Identification of Potential F1 Hybrids in Maize Responsive to Water Deficient Condition

The assessment of heterotic F1 combinations is a basic requisite for hybrid seed development. A set of 30 F1 hybrids along with their parental inbred lines were evaluated under both normal and water deficit conditions for various physiological and agronomic traits. Highly significant mean squares due to general combining ability, specific combining ability and reciprocal effect were observed for all traits under both water regimes. Components of variation exhibited greater estimates for GCA variance (б2g) than SCA variance (б2s) for majority of the traits under both normal and stress conditions depicting the predominant role of additive genetic component. Inbred lines NCIL-20-20, D-157 and OH-8 were found to be the best general combiner on the basis of performance regarding grain yield per plant under water deficit condition. The F1 combinations namely, NCIL-20-20 × D-109, NCIL-20-20 × OH-8 and D-157 × NCIL-20-20 were out-performers based on yield and yield attributes under water deficit conditions. On the basis of our results, we recommend these hybrids for further exploitation to assess their potential for commercial cultivation under water deficit condition.


Introduction
Maize (Zea mays L.), is a versatile plant amongst cereal crops with a wide range of agro-climatic adaptability.It is a multipurpose crop consumed as food for human, feed for poultry, fodder for animals and fuel for industry.In Pakistan, it is mostly cultivated in Punjab and KPK province in spring and autumn seasons [1].A number of biotic and abiotic stresses affect standing crop in the field.Overall one major factor that constrains crop growth in the world is water availability [2] and it is anticipated that by 2025 about one third of human population will be affected by water deficit [3].Due to water deficit conditions in maize about 24 million tons is being lost annually and high yield potential genotypes under potential regions cannot compensate the projected increase in demand for the next decade [4].Breeding cultivars tolerant to water deficit condition appear to be the only option for stress prone areas.Hybrids were found to be more tolerant to water deficit condition as compared to inbred lines [5].Interpretation of various agro-physiological traits that provide tolerance against water deficit is essential for fruitful selection of genotypes under stress environments [6].Existence of variability for drought tolerance in crop plant has already been reported [7].Combining ability analysis based on progeny test is useful and reliable approach for screening inbred lines and F 1 hybrids [8].Among the available conventional techniques, diallel cross analysis as developed by Hayman [9] [10] and Jinks [11] is an efficient tool furnishing information on genetic mechanism conditioning various plant traits in one generation.Therefore, combining ability estimates provide information on mechanism controlling quantitative characters and further help in selecting suitable parents for developing superior hybrids or varieties [12].General and specific combining ability provide estimates for additive and non-additive components, respectively [13] [14].In literature, additive [15]- [18] and non-additive [12] [19] genetic effects have been reported for grain yield and yield related traits under various environmental conditions.Therefore, information regarding genetic mechanism for water deficit tolerance is pre-requisite for development of maize hybrids and synthetics for sustainable agriculture.The present research 6 × 6 diallel analysis was pursued to evaluate maize inbred lines and F 1 hybrid for water deficit regimes using some agro-physiological traits.

