Cadmium toxicity is reduced by nitric oxide in rice leaves
Yi Ting Hsu and Ching Huei Kao*
Department of Agronomy, National Taiwan University, Taipei, Taiwan, Republic of China; *Author for correspondence (e-mail: kaoch@ccms.ntu.edu.tw; phone: +886-2-23698159; fax: +886-2-23620879)
Received 2 June 2003; accepted in revised form 15 December 2003
Key words: Active oxygen species, Cadmium, Lipid peroxidation, Nitric oxide, Oryza sativa L.
Abstract
We evaluate the protective effect of nitric oxide (NO) against Cadmium (Cd) toxicity in rice leaves. Cd toxicity of rice leaves was determined by the decrease of chlorophyll and protein contents. CdCl2treatment
resulted in (1) increase in Cd content, (2) induction of Cd toxicity, (3) increase in H2O2and malondialdehyde
(MDA) contents, (4) decrease in reduced form glutathione (GSH) and ascorbic acid (ASC) contents, and (5) increase in the specific activities of antioxidant enzymes (superoxide dismutase, glutathione reductase, ascorbate peroxidase, catalase, and peroxidase). NO donors [N-tert-butyl--phenylnitrone, 3-morpholino-sydonimine, sodium nitroprusside (SNP), and ASC + NaNO2] were effective in reducing CdCl2-induced
toxicity and CdCl2-increased MDA content. SNP prevented CdCl2-induced increase in the contents of H2O2
and MDA, decrease in the contents of GSH and ASC, and increase in the specific activities of antioxidant enzymes. SNP also prevented CdCl2-induced accumulation of NH4
+
, decrease in the activity of glutamine synthetase (GS), and increase in the specific activity of phenylalanine ammonia-lyase (PAL). The protective effect of SNP on CdCl2-induced toxicity, CdCl2-increased H2O2, NH4+, and MDA contents, CdCl2
-decreased GSH and ASC, CdCl2-increased specific activities of antioxidant enzymes and PAL, and
CdCl2-decreased activity of GS were reversed by
2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, a NO scavenger, suggesting that protective effect by SNP is attributable to NO released. Reduction of CdCl2-induced toxicity by NO in rice leaves is most likely mediated through its ability to
scavenge active oxygen species including H2O2.
Abbreviations: AOS – active oxygen species; APX – ascorbate peroxidase; ASC – ascorbic acid; CAT – catalase; c-PTIO – 2-(4-carboxy-2-phenyl)-4,4,5,5-tetra-methylimidazoline-1-oxyl-3-oxide; d.wt – dry weight; f.wt – fresh weight; GR – glutathione reductase; GS – glutamine synthetase; GSH – reduced glutathione; MDA – malondialdehyde; NO – nitric oxide; PAL – phenylalanine ammonia-lyase; PBN – N-tert-butyl--phenylnitrone; POX – peroxidase; SIN-1 – 3-morpholinosydonimine; SNP – sodium nitro-prusside; SOD – superoxide dismutase
Introduction
Cadmium (Cd) is a heavy metal that is toxic for humans, animals and plants, and is one of the widespread pollutants with a long biological half-life (Wagner 1993). This metal enters the environ-ment mainly from industrial process and phos-phate fertilizers and is transferred to animals and
humans through the food chain (Wagner 1993). Taken up in excess by plants, Cd directly or indir-ectly inhibits physiological processes such as respiration, photosynthesis, cell elongation, plant– water relationships, nitrogen metabolism, and mineral nutrition, resulting in poor growth and low biomass (Sanita´ di Toppi and Gabbrielli 1999).
Cd is a non-redox metal unable to produce active oxygen species (AOS) via Fenton and/or Haber– Weiss reactions (Sanita´ di Toppi and Gabbrielli 1999). However, several reports demonstrate that Cd can indirectly promote the generation of AOS (Sandalio et al. 2001; Schu¨tzendu¨bel et al. 2001; Olmos et al. 2003). Cd-increased lipid peroxidation has been demonstrated in Phaseolus vulgaris roots and leaves (Chaoui et al. 1997), Helianthus annuus leaves (Gallego et al. 1996), Pisum sativum shoot and root tissues (Lozano-Rodriguez et al. 1997), and Oryza sativa leaves (Chien et al. 2002). Thus, Cd leads to an oxidative stress in plant cells.
