUSION
Among the inbred lines that were used for the development of F hybrids under drought conditions, W-64SP, A-495, A-509 and A50-2 performed best for drought tolerant hybrid development.

Table 1 :
Genetic component for various agronomic traits of maize under normal irrigation conditions

Table 2 :
Genetic component for various agronomic traits of maize under drought conditions

Table 3 :
General combining ability for various agronomic traits of maize under normal conditions

Table 4 :
General combining ability for various agronomic traits of maize under drought conditions (2)6Journal of the National Science Foundation of Sri Lanka 44(2)

Table 6 :
Specific combining ability for various agronomic traits of maize under drought conditions

Table 5 :
Specific combining ability for various agronomic traits of maize under normal conditions Pandey et al. (2000)12;2013;ed byKhan 2016).(2014),Saeedetal. (2014)andAli et al. (2011a;2012;2013; 2014a; b;2016)for root and shoot length.Pandey et al. (2000)reported that increasing moisture stress was the major cause of decreases in crop growth rate, leaf area, shoot dry matter, plant height and harvesting index.Nigussie and Zelleke (2001) observed that specific combining ability effects were significant for plant height, days taken to tasseling, days taken to silking and grain yield per plant.The mid parent heterosis in this trial showed a range of -11.6 -21.9 % for grain yield per Similar findings were reported by Ali et al. (2013; 2014a;b) and Singh et al. (1998)concluded that moderate estimates of heritability and genetic advance, i.e. positive and significant genotypic correlations, were found for grain yield per plant vs. plant height, cob length, grains per cob, 100-seed weight and the number of cobs per plant.