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Identification of drought tolerant sweet potato (Ipomoea batatas (L.) Lam) cultivars

Prabawardani Saraswati 1, Mark Johnston2, Ross Coventry1 and Joseph Holtum1

1 School of Tropical Biology, James Cook University, Townsville 4811, Queensland Email Prabawardani.Saraswati@jcu.edu.au
2 National Agriculture Research Institute WLIP-Keravat, Kokopo, Papua New Guinea.

Abstract

Pot experiments were conducted to screen fifteen sweet potato cultivars for drought tolerance. Two levels of water regimes were applied, control (maintained by regular watering at or close to field capacity) and water stressed plants (saturated then allowed to dry progressively to permanent wilting point). Plant biomass, main stem length, internode diameter, internode length, leaf number and area, and root weight all decreased in response to the water stress. Leaf water potential decreased significantly when water supply was withheld, however leaf water potential was not statistically different among cultivars in both control and water stressed treatments. The cultivar Lole showed more strongly developed drought resistant characters than all the other cultivars.

Media summary

Of 15 sweet potato cultivars evaluated, Lole showed the greatest drought tolerance

Key words

Sweet potato (Ipomoea batatas L.), drought tolerance

Introduction

Sweet potato (Ipomoea batatas (L.) Lam) is grown over a broad range of environments and cultural practices and is commonly grown in low-input agriculture systems (Prakash 1994). The plant is sensitive to water deficits, particularly during the establishment period including vine development and storage root initiation (Indira and Kabeerathumma 1988). Sweet potato is considered to be moderately drought tolerant (Valenzuela et al 2000). However, drought is often a major environmental constraint for sweet potato production in areas where it is grown under rain fed conditions (Anselmo et al. 1998). Different cultivars may respond differently to limited quantities of soil water. Selection for good cultivar performance (growth rate; tuber development) under drought conditions is considered to be of major importance. Experiments were therefore conducted, to identify sweet potato cultivars associated with superior drought tolerance.

Methods

A pot experiment to evaluate drought tolerance of 15 cultivars of sweet potato was conducted in a glasshouse at James Cook University, Townsville, North Queensland. A factorial experiment with a randomised complete design consisted of water stress treatments and normal water supply for 15 cultivars (Table 1). Each treatment was replicated 4 times. Sixty pots with 5-L volume were used to determine the amount of water at wilting point, and another 60 pots were watered normally. Tip cuttings (25 cm long) of each cultivar were planted after soaking for 2 days in water. The plants were watered to saturation after which water was permanently withheld in the stress treatment. While water stress was induced gradually by withholding water, the unstressed set of control plants was watered to field capacity every other day. Growth parameters were measured and analysed using analysis of variance (SPSS 10.0) at a significance level of 5% (P<0.05). When the effects of various treatments were significant, post hoc comparisons were carried out using Bonferroni’s method.

Results and Discussions

Table 1. Plant growth parameters of sweet potato cultivars as affected by soil water condition.

Cultivar

Dry biomass (g plant-1)

Main stem length (cm plant-1)

Internode length (cm plant-1)

Internode dia-meter (mm plant-1)

Root dry weight (g plant-1)

Control

Stress

Control

Stress

Control

Stress

Control

Stress

Control

Stress

Beerwah Gold

37.6

10.0

98.0

76.0

3.6

3.5

3.8

2.9

4.6

1.4

Hawaii

26.2

9.4

118.0

101.8

5.8

5.8

2.5

2.0

3.6

1.1

Lole

22.9

6.9

70.6

60.5

2.5

2.5

2.8

2.5

3.4

1.1

Markham

36.7

10.3

200.5

165.4

7.7

7.6

3.4

3.0

5.5

1.7

Mariken

39.8

7.8

300.8

188.8

9.0

8.9

3.0

2.3

4.4

1.0

Wanmun

36.4

9.7

214.6

135.8

5.6

5.3

3.5

3.0

4.4

1.2

NG7570

31.5

8.3

160.8

127.8

6.3

6.0

2.3

2.0

3.9

1.2

LO323

39.4

9.0

162.0

109.8

7.1

6.8

2.9

2.0

3.7

1.2

L3

37.8

9.1

199.8

135.8

6.8

6.8

3.9

3.0

4.3

1.4

L11

32.4

9.7

172.8

148.4

7.2

7.0

3.7

2.8

4.8

1.2

L18

30.8

8.2

121.6

53.0

2.5

2.4

3.8

3.2

3.5

1.2

L46

32.7

9.4

163.1

111.3

5.4

5.0

3.8

3.1

4.4

1.4

L49

37.7

10.3

149.4

102.3

5.0

4.9

5.1

3.4

5.0

1.4

L131

24.4

8.8

197.0

149.8

6.1

5.9

3.0

2.0

4.5

1.3

L135

37.5

8.3

181.8

118.0

5.0

5.0

2.7

2.4

3.5

0.6

Table 2. Leaf growth parameters of sweet potato cultivars as affected by soil water condition.

