Genetic Analysis Of Yield

Published Date: 02 Nov 2017

Abstract

The genetic basis of salt tolerance was investigated in six bread wheat cultivars (Local white, Pavon, Pasban 90, Frontana, Tobari 66 and Chakwal 97) differing in salinity tolerance, and their F1 crosses made in a half diallel mating design. The F1s and parents were germinated in pots, and were subjected to 200 mM NaCl salt stress after one month. Most of the crosses had high heterosis for yield suggesting that breeding for high yield under salt stress is possible. Narrow sense (h2 N) heritability estimates ranged from 0 to 51%, whereas broad sense (h2 B) heritability estimates ranged from 25 to 84 % for the studied traits. Additive genetic effects were significant for days to heading, days to maturity, plant height and fertile tillers plant-1, suggesting that selection in early generations could be effective to bring desirable changes in these traits under NaCl stress. Dominance effects were significant for yield and yield contributing traits, indicating that selection for yield under NaCl stress would be effective in later generations.

Key words: diallel analysis, quantitative traits, bread wheat, salinity tolerance

Abbreviations: GCA; general combining ability, SCA; specific combining ability, AD; Additive-Dominance, AUP; Adjusted Unbiased Prediction, MPH; mid parent heterosis, BPH; better parent heterosis,

Introduction

Salinity is the oldest and severe abiotic stress which limits crop production in arid and semi arid areas of the world. Over 800 million hectares of land are salt affected throughout the world (Munns, 2005). In Pakistan, about six million hectares are salt affected (Chatrath et al., 2007). One of the possible ways to bring saline marginal soils under crop cultivation is to develop salt tolerant crop cultivars instead of pricey engineering approaches (Qureshi et al., 1990).

Salt tolerant crop varieties can be developed by conventional methods if genetic variation for salinity tolerance exists in the available germplasm. Genetic variation with high heritability is desirable for direct phenotypic selection. Only a few cycles of selection could result in significant improvement in salinity tolerance. Diallel cross designs are frequently used in plant breeding research to obtain information on genetic effects for a fixed set of parental lines or estimates of general combining ability (GCA) and specific combining ability (SCA), variance components and heritability for a population from randomly chosen parental lines. Diallel crossing, particularly half or partial diallels are extensively used in crop breeding programs (Yanchuk, 1996). Knowledge of broad sense heritability, narrow sense heritability, GCA and SCA is useful in the choice of parental genotypes. Combining ability studies assist in the identification of parents with greater GCA values and parental combinations with greater SCA values.

High heritability values of traits which confer salinity tolerance in spring wheat indicated that a major progress in salinity tolerance may be possible through selection by the imposition of high selection pressure (Ashraf, 1994). Selection of plants exhibiting better combination of desirable traits is easy if variation is controlled by additive gene effects. Stuber (1994) reported that assessment of additive and non-additive gene action could be helpful in determining the possibility of commercial utilization of heterosis and isolation of pure lines among the progenies of the good hybrids. Singh & Singh (2000) reported that salinity sensitive parent response was partially dominant, whereas the tolerant parent showed partial dominance for yield potential. Salinity tolerance and yield potential appeared to be controlled by different gene complexes.

The present study was initiated to estimate the morphological traits related to salt tolerance and to determine the type of gene action controlling salt tolerance in wheat under 200 mM NaCl stress.

Materials and Methods

2.1. Plant material and growth conditions

Seeds of the six bread wheat varieties (Local white, Pavon, Pasban 90, Frontana, Tobari 66 and Chakwal 97) differing in salinity tolerance were taken from the gene bank of Plant Genetic Resources Program, NARC Islamabad. These cultivars were crossed in a one-way diallel mating design to obtain a total of 15 [(6(6–1)/2] cross combinations. F1 seeds along with their parents were sown in pots in a randomized complete block design with three replications. After one month of germination, 200 mM NaCl salt stress was given to all F1 and parents. Two plants were maintained in each pot. One pot represented one replication. Data were recorded for plant height, No. of tillers plant-1, days to heading, days to maturity, spike length, number of spikelets spike-1, number of grains spike-1, 100 gain weight and grain yield plant-1.

2.2. Data analysis

The data were analyzed using the Mixed Procedure in SAS (SAS Institute, 2003). Likelihood ratios were constructed as differences between the –2 Residual Log Likelihood values of the reduced covariance model (without the effect being tested) and the full covariance model (with the effect being tested) as described by Iqbal et al. (2007). Diallel analyses were performed on the mean values of parents and F1 crosses, employing an Additive-Dominance (AD) genetic model for the analyses of F1 diallels (Zhu, 2003). The genetic variance components were estimated based on an AD model using a mixed linear model approach, minimum norm quadratic unbiased estimation (Rao, 1971).The genetic effects were predicted using the Adjusted Unbiased Prediction (AUP) method (Zhu & Weir, 1996). Jackknifing over genotype was used to estimate standard errors of variances and the predicted genetic effects.

