Selection of Rice (<i>Oryza sativa</i> L.) Cultivars Tolerant to Drought Stress at the Vegetative Stage under Field Conditions

doi:10.4236/ajps.2013.49207

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American Journal of Plant Sciences, 2013, 4, 1701-1708

http://dx.doi.org/10.4236/ajps.2013.49207 Published Online September 2013 (http://www.scirp.org/journal/ajps)

Selection of Rice (Oryza sativa L.) Cultivars Tolerant to

Drought Stress at the Vegetative Stage under Field

Conditions

Sumontip Bunnag*, Prapaporn Pongthai

Genomics and Proteomics Research Group for Improvement of Salt-Tolerant Rice, Department of Biology, Faculty of Science, Khon

Kaen University, Khon Kaen, Thailand.

Email: *[email protected]

Received June 21st, 2013; revised July 21st, 2013; accepted August 4th, 2013

Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is

properly cited.

ABSTRACT

Water status is one of the critical factors affecting rice production. Rice cultivars tolerant to drought stress at the vegeta-

tive stage under field conditions were selected. Seven rice cultivars, namely, KDML 105, IR58821, CT9993, IR62266,

IR57514, IR52561 and BT, were included in this study. The plant height, number of tillers per plant, leaf rolling, leaf

death, leaf water potential, relative leaf water content and proline content in plants subjected to drought stress for 0, 20

and 60 days were recorded. Based upon the levels of water stress tolerance, three groups of rice cultivars were recog-

nized, as follows: highly drought-tolerant, moderately drought-tolerant and drought-sensitive cultivars. The CT9993

rice cultivar was considered to be a highly drought-tolerant cultivar. The moderately drought-tolerant cultivars included

KDML 105, IR58821, IR57514, IR52561 and BT. The IR62266 cultivar was considered sensitive to drought.

Keywords: Proline; Leaf Rolling; Leaf Death; Leaf Water Potential

1. Introduction

Rice (Oryza sativa L.) is one of the most widely con-

sumed cereal crops, providing a staple diet for almost

half of the world’s population [1]. Rice-growing areas

occupy the tropics, subtropics, semiarid tropics and tem-

perate regions of the world. More than 90% of the

world’s rice is grown and consumed in Asia, where rice

is cultivated on 135 million ha with an annual production

of 516 million tonnes [2]. In Thailand, a total of 11.116

million ha is dedicated to growing rice [3], and the north-

eastern and northern regions of the country are the major

rice-growing areas. The predominant rice-growing areas

in the two regions are often threatened by severe water

deficit, partly due to low-input irrigation systems. In ad-

dition, emerging water shortages resulting from eco-

nomic development and urbanization are leading to ra-

tioning of water in regions where irrigated lowland rice

has traditionally been grown, and these production sys-

tems are also becoming water-limited. Accordingly, rice

yield in these regions is low and fluctuates [4]. Water de-

ficit may occur early in the growing season or at any time

from flowering to grain filling, and the intensity of the

stress depends on the duration and frequency of the water

deficit [5]. Drought stress suppresses leaf expansion, till-

ering and midday photosynthesis [6], and it reduces the

photosynthetic rate and leaf area due to early senescence

[7]. All of these factors are responsible for a reduction in

grain yield under drought conditions. Furthermore, water

deficit also increases the formation of reactive oxygen

species (ROS), resulting in lipid peroxidation, protein de-

naturation and nucleic acid damage with severe conse-

quences affecting the overall metabolism [8], thereby lead-

ing to a reduction in grain yield.

Rice is most susceptible to drought stress at both the

vegetative and reproductive stages [9,10]. A dramatic re-

duction in grain yield occurs when drought stress coin-

cides with irreversible reproductive processes [10,11].

Early-season drought occurs in most areas, affecting the

timely transplanting of seedlings and the growth of di-

rect-seeded rice. Late-season drought develops in most

years at the end of the rainy season before crop matura-

tion, particularly in paddy rice in a high toposequence

*Corresponding author.

Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress

at the Vegetative Stage under Field Conditions

1702

position [9,12].

The phenology, particularly at the reproductive stage,

is a major determinant of grain yield in rain-fed lowland

rice, and any attempt to screen for drought resistance

needs to consider variation at the reproductive stage [13].

However, the vegetative stage is another critical deter-

minant of the growth and maturation of rice. Therefore,

selecting rice cultivars that confer drought resistance

from different cultivars with contrasting drought toler-

ance at the vegetative or reproductive stages will bring

new insights for the breeding of rice.