Nitric oxide (NO) is a bioactive free radical implicated in a number of physiological functions, including intra-cellular mediation of some animal responses (Anbar 1995). In plants, NO is involved in many physiological responses, such as pathogen response, programmed cell death, growth, germina-tion, root organogenesis, phytoalexin producgermina-tion, internal iron availability, and abscisic acid-dependent stomatal closure (Lamattina et al. 2003; Neill et al. 2003).
Several reports convincingly demonstrate that NO is able to counteract the toxicity of paraquat and diquat, which are known to generate AOS, in potato and rice leaves (Beligni and Lamattina 1999;
Hung et al. 2002), and block H2O2 production
induced by jasmonic acid in tomato leaves (Orozco-Ca´rdenas and Ryan 2002). Thus, a possi-ble participation of NO in antioxidant system in plants, as it does in animals (d’Ischia et al. 2000), is suggested.
In rice leaves, we have shown that NO counter-acts oxidative stress induced by paraquat, dehydra-tion, and polyethylene glycol (Cheng et al. 2002; Hung et al. 2002). More recently, we have shown that the promotion of rice leaf senescence caused by abscisic acid, which induces H2O2 production
and lipid peroxidation, can be counteracted by NO donors (Hung and Kao 2003). In the present inves-tigation, we examined the effect of NO on Cd-induced toxicity of rice leaves.
Material and methods Plant material and chemicals
Rice (O. sativa L., cv. Taichung Native 1) was sterilised 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 paper at 37 C under
Figure 1. Changes in the contents of chlorophyll, protein, MDA, and Cd in rice leaves treated with CdCl2. Detached rice leaves were
dark condition. After 48 h incubation, uniformly germinated seeds were selected and cultivated in a 500 ml beaker containing half-strength Kimura B solution as described previously (Chu and Lee 1989). The hydroponically cultivated seedlings were grown for 12 days in a Phytotron with natural light 30C day (12 h)/25C night (12 h) and 90% relative humidity. The apical 3 cm of the third leaf was used in all experiments. Incubation was carried out at 27C in the light (40 mol m2s1).
Test solutions included CdCl2, NO donors, and
a NO scavenger [2-(4-carboxy-2-phenyl)-4,4,5, 5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO)]. N-tert-butyl--phenylnitrone (PBN), 3-morpholi-nosydonimine (SIN-1), and sodium nitroprusside (SNP) were used as NO donors. We also used a solution containing ascorbic acid (ASC) and
NaNO2as another NO donor. All chemicals were
purchased from Sigma Co. (St. Louis, MO, USA).
Determination of chlorophyll, protein, H2O2,
lipid peroxidation, GSH, ASC, NH4+, and Cd
Chlorophyll was determined according to Wintermans and De Mots (1965) after extraction in 96% (v/v) ethanol. For protein determination, leaf segments were homogenised in a 50 mM sodium phosphate buffer (pH 6.8). The extracts were centrifuged at 17,600 g for 20 min, and the supernatants used for determination of protein by the method of Bradford (1976) and antioxidant enzyme activities. The H2O2content was measured
colorimetrically as described by Jana and
Figure 2. Changes in the contents of H2O2and the specific activities of SOD, APX, GR, CAT, and POX in rice leaves treated with
CdCl2. Detached rice leaves were treated with either water or 5 mM CdCl2in the light. * and ** represent values that are significant at
Choudhuri (1981). H2O2was extracted by
homo-genising leaf tissue with phosphate buffer (50 mM, pH 6.5) containing 1 mM hydroxylamine. The homogenate was centrifuged at 6000 g for 25 min. To determine H2O2 content, extracted
solution was mixed with 0.1% titanium chloride in 20% (v/v) H2SO4. The mixture was then
centri-fuged at 6000 g for 25 min. The absorbance was measured at 410 nm. The H2O2 content was
calculated using the extinction coefficient 0.28 mol1 cm1. MDA, routinely used as an indicator of lipid peroxidation, was extracted with 5% (w/v) trichloroacetic acid and determined according to Heath and Packer (1968). GSH in 3% sulfosalicylic acid extract and ASC in 5% (w/v) trichloroacetic acid extract were determined as described by Smith (1985) and Laws et al. (1983), respectively. NH4+was extracted and its
concentra-tion determined as described previously (Chien and Kao 2000). For determination of Cd, leaves were dried at 65C for 48 h and the dried material ashed