Narrow-sense heritability was estimated as h2 N = VA/VP, and broad-sense heritability across environments as h2 B = (VA+VD)/VP. The significance of variance components were tested using one-tailed t-test, whereas those of genetic effects were tested using two-tailed t-tests. All genetic analyses were performed in the software ‘‘QGA Station 1.0’’ (Chen. & Zhu, 2003). Heterosis was determined for each cross as the percentage deviation of F1 means from mid parent means (MP) and better parent (BP) and expressed as percentages following the formulae given by Dreisigacker et al. (2005).

MPH (%) = (F1−MP)/MP × 100.

BPH (%) = (F1−BP)/BP × 100.

F1 hybrid performance, MPH and BPH was tested for significance by an ordinary t-test (Dreisigacker et al., 2005). Genetic and phenotypic correlations among the traits, and their standard errors, were estimated using multivariate REML implemented in the MIXED procedure of SAS (SAS Institute, 2003).

Results

Genotypes (F1 and parents) differed significantly (P < 0.05) for all traits studied. Genotypes accounted for more than 50% of the total variation for all the traits except fertile tillers plant-1 and spikelets spike-1 (Table 1). Among the components of genotypic variance in the F1 generations, all the parents were significant (P < 0.05) for all traits except for grain spike-1. Genotypic variance for crosses were also significant (P < 0.05) for all traits except fertile tillers plant-1 and spikelets spike-1. Parents vs. crosses effects were significant (P < 0.01) for spike length, 100 grain weight and yield plant-1 (Table 1).

Local white was the latest of the six cultivars followed by Frontana, Pavon, Pasban 90, Chakwal 97 and Tobari 66 in descending order of days to heading and maturity. The F1 crosses involving Frontana and Local white also matured later than other crosses (Table 2). Local white was the tallest parent followed by Frontana. Pasban 90 and the crosses involving it had the shortest plants. The F1 crosses involving Local white and Frontana were also taller than the crosses not involving these parents. Local white and Pasban 90, and the crosses involving these produced the maximum fertile tillers plant-1. The longest spikes were produced by Pavon, Local white and Pasban 90. The F1 crosses involving Pavon, Local white and Pasban 90 also produced the longest spikes (Table 2). Local white and its crosses had the maximum spikelets spike-1. Tobari 66 and two of its F1 crosses had maximum grains spike-1. Pavon and Frontana, and the crosses involving these had the highest 100 grain weight. The F1 crosses of these parents also produced the highest grain spike-1. Local white and its crosses had the highest yield plant-1 (Table 2). Local white, Pavon and Pasban 90, and the crosses involving one of these performed better than the other three parents under 200 mM NaCl stress.

Significant (P < 0.01) additive genetic effects were observed for days to heading and maturity, plant height and fertile tillers plant-1 (Table 3). Additive genetic effects accounted for >50% of the total phenotypic variation in days to maturity and plant height but were not significant for spike length, spikelets spike-1, grain spike-1, 100 grain weight and yield plant-1. Dominance effects were significant (P < 0.05) for all traits studied. Dominance effects were > 75 % of the total variability in 100 grain weight and yield plant-1 (Table 3).

Narrow sense (h2 N) heritability estimates were significant (P < 0.01) for days to heading and maturity, plant height and fertile tillers plant-1 (Table 3). Narrow sense (h2 N) heritability estimates ranged from 0 to 51 %. Broad sense (h2 B) heritability estimates were significant (P < 0.01) for all the traits studied and ranged from 25 to 84 %. Broad sense heritability estimates were relatively low for spikelets spike-1 (27%) and fertile tillers plant-1 (41%), but higher for the rest of the traits studied (Table 3).

Analysis of the relative importance of general combining ability (GCA) and specific combining ability (SCA) effects provides an indication of the type of gene action involved in the expression of traits. Frontana (2.12) and Local white (2.33) had significant (P < 0.01) positive GCA for days to heading, whereas Tobari 66 (-1.87) and Pasban 90 (-1.73) exhibited significant (P < 0.05) negative GCA effects. High positive SCAs for days to heading were recorded for the crosses ‘Local white × Pavon’ (7.10) and ‘Tobari 66 × Pasban 90’ (4.17), whereas negative SCAs were recorded for the crosses ‘Local white × Tobari 66’ (-4.32) and ‘Local white × Pasban 90’ (-6.79).