2. Materials and Methods

2.1. Drought Stress Treatments and

Measurements

Seven rice cultivars, KDML 105, IR58821, CT9993,

IR62266, IR57514, IR52561 and BT, were used in this

study. Field experiments were conducted at the experi-

mental farm (Muaeng district, Khonkaen province, Thai-

land; 102˚49'E, 16˚25'N, and 152 m above sea level) at

the Khonkaen Rice Experiment Station from December

1999 to April 2000. The experiments were conducted in

a 2.20 m × 16.40 m concrete box filled with local lateritic

soil, and the box was located under a rainout shelter with

removable roof panels. Three to four seeds of each rice

cultivar were directly sown in each burrow prepared

within the concrete box in 20 cm rows with an isolation

distance of 15 cm from any other rice seed. After 14 days

of germination, only one plantlet was left to grow in each

burrow, whereas the other rice plantlets were removed.

Pesticides were applied biweekly at the manufacturers’

recommended rates. The plants also received fertilizers

biweekly with an N-P2O5-K2O ratio of 8-8-8 (kg/rai), and

water was drained 42 days after seed germination.

The experiments were laid out in a randomized com-

plete block design in which each treatment was repli-

cated three times. For comparison, a well-watered treat-

ment was also included in the experiment as the control

treatment. At the end of the drought treatment, the con-

trol and drought-stressed plants were sampled. The plant

height and number of tillers per plant were measured at

the beginning and end, respectively, of the drought-stress

treatment. The plant height was measured from the stem

base to the highest leaf tip.

A comparison of the degree of leaf rolling between the

plants subjected to drought stress and those under normal

irrigation during the same period was determined based

on a standard chart presented by O’Toole and Cruz (1980)

[14]. A visual score was assigned to indicate the degree

of leaf rolling found on the sample leaf using a scale

ranging from 1 to 5, with 1 indicating the first evidence

of rolling and 5 indicating a closed cylinder. The leaf

death (drought score) of the plants subjected to drought

stress was compared with that of the control plants under

normal irrigation during the same period using standard

criteria proposed by a standard evaluation system for rice

[15]. A visual score was assigned for the degree of leaf

death found on the sample leaf using a scale ranging

from 0 to 9, with 0 indicating no symptoms and 9 indi-

cating apparent death.

2.2. Measurements of Leaf Water Potential and

Relative Leaf Water Content

Leaf water potentials were measured using the pressure

chamber technique described by Turner (1981) [16] by

pressurizing the chamber with N gas until plant sap ac-

cumulated at the cut end of the leaf. The leaf water po-

tential was determined at three intervals. The first test

was performed when the 40-day-old plants were in non-

stress conditions. The second leaf water potential deter-

mination was performed on 60-day-old plants subjected

to drought stress for 20 days (mild stress), and the last

interval test was conducted after the 100-day-old plants

were subjected to drought stress for 60 days (severe

stress).

The relative leaf water contents were also measured

based on the method described by Turner (1981) [16].

The relative leaf water content was determined in the

fully expanded leaf. The fresh weights of the sample

leaves were recorded, and the leaves were immersed in

distilled water in a Petri dish. After 2 h, the leaves were

removed, the surface water was blotted off, and the tur-

gid weight was recorded. The samples were then dried in

an oven at 70˚C to constant weight. The relative leaf wa-

ter content was calculated using the following formula:







 

RLWC%FWDW TWDW100 



;

where FW is the fresh weight; DW is the dry weight; and

TW is the turgid weight.

2.3. Determination of Proline Content

The proline content in the leaves was estimated based on

the method described by Bates et al. (1973) [17]. Briefly,

0.1 g of rice leaves was ground with 5 ml of 3% sul-

fosalicylic acid, and the mixture was then filtered. To 2

ml of the filtered mixture in a test tube, 2 ml of acid-

ninhydrin and 2 ml of glacial acetic acid were added. The

mixture was mixed with a Vortex mixer and boiled at

100˚C for 1 h. The mixture was then frozen in ice and

combined with 4 ml of toluene, mixed, and then left to

stand for 5 - 10 min. Absorbance of the reddish pink up-

per phase was recorded at 520 nm against a toluene

blank.

Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress

at the Vegetative Stage under Field Conditions

1703

2.4. Statistical Analysis

The physiological results were exported to the Statistical

Package for Social Sciences v 17.0 software (SPSS Inc.,

Chicago, Illinois, USA), and analysis of variance

(ANOVA) was used for statistical analysis. The means of

the various results were tested for level of significance by

Duncan’s multiple range test (DMRT). Statistical sig-

nificance was accepted at p < 0.05.