at 550C for 20 h. The ash residue was incubated with 31% HNO3and 17.5% H2O2at 72C for 2 h,
and dissolved in 0.1 N HCl. Cd was then quantified using an atomic absorption spectrophotometer (Model AA-6800, Shimadzu, Kyoto, Japan). Enzyme assays
Peroxidase (POX) activity was measured using a modification of the procedure of MacAdam et al. (1992). Activity was calculated using the extinction coefficient [26.6 mM1 cm1 at 470 nm] for tetra-guaiacol. Catalase (CAT) activity was assayed by measuring the initial rate of disappearance of H2O2
(Kato and Shimizu 1987). The decrease in H2O2
was followed as the decline in absorbance at 240 nm, and activity was calculated using the extinction coefficient [40 mM1 cm1 at 240 nm] for H2O2 (Kato and Shimizu 1987). Superoxide
dismutase (SOD) was determined according to Paoletti et al. (1986). Ascorbate peroxidase (APX) was determined according to Nakano and Asada (1981). The decrease in ASC concentration was followed as the decline in absorbance at 290 nm and activity was calculated using the extinction coefficient [2.8 mM1 cm1 at 290 nm] for ASC. Glutathione reductase (GR) was determined by the method of Foster and Hess (1980). One unit of activity for CAT, POX, SOD, APX, and GR was defined as the amount of enzyme which degraded 1 mol H2O2 per min, caused the formation of
1 mol tetraguaiacol per minute, inhibited 50% the rate of NADH oxidation observed in control, degraded 1 mol of ASC per minute, and decreased 1 A340per minute, respectively.
For extraction of glutamine synthetase (GS), leaf samples were homogenised with 10 mM Tris–HCl buffer (pH 7.6, containing 1 mM MgCl2, 1 mM
EDTA and 1 mM 2-mercaptoethanol) using a chilled pestle and mortar. The homogenate was centrifuged at 15,000 g for 30 min and the result-ing supernatant was used for determination of GS activity. The whole extraction procedure was carried out at 4C. GS was assayed by the method of Oaks et al. (1980). The reaction mixture con-tained in a final volume of 1 ml was 80 mol Tris– HCl buffer, 40 molL-glutamic acid, 8 mol ATP,
24 mol MgSO4, and 16 mol NH2OH; the final
pH was 8.0. The reaction was started by addition of the enzyme extract and, after incubation for 30 min
Figure 3. Changes in the contents of ASC and GSH in rice leaves treated with CdCl2. Detached rice leaves were treated
with either water or 5 mM CdCl2 in the light. * and **
represent values that are significant at P < 0.05 and P < 0.01, respectively.
at 30C, was stopped by adding 2 ml 2.5% (w/v) FeCl3 and 5% (w/v) trichloroacetic acid in 1.5 N
HCl. After centrifugation the absorbance of the supernatant was read at 540 nm. One unit of GS activity is defined as 1 molL-glutamate -mono-hydroxamate formed per minute. Phenylalanine
ammonia-lyase (PAL) was extracted and deter-mined according to Hyodo and Fujinami (1989). The calculation was based on the extinction coeffi-cient (9500 M1 cm1) for trans-cinnamic acid. One unit of activity for PAL was defined as the amounts of enzyme which caused the formation of 1 mol trans-cinnamic acid per hour.
Statistical analysis
Statistical differences between measurements (n ¼ 4) on different treatments or on different times were analyzed following the Duncan’s multiple range test or Student’s t-test.
Results
In the present investigation, Cd toxicity in detached rice leaves caused by excess Cd was assessed by a decrease in chlorophyll and protein contents. In previous work, we observed that increasing concentration of CdCl2 from 0.1 to
5 mM progressively decreased chlorophyll and pro-tein contents in detached rice leaves in the light and no further decrease was observed at 10 mM CdCl2
(Chien and Kao 2000). Thus, 5 mM CdCl2 was
used in the present investigation. Figure 1 shows the time courses of chlorophyll, protein and Cd contents in detached rice leaves treated with either water or 5 mM CdCl2in the light. Cd content in
control leaves remained unchanged during 24 h of incubation in the light. However, Cd contents in CdCl2-treated detached rice leaves increased with
increasing duration of incubation. The increase in Cd content in CdCl2-treated detached rice leaves
was evident at 4 h after treatment. The promotion
Table 1. Effect of NO donors on chlorophyll, protein, and MDA contents in rice leaves treated with CdCl2.