Highest positive GCA effects for days to maturity were observed for Local white (5.56) and Frontana (3.46). Tobari 66, Pasban 90 and Chakwal 97 exhibited negative GCA effects for days to maturity. Significant (P < 0.05) SCAs were observed for days to maturity for the crosses ‘Pasban 90 × Chakwal 97’ (8.31), ‘Frontana × Local white’ (7.51), and ‘Tobari 66× Pasban 90’ (3.51), whereas significant (P < 0.05) negative SCAs were found for the crosses ‘Frontana × Pasban 90’ and ‘Local white × Pasban 90’. Significant (P < 0.01) positive GCA effects were observed for Local white (9.20) and Frontana (2.84), whereas Pavon, Pasban 90 and Chakwal 97 showed significant (P < 0.01) negative GCA for plant height. Significant (P < 0.05) positive SCAs for plant height were observed for the crosses ‘Frontana × Tobari 66’ (8.62) and ‘Local white × Tobari 66’ (7.92), whereas significant (P < 0.05) negative SCAs were displayed by the crosses ‘Frontana × Local white’, ‘Frontana × Pasban 90’ and ‘Local white × Pasban 90’. Local white (0.74), Pavon (-0.66) and Chakwal 97 (0.50) exhibited significant (P < 0.01) GCA for fertile tillers plant-1. GCAs for other traits were non significant due to small proportion of the additive variance.

Significant (P < 0.05) positive SCAs were observed for spike length for the crosses ‘Frontana × Chakwal 97’ (0.96), ‘Local white × Tobari 66’ (1.53), ‘Pavon × Pasban 90’ (0.55), ‘Tobari 66 × Chakwal 97’ (1.12) and ‘Pasban 90 × Chakwal 97’ (0.98). Significant (P < 0.05) positive SCAs for spikelets spike-1 were possessed by the crosses ‘Local white × Tobari 66’ (2.10), ‘Pavon × Chakwal 97’ (1.35) and ‘Pasban 90 × Chakwal 97’ (0.73), whereas significant (P < 0.05) negative SCAs were detected for crosses ‘Local white × Pasban 90’ (0.95) and ‘Local white × Chakwal 97’ (1.40). Significant (P < 0.01) positive SCAs were found for grains spike-1 for the crosses ‘Local white × Tobari 66’ (15.07) and ‘Tobari 66 × Chakwal 97’ (11.26), whereas significant (P < 0.05) negative SCAs were detected for crosses ‘Local white × Pavon’ (10.44), ‘Local white × Chakwal 97’ (5.01), ‘Pavon × Chakwal 97’ (10.60) and ‘Tobari 66 × Pasban 90 (12.53). Significant (P < 0.01) positive SCAs were observed for 100 grain weight for the crosses ‘Frontana × Tobari 66’ (0.83), ‘Frontana × Pasban 90’ (0.67), ‘Local white × Pavon’ (0.75), ‘Local white × Tobari 66’ (0.99), ‘Local white × Pasban’ (0.78), ‘ Pavon × Pasban 90’ (0.61) and ‘Pasban 90 × Chakwal 97’ (0.96). Significant (P < 0.05) positive SCAs for yield plant-1 were detected for crosses ‘Local white × Tobari 66’ (3.92), ‘Pavon × Pasban 90’ (0.80), ‘Tobari 66 × Chakwal 97’ (2.88) and ‘Pasban 90 × Chakwal 97’ (1.42), whereas significant (P < 0.01) negative SCA were detected for the cross ‘Tobari 66 × Pasban 90 (3.41).

MPH estimates for days to heading, days to maturity, plant height and fertile tillers plant-1 were less than 15% (Table 4). This indicated that improvement in these traits through hybridization is difficult. The cross ‘Tobari 66 × Chakwal 97’ had highest (33.6%) MPH for spike length. This cross also had the maximum (27.1%) MPH for spikelets spike-1, 37.8% for grains spike-1, 64% for 100 grain weight and 270.6% yield plant-1. Highest (56%) MPH for grains spike-1 was recorded for cross ‘Local white × Tobari 66’. This cross also had high MPH for 100 grain weight (103%) and yield plant-1 (158%). Fourteen crosses had >50% MPH for 100 grain weight. This indicated that there is a great potential in the cultivars to improve 100 grain weight which is an important yield contributing trait. Eight crosses showed > 50% MPH for yield plant-1. Four crosses including ‘Frontana × Tobari 66’ (141%), ‘Frontana × Chakwal 97’ (107%), ‘Local white × Tobari 66’ (89%) and Tobari 66 × Chakwal 97 (213) showed > 50% BPH for yield plant-1. The highest heterosis (271%) for yield was recorded for cross ‘Tobari 66 × Chakwal 97’. High heterosis estimates for yield suggested that breeding for high yield under salt stress is possible if parents with better combining ability are crossed.