3. Results

3.1. Phenotypic Variations

The well-watered plants showed normal growth of the

stems. Under mild drought stress conditions (after 20

days of the treatment), the plants showed a slight reduc-

tion in the growth rate of the stems, and the growth rate

reduction became more dramatic under severe stress (af-

ter 60 days of the treatment). IR52561 showed the high-

est plant height with an average of 93 cm followed by

IR58821 (88 cm), BT (72 cm) and IR62266 (72 cm) as

presented in Figure 1(b). The plants subjected to drought

stress showed a decrease in tillering rates compared with

the well-watered plants. Under mild drought stress, the

plants showed a slight reduction in tillering rates, and the

rates became more dramatic when the plants were subject

to severe drought stress. IR62266 showed the highest

number of tillers per plant, at 8, and CT9993 displayed

the lowest number of tillers per plant at 3. The other cul-

tivars showed the number of tillers per plant as 5 as

shown in Figure 1(c).

Leaf rolling, which is a visible sign of drought stress,

0 20 406080100120

Days after germination

(c)

KDML 105

IR57514

IR58821

IR62266

IR52516

CT9993

Number of tillers per plant

(a)

Soil humidity (% vy weight)

140

160

(b)

Plant height (cm)

120

100

Figure 1. Soil humidity (a), plant height (b) and number of tillers per plant (c) of the seven rice cultivars subjected to drought

stress.

Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress

at the Vegetative Stage under Field Conditions

1704

was observed in all the cultivars after 7 days of the

drought treatment. The controls remained unrolled, but

the degree of leaf rolling in all the other cultivars became

progressively more pronounced with the drought stress

treatment. The responses of the real rolling score to soil

water stress are presented in Figure 2. After 7 days of

the drought treatment, all the cultivars showed a low de-

gree of leaf rolling (Degree 1). When the soil water be-

came more deficient, different degrees of leaf rolling

were observed among the cultivars. After 27 days of the

treatment, IR62266 and IR52561 progressively devel-

oped Degree 4 of the leaf rolling symptom, and the other

cultivars were at Degree 3. After 47 days of the treatment,

CT9993 was found to develop Degree 4 of the leaf roll-

ing symptom, and the other cultivars showed leaf rolling

at Degree 5. Leaf death, which is another visible sign of

drought stress, developed from the leaf tips and extended

to all the plant parts and finally to all the tillers. After 14

days of the treatment, all the cultivars developed Degree

2 of the leaf death symptom in response to drought. Af-

terwards, different degrees of leaf death were found in

the different cultivars. KDML 105, IR62266 and IR52561

developed the symptom more rapidly than the other cul-

tivars under severe drought stress conditions. CT9993

showed 70% leaf death (Degree 7) after 47 days of the

treatment, and the other cultivars developed leaf death

symptoms at percentages greater than 70% (Degree 8).

IR62266 had the highest degree of leaf death as shown in

Figure 3.

3.2. Acclimation to Water Stress: Leaf Water

Potential, Relative Leaf Water Content and

Proline Content

The drought treatment of the seven rice cultivars resulted

in water deficit. The responses of the flag leaves of all

the cultivars towards water deficit were compared by

analyzing the leaf water potential (ΨL) in both the day-

time and nighttime. During the daytime, none of the cul-

tivars from the controls or the plants subjected to mild

drought stress showed significant differences in the ΨL,

but significant differences in the ΨL became more evi-

dent when the plants were subjected to severe stress

(Figure 4(a)).

The ΨL was reduced to −1.76, −1.88, −1.95 and −2.66

in CT9993, BT, IR58821 and IR62266, respectively

(Figure 4(a)). During the nighttime, the ΨL was not sig-

nificantly different among all the cultivars. The ΨL re-

duced the most under severe drought stress. The ΨL

dropped to −0.56, −0.57 and −1.28 in BT, CT9993 and

IR62266, respectively (Figure 4(b)).

The relative leaf water content (RLWC) of all the cul-

tivars, subjected to different drought stress conditions,

was compared in both the daytime and nighttime. During

the daytime, no significant differences were found in the

RLWC among the cultivars prior to being subjected to

drought stress with results ranging from 99.5% - 99.8%.

