Advances in ISSN: 2373-6402APAR

Plants & Agriculture Research
Research Article
Volume 4 Issue 1 - 2016
Indole-Acetic Acid Oxidase Enzyme Activity in Three Wheat Cultivars under Salt Stress Conditions at the Early Seedling Stage
Ravi Sharma*
Department of Post-graduate Studies, K R College Mathura, India
Received: March 30, 2016 | Published: July 05, 2016
*Corresponding author: Ravi Sharma, Department of Post graduate studies, KR College Mathura, India, Email:
Citation: Sharma R (2016) Indole-Acetic Acid Oxidase Enzyme Activity in Three Wheat Cultivars under Salt Stress Conditions at the Early Seedling Stage. Adv Plants Agric Res 4(1): 00123. DOI: 10.15406/apar.2016.04.00123

Abstract

Indole-acetic acid enzyme activity in three wheat cultivars differing in their salt response was studied at the early seedling stage. It was found that the IAA oxidase enzyme activity gradually decreased with increasing EC levels in both the salt-sensitive cultivar IWP-72 and moderately salt-tolerant cultivar Sharbati sonora while in the tolerant cultivar HD-2160 the enzyme activity increased till 96 hours up to 12 EC while at 120 hours at all EC levels the activity decreased. The magnitude was, however, higher in the salt-sensitive cultivar. The IAA oxidase activity increased with advancement in seedling age but the degree of increase varied with the cultivars. Whereas in the tolerant and moderately tolerant cultivars a sharp increase was observed at 72 and 96 hours which later declined at 120 hours, on the other hand, in the sensitive cultivar it increased from 24 hours reaching a peak level at 120 hours. Also from the observations it is noticed that IAA oxidase enzyme activity was affected more in the salt-sensitive cultivar (IWP-72) as compared with the moderately salt-tolerant (Sharbati sonora) and the salt-tolerant cultivar (HD-2160).

Keywords: Wheat (Triticum aestivum L); NaCl salt stress; Salt-tolerant cultivar (HD-2160); Moderatety salt-tolerant cultivar (Sharbati sonora); Salt-sensitive cultivar (IWP-72); IAA oxidase enzyme activity

Introduction

Plant germination and growth is regulated by the balance of plant hormones such as IAA, GA, CK etc., [1-5] and any disturbance in the balance of plant hormones retard the plant growth in which IAA plays a predominant role whose level, of course, is maintained endogenously by the enzyme activity of IAA oxidase [4-6]. Several workers [7-22] have shown genotypic variation among the same species of crop plants in relation to salt stress. In the present work an attempt has been made to find out a relationship existing between salt tolerance and sensitivity with IAA oxidase enzyme activity, if any, in three cultivars of wheat, salt-tolerant (HD-2160), moderately salt-tolerant (Sharbati sonora) and salt-sensitive (IWP-72). Thus, the objective of the study is to correlate relationship between salt tolerance and IAA oxidase enzyme activity.

Materials and Methods

Forty two wheat cultivars (Triticum aestivum L) procured from Wheat Directorate, Cummings Laboratory, Division of Genetics and Plant Breeding, Indian Agricultural Research Institute, New Delhi and Chandra Sekhar Azad University of Agriculture and Technology, Kanpur (UP), India [22] were subjected to screening for salt resistance by Garrad’s Technique [23] modified by Sarin & Rao [24] and Sharma [8,22] and as per method of Sheoran and Garg [25]. The shoot and root lengths of seedlings were recorded at definite interval of 24 hours using test tubes of uniform size (30 ml capacity) fitted with rolls of filter paper folded at the top into a cone to support the seeds. The tubes were filled to one-third part with the test solutions so that the solution might not come in direct contact with the growing roots and to ensure that the salt solution being supplied to the roots through capillary action of the filter paper. Distilled water (represented the mean loss of water from the blanks) was added to each test tube after every 24 hours of interval in order to maintain salt concentration near the target levels throughout the germination period. The seeds were initially sterilized with 0.1 percent mercuric chloride (HgCl2) solution and later washed thoroughly with distilled water. Three seeds per tube were then transferred to the edge of the filter paper cone and were allowed to grow between the filter paper roll and the wall of the test tube in dark growth chamber at 25±20C. Fifteen replicates (five tubes each having three seeds) were maintained for each treatment including the controls (half-strength Hoagland solution grown). Observations on the influence of salinity levels at 4, 8, 12 and 16 EC dsm-1 of salt solution and the controls on the total length of shoot and root at early seedling stage were recorded at 24 hour intervals from 48 hours after sowing up to the end of 120 hours under green safe light. The relative tolerance of different cultivars was evaluated on the basis of the percentage reduction in shoot growth at 12 EC [22].

