Botanical Studies (2007) 48: 435-444.
*
Corresponding author: E-mail: pliu99@163.com; Fax:
+86-579-2299168.
INTRODUTION
Ecological impact assessment of contamination stresses
on plants has been an interesting area in the last few years
as restoration of the natural environment and remediation
of contaminated soil and/or groundwater require a better
understanding of the relationship between ecological
health and environmental contamination. Metals are
among the major contaminants found in both contaminated
lands and natural soils. Aluminium is a light metal that
makes up 7% of the earth¡¦s crust, occurring in the form of
harmless oxides and aluminosilicates. If the soil becomes
acidic, Al is solubilized into toxic forms like [Al (H
2
O)
6
]
3+
,
generally referred to Al
3+
,
which is now present in 40%
of the arable lands in the world. Excess Al
3+
in soil enters
roots, resulting in reduced plant vigor and yield (Delhaize
and Ryan, 1995; Matsumoto, 2000; Ciamporova, 2002).
The initial symptom of Al toxicity is the inhibition of root
elongation, which has been proposed to be caused by a
number of different mechanisms, including Al interactions
within the cell wall (Massot et al., 1999), the plasma
membrane (Pi.eros and Kochian, 2001), or the symplast
(Kochian, 1995).
Given the increasing frequency of acid rain, a strategy
should be established to protect soybean plants from
aluminum toxicity stress in high Al soils. This requires
a better understanding of the physiological responses
of soybean to this stress. Good progress in this field has
Effect of Al in soil on photosynthesis and related
morphological and physiological characteristics of two
soybean genotypes
Xiao-Bin ZHANG
1
, Peng LIU
1,
*, Y.S. YANG
2
, and Gen-Di XU
1
1
Key Laboratory of Botany, Zhejiang Normal University, Jinhua 321004, P.R. China
2
School of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK
(Received September 13, 2006; Accepted April 16, 2007)
ABSTRACT.
Ecological impact assessments of contaminated soil on plants has been an interesting area
in the last few years as restoration of contaminated environments for better ecological health is addressed.
In this study, the photosynthetic and related morphological and physiological characteristics of two soybean
(Glycine max Merrill.) varieties were evaluated in response to aluminum (Al) stress in soil. The pot-grown
soybean plants were cultured with different supplemental aluminum, and measurements was conducted during
the 5-foliate period. Results indicate that Al at low concentrations in the soil is helpful to growth, and Al is
toxic to plants only when the concentration exceeds a certain threshold. Increased leaf area, root surface area,
specific leaf weight (SLW), and lower malondialdehyde levels were found in soybean plants under a 200
mg/kg Al
3+
treatment. However, higher aluminum concentrations (800 mg/kg) caused declining chlorophyll
contents, depressed photosynthesis rates (P
N
), enhanced transpiration rates, and decreased PAR utilization
efficiency (PUE) and water utilization efficiency. No significant difference in stomatal conductance or leaf
water potential was observed among soybean plants under the various aluminum treatments. Moreover, higher
aluminum concentration significantly increased lipid peroxidation, decreased cell membrane stability, and
changed the activities of superoxide dismutase (SOD) in the leaves of both plants. It is concluded that soybean
plants maintain relatively higher SLW, P
N
, PUE, WUE, SOD activity to cope with high aluminum stress. Our
study produces insights into plant behavior under contamination stress, which may be useful in the selection
and breeding of aluminum-tolerant soybean cultivars for the sustainable development of agriculture and
ecology.
Keywords: Aluminium; Photosynthetic characteristics; Physiological characteristics; Soybean.
