Botanical Studies (2008) 49: 335-342.

*

Corresponding author: E-mail: kaoch@ntu.edu.tw; Tel:

+886-2-33664757; Fax: +886-2-23620879.

INTRODUCTION

Cadmium (Cd) is a pollutant and its presence in

the environment is essentially due to anthropogenic

activities (Sanita di Toppi and Gabbrielli, 1999). Major

sources of Cd pollution are industrial processes and

phosphate fertilizers (Pinot et al., 2000). Because of

its long biological half-life, Cd, which belongs to the

group of non-essential transition metals, is highly toxic.

Taken up in excess by plants, Cd directly or indirectly

inhibits physiological processes, such as respiration,

photosynthesis, cell elongation, plant-water-relationship,

nitrogen metabolism, and mineral nutrition, resulting

in poor growth and low biomass (Sanita di Toppi and

Gabbrielli, 1999; Kuo and Kao, 2004; Sun et al., 2007).

Global warming, accompanied an increased frequency

of periods with exceptionally high temperature, is one

of the most important characteristics of the accelerated

climatic changes. Global surface temperatures are

projected to increase by 1.4 to 5.8¢XC by 2100, in relation

to rising concentrations of greenhouse gases in the

atmosphere (Houghton et al., 2001; Cui et al., 2006). Peng

et al. (2004) analyzed weather data at the International

Rice Research Institute Farm from 1979 to 2003 to

examine temperature trends and reported that annual mean

maximum and minimum temperatures have increased by

0.35 to 1.13¢XC, respectively. Hence, plants will be more

often exposed to high temperature conditions. It has been

shown that Cd toxicity (the reduction of plant height, the

decrease in chlorophyll content, and the accumulation

of phenolic compounds and proline) to wheat seedlings

increases parallel to temperature increases (o ncel et

al., 2000). Temperature is a key factor in altering ion

accumulation (Chawla et al., 1991; Hooda and Alloway,

1993; Macek et al., 1994; Mautsoe and Backett, 1996).

The high toxicity of Cd induced by high temperature may

be a result of much more Cd uptake from the medium.

The plant hormone abscisic acid (ABA) is a

sesquiterpenoid derived from xanthophyll (Seo and

Koshiba, 2002; Hsu and Kao, 2004; Nambara and

Marion-Poll, 2005; Zhang et al., 2005) and appears to

influence several physiological and developmental events

(Zeevaart and Creelman, 1988; Seo and Koshiba, 2002).

Heavy metals such as Cd, Ni, Zn, and Al (Rauser and

Dumbroff, 1981; Poschenrieder et al., 1989; Hollenbach

et al., 1997; Foy, 1998; Fediuc et al., 2005) have been

shown to increase ABA contents in plants. Fediuc et al.

(2005) demonstrated that Cd-induced ABA accumulation

was observed in roots, but not in shoots, of Typha and

Phragmites plants.

Distinct roles of abscisic acid in rice seedlings during

cadmium stress at high temperature

Yi Ting HSU and Ching Huei KAO*

Department of Agronomy, National Taiwan University, Taipei, Taiwan, Republic of China

(Received November 16, 2007; Accepted May 7, 2008)

ABSTRACT.

Cd toxicity was judged by the decrease in chlorophyll and protein contents. Twelve-day-

old seedlings of rice cultivars [Tainung 67 (TNG67) and Taichung Native 1 (TN1)] were treated with or

without CdCl

2

at high temperature (35/30¢XC day/night). The results indicated that at high temperature, TNG

67 seedlings are a Cd-tolerant cultivar while TN1 seedlings are Cd-sensitive. On treatment with CdCl

2

,

the abscisic acid (ABA) contents increased in the leaves of both TNG67 and TN1 seedlings grown at high

temperature. Fluridone (Flu), an inhibitor of carotenoid biosynthesis, treatment, reduced ABA accumulation,

increased transpiration rate and Cd content, and decreased Cd tolerance of TNG67 seedlings grown at high

temperature. Flu¡¦s effect on Cd toxicity of TNG 67 seedlings was reversed by the application of ABA. For

TN1 rice seedlings grown at high temperature, Flu treatment resulted in less Cd-induced ABA accumulation,

as well as toxicity. These Flu effects were reversible by application of ABA. However, Flu treatment did not

reduce Cd content in the leaves of TN1 seedlings grown at high temperature. Exogenous application of ABA

at high temperature provoked chlorosis, a symptom of Cd toxicity, in the leaves of TN1, but not in TNG67

seedlings. Suggested roles for endogenous ABA in Cd tolerance of TNG67 seedlings and Cd toxicity of TN1

seedlings are discussed.