Three selected cultivars viz., salt-tolerant (HD-2160), moderately salt-tolerant (Sharbati Sonora) and salt-sensitive (IWP-72) from the entire screening were studied for IAA oxidase enzyme activity under salt-stressed conditions. The seeds were germinated and seedlings grown in sterilized petridishes on moistened filter paper kept in a dark growth chamber at 25±20C. The ‘treated sets’ contained the salt (NaCl) solution at different EC (4, 8, 12 and 16) levels and the ‘controls’ half-strength Hoagland solution. Samples were collected at 24 hour intervals from 48 hours after sowing up to the end of 120 hours. The seedlings were raised according to the method described previously [4,26] and IAA oxidase enzyme activity was determined according to Rabin & Klein [27] technique as described by Witham et al., [28] and Sharma et al., [4]. The enzyme assay was conducted in cold room. One g of the seedlings was homogenized with 10 ml of cold distilled water in a chilled mortar. The slurry was passed through a layer of cheese cloth. The filtrate thus obtained was centrifuged at 10,000 rpm for 15 minutes. To the supernatant excess of acetone was added so that the final acetone concentration was brought to 40 percent by volume. It was centrifuged at 1,000 rpm for 15 minutes. After discarding the supernatant the pellet of precipitate was re-suspended in 4 ml of 0.2 M phosphate citrate buffer (pH 5.6)* containing 0.5 ml10-4M MnCl2 and 0.5 ml 10-3 M 2,4 dichlorophenol. The precipitate was shaken vigorously to dissolve it and the resulting solution was used as enzyme preparation. The reaction mixture containing 2 ml of the enzyme preparation, 7 ml of 0.2 M phosphate citrate buffer (pH 5.6) and 1 ml of indole-acetic acid (200 µg/ml) was incubated for one hour at 300C. After incubation residual IAA was estimated by taking 1 ml of the reaction mixture and adding 4 ml Salkowski reagent and was left to stand for 30 minutes at room temperature. Pink colour of IAA was read at 540 nm against a reagent blank. The results are expressed as µg IAA destroyed per g fresh wt per hr.

*Phosphate citrate buffer (0.2 M pH 5.6): was prepared from - A. 19.21 g citric acid dissolved in 1000 ml of distilled water; B. 53.65 g sodium hydrogen phosphate (Na2HPO4.7H2O) dissolved in 1000 ml of distilled water. Working Solution was prepared by mixing 21.0 ml of solution A with 29.0 ml of B diluted to a total of 100 ml and adjusted pH to 5.6. All parameters were analyzed by ‘Analysis of Variance’ (ANOVA) method as given by Panse & Sukhatme [29].