Cultivar

Leaf weight (g plant-1)

Leaf area (cm2 plant-1)

Leaf number/plant

Control

Stress

Fresh

Dry

Fresh

Dry

Control

Stress

Control

Stress

Beerwah Gold

89.4

12.6

11.8

4.3

3891

771

67

25

Hawaii

71.1

9.4

14.2

3.9

3216

839

62

31

Lole

51.8

9.2

13.9

3.4

2114

756

88

38

Markham

70.8

11.8

13.9

3.9

3716

889

54

20

Mariken

66.9

11.1

12.0

2.6

3524

771

46

18

Wanmun

71.8

12.1

12.7

2.9

3489

823

57

23

NG7570

77.4

11.8

14.7

2.6

4065

868

85

33

LO323

78.9

13.2

12.3

4.1

4160

851

50

25

L3

76.6

12.8

12.4

2.9

3871

818

54

22

L11

73.0

10.5

14.3

3.3

3682

819

43

20

L18

77.1

12.1

13.0

2.6

3609

762

93

52

L46

69.7

12.0

14.5

3.9

3809

842

38

18

L49

77.2

12.1

13.2

3.4

2972

814

51

24

L131

52.4

10.0

12.4

3.7

2348

824

53

30

L135

64.7

12.0

12.3

3.6

2773

798

46

23

Table 3. Morpho-physiology and water relations parameters of sweet potato cultivars as affected by soil water conditions.

Cultivar

Leaf dry matter content (g kg-1)

Specific leaf area (cm2 g-1)

Leaf water potential at permanent wilting point (bar)

Final soil water content
(g water pot-1)

Number of days to permanent wilting point

Control

Stress

Control

Stress

Control

Stress

Beerwah Gold

365.3

144.7

309

180

-2.4

-14.2

240

19

Hawaii

275.9

132.6

344

215

-1.9

-11.3

174

25

Lole

240.6

178.2

229

227

-2.3

-11.8

170

27

Markham

278.1

167.4

312

235

-2.3

-12.2

191

23

Mariken

200.3

166.1

317

296

-2.4

-12.5

179

21

Wanmun

228.2

168.3

289

286

-2.8

-12.3

163

21

NG7570

179.0

152.8

346

340

-2.3

-11.7

163

21

LO323

333.6

167.4

316

209

-2.8

-13.0

239

21

L3

215.9

167.9

305

294

-2.3

-12.2

199

21

L11

228.0

143.3

355

253

-2.3

-11.3

181

23

L18

199.7

157.2

298

297

-2.4

-12.0

158

23

L46

270.0

171.8

318

219

-2.2

-11.8

179

23

L49

243.7

156.3

245

244

-2.3

-11.4

174

23

L131

209.1

192.9

238

221

-2.2

-12.5

173

23

L135

249.2

184.8

233

226

-2.4

-11.5

172

23

There were significant interactions between water treatments and cultivars for most of the parameters observed except for root dry weight, suggesting that the general responses of the cultivars to the water treatments were statistically similar. All cultivars significantly responded to drought by a reduction in overall plant growth apart from the internode length (Table 1-3).

However, a number of cultivars showed differential growth responses to drought. Drought reduced the total plant dry weight from 31% to 46% with respect to the controls. The lowest biomass reduction was in cultivars L131, Hawaii, and Lole (Table 1). Although Lole had the lowest biomass weight, it produced the smallest reduction in the total dry biomass compared to other cultivars (Table 1). In all cultivars, stem length grew significantly slower after water was withheld (Table 1). The reduction of stem length varied from control to drought stress treatments. The greatest reduction in stem length was found in cultivar L18 (46.0%), while Lole showed the lowest reduction (16.1%; Table 1). Drought decreased internode diameter but not internode length in all cultivars. Internode diameter was slightly reduced in Lole (12%), whereas cultivars LO323, L49, and L131 produced the greatest reduction (45-50%; Table1). This reduction is in agreement with the findings of Kirnak et al. (2001) who showed that water stress reduced both stem height and internode diameter in eggplants by 46% and 51% under 40 % field capacity compared to the control (100% field capacity). Drought affected root dry weight irrespective of cultivars. The smallest reduction was in cultivars L18 (65.6%), followed by L3 (66.7%), LO323 (68.2%), Lole (68.6%), and L46 (68.7%; Table 1).