Keywords: Abscisic acid; Cadmium; High temperature; Oryza sativa.

phySIOlOgy

336

Botanical Studies, Vol. 49, 2008

Plants grown in the field may encounter several abiotic

stresses, rather than a single stress. Recently, Mittler

(2006) emphasized the importance of focusing the research

programs on the response of plants to a combination of

two different abiotic stresses. Previously, we have shown

that, on treatment with CdCl

2

, the ABA content increased

in the leaves in the rice seedling leaves of the Tainung

67 cultivar (TNG67) but not in Taichung Native 1 (TN1)

grown at normal temperature (30/25¢XC, day/night) (Hsu

and Kao, 2003). Our recent work demonstrated that Cd

treatment resulted in an increase in ABA content in the

leaves of TN1 seedlings grown at high temperature (35/30

¢XC, day/night) (Hsu et al., 2006). It is not known whether

CdCl

2

also increases ABA content in TNG67 leaves at

high temperature. In this work, we shall examine the roles

of endogenous ABA in TN1 and TNG67 seedlings during

Cd stress at high temperature.

MATERIAlS AND METhODS

plant materials and treatments

Rice (Oryza sativa L., cv. TN1 and TNG67) seeds were

sterilized with 2.5% sodium hypochlorite for 15 min and

washed extensively with distilled water. These seeds were

then germinated in Petri dishes with wetted filter papers

at 3 7¢XC in t he d ark. A fter 48 h incubation , u niform ly

germinated seeds were selected and cultivated in a 250

ml beaker containing half-strength Kimura B solution

containing the following macro- and micro-elements:

182.3 £gM (NH

4

)

2

SO

4

, 91.6 £gM KNO

3

, 273.9 £gM MgSO

4

.

7H

2

O, 91.1 £gM KH

2

PO

4

, 182.5 £gM Ca(NO

3

)

2

, 30.6 £gM

Fe-citrate, 0.25 £gM H

3

BO

3

, 0.2 £gM MnSO

4

.H

2

O, 0.2

£gM ZnSO

4

.7H

2

O, 0.05 £gM CuSO

4

.5H

2

O, and 0.07 £gM

H

2

MoO

4

. Kimura B solution contains the desired nutrient

elements (the concentrations of N, P, K, S, Ca and Mg in

half-strength Kimura B solution are 11.5, 2.9, 7.2, 15.0,

7.4 and 8.7 £gg ml

-1

) and has been widely used for growing

rice plants. Since young rice seedlings were used for the

present study, the nutrient solution contained no silicon,

although silicon is essential for rice. The nutrient solutions

(pH 4.7) were replaced every 3 days. The hydroponically

cultivated seedlings were grown in a Phytotron

(Agricultural Experimental Station, National Taiwan

University, Taipei, Taiwan) with natural sunlight at 30/25

¢XC day/night and 90% relative humidity. Twelve-day-old

seedlings with three leaves were moved to a Phytotron

with temperature controlled at 35/30¢XC (day/night) and

grown in the Kimura B solution with or without 0.5 mM or

30 £gM CdCl

2

. For the experiments in which the effect of

exogenous ABA was examined, various concentrations of

ABA (mixed isomers, 5-40 £gM) were added directly to the

Kimura B solution. In experiments to understand the role

of endogenous ABA, 0.2 mM fludidone (Flu), which is

known to block the conversion of phytoene to phytofluene

in the carotenoid biosynthesis pathway (Kowalczyk-

Schroder and Sandmann, 1992), was added directly to the

Kimura B solution.

Cd determination

For determination of Cd, leaves were dried at 65¢XC for

48 h. Dried material was ashed at 550¢XC for 20 h. The ash

residue was incubated with 31% HNO

3

and 17.5% H

2

O

2

at

72¢XC for 2 h and dissolved in distilled water. Cd was then

quantified using an atomic absorption spectrophotometer

(Model AA-6800, Shimadzu, Kyoto, Japan). Cd amounts

are expressed on a dry weight (DW) basis.