Results

Wheat seedling growth

In the screening of the forty two wheat cultivars for salinity tolerance at the early seedling stage shoot and root lengths under varying salinity levels (0, 4, 8, 12 and 16 dsm-1) induced by NaCl were observed. In the ANOVA analysis all the main effects viz., variety (V), treatment (T) and seedling age (D) and their interactions (V x D; V x T; D x T and V x D x T) were found to be highly significant at 0.01 probability with significant differences noticed in the shoot and root growths of all the cultivars studied [22]. With this, relative shoot and root growths of the cultivars showed that shoot growth was affected more as compared with root growth under salt stress. On the other hand, all the cultivars showed an increase in shoot and root growths with seedling age. It was evident that the different cultivars exhibited marked differences in their early seedling growth with increasing age of the seedling and that with advancement of seedling age the effect of salt declined and that, in general, tolerance to salinity increased (Figure 1). As indicated earlier [22] only 11 cultivars showed less than 60 percent reduction in shoot growth while majority of the 31 cultivars had more than 60 percent reduction at 16 EC. This is in contrast with root growth where almost a reverse trend was noticed, i.e., out of the 42 cultivars only 15 showed more than 60 percent reduction at 16 EC whereas 27 had less than 60 percent reduction (Table 1). The observations recorded clearly indicated that the shoot is more sensitive to salt stress than the root and that shoot growth is a better index of relative salt tolerance of different cultivars of the same species at early seedling stage with this also 12 EC salinity level was found to be a critical level for majority of the cultivars. Thus, on the basis of the percent reduction in shoot growth at 12 EC level over respective control all the cultivars were categorized into three groups, viz., salt-tolerant, moderately salt-tolerant and salt-sensitive, showing less than 40%, 40–60% and more than 60% reduction respectively (Table 2).

Figure 1: Relative Shoot and Root Growths of Certain Wheat (Triticum aestivum L) Cultivars Under Salt Stress at the Early Seedling Stage [22].

Further, the different rates of shoot growth of the three groups as affected by increasing level of salinity showed a gradual decline in both the salt tolerant and moderately salt–tolerant cultivars. On the other hand, the salt–sensitive cultivars had a sharp decline in growth with increasing salt concentrations. With this the relative comparison of seedling growth between three wheat cultivars studied indicated better performance of HD–2160 (salt-tolerant) at almost all levels of salinity when compared with controls. It showed highest tolerance to salinity (i.e., 85.219 percent), IWP-72 (salt-sensitive) showing highest inhibition (i.e., only 7.818 percent) and Sharbati sonora (moderately salt-tolerant) showing (48.574) percent shoot growth at 12 EC over control (Figure 2).

Figure 2: Relative Salt Tolerance of Three Groups (salt-tolerant, moderately salt-tolerant and salt-sensitive) Wheat (Triticum aestivum L) Cultivars Under Salt Stress at the Early Seedling Stage (Data Expressed as Percent Over Control).