Materials and Methods
The research work was conducted in the Department of Plant Breeding and Genetics, University of Agriculture Faisalabad during the years 2010-2011.Six inbred lines including M-14, OH-8, D-157, D-114, D-109 and NCIL-20-20 were selected out of fifty collected inbred lines at seedling stage using physio-agronomic parameters under water deficit conditions.Selected inbred lines were crossed in all possible combinations in the field during spring 2011 in a complete diallel mating design.Both male and female inflorescences of maize plants were covered with kraft paper bag and butter paper bags at the time of inflorescence, respectively, to make controlled crosses.Pollens collected in petri dish were applied on silks with the help of camel hair brush.Silks were pollinated twice on consecutive days to ensure required seed setting.After pollination, the inflorescences were again covered with their respective bags.The instruments were sterilized after each pollination.The F 1 and their reciprocal crosses along with the parents were planted in the research field in autumn 2011 under normal and water deficit stress conditions using Randomized Complete Block Design (RCBD) with three replications.Each experimental unit comprised of two rows of 5.3 m each keeping row to row distances of 75 cm while plant to plant spacing was 23 cm.Two seeds were dibbled per hill to ensure good plant population.Thinning was done to keep one healthy plant per hill after 15 days of sowing.Six rows of non-experimental lines were planted on each side of experimental area to minimize border effect.Recommended insecticide was applied to counter for shoot fly and stem borer.Except for irrigation schedule, all recommended agronomic, cultural practices and plant protection measures were kept uniform.Normal experimental set received standard irrigation whereas 50% of normal irrigation was supplied to the water deficit set [20].Ten equally competitive plants were ear-marked from each entry from both sets and data for pertaining to various physio-agronomic traits like cell membrane thermostability [21], Stomatal conductance (Steady state porometer, Model L-1 1600 SSP1674 Li cor.Ink, USA), plant height, number of days to 50% tasseling, number of days to 50% silking, anthesis-silking interval and grain yield per plant.Data relating to various agro-physiological traits were enumerated and compared using statistical analysis according to Steel et al. [22].Combining ability studies were performed by using Method I Model I following Griffing's approach [23].Genetic variability in the material was divided into components of general combining ability (GCA), specific combining ability (SCA), reciprocal effects and error mean squares for the traits.Sum of squares for these components were calculated as under: Thus, SS due to error = SS (error) in ANOVA/r where, r = number of replications.
Keeping in view the probability of the mean squares for fixed model I, estimates of genetic components due to SCA, GCA and reciprocals are obtained as under:

ANOVA for combining ability (Griffing method I model I)
Estimation of components of variation was carried out as below: where, 2 g σ , 2 s σ , 2 r σ and 2 e σ are the estimates of variance due to general combining ability (GCA), specific combining ability (SCA), reciprocal effects and environment, respectively.
General combining ability (GCA) effects were calculated using the expression: Specific combining ability effects (SCA) were calculated using the expression: Reciprocal effects were calculated following: ( ) Variances were calculated as under: ( ) ( ) Critical difference between two parents = S.E × T tab at 0.05 probability.OR = (σ 2 ) 0.5 × T tab at 0.05 probability Morpho-physiological characteristics of selected maize inbred lines

Cell Membrane Thermostability
Estimation of combining ability revealed highly significant estimates for general and specific combining ability revealing the significance of both additive and non-additive gene action for the expression of trait under normal and water deficit conditions (Table 1).General combining ability (GCA) variance (б 2 g) was found higher than Inbred NCIL-20-20 proved to be the best general combiner under both conditions while inbred D-109 reflected poor combiner among the parents.Cross D-109 × NCIL-20-20 exhibited maximum SCA (2.04) effects under stress condition.Among the reciprocal crosses, cross D-157 × OH-8 displayed maximum positive (0.15) effects while combination D-109 × OH-8 showed maximum negative effects (−0.39) under water deficit condition.Chohan et al. [17] reported additive type of gene action, whereas and Akbar [24] reported non-additive gene action for this trait.In both environment, inbred NCIL-20-20 showed maximum value and may be used as donor parent for developing drought tolerant genotypes.

Stomatal Conductance
General combining ability (GCA) effects were observed more under normal condition than specific combining ability (SCA) effects revealing the role of predominant additive genetic effect (Table 1).Variance due to general combining ability (GCA) (б 2 g) was more than specific combining ability (SCA) variance (б 2 s) indicated additive genetic effects (Table 2).Under normal conditions, half parental lines showed positive and rest showed negative effects (Table 3

Days to Tasseling
Mean squares for general combining ability (GCA) and specific combining ability (SCA) indicated the presence of both additive and non-additive gene action for the trait (Table 1).High estimates of GCA than SCA under both conditions indicated the role of additive gene action for inheritance of trait.Greater estimates of GCA variance (б 2 g) (Table 2) than SCA variance (б 2 s) indicated additive genetic effects.Prakash and Ganguli [30], Bello and Olaoye [15], Chohan et al. [17] and Iqbal et al., [18] reported additive gene action while Akbar et al. [24] reported non-additive gene action for this trait.Half of the parents displayed positive value while the rest showed negative GCA effects (Table 3(