Abbreviations: LA, leaf area; RSA, root surface area; SLW, specific leaf weight; P
N
, net photosynthetic
rate; TR, transpiration rate; PAR, Photosynthetically Active Radiation; g
s
, stomata conductance; r
s
, stomata
resistance; Chl, Chlorophyll; PUE, PAR utilization efficiency; WUE, water utilization efficiency; LW P, leave
water potential; MDA, malondialdehyde; SOD, superoxide dismutase.
phySIOlOgy
pg_0002
436
Botanical Studies, Vol. 48, 2007
been made during the last few decades, and competent
compilations and critical reviews on several aspects
of this field have been published, e.g. Clarkson, 1969;
Foy et al., 1978; Foy, 1984; Haug, 1984; Taylor, 1988;
Rengel, 1992; Kochian, 1995; Delhaize and Ryan, 1995;
Horst, 1995; Barcelo et al., 1996; Matsumoto, 2000; Ma,
2000; Ma et al., 2001; Ryan et al., 2001; Barcelo and
Poschenrieder, 2002. Most of the mechanisms studied
are related to limited root growth and development or
their consequences; however, no breakthrough in these
mechanisms has been made. Hence, it is suggested
that more attention should be paid to aerial tissues in
future studies, which are important in revealing Al
toxicity and mechanisms of plant tolerance to Al stress.
The knowledge obtained from aerial plant tissues in
acclimation to aluminum toxicity in previous studies
has been limited. Leaf necrosis as a visible symptom of
Al stress, was found to be accompanied by decreasing
chlorophyll concentrations and photosynthetic rates in
Picea abies (Schlegel and Godbold, 1991), rice (Shi,
2004), and Dimocarpus Longana (Xiao, 2002). Due to
reduction in absorbing surfaces (Stienen, 1986), root-water
permeability (Zhao et al., 1987) and stomata aperture
(Hampp and Schnabl, 1975), Al has generally been found
to decrease transpiration rates. In contrast, Schlegel and
Godbold (1991) observed enhanced transpiration rates
of spruce needles due to Al. Further investigations on
aerial tissues are needed to seek better stress elimination
solutions or strategies for growing soybean plants in Al
contaminated soil.
In this study, we compared photosynthetic and related
morphological and physiological characteristics in two
soybean varieties Huachun No.18 and Zhechun No.3 in
the 5-trifoliate period. We tried to elucidate the responses
of the soybean aerial tissues to the aluminum toxicity in
the soil, reveal causes involved in these processes, and
thus provide a foundation for selection and breeding of
aluminum-tolerant soybean cultivars. This research will
be useful in obtaining insights into physiological behavior
under Al contamination stress, which would aid in the
selection and breeding of aluminum-tolerant soybean
cultivars for sustainable agricultural and ecological
development.
MATERIAlS AND METhODS
plant materials and treatments
This research was conducted in the greenhouses of
Zhejiang Normal University, Jinhua, China. Two soybean
cultivars (Glycine max Merrill.) "Huachun No.18" and
"Zhechun No.3" are widely planted in Zhejiang. The
selected seeds of soybean were disinfected with 0.01%
(w/v) HgCl
2
solution for 5 min and then rinsed five
times with de-ionized water. Subsequently, the seeds
were soaked in de-ionized water for 2 h and germinated
in salvers covered with sterilized gauze at 25
¢X
C. Both
soybean cultivars (Glycine max Merrill.) "Huachun No.18"
and "Zhechun No.3" were grown in pots (height 23 cm
and diameter 18 cm) under natural irradiance in the middle
of May 2004 after the seeds had germinated. Each pot was
filled with 5 kg of soil, the physicochemical characteristics
of which can be given as follows: pH=5.96, water content
3.04%, organic C 8.1 g¡Pkg
-1
, total N 0.205 g¡Pkg
-1
, NO
3
-N
20.2 mg¡Pkg
-1
, NH
4
+
-N 30.5 mg¡Pkg
-1
, total P 0.423g¡Pkg
-1
,
available P 8.15 mg¡Pkg
-1
, available K 98.2 mg¡Pkg
-1
. The
soils were supplied with five Al concentration treatments:
Al
2
(SO
4
)
3
¡P18H
2
O: 0 g¡Pkg soil
-1
(CK), 0.2 g¡Pkg soil
-1
(R1),
0.4 g¡Pkg soil
-1
(R2), 0.6 g¡Pkg soil
-1
(R3), 0.8 g¡Pkg soil
-1
(R4),
respectively. Four repetitions (pots) with five plants in
each pot were performed in each treatment. After a 25-day
treatment, the plants had entered a 5-foliate period, and the
topmost three fully-expanded leaves of each plant were
sampled. Each experiment was carried out at least three
times.