S.No

Cultivar

Shoot Growth

Root Growth

4EC

8EC

12EC

16EC

4EC

8EC

12EC

16EC

1

HD-2236

113.561**

56.847

11.491

5.599

111.150**

84.029

22.589

12.905

2

WL-410

113.259**

74.804

43.746

18.731

114.330**

92.463

75.146

47.188

3

*Sharbati sonora

90.591

80.876

48.574

40.3

98.463

95.181

80.079

59.912

4

Moti

95.025

66.374

11.423

9.995

87.94

70.758

37.162

29.576

5

Sonalika

86.523

70.728

48.179

43.217

95.222

89.47

80.958

66.694

6

*HD-2160

95.135

91.113

85.219

82.6

94.623

83.76

77.584

71.188

7

HD-2135

73.548

52.043

35.555

9.892

81.956

65.602

43.14

28.569

8

IWP-503

80.606

59.617

29.346

13.939

93.087

80.71

60.972

31.834

9

HS-43

92.583

64.833

43.948

29.261

92.614

75.133

59.131

40.818

10

UP-262

78.452

59.844

28.956

8.15

85.716

66.802

44.781

26.168

11

HD-2177

79.002

58.31

28.527

6.716

109.224**

89.42

55.588

36.919

12

WG-1559

83.288

25.802

20.454

9.358

83.98

20.319

15.742

6.588

13

HD-2267

89.577

49.142

11.873

8.245

79.032

28.403

16.921

2.87

14

*IWP-72

82.921

39.094

7.818

5.144

84.874

48.701

14.736

5.826

15

HD-2282

95.6

89.584

57.47

35.213

95.272

93.368

65.889

56.434

16

WL-711

92.973

89.329

71.437

46.128

97.498

88.607

74.932

54.35

17

Raj-1482

96.668

71.829

38.573

32.378

96.239

78.587

48.426

44.231

18

HD-2260

77.059

72.661

70.284

22.894

92.593

89.221

77.579

40.201

19

WH-246

92.266

59.304

43.644

33.929

114.221**

77.774

56.468

49.96

20

WL-2200

62.53

112.53

46.654

35.279

84.951

105.439**

71.072

60.244

21

K-7634

96.841

92.545

80.353

52.179

85.271

90.808

80.487

59.321

22

Raj-1556

81.372

66.86

52.875

44.063

81.503

74.521

59.752

55.695

23

UP-154

90.714

75.952

60.714

49.523

91.303

86.454

75.083

68.645

24

HD-1977

83.333

54.973

43.01

40.456

88.177

54.75

48.064

41.449

25

WG-1558

94.865

71.393

43.276

35.207

130.566**

120.546**

65.282

43.319

26

HD-2204

65.638

76.945

42.878

34.948

77.622

91.666

62.237

55.594

27

WL-1531

90.816

84.081

71.02

33.061

91.401

79.841

66.907

40.895

28

K-7631

73.214

68.75

56.25

47.321

76.935

72.955

63.467

58.887

29

Raj-1409

69.243

46.546

38.98

17.708

81.585

59.013

44.596

27.941

30

Raj-1493

75.996

64.855

50.815

26.721

73.242

67.101

60.822

37.093

31

Raj-1494

44.152

39.379

28.353

11.217

58.24

42.093

35.408

22.251

32

WL-903

75.341

68.003

59.726

38.822

93.159

85.064

69.72

57.263

33

UP-171

82.361

60.073

17.195

14.612

80.992

59.594

38.173

28.072

34

HD-2275

83.125

64.034

25.738

17.329

79.938

64.834

43.078

37.768

35

HD-1593

30.959

45.903

27.746

14.944

44.547

56.477

34.855

20.465

36

HD-2252

106.760**

75.329

65.759

35.381

109.373**

76.631

71.436

42.741

37

HP-1303

86.174

56.969

54.166

39.09

79.185

68.89

67.294

42.038

38

UP-115

85.802

72.619

66.535

40.169

89.683

81.821

58.298

42.637

39

HD-1980

94.002

60.403

54.406

44.369

93.379

58.454

52.941

40.599

40

CC-464

57.12

51.009

46.866

24.08

72.834

69.253

61.864

47.878

41

HD-2009

88.962

60.902

57.321

39.097

83.056

69.413

63.068

50.406

42

Kharchia

69.192

49.329

43.035

30.542

90.201

70.451

65.195

55.269

CD at 5% P = 0.064 S.Em. ± 0.023

CD at 5% P = 0.351 S.Em. ± 0.126

Table 1: Shoot and Root Growth of Forty two Wheat Cultivars at Different Salinity Levels
(Data expressed as percent over control) (Cultivars with asterisk* used in the present study)
(Cultivars with ** showed stimulation observed in growth at moderate levels of salinity).

