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Genetic analysis of seedling characters in bread wheat

M. Fazel Najafabadi, M.R. Ghanadha, A.A. Zali and B. Yazdi Samadi

Department of Agronomy and Plant Breeding, Faculty of Agriculture, Tehran University, Karaj, Iran. Corresponding author email: mfazel2000@yahoo.com

Abstract

For estimating gene action in germination and seedling stage, one drought-resistant and one susceptible genotype were selected from a prior experiment. F1, F2 and backcross progeny were produced and these generations as well as their parents were sown at two osmotic potentials, 0 and -0.4 Mpa, provided using PEG-6000 at 22C. A factorial experiment based on completely randomized design with different replications for each generation was used. First factor was different generations and the second was osmotic potential. Measurements were made of rootlet number and length, and coleoptile length after one week. The additive component of genotypic variation in segregating generations was larger than the dominance component in most cases. This indicated that selection for better performing individuals in early generations will lead to improvement of germination characteristics. Generation mean analysis did not fit an additive-dominance model for any trait with additive additive and dominance dominance epistatic effects predominating in most osmotic potentials for most traits.

Keywords

Seed germination, Osmotic stress, PEG, Generation mean analysis, Gene action.

Introduction

Drought stress is one of the most limiting factors to crop production worldwide. Autumn droughts often result in reduced seed germination and seedling establishment (Schonfeld et al., 1988). Semi-arid areas like those of the Middle East are characterized by limited and erratic precipitation frequently resulting in droughts at different periods during the growing season. Most studies on drought tolerance of wheat and traits contributing to tolerance have focused on the late-vegetative and reproductive periods of growth, and the effects of stress during those periods on yield (Baalbaki et al., 1998). Crop improvement in a breeding program depends on identification of good genotypes and an understanding of gene action for economic characters. This experiment was conducted in order to estimating the heritability and gene action of some germination characters in wheat.

Materials and methods

Two bread wheat genotypes, one resistant ‘Tabasi’ and one susceptible ‘518’ were crossed in field condition. Different generations such as F2, BC1 and BC2 were obtained from F1 selfing and F1 crossed with each parent, respectively (Fig 1). A factorial experiment with unequal replication size and two factors based on CRD design was performed. The first factor was generations (P1, P2, F1, F2, BC1 and BC2), and the second was two different osmotic potentials (0 as control potential and 0.4 Mpa as an osmotic stress). Control osmotic potential was distilled water and osmotic stress provided by a 0.4 Mpa osmotic potential solution using polyethylene glycol 6000 (Michael & Kaufmann 1973). Numbers of individuals for parents, F1, F2, and BC progenies were 3, 2, 25, and 5 respectively. Uniform seeds were surface sterilized in 1% Clorox (sodium hypochlorite) solution for five minutes and washed with distilled water. Each 10 seeds were germinated on a Whatman filter paper #1 in 2cm10cm plastic test tubes with 10 ml of test solution maintained at 22 1C and twelve-hour photoperiod. A seed was considered germinated if the radical had emerged and grown to 2 mm in length. After one week, three characters namely number of seed rootlets, rootlet and coleoptile length were measured on individual plantlets. Data were subjected to Generation Mean Analysis (GMA) using a program in Excel.

Fig 1. Diagram of producing different generations from two extreme parents for germination traits

Results and Discussion

Phenotypic variation was large for all traits in segregating generations (e.g. F2 progeny in Fig.2) as shown in Table 1. When H/D is less than one, dominance is small and efficiency of selection in segregating progenies for more tolerant and/or less drought susceptibility is greater. That is, additive genetic variance is greater and breeders can select individuals with high performance. The exception is coleoptile length under osmotic stress where dominance (H) was large. In all other cases, this ratio is low and approximately equal in control and stress conditions.

Both broad and narrow sense heritabilities were high in different osmotic potentials (Table 1) in agreement with high additive genetic variance.

Fig 2. Germination of F2 progenies in different osmotic potentials (control – left and osmotic stress – right). Differences between individuals in rootlet and coleoptile length are considerable in osmotic stress condition.

Table 1. Different parts of phenotypic variance and heritabilities of three characters in bread wheat seed germination.