Severe drought stress resulted in significant differences

in the RLWC among the cultivars. The RLWC was re-

duced to 92.9, 92.3 and 86.1% in CT9993, BT and

Days after germination

KDML 105

IR57514

IR58821

IR62266

IR52516

CT9993

Degree of leaf rolling

40 47 5460677480 87

Figure 2. Degree of leaf rolling in the seven rice cultivars subjected to drought stress.

Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress

at the Vegetative Stage under Field Conditions

1705

Days after germination

KDML 105

IR57514

IR58821

IR62266

IR52516

CT9993

Degree of leaf death

4047 5460 67 7480 87

Figure 3. Degree of leaf death in the seven rice cultivars subjected to drought stress.

Days after germination

KDML 105

IR57514

IR58821

IR62266 IR52516

CT9993

Leaf water potential (MPa)

-3.0

KDML 105

IR57514

IR58821

IR62266 IR52516

CT9993

Days after germination

60 100

40 60 100

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Leaf water potential (MPa)

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

(a)

(b)

Figure 4. Leaf water potential during the daytime (a) and nighttime (b) in seven rice cultivars subjected to different drought

tress conditions. s

Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress

at the Vegetative Stage under Field Conditions

1706

by reducing plant height and the number of tillers per

plant because plants are unable to absorb soil water when

soil water becomes inadequate, resulting in the essential

elements being less available to the plants. The plant

cells become less turgid, leading to a reduction in cell di-

vision and expansion. Therefore, the growth of the stems

is retarded [18]. Gupta (1997) [19] found that plants sub-

jected to drought stress have smaller-sized stomata, which

results in less carbon dioxide being introduced. Further-

more, the amount of active chlorophyll is also reduced,

lowering photosynthesis activity and thereby resulting in

inadequate photosynthetic products for the activities wi-

thin the plant cells leading to retarded growth of the

plants.

IR62266, respectively (Figure 5(a)). During the night-

time, no significant differences in the RLWC were ob-

served among all the cultivars under normal conditions

with values ranging from 99.7% - 99.9%. Significant dif-

ferences in the RLWC were found under severe drought

stress conditions. The RLWC dropped to 92.7% in

KDML 105, CT9993 and IR52561, and the RLWC was

reduced to 99.3% in IR62266 (Figure 5(b)).

The proline contents in all the cultivars under both

normal and mild drought stress conditions were signifi-

cantly different. However, severe drought stress condi-

tions produced non-significant differences among the

rice cultivars. KDML 105, IR62266 and IR52561 accu-

mulated a large amount of proline compared with the

normal level of proline found in CT9993 and BT, result-

ing in no significant differences among the cultivars

(Figure 6).

Datta (1975) [20] found that rice grown under drought

stress ranging from −0.7 to −0.8 MPa show slightly re-

tarded height at the vegetative and flowering stages. Un-

der drought stress ranging from −1.8 to −1.9 MPa, dra-

matically reduced growth of plant height is observed.

4. Discussion Plants also respond to water deficit by developing the

leaf rolling symptom. All the cultivars showed higher

Drought stress directly affects the growth of rice plants

Days after germination

Relative leaf water content (%)

KDML 105

IR57514

IR58821

IR62266IR52516

CT9993

40 60 100

108

Relative leaf water content (%)

104

100

108

104

100

(b)

(a)

40 60100

KDML 105 IR58821 CT9993 IR62266

IR57514 IR52516 BT

Figure 5. Relative leaf water content during the daytime (a) and nighttime (b) in seven rice cultivars subjected to drought

stress.

Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress

at the Vegetative Stage under Field Conditions

1707

Days after germination

Proline content (μg/g fresh weight)

KDML 105

IR57514

IR58821

IR62266 IR52516

CT9993

40 60 100

250

200

150

100

Figure 6. Proline content measured in the leaves from seven rice cultivars subjected to drought stress.

degrees of leaf rolling when the levels of water deficit

were elevated. Based on the results from this study,

CT9993 showed a lower degree of leaf rolling than the

other cultivars. These findings were in agreement with

the results of Dingkuhn et al. (1991) [21], who reported

that leaf rolling is one of the mechanisms found in plants

to escape drought. This mechanism can be explained by

the plants adjusting their leaf water potential to allow

them to absorb soil water better than other plants with

low capabilities to adjust their leaf water potential under

drought stress. As a result, leaf rolling is uncommon in

these better-adjusting plants, so their photosynthesis was

not prohibited.

Leaf death is another visible sign of drought stress that

plants initially develop from the leaf tips. According to

the findings obtained in this study, CT9993 showed the

lowest degree of leaf death. Sigari et al. (1997) [22] re-

ported that the degree of leaf rolling is related to leaf

death. Plants with a good capacity for leaf water potential

adjustment can keep their leaves expanded such that the

symptoms of leaf rolling and death are not developed,

suggesting that the plants can rapidly recover from

drought.