It is evident from the Tables 3 and 4 that the IAA oxidase enzyme activity gradually decreased with increasing salt stress created by NaCl in both the salt-sensitive cultivar IWP-72 and moderately salt-tolerant cultivar Sharbati sonora while in the tolerant cultivar (HD-2160) the enzyme activity increased till 96 hours up to 12 (dsm-1) EC salinity level, but at 120 hours at all EC levels the activity decreased, although, the magnitude of the enzyme activity was higher in the sensitive cultivar IWP-72. The IAA oxidase activity increased with advancement in seedling age with varying degree in the three cultivars. Whereas in the tolerant and moderately tolerant cultivars a sharp increase was observed at 72 and 96 hours declining later at 120 hours, on the other hand, in the sensitive cultivar enzyme activity increased from the very beginning at 48 hours reaching peak level at 120 hours. Also from the observations it is noticed that IAA oxidase enzyme activity was affected more in the salt-sensitive cultivar (IWP-72) as compared with the moderately salt-tolerant (Sharbati sonora) and the salt-tolerant cultivar (HD-2160). As shown in the Table 5 in the three cultivars there is direct correlation found in the shoot growth, root growth and IAA oxidase enzyme activity as with increasing salt concentration there is decrease in root and shoot growth and also the enzyme activity. On the other hand, with seedling age as the root and shoot growth increased the IAA oxidase activity also increased but till 96 hours and at 120 hours it slightly declined (Table 6).

Group I Salt-Tolerant (Less than 40% Reduction)

Group II Moderately Salt-Tolerant (40 – 60% Reduction)

Group III Salt-Sensitive (More than 60% Reduction)

Wheat Cultivars

1. HD-2160*

85.219

1. WL-903

59.726

1. Raj-1409

38.98

2. K-7634

80.353

2. HD-2282

57.47

2. Raj-1482

38.573

3. WL-711

71.437

3. HD-2009

57.321

3. HD-2135

35.555

4. WL-1531

71.02

4. K-7631

56.25

4. IWP-503

29.346

5. HD-2260

70.284

5. HD-1980

54.406

5. UP-262

28.956

6. UP-115

66.535

6. HP-1303

54.166

6. HD-2177

28.527

7. HD-2252

65.759

7. Raj-1556

52.875

7. Raj-1494

28.353

8. UP-154

60.714

8. Raj-1493

50.815

8. HD-1593

27.746

9. Sharbati Sonora*

48.574

9. HD-2275

25.738

10. Sonalika

48.179

10. WG-1559

20.454

11. CC-464

46.866

11. UP-171

17.195

12.WL-2200

46.654

12. HD-2267

11.873

13. HS-43

43.948

13. HD-2236

11.491

14. WL-410

43.746

14. Moti

11.423

15. WH-246

43.644

15. IWP-72*

7.818

16. WG-1558

43.276

17. Kharchia

43.035

18. HD-1977

43.01

19. HD-2204

42.878

Table 2: Relative Tolerance of Certain Cultivars of Wheat Based on the Percent Reduction in Shoot Growth at 12 EC (dsm-1) Salinity Level.

Variety

Treatment NaCl Conc. (EC dsm-1)

Shoot Length (cm)

Root Length (cm)

IAA oxidase activity (µg IAA destroyed/g fresh wt/hr)

Duration (hours)

Duration (hours)

Duration (hours)

48hrs

72hrs

96hrs

120hrs

48hrs

72hrs

96hrs

120hrs

48hrs

72hrs

96hrs

120hrs

C1 HD-2160 (Salt-tolerant)

Control

0.27

0.49

0.95

2.56

1.14

2.3

3.7

7.36

224

262

262

248

4 EC

0.27

0.41

0.85

2.53

0.96

2.18

3.68

6.9

240

248

312

220

8 EC

0.25

0.41

0.84

2.39

0.78

2.05

3.57

5.75

281

292

292

232

12 EC

0.23

0.39

0.83

2.19

0.68

1.89

3.19

5.49

262

278

286

224

16 EC

0.23

0.37

0.78

2.15

0.62

1.5

2.9

5.3

232

250

256

198

C2 SHARBATI SONORA (Mod. Salt-tolerant)

Control

0.95

1.45

3.96

6.67

1.75

4.33

6.87

8.73

282

296

320

232

4 EC

0.76

1.36

3.26

6.43

1.45

4.31

6.3

8.66

262

276

282

198

8 EC

0.51

1.02

2.9

6.12

1.43

4.26

6.04

8.32

242

242

256

195

12 EC

0.08

0.61

1.98

3.68

0.69

3.11

5.75

7.28

198

213

232

122

16 EC

0.06

0.61

1.74

2.85

0.55

2.88

3.41

5.79

120

140

146

127

C3 IWP-72 (Salt-sensitive)