Determination of chlorophyll, protein, and ABA

Chlorophyll content was determined according to

Wintermans and De Mots (1965) after extraction in 96%

(v/v) ethanol. For protein determination, leaves were

homogenized in a 50 mM sodium phosphate buffer (pH

6.8). The extracts were centrifuged at 17,600 g for 20

min, and the supernatants were used for determination by

the method of Bradford (1976). Chlorophyll and protein

contents are expressed on the basis of initial fresh weight

(FW).

For extraction of ABA, leaves were homogenized with

a pestle and mortar in extraction solution (80% methanol

containing 2% glacial acetic acid). To remove plant

pigments and other non-polar compounds which could

interfere in the immunoassay, extracts were first passed

through polyvinylpyrrolidone column and C18 (Sep-

Pak Vac) cartridges (Waters, Milford, MA). The eluates

were concentrated to dryness by vacuum-evaporation and

resuspended in Tris-buffered saline before enzyme-linked

immunosorbent assay (ELISA). ABA was quantified by

ELISA (Walker-Simmons, 1987). The ABA immunoassay

detection kit (PGR-1) purchased from Sigma Chemical Co.

(St. Louis, MO) is specific for (+)-ABA. By evaluating

3

H-ABA recovery,

3

H-ABA loss was less than 3% by the

method described here. ABA content is expressed on the

basis of FW.

Transpiration rate

The transpiration rate was measured according to

Greger and Johansson (1992). The weight of rice

seedlings grown in hydroponic solution was determined

at the beginning and end of the interval, respectively. The

transpiration rate was calculated for the water loss during

each interval and converted to a per day per seedling basis.

Statistical analysis

Absolute levels of each measurement varied among

experiments because of seasonal effects. However, the

patterns of responses to CdCl

2

were reproducible. For all

measurements, each treatment was performed four times.

All experiments were performed at least thrice. Similar

results and identical trends were obtained each time. The

data reported here are from a single experiment. Statistical

differences between measurements (n = 4) on different

treatments or on different times were analyzed following

LSD test.

HSU and KAO ¡X Distinct roles of abscisic acid in rice seedlings during cadmium stress at high temperature

337

RESUlTS

Evaluation of Cd toxicity

In plants, the most general symptom of Cd toxicity is

chlorosis (Das et al., 1997). In our previous work, we

observed that chlorosis first occurred in the second leaves

of TN1 seedlings treated with CdCl

2

(Hsu and Kao, 2003).

Thus, in the present study, Cd toxicity in the second leaves

by 0.5 mM CdCl

2

was assessed by decreases in chlorophyll

and protein contents. A marked decrease in chlorophyll

and protein was observed in TN1 seedlings after CdCl

2

treatment at high temperature (Figure 1A , B). However,

Cd had a slight effect on reducing chlorophyll and protein

contents in the second leaves of TNG67 seedlings at high

temperature (Figure 1D, E).

ABA accumulation

CdCl

2

treatment resulted in a 4-fold increase in ABA

content in the second leaves of both TN1 and TNG67

seedlings grown at high temperature (Figure 1C, F).

Fluridone effect

Fluridone (Flu) is known to block the conversion of

phytoene to phytofluene in the carotenoid biosynthesis

pathway (Kowalczyk-Schroder and Sandmann, 1992).

When Flu was added to the nutrient solutions, reduction

of Cd-induced ABA accumulation in the second leaves,

and Cd tolerance of TNG67 seedlings grown at high

temperature was observed (Figures 2B and 3B, D). The

effect of Flu on the reduction of Cd tolerance of TNG67

seedlings at high temperature was reversed by exogenously

applied ABA (Figure 3B, D). ABA per se did not affect

chlorophyll and protein contents in TNG67 leaves at high

temperature (Figure 3B, D).

Flu was also observed to inhibit Cd-induced ABA

accumulation in the second leaves of TN1 seedlings at

Figure 1. Effect of CdCl

2

(0.5 m M) on the con ten ts of

chlorophyll, protein, and ABA in the second leaves of TN1 (A,

B, C) and TNG67 (D, E, F) rice seedlings at high temperature

(35/30¢XC). All measurements were made 2 days after treatment

(C, D). Bars indicate standard error (n = 4). Values with the

same letter are not significantly different at P < 0.05.

Figure 2. Effect of fluridone (Flu, 0.2 mM) on the contents

of ABA in the second leaves of TN1 (A) and TNG67 (B) rice

se edlings t re ated with or without CdCl

2

(0.5 mM) at high

temperature (35/30¢XC). ABA contents were determined 2 days

after treatment. Bars indicate standard error (n = 4). Values

with the same letter are not significantly different at P < 0.05.