Days to Silking
Variance for general combining ability (б 2 g) was observed greater than specific combining ability effects (б 2 s) (Table 2) indicated the presence of additive genetic effects.The results are compatible with those of Reddy et al. [34], Olaoye [35], Gichuru et al. [16], Chohan et al. [17], and Iqbal et al. [18] who reported additive gene effects for days to silking under both conditions.Half inbred lines depicted positive while the rest showed negative GCA effects under normal water condition (Table 3(

Anthesis-Silking Interval
Analysis of variance for anthesis-silking interval (ASI) revealed significant mean squares due to general combining ability (GCA) and specific combining ability (SCA) effects (Table 1).GCA variance (б 2 g) was observed more than SCA variance (б 2 s) under both conditions which indicated the significance of additive genetic effects for the inheritance of the trait (Table 2).Similar findings have been reported by Ahmad [36], Bello and Olaoye [16], Chohan et al. [17] and Iqbal et al., [18] who reported additive gene action for this trait.Half inbred lines showed positive GCA value while other three parents displayed negative GCA value under normal water condition (

Conclusion
Highly significant mean square estimates due to specific combining ability (SCA), general combining ability (GCA) and reciprocal effects for the traits under both conditions suggested significant contribution of genetic components of variation attributable to general combining ability, specific combining ability and reciprocal effects.Components of variation exhibited greater estimates for GCA variance (б 2 g) than SCA variance (б 2 s) for majority of the traits under both conditions depicting the predominant role of additive genetic component except for days to silking under water deficit condition which displayed more SCA variance (б 2 s) than GCA variance (б 2 g).Inbred lines NCIL-20-20, D-157 and OH-8 were recorded as the best general combiner on the basis of performance regarding grain yield per plant under both conditions i.e. normal and water deficit condition.These inbred lines can be exploited and utilized in future breeding program.On the basis of mean grain yield per plant the best combination was NCIL-20-20 × D-109 followed by NCIL-20-20 × OH-8 and D-157 × NCIL-20-20, respectively under normal and water stress condition.These well performing combinations can be utilized for developing new hybrids for drought affected areas.Prediction of additive gene action would be expected to be more reliable as compared to the traits which were controlled by non-additive type of gene action.
Y i. and Y. j = total of the ith and jth arrays in the mean table Y. = Grand total of the mean table Yij = mean value of the cross of ith parent with jth parent Yji = mean value of the cross of jth parent with ith parent (reciprocal cross) n = number of parents Sum of Square due to error Mean sum of squares due to error obtained in the Analysis of variance (ANOVA) were used after dividing by number of replications because mean values are used here.

Table 1 .
Mean combining ability (SCA) variance (б 2 s) indicated additive gene action under both normal and water deficit conditions (Table2).Higher additive genetic variance for a trait suggested early selection with significant genetic gain.Half of the parents showed positive GCA estimates while half parents displayed negative GCA estimates.Inbred line NCIL-20-20 showed maximum general combining ability effect (5.438) whereas inbred D-109 (-6.589) exhibited lowest GCA effects under normal condition (Table3(a)).NCIL-20-20 proved to be the best general combiner on the basis of GCA effects.Among the crosses, most useful combination was OH-8 × D-114 (2.270) with maximum specific combining ability (SCA) effects whereas combination M-14 × D-114 showed negative effects (−2.12).Regarding reciprocal crosses, 6 were observed with positive while 9 crosses displayed negative effects.Maximum positive effects (0.673) were produced by cross D-114 × D-157 while maximum negative value (−1.11) recorded for cross NCIL-20-20 × D-157 under normal condition for cell membrane thermo-stability.
squares due to general combining ability (GCA), specific combining ability (SCA) and reciprocal effects under normal (N) & moisture stress (S) condition.**= Highly Significant.specific

Table 3
(F)).Inbred line D-109 (0.44) showed maximum value and proved to be the best general combiner while inbred line M-14 (−0.33) indicated minimum value depicting poor combiner for the trait.Cross OH-8 × D-157 showed maximum SCA value (0.19) while M-14 × D-157 showed minimum (−0.19)SCA effects.Maximum reciprocal effect was observed for D-114 × M-14 (1.16) whereas minimum value was shown by cross