Under severe drought stress, a reduction was found in

the ΨL in all the rice cultivars. Based on the present study,

CT9993 was the only cultivar that was able to maintain

its leaf water potential. Munns (2002) [23] reported that a

reduction in leaf water potential occurs when soil water

becomes deficient, resulting in the plants being less able

to absorb water and leading to a decrease in turgidity and

leaf water potential. The RLWC is another parameter to

determine the ability of plants to withstand drought. The

present study indicated that various cultivars can resist

drought differently. Nguyen et al. (1997) [24] stated that

drought-tolerant cultivars can maintain the water status in

their leaves, which demonstrates their ability to cope

with drought stress.

Accumulation of proline in plants is a result of drought

stress. In the present study, plants tended to increasingly

accumulate proline under severe drought stress.

5. Conclusion

In summary, the studied rice cultivars displayed different

abilities to resist drought. Based on the present study, the

rice cultivars could be classified into 3 categories based

on their responses to drought. CT9993 was a highly

drought-tolerant cultivar. KDML 105, IR58821, IR57514,

IR52561 and BT were considered to be moderately

drought-tolerant cultivars, and IR62266 was considered

to be sensitive to drought.

6. Acknowledgements

This work was financially supported by the Graduate

School of Khon Kaen University, Thailand. The authors

thank to KKU Publication Clinic for providing profes-

sional editing service from American Journal Experts.

We are indebted to the Department of Biology of the

Faculty of Science of Khon Kaen University for granting

us permission to use the devices necessary for the study.

REFERENCES

[1] Z. P. Song, B. R. Lu, Y. G. Zhung and J. K. Chen, “Gene

Flow from Cultivated Rice to the Wild Species Oryza

rufipogon under Experimental Field Conditions,” New

Phytologist, Vol. 157, No. 3, 2003, pp. 657-665.

doi:10.1046/j.1469-8137.2003.00699.x

Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress

at the Vegetative Stage under Field Conditions

1708

[2] R. N. Roy and R. V. Misra, “Economic and Environ-

mental Impact of Improved Nitrogen Management in

Asian Rice-Farming Systems,” Proceedings of the 20th

Session of the International Rice Commission, 2002, pp.

23-26.

[3] OAE (Office of Agricultural Economics), “Report on

Survey of Rice Growing Areas in Thailand Crop Year

2012/2013,” Center for Agricultural Information, Office

of Agricultural Economics, Ministry of Agriculture and

Cooperation, Bangkok, 2013.

[4] B. Jongdee, “Designing a National Breeding Program for

Developing Drought-Tolerant Rainfed Lowland Varieties:

The Thailand Experience,” In: K. S. Fischer, R. Lafitte, S.

Fukai, G. Atlin and B. Hardy, Eds., Breeding Rice for

Drought-Prone Environments 64-69, International Rice

Research Institute, Los Banõs, The Philippines, 2003.

[5] L. J. Wade, C. G. McLaren, L. Quintana, D. Harnpichit-

vitaya, S. Rajatasereekul, A. K. Sarawgi, A. Kuma, H. U.

Ahmed, A. K. Sarwoto, R. Singh, J. Rodriguez, Sio-

pongco and S. Sarkarung, “Genotype by Environment In-

teractions across Diverse Rainfed Lowland Rice Envi-

ronments,” Field Crops Research, Vol. 64, No. 1, 1999,

pp. 35-50. doi:10.1016/S0378-4290(99)00049-0

[6] P. J. Kramer and J. S. Boyer, “Water Relations of Plant

and Soil,” Academic Press, San Diego, 1995.

[7] L. D. Nooden, “The Phenomena of Senescence and Ag-

ing,” In: L. D. Nooden and A. C. Leopald, Eds., Senes-

cence and Aging in Plants, Academic Press, San Diego,

1988, pp. 1-50.

[8] J. M. Hansen, Y. M. Go and D. P. Jones, “Nuclear and

Mitochondrial Compartmentation of Oxidative Stress and

Redox Signaling,” Annual Review of Pharmacology and

Toxicology, Vol. 46, 2006, pp. 215-34.

doi:10.1146/annurev.pharmtox.46.120604.141122

[9] B. Jongdee, “New Rice Breeding Methods for Rainfed

Lowlands of North and Northeast Thailand,” Proceedings

of an International Workshop: Increased Lowland Rice

Production in the Mekong Region, Vientiane, 30 Octo-

ber-2 November 2000, pp. 221-228.