Treatment Chlorophyll (mg g1f.wt) Protein (mg g1f.wt) MDA (nmol g1f.wt)
H2O 4.36 ± 0.01 (a) 51.35 ± 1.27 (a) 33.80 ± 2.89 (c) CdCl2 3.07 ± 0.06 (c) 27.53 ± 0.98 (c) 49.78 ± 1.95 (a) CdCl2+ PBN 3.52 ± 0.11 (b) 34.72 ± 1.88 (b) 42.55 ± 0.60 (b) CdCl2+ SIN-1 3.34 ± 0.04 (b) 33.39 ± 0.57 (b) 42.75 ± 1.11 (b) CdCl2+ SNP 3.35 ± 0.10 (b) 33.71 ± 2.21 (b) 43.15 ± 2.50 (b) CdCl2+ ASC + NaNO2 3.44 ± 0.09 (b) 33.62 ± 1.75 (b) 44.24 ± 1.07 (b)
The concentrations of CdCl2, PBN, SIN-1, SNP, ASC, and NaNO2were 5 mM, 100, 100, 100, 100, and 200 M, respectively. All
measurements were determined 24 h after treatment in the light. Values with the same letter are not significantly different at P < 0.05, according to Duncan’s multiple range test.
Figure 4. Effect of SNP concentrations on chlorophyll and protein contents in rice leaves treated with CdCl2. The
concentration of CdCl2 was 5 mM. Chlorophyll and protein
contents were determined 24 h after treatment in the light. * and ** represent values that are significant at P < 0.05 and P < 0.01, respectively.
of the loss of chlorophyll and protein by CdCl2was
evident 8 and 4 h after treatment, respectively. MDA content in CdCl2-treated detached rice
leaves was observed to be greater than that in water-treated controls, throughout the entire dura-tion of incubadura-tion (Figure 1). This showed that Cd toxicity in detached rice leaves was linked to lipid peroxidation. Lipid peroxidation is caused by AOS (Thompson et al. 1987). CdCl2 treatment also
caused an increase in H2O2 content (Figure 2).
All these results support the involvement of AOS as the chemical species inducing Cd toxicity in rice leaves.
Plants have evolved a complex antioxidant sys-tem to prevent the harmful effects of AOS. The plant antioxidant system is composed of non-enzymatic and non-enzymatic components. GSH and ASC are the two most important water-soluble
non-enzymatic antioxidants (Noctor and Foyer 1998). SOD, APX, GR, CAT, and POX are key antioxidant enzymes (Foyer et al. 1997). The strik-ing increase in lipid peroxidation seen in rice leaves treated with CdCl2 may be a reflection of the
changes in the specific activities of antioxidant enzymes and the contents of antioxidants. As shown in Figure 2, CdCl2-treated rice leaves higher
specific activities of SOD than the controls at 12 h after treatment. Higher specific activities of APX, GR, POX, and CAT were observed at 24 h after treatment. GSH and ASC contents were observed to be lower than the controls at 4 and 8 h after treatment, respectively (Figure 3). The increased specific activities of antioxidant enzymes and the decreased contents of ASC and GSH in response to CdCl2are further suggestive of strong induction of
oxidative stress.
Figure 5. Effect of SNP on the contents of chlorophyll, protein, MDA, and Cd in CdCl2-treated rice leaves in the presence or absence of
c-PTIO. The concentrations of CdCl2, SNP, and c-PTIO were 5 mM, 100, and 100 M, respectively. All measurements of were
determined 24 h after treatment in the light. Values with the same letter are not significantly different at P < 0.05, according to Duncan’s multiple range test.
There are reports that NO counteracts oxidative stress in plants (Beligni and Lamattina 1999; Beligni et al. 2002; Cheng et al. 2002; Hung et al. 2002; Hung and Kao 2003). Thus, it is of great interest to know whether the protective role of NO is also active in CdCl2-induced toxicity in rice
leaves. Consequently, detached rice leaves were treated with CdCl2 in the presence or absence
of NO donors, such as PBN, SIN-1, SNP, and a
mixture of ASC and NaNO2for 24 h in the light.
As indicated in Table 1, all NO donors used are effective in reducing Cd toxicity and Cd-induced lipid peroxidation in rice leaves.
SNP alone had no effect on protein content and slightly decreased chlorophyll content at 100– 200 M (Figure 4). When applied together with CdCl2, SNP concentration at 100 M had the
high-est protective effect on Cd toxicity (Figure 4).