Figure 3. Effect of fluridone (Flu, 0.2 mM) and ABA (5 £gM)

on the contents of chlorophyll and protein in the second leaves

of TN1 (A, C) and TNG67 (B, D) rice seedlings treated with

or without CdCl

2

(0.5 mM) at high temperature (35/30¢XC). All

measurements were made 2 days after treatment. Bars indicate

standard error (n = 4). Values with the sam e letter are not

significantly different at P < 0.05.

338

Botanical Studies, Vol. 49, 2008

high temperature (Figure 2A). However, Flu reduced

Cd toxicity of TN1 seedlings at high temperature

(Figure 3A, C). The effect of Flu on the reduction of Cd

toxicity of TN1 seedlings grown at high temperature was

reversed by adding ABA (Figure 3A, C). ABA treatment

alone also clearly decreases chlorophyll and protein

contents in leaves of TN1 seedlings at high temperature

(Figure 3A, C). On treatment with CdCl

2

, ABA content

significantly increased in the leaves of both Cd-tolerant

cultivar (TNG67) and Cd-sensitive cultivar (TN1) at high

temperature (Figure 1C, F). Thus, seedlings treated with

CdCl

2

+ ABA were not included in experiments of Figure

3 or subsequent experiments (Figures 4 and 5).

Cd concentration and transpiration rate

Cd concentration in the second leaves of TN1 seedlings

treated with Flu plus CdCl

2

at high temperature is similar

to that with CdCl

2

alone (Figure 4A). In contrast, Flu

caused an increase in Cd concentration in Cd-treated

TNG67 seedlings (Figure 4B). The Flu effect on the

increase in Cd concentration in leaves of TNG67 seedlings

at high temperature was reversed by adding ABA (Figure

4B).

Cd has been shown to decrease transpiration rate in

several plants (Kirkham, 1978; Lamoreaux and Chaney,

1978; Hagemeyer et al., 1986; Schlegel et al., 1987). We

also observed that Cd decreased the transpiration rate of

TN1 and TNG67 seedlings at high temperature (Figure 4C,

D). Flu treatment resulted in an increase in transpiration

rate in CdCl

2

-treated TNG67 seedlings (Figure 4D). The

Flu effect on the increase in transpiration rate at high

temperature was reversed by exogenously applied ABA

(Figure 4D). However, transpiration rate of TN1 seedlings

treated with Flu plus CdCl

2

at high temperature is similar

to that with CdCl

2

alone (Figure 4C).

Effect of lower CdCl

2

concentrations

The concentration of CdCl

2

used in the aforementioned

study was 0.5 mM. We also conducted experiments with

lower CdCl

2

(30 £gM) applied over a longer period (6 days).

At high temperature, CdCl

2

(30 £gM) treatment resulted

in an approximately 2-fold increase in ABA content in

the second leaves of TN1 and TNG67 seedlings (Figure

5A, B). When Flu was added to the nutrient solutions,

reduction of Cd-induced ABA accumulation in the second

leaves and Cd tolerance of TNG67 seedlings at high

temperature was observed (Figure 5B, D, F). The effect of

Flu on the reduction of Cd tolerance of TNG67 seedlings

to high temperature was reversed by exogenously applied

Figure 4. Effect of fluridone (Flu, 0.2 mM) and ABA (5 £gM)

on Cd concentrations in the second leaves and the transpiration

rate of TN1 (A, C) and TNG67 (B, D) rice seedlings treated with

or without CdCl

2

(0.5 mM) at high temperature (35/30¢XC). All

measurements were made 2 days after treatment. Bars indicate

standard error (n = 4). Values with the sam e letter are not

significantly different at P < 0.05.

Figure 5. Effects of fluridone (Flu, 0.2 mM) on the contents of

ABA (A, B) and of fluridone and ABA (5 £gM) on the contents

of chlorophyll (C, D) and protein (E, F) in the second leaves

of TN1 and TNG67 rice seedlings treated with or without low

concentration of CdCl

2

(30 £gM) for 6 days at high temperature

(35/30¢XC). Bars indicate standard error (n = 4). Values with the

same letter are not significantly different at P < 0.05.