Characters

Osmotic Potentials (Mpa)

D

H

Ew

(H/D)1/2

Hb

Hn

Number of Seed Rootlets

0

1.34

0.16

0.05

0.35

0.93

0.88

0.4

3.30

0.32

0.09

0.31

0.95

0.91

Rootlet Length

0

20.56

7.44

5.80

0.60

0.68

0.57

0.4

27.82

10.08

3.02

0.60

0.84

0.72

Coleoptile Length

0

0.40

0.12

0.04

0.55

0.86

0.76

0.4

0.58

7.52

0.81

3.60

0.73

0.10

Where D is additive, H is dominance and Ew is environmental variances. (H/D)1/2 is dominance ratio and Hb and Hn are broad and narrow sense heritabilities.

Generation mean analysis did not fit models with less than 6 parameters in most cases (Table 2). An exception was the control osmotic potential for rootlet length where the additive-dominance model fitted the data. In all other cases, a six-parameter model was required (Mather and Jinks 1977). Data transformation was not effective for improving model fitting. These larger parameter models suggest that complex epistatic effects are important in controlling seed germination characters. In addition, it suggests that genetic parameters other than additive, dominance and their interaction effects may be important. Further generations and environments may be required for specifying even higher orders of epistatic interaction.

For all traits under osmotic stress condition, [d] was positive indicating F1 means are more like those of the tolerant parent, Tabasi (Table 2). In contrast, [d] was negative for number of rootlets and coleoptile length in the control indicating F1 means are less than the mid-parent and more similar to the susceptible parent, 518. For number of rootlets in both conditions, there was evidence of significant additive dominant genic interaction indicating duplicate epistasis. For other traits, it seems there are different epistatic interactions but mostly of a complementary nature. Overall, there are more additive additive and dominance dominance genic interactions than additive dominance genic interactions for these traits.

Table 2. Analyzing of characters means into different parameters in two osmotic conditions.

Characters

Osmotic Potential (Mpa)

M

[d]

[h]

[i]

[j]

[l]

χ2

Number of Seed Rootlets

0

8.200.34**

1.500.03**

-10.470.90**

-3.730.34**

-2.640.27**

5.210.58**

-

0.4

1.540.54*

1.440.06**

14.681.40**

3.100.53**

-1.960.42**

-13.140.88**

-

Rootlet Length

0

11.080.31**

1.100.31*

1.340.61*

-

-

-

4.76

0.4

4.451.86**

1.690.44*

61.504.98**

16.501.80**

-4.911.67*

-47.053.17**

-

Coleoptile Length

0

6.030.22**

0.300.03**

-4.180.58**

-1.980.21**

-0.270.18

1.870.38**

-

0.4

2.690.31**

0.210.17

27.320.44**

8.270.33**

-

-19.633.23**

0.01

Where m is mean parents, [d] is additive, [h] is dominance, [i] is additive additive, [j] is additive dominance and [l] is dominance dominance parts of characters estimated based on six parametric model and χ2 is joint scaling test statistic. Parameters with * and ** are significant at 5 and 10 percents of probability level.

References

Abayomi, Y. A. and D.Wright. 1999. Osmotic potential and temperature effects on germination of spring wheat genotypes. Tropical Agriculture. 76(2): 114-119.

Baalbaki, R. Z., R. A. Zurayk, M. M. Bleik and S. N. Talhouk. 1998. Germination and seedling development of drought tolerance and susceptible wheat under moisture stress. Seed Sci. & Techno. 27: 291-302.

Mather, K. and J. Jinks. 1977. Introduction to biometrical genetic. Cornell Univ. press, Cambridge, Great British.

Mather, K. and J. Jinks. 1982. Biometrical Genetics-The study of continuous variation. Chopman and Hall.

Michel, B. E. and M. R. Kaufman. 1973. The osmotic potential of polyethylene glycol-6000. Plant Physiology. 51: 914-916.

Schonfeld, A. M., R. C. Johnson, B. F. Carver, and D. W. Mornhinweg, 1988. Water relations in winter wheat as drought resistance indicators. Crop Sci. 28: 526-531.

Scott, S. J., R. A. Jones, and W. A. Williams. 1983. Review of Data Analysis Method for seed germination. Crop Sci. 27: 1192-1199.

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