Figure 6. Effect of SNP on the contents of H2O2and the specific activities of SOD, APX, GR, CAT, and POX in CdCl2-treated rice
leaves in the presence or absence of c-PTIO. The concentrations of CdCl2, SNP, and c-PTIO were 5 mM, 100, and 100 M, respectively.
All measurements of were determined 24 h after treatment in the light. Values with the same letter are not significantly different at P < 0.05, according to Duncan’s multiple range test.
To investigate whether the protective effect induced by SNP treatment was the result of the production of NO, 100 M c-PTIO, a NO-specific scavenger, was applied along with 100 M SNP. The effect of SNP on CdCl2-induced toxicity and
increase in MDA and H2O2 contents could be
reversed by c-PTIO (Figures 5 and 6). Figure 5 also shows that SNP treatment was ineffective in reducing CdCl2-induced Cd content in rice leaves,
indicating that the effect of SNP on Cd toxicity is unlikely due to the decrease in Cd content in rice leaves. We also observed that SNP counteracted CdCl2-induced increase in the specific activities
of antioxidant enzymes (SOD, APX, GR, CAT, and POX) and c-PTIO reversed the effect of SNP-decreased specific activities of antioxidant enzymes (Figure 6). Furthermore, the effect of
SNP on CdCl2-decreased ASC and GSH contents
could be reversed by c-PTIO (Figure 7). Clearly, the effect of NO donor SNP is attributable to NO released.
Figure 8 shows that the increase in NH4+content
by CdCl2was evident at 8 h after treatment. NH4+
accumulation was observed to be correlated with the decrease in activity, rather than the specific activity of GS, a key enzyme in ammonia assimila-tion (Miflin and Lea 1976). Since NH4+is known to
be released through the action of PAL, the first enzyme in the phenylpropanoids (Hahlbrock and Grisebach 1979), it is possible that CdCl2-induced
NH4+accumulation is associated with the increase
in the activity or the specific activity of PAL in rice leaves. Indeed, we have shown that NH4 +
accumulation caused by CdCl2is associated with
the increase in specific activity of PAL (Figure 9). Our previous results showed that the decrease in GS activity and the accumulation of NH4+ in
detached rice leaves are a consequence of oxidative stress caused by excess Cd (Chien and Kao 2002). Furthermore, it has been reported that the specific activity of PAL induced by wounding is related to the ability to produce superoxide radicals in potato tuber (Kumar and Knowles 2003). These results strongly suggest that oxidative stress is involved in NH4+accumulation in CdCl2-treated rice leaves.
If this suggestion is correct, then SNP treatment is expected to counteract CdCl2-induced increase in
NH4 +
content, decrease in the activity of GS, and increase in the specific activity of PAL and c-PTIO
Figure 7. Effect of SNP on the contents of ASC and GSH in CdCl2-treated rice leaves in the presence or absence of c-PTIO.
The concentrations of CdCl2, SNP, and c-PTIO were 5 mM,
100, and 100 M, respectively. All measurements of were determined 24 h after treatment in the light. Values with the same letter are not significantly different at P < 0.05, according to Duncan’s multiple range test.
Figure 8. Changes in the contents of NH4+in rice leaves treated
with CdCl2. Detached rice leaves were treated with either water
or 5 mM CdCl2in the light. * and ** represent values that are
is expected to reverse these SNP effects. As indi-cated in Figure 10, this indeed is the case.
Discussion
It has been shown that Cd can cause an increased production of H2O2 (Schu¨tzendu¨bel et al. 2001;
Olmos et al. 2003) and induce lipid peroxidation (Gallego et al. 1996; Chaoui et al. 1997; Lozano-Rodriguez et al. 1997; Chien et al. 2002). These suggest that Cd treatment causes an oxidative stress in plants. Our results not only have shown that CdCl2increased the content of H2O2(Figure 2)
and the specific activities of SOD, APX, GR, CAT, and POX (Figure 2), but also demonstrate that caused a decrease in GSH and ASC contents (Figure 3). Meanwhile, chlorophyll and protein loss and lipid peroxidation were observed in CdCl2-treated rice leaves (Figure 1). All these
results suggest that CdCl2 causes an oxidative
stress and that CdCl2-induced toxicity in rice leaves
in mediated through oxidative stress.