HSU and KAO ¡X Distinct roles of abscisic acid in rice seedlings during cadmium stress at high temperature

339

ABA (Figure 5D, F). Flu was also observed to inhibit Cd-

induced ABA accumulation in the second leaves of TN1

seedlings at high temperature (Figure 5A). However,

it was observed to reduce Cd toxicity in leaves of TN1

seedlings at high temperature (Figure 5C, E). The effect of

Flu on the reduction of Cd toxicity in the second leaves of

TN1 seedlings grown at high temperature was reversed by

adding ABA (Figure 5C, E). The contents of chlorophyll

and protein were observed to be decreased by ABA

alone (Figure 5C, E). Thus, the responses to lower CdCl

2

concentrations are basically in accordance with responses

to higher ones.

Exogenous application of ABA

Figure 6 shows the effect of ABA concentrations, in the

range from 5 to 40 £gM, applied over a period of 2 days,

on the chlorosis of the second leaves of rice seedlings at

high temperature. It is clear that increasing concentration

of ABA progressively promotes chlorosis of the second

leaves of TN1, but not of TNG67 seedlings at high

temperature. It appears that in terms of leaf chlorosis,

TNG67 is ABA-insensitive and TN1 is ABA-sensitive.

DISCUSSION

The Cd concentration in the shoot increased at high

temperature with increasing light intensity (Greger,

1999). In the present study, rice seedlings were grown in

a Phytotron with natural sunlight at 30/25¢XC or 35/30¢XC.

Under natural sunlight conditions, light intensity varied

daily or seasonally. Based on our experience from the

experiments of Cd effect on rice seedlings, consistent

Cd toxicity was observed in 2 and 6 days, respectively,

when 0.5 mM and 30 £gM CdCl

2

were used. However, no

consistent Cd toxicity of rice seedlings exposed to CdCl

2

concentrations lower than 30 £gM was observed. For this

reason in this study, 0.5 mM or 30 £gM CdCl

2

were added

directly to nutrient solution. In a study on 64 soils (urban,

forest and agricultural soils) containing various levels

of Cd contamination, free dissolved Cd concentrations

ranged from 0.1 to 2,000 nM (Sauve et al., 2000). Thus,

the concentrations of Cd used in our experiments can

be considered very high. Basically, rice seedlings in the

present study were considered to be suffering from acute

Cd toxicity.

In the absence of CdCl

2,

the DW of shoot and roots of

TN1 or TNG67 seedlings grown at normal temperatures

for 2 days was similar to that at high temperatures (data

not shown). High temperature per se does not seem to

exert stress effects on TN1 or TNG67 seedlings.

In the present study, we evaluated Cd toxicity by the

decrease in chlorophyll and protein contents. On the basis

of these criteria, we demonstrated that at high temperature,

TNG67 seedlings are a Cd-tolerant cultivar while TN1

Figure 6. Effect of ABA on chlorosis of the second leaves of TN1 and TNG67 rice seedlings grown at high temperature (35/30¢XC).

Arrows indicated the second leaves, and pictures were taken 2 days after treatment.

340

Botanical Studies, Vol. 49, 2008

seedlings are Cd-sensitive (Figure 1). It has also been

described previously that TNG67 seedlings grown at

normal temperature are more tolerant to Cd than TN1 (Hsu

and Kao, 2003).

Plants have several potential mechanisms at the cellular

level that might be involved in the detoxification and thus

tolerance to heavy metals. These all appear to be involved

primarily in avoiding a build-up of toxic concentrations at

sensitive sites within the cell and thus preventing damage

(Hall, 2002). In this connection, a reduced translocation

of Cd to the shoot appears to be possible mechanism of Cd

tolerance in the shoot. Some hold this translocation to be

driven by transpiration (Salt et al., 1995). The mechanism

of Cd tolerance of TNG67 seedlings grown at high

temperature is basically related to the inhibitory effect of

ABA on transpiration rate and Cd uptake. This conclusion

was based on observations: (i) ABA accumulated in the

second leaves of TNG67 seedlings under Cd stress at

high temperature (Figure 1F); (ii) Flu treatment led to a

decrease in the ABA content (Figure 2B), an increase in

transpiration rate and Cd concentration (Figure 5B, D), and

decrease in Cd tolerance of TNG67 seedlings treated with

CdCl

2

at high temperature (Figure 3B, D); and (iii) the

effects of Flu on the transpiration, Cd concentration, and

Cd toxicity of TNG67 seedlings grown at high temperature

were reversed by application of ABA (Figures 3B, D and

4B, D). Cd tolerance of TNG67 seedlings grown at high

temperature appears to be mediated through ABA-induced

inhibition of Cd uptake.