Leaf fresh weight was reduced from 60% to 80% among cultivars (Table 2). The lowest leaf fresh weight reduction was in Lole (73.2%; Table 2). When water was withheld, cultivar L18 produced the highest leaf dry weight reduction (78.9%), while the lowest reduction was in Hawaii (58.5%); leaf area was reduced from 80.2 % in Beerwah Gold to 64.2 % in Lole, and leaf number reduced significantly from 62% in Markham and Beerwah Gold to 50 % in Hawaii, L135, and LO323. The highest leaf area reduction was in Beerwah Gold, while Lole produced the lowest percentage of leaf area reduction. The reduction of leaf weight and area was strongly correlated with the reduction of leaf water potential (r2= 0.89 and r2= 0.91, respectively). Lole had the smallest leaf area, lowest specific leaf area, and greater drought tolerance characters; transpiration was therefore expected to be lower. Leaf size of sweet potato has been reported to have a negative correlation with apparent photosynthesis, suggesting that cultivars with small leaves have an advantage in the field (Bhagsari and Brown 1986).

Leaf dry matter reflects a fundamental trade off in plant functioning between a rapid production of biomass (high specific leaf area, low leaf dry matter content) and an efficient conservation of nutrients (low specific leaf area, high leaf dry matter content, Garnier et al. 2001). In the controls, cultivar Beerwah Gold produced the greatest leaf dry matter content, whereas cultivar L131 produced the highest leaf dry matter content in the water stressed treatment. NG7570 produced the greatest specific leaf area in both the control and water stressed conditions; Lole, on the other hand, produced the lowest specific leaf area in the controls.

In this trial, leaf water potential was not statistically different among cultivars. However, it progressively declined when water was withheld. The decrease in leaf water potential caused by water stress is well documented in other crop species; Siddique et al. (2000) found that leaf water potential of wheat cultivars decreased from –0.63 MPa in well watered plants to – 2.00 MPa in stressed plants, and as a consequence photosynthetic rate was extremely affected. In cassava, leaf wilting occurred at leaf water potential of less then –9 bars (Ike and Thurtell, 1981). Lole transpired less water than the other cultivars, and it was also withstood a prolonged dry period (Table 3). Plant growth usually decreased, as soil water availability becomes more limited due to turgor loss in expanded cells (Kirnak et al. 2001). Of the15 cultivars in this study, Lole was found to be relatively drought tolerant; its growth was less affected by drought, it delayed wilting, and it had higher leaf water content compared to the other cultivars. Lole used less water simply as a function of differences in leaf areas; therefore it has a greater capacity to retain moisture. This is in agreement with the results of Monneveux and Belhassen (1996) who showed that water loss at the plant level largely depends upon the size of the evaporating areas (leaves, stems).

Conclusion

Drought gradually leads to a decrease in soil water content and affected water relations and growth in all sweet potato cultivars. Among the 15 sweet potato cultivars evaluated Lole is considered to have the greatest tolerance to water stress. Drought tolerance in Lole is associated with the lowest percentage of growth reduction, less water consumption, higher leaf water potential, and delayed wilting under imposed drought stress conditions.

References

Anselmo BA, Ganga ZN, Badol EO, Heimer YM, Nejidat A (1998). Screening sweet potato for drought tolerance in the Philippine highlands and genetic diversity among selected genotypes. Tropical Agriculture (Trinidad) 75, 189-196.

Bhagsari AS, Brown RH (1986). Leaf photosynthesis and its correlation with leaf area. Crop Science. 26, 127-132.

Garnier E, Shipley B, Roumet C, Laurent G (2001). A Standardized protocol for the determination of specific leaf area and leaf dry matter content. Technical Report. Functional Ecology 15, 688-695.

Ike IF and Thurtell GW (1981). Response of indoor-grown cassava to water deficits and recovery of leaf water potential and stomatal activity after water stress. J. Exp. Botany 32, 1029-1034.

Indira P, Kabeerathumma S (1988). Physiological response of sweet potato under water stress. 1. Effect of water stress during the different phases of tuberization. Journal of Root Crops 14(2), 37-40.

Kirnak H, Kaya C, Tas I, Higgs D (2001). The influence of water deficit on vegetative growth, physiology, fruit yield and quality in eggplants. Bulgarian Journal Plant Physiology 27(3-4), 34-46.

Monneveux P and Belhassen E (1996). The diversity of drought adaptation in the wide. Plant Growth Regulation 20, 85-92.

Prakash CS (1994). Sweet potato biotechnology: Progress and potential. Biotechnology and development monitor 18:18-19 http://www.pscw.uva.nl/monitor/1811.htm

Siddique MRB, Hamid A and Islam MS (2000). Drought stress effects on water relations of wheat. Botany Bull. Acad. Sinica 41, 35-39.

Valenzuela H, Fukuda S, Arakaki A (2000) Sweetpotato production guidelines for Hawaii. http://www.extento.hawaii.edu/kbase/reports/sweetpot_prod.htm

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