[10] G. Pantuwan, S. Fukai, M. Cooper, S. Rajatasereekul and

J. C. O’Toole, “Yield Response of Rice (Oryza sativa L.)

Genotypes to Drought under Rainfed Lowlands 2. Selec-

tion of Drought Resistant Genotypes,” Field Crops Re-

search, Vol. 73, No. 2-3, 2002, pp. 169-180.

doi:10.1016/S0378-4290(01)00195-2

[11] A. Price and B. Courtois, “Mapping QTLs Associated

with Drought Resistance in Rice: Progress, Problems and

Prospects,” Plant Growth Regulation, Vol. 29, 1999, pp.

123-133. doi:10.1023/A:1006255832479

[12] B. Jongdee, “Designing a National Breeding Program for

Developing Drought Tolerant Rainfed Lowland Varieties:

The Thailand Experiences,” In: K. S. Fischer, R. Laﬁtte,

S. Fukai, G. Atlin and B. Hardy, Eds., Breeding Rice for

Drought-Prone Environments, International Rice Research

Institute, Los Banõs, 2004. pp. 64-69.

[13] G. Pantuwan, S. Fukai, M. Cooper, S. Rajatasereekul and

J. C. O’Toole, “Yield Response of Rice (Oryza sativa L.)

Genotypes to Drought under Rainfed Lowlands. 1. Grain

Yield and Yield Components,” Field Crop Research, Vol.

73, 2001, pp. 153-168.

doi:10.1016/S0378-4290(01)00187-3

[14] J. C. O’Toole and R. Cruz, “Response of Leaf Water

Potential, Stomatal Resistance and Leaf Rolling to Water

Stress,” Plant Physiology, Vol. 65, No. 3, 1980, pp. 428-

432. doi:10.1104/pp.65.3.428

[15] IRRI, “Annual Report for 1975,” International Rice Re-

search Institute, Los Banõs, 1976, p. 418.

[16] N. C. Turner, “Techniques and Experimental Approaches

for the Measurement of Plant Water Status,” Plant Soil,

Vol. 58, No. 1, 1981, pp. 339-366.

doi:10.1007/BF02180062

[17] L. S. Bates, R. P. Waldren and I. D. Teare, “Rapid De-

termination of Free Proline for Water Stress Studies,”

Plant and Soil, Vol. 39, No. 1, 1973, pp. 205-208.

doi:10.1007/BF00018060

[18] T. C. Hsiao, J. C. O’Toole, E. B. Yambao and N. C.

Turner, “Influence of Osmotic Adjustment on Leaf Roll-

ing and Tissue Death in Rice (Oryza sativa L.),” Plant

Physiology, Vol. 75, No. 1, 1984, pp. 338-341.

doi:10.1104/pp.75.2.338

[19] U. S. Gupta, “Crop Improvement: Stress Tolerance,” Sci-

ence Publishers, Inc., New Hamphire, 1997.

[20] S. K. Datta, “Drought Tolerance in Rice in Relation to

Growth Stages,” IRRI, Los Banõs, 1975.

[21] M. Dingkuhn, R. T. Cruz, C. O. O’Toole, N. C. Turner

and K. Doerffling, “Responses of Seven Diverse Rice

Cultivars to Water Deficits. III. Accumulation of Abscisic

Acid and Proline in Relation to Leaf Water Potential and

Osmotic Adjustment,” Field Crops Research, Vol. 27, No.

1-2, 1991, pp. 103-117.

doi:10.1016/0378-4290(91)90025-Q

[22] A. Sigari, L. J. Wadel and A. Yamauch, “Response of

Roots Traits of Rainfed Lowland Rice to Drought and

Rewatering,” In: J. Abe and S. Morita, Eds., Root System

Management that Leads to Maximize Rice Yields, JSRR,

Tokyo, 1997.

[23] R. Munns, “Comparative Physiology of Salt and Water

Stress,” Plant, Cell and Environment, Vol. 25, No. 2,

2002, pp. 239-250.

doi:10.1046/j.0016-8025.2001.00808.x

[24] H. T. Nguyen, R. C. Babu and A Blum, “Breeding for

Drought Resistance in Rice: Physiology and Molecular

Genetics Considerations,” Crop Science, Vol. 37, No. 5,

1997, pp. 1426-1434.

doi:10.2135/cropsci1997.0011183X003700050002x