ASC is a major antioxidant in photosynthetic and non-photosynthetic tissues which reacts directly with AOS and is utilised as a substrate for
APOD catalysed H2O2detoxification (Noctor and
Foyer 1998). GSH is involved in ASC regeneration and functions also as a direct antioxidant of AOS (Noctor and Foyer 1998). In the present investiga-tion, we observed that the decrease in GSH content is one of the earliest steps in oxidative stress induced by CdCl2in rice leaves, which occurred at
4 h after treatment (Figure 3). It may be suspected that the decrease in GSH may favor the accumula-tion of AOS in rice leaves. In a recent review, Schu¨tzendu¨bel and Polle (2002) also suggest that the depletion of GSH is apparently a critical step in Cd toxicity.
There is only limited information about the mechanism of Cd-induced H2O2 production.
Olmos et al. (2003) reported that NADPH oxi-dase-like enzyme was possibly involved in H2O2
production in Cd-treated tobacco cells. In the pre-sent study, we have not investigated whether H2O2
production by CdCl2in rice leaves is augmented by
the stimulation of plasma-bound NADPH oxi-dases as in tobacco cells. Further research is neces-sary to clarify this point.
In the present investigation, we found that NO reduced CdCl2-increased the content of MDA and
the specific activities of antioxidant enzymes in rice
Figure 9. Changes in the activities or specific activities of GS and PAL in rice leaves treated with CdCl2. Detached rice leaves were
leaves (Figures 5 and 6). These results are in agree-ment with our previous work, in which we demon-strated that NO counteracted paraquat-increased the content of MDA and the specific activities of
antioxidant enzymes (Hung et al. 2002). Because lipid peroxidation and the increase in the specific activities of antioxidant enzymes are the conse-quence of AOS overproduction (Thompson et al. 1987) and NO acts as an AOS scavenger, therefore, the reduction of the content of MDA and the spe-cific activities of antioxidant enzymes could be a result of low levels of AOS including H2O2in rice
leaves treated with NO and CdCl2. The fact that
NO counteracted CdCl2-decreased GSH and ASC
(Figure 7) may also results in an increase in the capacity of NO to scavenge AOS in rice leaves treated with NO and CdCl2and might account in
part for the lower contents of H2O2observed in rice
leaves treated with NO and CdCl2(Figure 6).
APX, CAT, and POX have been shown to be inhibited by NO (Clark et al. 2000). However, our results show that SNP treatment alone did not affect the specific activities of antioxidant enzymes in rice leaves (data not shown). Thus, the reduction of CdCl2-induced increase in the specific activities
of antioxidant enzymes by NO is unlikely due to a direct NO-mediated inhibition of the enzymes.
AOS can react with NO to form peroxynitrite (Kim and Han 2000; Martinez et al. 2000). Peroxynitrite has been shown to react with H2O2
to yield nitrite ion and oxygen (Martinez et al. 2000). This reaction has been suggested to be the mechanism of NO cytoprotective actions in animals (Wink et al. 1993). It appears that this mechanism is operating in rice leaves.
Of particular interest in the present investigation is the finding that CdCl2treatment resulted in an
increase in the specific activity of PAL and a decrease in the activity of GS in rice leaves (Figure 9). NH4+ released from PAL reaction is
known to be trapped in glutamine molecule by the action of GS (Sakurai et al. 2001). It appears that CdCl2-induced NH4+ accumulation is
mediated through the increase in the specific activ-ity of PAL and the decrease in the activactiv-ity of GS in rice leaves. It has been shown that the decrease of the activity of GS is caused by oxidative damage (Chien et al. 2002) and the specific activity of PAL induced by wounding is related to the ability to produce superoxide radicals (Kumar and Knowles 2003). The fact that NO counteracts the CdCl2
-induced decrease in the activity of GS and increase in the specific activity of PAL in the rice leaves (Figure 10), further strengthens the idea
Figure 10. Effect of SNP on the content of NH4+, the activity of
GS, and the specific activity of PAL in CdCl2-treated rice leaves
in the presence or absence of c-PTIO. The concentrations of CdCl2, SNP, and c-PTIO were 5 mM, 100, and 100 M,
respectively. All measurements of were determined 24 h after treatment in the light. Values with the same letter are not significantly different at P < 0.05, according to Duncan’s multiple range test.
that antioxidant properties of NO are operating for counteracting oxidative stress in rice leaves. Acknowledgements
This work was supported by grant NSC 91–2312-B-002–364 from the National Science Council of the Republic of China.
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