In contrast, we show that ABA is involved in Cd

toxicity of TN1 seedlings grown at high temperature. This

conclusion was based on observations that (i) the increase

in endogenous ABA content in response to Cd in leaves of

rice seedlings was observed at high temperature (Figure

1C); (ii) exogenous application of ABA to TN1 seedlings

at high temperature increased ABA content (Hsu et al.,

2006) and resulted in chlorophyll loss (Figures 3A and 5C)

and chlorosis (Figure 6); (iii) Flu treatment reduced the

ABA content, as well as the Cd toxicity to TN1 seedlings

grown at high temperature (Figures 2A, 3A, C, and 5C,

E); and (iv) the effect of Flu on the Cd toxicity of TN1

seedlings grown at high temperature can be reversed by

the application of ABA (Figures 3A, C and 5C, E).

The fact that Flu treatment reduced ABA content,

but did not reduce Cd content in the second leaves and

transpiration rate of TN1 seedlings in response to CdCl

2

at high temperature (Figures 1A and 4A) suggests that

Flu effect on the reduction of Cd toxicity is attributable to

reduction of ABA but not Cd concentration in the second

leaves and transpiration rate of TN1 seedlings grown at

high temperature. These results strengthen further our

conclusion that Cd toxicity of TN1 seedlings at high

temperature is mediated through ABA accumulation.

Recently, Fediuc et al. (2005) reported that ABA mediated

the Cd-induced stimulation of O -acetylserine (thiol)

lyase (OASTL), the enzyme responsible for cysteine

biosynthesis. However, in the case of Arabidopsis growth

(root length and seedling fresh weight), the magnitude

of Cd-induced inhibition in ABA-insensitive mutant was

generally comparable to that in the wild type (Sharma and

Kumar, 2002)

Cd is known to inhibit the transpiration rate of several

plants (Lamoreaux and Chaney, 1978; Hagemeyer et al.,

1986; Schlegel et al., 1987). The fact that Flu treatment

had no effect on the transpiration rate of TN1 seedlings

in response to CdCl

2

at high temperature suggests that

the decrease in transpiration rate caused by CdCl

2

is not

attributable to increases in ABA but to increases in Cd

concentration (Figure 5A).

At high temperature, ABA increased at about the same

magnitude (about 4-fold) in the leaves of Cd-tolerant

TNG67 and Cd-sensitive TN1 seedlings in response to

Cd (Figure 1C, F). Clearly, ABA may exert distinct roles

during Cd stress. In terms of chlorophyll and protein loss

and chlorosis, we observed that rice seedlings of cultivar

TN1 are ABA-sensitive, and those of cultivar TNG67

are ABA-insensitive (Figures 3A-D and 6). This would

explain why ABA accumulating in leaves of TNG67

seedlings grown at high temperature plays a protective

role against Cd stress, while the leaves of TN1 seedlings

show toxic effects of Cd stress. In our previous work,

we showed that at normal temperatures CdCl

2

treatment

resulted in ABA content increases in leaves of TNG67 but

not in TN1 (Hsu and Kao, 2003). The data of the present

study together with previous work (Hsu and Kao, 2003)

clearly indicate that ABA could participate in regulation of

Cd toxicity/tolerance of rice seedlings as diagrammatically

shown in Figure 7.

It is well established that ABA is derived from

xanthophylls (Nambara and Marion-Poll, 2005). Here, we

show that ABA accumulation in rice leaves of both TN1

and TNG67 seedlings is induced by Cd at high temperature

(Figure 1C, F). ABA in high plants is derived from C

40

-

carotenoid (Nambara and Marion-Poll, 2005). As Flu

Figure 7. Regulation of Cd tolerance/toxicity by endogenous

ABA in rice seedlings grown at normal and high temperatures.

HSU and KAO ¡X Distinct roles of abscisic acid in rice seedlings during cadmium stress at high temperature

341

is an inhibitor of carotenoid biosynthesis (Kowalczyk-

Schroder and Sandmann, 1992), the effect of this inhibitor

on Cd-induced ABA accumulation in the second leaves of

TN1 and TNG67 seedlings at high temperature may imply

that ABA biosynthesis pathway in response to Cd at high

temperature appears to be the same as that established in

other stress conditions (Zeevaart and Creelman, 1988;

Nambara and Poll, 2005).

Acknowledgements. This research was supported by the

National Science Council of the Republic of China.

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