Control

0.38

1.05

2.86

5.42

1

2.65

4.66

8.14

240

290

320

356

4 EC

0.3

0.94

2.2

4.62

0.71

2.5

4.45

6.3

202

218

230

290

8 EC

0.2

0.36

1.12

2.12

0.41

1.65

2.54

3.4

188

226

240

272

12 EC

0.12

0.16

0.2

0.28

0.1

0.25

0.65

1.42

122

134

218

218

16 EC

0.1

0.12

0.12

0.16

0.04

0.12

0.22

0.58

90

120

204

218

SEm

±0.053

±0.065

±9.332

Table 3: Effect of Different Levels of Salt Stress (NaCl) Exposure on Shoot Length, Root Length and IAA Oxidase Enzyme Activity in three Wheat Cultivars.

Variety

Treatment NaCl Conc. (EC dsm-1)

Shoot Length (cm)

Root Length (cm)

IAA oxidase activity(µg IAA destroyed/g fresh wt/hr)

Duration (hours)

Duration (hours)

Duration (hours)

48hrs

72hrs

96hrs

120hrs

48hrs

72hrs

96hrs

120hrs

48hrs

72hrs

96hrs

120hrs

C1

Control

0.27

0.49

0.95

2.56

1.14

2.3

3.7

7.36

224

262

262

248

HD-2160

4 EC

0.27

0.41

0.85

2.53

0.96

2.18

3.68

6.9

240

248

312

220

(Salt-tolerant)

8 EC

0.25

0.41

0.84

2.39

0.78

2.05

3.57

5.75

281

292

292

232

12 EC

0.23

0.39

0.83

2.19

0.68

1.89

3.19

5.49

262

278

286

224

16 EC

0.23

0.37

0.78

2.15

0.62

1.5

2.9

5.3

232

250

256

198

C2 SHARBATI SONORA

Control

0.95

1.45

3.96

6.67

1.75

4.33

6.87

8.73

282

296

320

232

(Mod. Salt-tolerant)

4 EC

0.76

1.36

3.26

6.43

1.45

4.31

6.3

8.66

262

276

282

198

8 EC

0.51

1.02

2.9

6.12

1.43

4.26

6.04

8.32

242

242

256

195

12 EC

0.08

0.61

1.98

3.68

0.69

3.11

5.75

7.28

198

213

232

122

16 EC

0.06

0.61

1.74

2.85

0.55

2.88

3.41

5.79

120

140

146

127

C3

Control

0.38

1.05

2.86

5.42

1

2.65

4.66

8.14

240

290

320

356

IWP-72

4 EC

0.3

0.94

2.2

4.62

0.71

2.5

4.45

6.3

202

218

230

290

(Salt-sensitive)

8 EC

0.2

0.36

1.12

2.12

0.41

1.65

2.54

3.4

188

226

240

272

12 EC

0.12

0.16

0.2

0.28

0.1

0.25

0.65

1.42

122

134

218

218

16 EC

0.1

0.12

0.12

0.16

0.04

0.12

0.22

0.58

90

120

204

218

SEm

±0.053

±0.065

±9.332

Table 4: Effect of Different Levels of Salt Stress (NaCl) Exposure on Shoot Length, Root Length and IAA Oxidase Enzyme Activity in Three Wheat Cultivars (Data Expressed as Percent Over Control).

Treatment NaCl (EC dsm-1)

Shoot Length (cm)

Root Length (cm)

IAA oxidase activity (µg IAA destroyed/g fresh wt/hr)

Control

2.251

4.386

277.667

4 EC

1.994

4.033

248.167

8 EC

1.52

3.35

246.528

12 EC

0.896

2.542

208.917

16 EC

0.774

1.992

175.111

SEM

± 0.015

± 0.019

± 2.694

Table 5: Effect of Different Levels of Salt Stress (NaCl) Exposure on Shoot Length, Root Length and IAA Oxidase Enzyme Activity in Three Wheat Cultivars (TREATMENT).

