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: *sumbun@kku.ac.th
Received June 21st, 2013; revised July 21st, 2013; accepted August 4th, 2013
Copyright © 2013 Sumontip Bunnag, Prapaporn Pongthai. This is an open access article distributed under the Creative Commons
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.
Copyright © 2013 SciRes. AJPS
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.
Copyright © 2013 SciRes. AJPS
Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress
at the Vegetative Stage under Field Conditions
Copyright © 2013 SciRes. AJPS
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
0
2
4
6
8
10
12
14
16
18
20
(c)
KDML 105
BT
IR57514
IR58821
IR62266
IR52516
CT9993
Number of tillers per plant
0
2
4
6
8
10
12
14
16
18
20
(a)
Soil humidity (% vy weight)
0
20
40
60
140
160
(b)
Plant height (cm)
120
100
80
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
BT
IR57514
IR58821
IR62266
IR52516
CT9993
Degree of leaf rolling
40 47 5460677480 87
0
1
2
3
4
5
6
Figure 2. Degree of leaf rolling in the seven rice cultivars subjected to drought stress.
Copyright © 2013 SciRes. AJPS
Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress
at the Vegetative Stage under Field Conditions
1705
Days after germination
KDML 105
BT
IR57514
IR58821
IR62266
IR52516
CT9993
Degree of leaf death
4047 5460 67 7480 87
0
2
4
6
8
10
Figure 3. Degree of leaf death in the seven rice cultivars subjected to drought stress.
Days after germination
KDML 105
BT
IR57514
IR58821
IR62266 IR52516
CT9993
Leaf water potential (MPa)
40
-3.0
KDML 105
BT
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
Copyright © 2013 SciRes. AJPS
Selection of Rice (Oryza sativa L.) Cultivars Tolerant to Drought Stress
at the Vegetative Stage under Field Conditions
Copyright © 2013 SciRes. AJPS
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
BT
IR57514
IR58821
IR62266IR52516
CT9993
40 60 100
108
Relative leaf water content (%)
104
100
96
92
88
84
108
104
100
96
92
88
84
(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
BT
IR57514
IR58821
IR62266 IR52516
CT9993
40 60 100
250
200
150
100
50
0
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.
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at the Vegetative Stage under Field Conditions
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