Generally genotypic correlation coefficients were higher than phenotypic correlation coefficients (Table 5). All the studied traits were positively correlated with yield plant-1 except days to heading. Similar findings were also reported by previous workers (Ali, 2001; Yagdi, 2009).

Discussion

The present study revealed that additive genetic effects played a major role in the inheritance of days to heading and maturity, plant height and fertile tiller plant-1 under 200 mM NaCl stress. The major role of additive genetic effects in these traits suggested that selection in early segregating generations could be effective to bring desirable changes in these traits. The contribution of additive genetic effects in the inheritance of days to maturity in spring wheat was previously reported by Iqbal et al. (2007). The broad sense heritability values for all yield contributing traits except fertile tillers plant-1 and spikelets spike-1 are within the requisite range for an effective selection for the improvement of salinity tolerance in spring wheat. The high values of heritabilities estimated for a number of variables indicated that significant advances in salinity tolerance in wheat may be possible through selection. Higher SCA values indicated dominance gene effects and higher GCA values refer to a greater role of additive gene effects that control these traits. If both the GCA and SCA values are non significant, it indicates epistatic gene effects (Fehr, 1993). The SCA represents the dominance and epistatic interaction, which can be related with heterosis. However, in self-pollinated crops like wheat, the additive × additive type of interaction component is fixable in later generations. In accordance with earlier studies in wheat, GCA variances were more important than SCA variances, indicating the predominance of additive effects. Number of fertile tillers directly contributes to crop yield. ‘Local white’ proved to be the best general combiner to increase fertile tillers plant-1. Negative GCA effects of two parents ‘Tobari 66’ and ‘Pasban 90’ indicating that these parents could be used to incorporate early maturity in wheat.

Early heading is desirable because it provides sufficient time for grain formation and filling. Seven crosses showed negative heterosis for days to heading. Genotypes with early maturing habits are generally required to avoid stress conditions. Therefore, negative heterosis for days to heading and maturity is useful. Five F1 crosses had negative MPH and could be used to incorporate earliness. Workers (Dreisigacker, 2005; Sadeque, 1991; Inamullah, 2006) also reported negative heterosis for days to heading and maturity and reported the importance of heterotic studies for incorporating earliness in wheat. Tall plants expected to lodge quite often. They require more energy to translocate solutes to the grain and have lower grain weight. Short stature wheat is therefore preferred and negative heterosis is desirable. Seven crosses showed negative heterosis for plant height. Workers (Dreisigacker, 2005; Sadeque, 1991) had reported negative as well as positive heterosis for plant height.

Spike length is directly associated with number of grains. It has been reported positive MPH (Thakur, 1991), while negative MPH was reported for spike length in wheat genotypes (Sadeque, 1991). Most of the crosses had positive MPH for grains spike-1, 100 grain weight and yield plant-1. This showed the effectiveness of heterosis for increased grain yield under salinity stress. Similar findings were reported by Afia (2000). Above 50% MPH of eight crosses and BPH for four crosses suggested that hybrids could yield better under salt stress conditions. Our results showed high heterosis for yield plant-1 which was particularly observed under salinity stress. Earlier Walton (1971) reported 92% MPH for grain yield. Our result showed that heterosis could improve grain yield and provides sufficient chances to select the desired combinations under salinity stress.

Positive correlation of days to heading and maturity with plant height and fertile tillers plant-1 suggested that delayed crop maturity leads to better crop growth. Negative correlation between days to heading and yield plant-1 suggested that early maturity helps plant to avoid stress and increased crop yield under salinity stress. All the yield attributing traits displayed positive correlation at both genotypic and phenotypic level with yield plant-1, indicating that all these traits contributed towards yield under NaCl stress.

Three parents, ‘Local white’, ‘Pavon’ and ‘Pasban 90’, and their F1 crosses performed better on the basis of yield and yield components under NaCl stress. These parents can, therefore, serve as donor parents for developing salt tolerant wheat varieties. Significance of additive effects for days to heading, days to maturity, plant height and fertile tillers plant-1 suggested that selection in early segregating generations could be effective to bring desirable changes in these traits under salt stress. High dominance effects for yield and yield contributing traits indicated that selection for yield and yield components under salt stress could be effective in later generations. Above 50% MPH of eight crosses and BPH for four crosses suggested that hybrids could yield better under salt stress conditions. Narrow sense (h2 N) heritability estimates ranged from 0-51%, whereas broad sense (h2 B) heritability estimates ranged from 25-84% for the studied traits. On the basis of yield and yield components, crosses ‘Local white × Tobari 66’, ‘Tobari 66 × Chakwal 97’, ‘Frontana × Local white’, ‘Local white × Pasban 90’, ‘Pavon × Pasban 90’ might be better choice to get high yield under NaCl stress conditions.