SD: Standard Deviation; BMI: Body Mass Index; WC: Waist Circumference; AC: Abdominal Circumference; HC: Hip Circumference; RER: Respiratory Exchange Ratio; HR: Hear Rate.

Duration (hours)

Shoot Length (cm)

Root Length (cm)

IAA oxidase activity (µg IAA destroyed/g fresh wt/hr)

48 hrs

0.314

0.821

212.333

72 hrs

0.65

2.399

232.333

96 hrs

1.639

3.862

257.067

120 hrs

3.345

5.961

223.378

SEM

± 0.014

± 0.017

± 2.409

Table 6: Effect of Different Levels of Salt Stress (NaCl) Exposure on Shoot Length, Root Length and IAA Oxidase Enzyme Activity in Three Wheat Cultivars (DURATION).

Discussion

One of the most common effects of salinity is stunting of growth often without any other sign of damage. This and other modifications of growth habit have suggested that growth hormones may be involved in the responses of plants to salinity [30]. Germinating seeds have been found to be rich sources of hormones such as IAA, GA, Kinins and some inhibitors also [31-35]. It has been observed that the salinity inhibits seedling growth and that the inhibition is directly correlated to the increase in salinity. To further understand this growth inhibition the pattern of hormonal regulation has been studied as IAA and GA contents along with the enzymes which regulate these systems [5,8,7,36,37]. Chen [3] suggested that these endogenous levels of plant hormones play a predominant role in growth regulation which depends upon the balance between the growth promoters [e.g., GA, Kinins etc.,] and the inhibitors [e.g., ABA or in some instances IAA]. It had been observed that with increasing salinity IAA oxidase activity increased in the tolerant cultivar, while it decreased in the sensitive cultivar. Also IAA oxidase activity increased with seedling age and reached a peak at 96 hours and later it declined, in the three cultivars. Verga & Balint [36] have also reported similar behavior of IAA oxidase activity during early seedling growth. Pilet & Gastner [38] have observed that one of the mechanisms controlling growth via endogenous auxins is through enzymatic oxidations. IAA oxidase is regarded to oxidize the IAA, thereby controlling the endogenous level in the seedlings. Interestingly it was observed in the tolerant cultivar while the auxin-like promoters were highest at 24 hours the IAA oxidase activity was lowest (Table 6). On the other hand, the sensitive cultivar showed peak auxin levels at 48 hours it also exhibited a higher level of IAA oxidase. Therefore, it is likely that the reduction in auxin-like promoters observed in the present investigation as a result of salt stress is mediated through the enzyme IAA oxidase. Goyal & Baijal [26] reported that IAA oxidase enzyme activity of the different cultivars of the same species is regulated by the specific gene in each cultivar, therefore, a genetic diversity was found in the growth behavior of the three cultivars differing in salt tolerance as well as in IAA oxidase enzyme activity. From our laboratory workers [7-22] have shown genotypic variation among the same species of crop plants in relation to salt stress as shown in the present work.

Conclusion

From the results presented, it appears that the growth under salt stress condition is mediated by endogenous IAA level at different state regulated by IAA oxidase enzyme system indirectly affecting metabolism. Thus, resulting in differential growth response of the three wheat cultivars viz., salt-tolerant (HD-2160), moderately salt-tolerant (Sharbati sonora) and salt-sensitive (IWP-72) in relation to salt stress and that the tolerance of a cultivar depends mainly on the endogenous IAA at the effective level.

Acknowledgements

Author is indebted to Dr. BD Baijal (Retired Professor Plant Physiology Department of Botany Agra College, Agra) for expert comments and to the Principal KR College, Mathura, India for providing necessary facilities.

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