Measurements
Leaf area (LA) was determined from the fully expanded
leaves (abaxial surface) of soybean plants using a leaf area
analysis system (Win/Mac FOCTA STD l200P, Regent,
Canada), and the root surface area (RSA) of soybean
plant roots was determined using a scanner connected to
an image-analysis system (Win/Mac RHIZO STD l600
+
,
Regent, Canada). Specific leaf weight (SLW, leaf mass per
area) was calculated as the ratio of leaf dry weight to LA;
Chlorophyll (Chl) was extracted using ethanol and acetone
according to Zhang et al. (1983). The concentrations of Chl
a and Chl b in extracts were determined from absorbance
at 663 and 645 nm, respectively, with a Leng-Guang 752
spectrophotometer (Leng-Guang, Shanghai, China). At
least three repetitions were used in determination of the
LA, SLW and Chl contents.
Photosynthetic rate (P
N
), transpiration rate (TR),
photosynthetic active radiation (PAR) on leaf surface,
stomatal conductance (g
s
), and stomatal resistance (r
s
)
were monitored using a portable photosynthesis and
transpiration measurement system (LCA-4, ADC, UK)
between 9:00 am and 11:00 am under field conditions.
Measurements were conducted on attached and fully
expanded leaves (abaxial surface) of soybean plants. All
parameters were calculated by the software operating
in LCA-4, and at least ten repetitions were carried out
for determination. Water utilization efficiency (WUE)
was calculated as WUE = P
N
/ Tr; and PAR utilization
efficiency (PAR) was calculated as PUE = P
N
/ PAR.
Leaf water potential was determined from the topmost
three attached and fully-expanded leaves (abaxial surface)
of soybean plants with a PSYPRO water potential
measurement system (PSYPRO, Wescor, America), with at
least ten repetitions for each measurement.
The level of lipid peroxidation in leaf tissue was
measured in terms of malondialdehyde (MDA) content,
which was determined by the thiobarbituric acid reaction
following the procedure of .pundova et al. (2003). The
activity of SOD (superoxide dismutase) was assayed
pg_0003
ZHANG et al. ¡X Effect of Al on photosynthesis and related characteristics of two soybean genotypes
437
according to Wang et al. (1983) in terms of its ability
to inhibit the photochemical reduction of nitro blue
tetrazolium (NBT). One unit of the SOD activity is
defined as the amount of enzyme required to cause a 50%
inhibition of NBT auto-oxidation under assay conditions.
The SOD activity was expressed as a unit per milligram
of protein in the soybean leaf. MDA content and SOD
activity was also determined with at least three repetitions.
Statistical analysis
Analysis of variance (one-way ANOVA) was performed
to test significant variations in response to different
temperatures. In order to evaluate significant differences
between treatments and varieties, the LSD-test (Least
Significant Difference) was used on a significance level of
P<0.05. Data are expressed as the means ¡Ó standard error
of at least eight measurements.
RESUlTS
Effect of Al on lA, RSA and SlW
The effect of aluminium in soil on leaf area, root
surface, and specific leaf weight of the soybean plants
is described in this section. It can be seen from Figure 1
and Figure 2 that leaf area and root surface area increased
and then decreased in soybean plants as Al concentration
in soil rose while the effects on SLW are less obvious.
Results in Figure 1A show that SLW in low concentration
Al
treatments (200 or even 400 mg¡Pkg
-1
Al) was higher than
in control, which indicates that Al at a low concentration
might be helpful to leaf growth. Higher Al
3+
treatments
(600 and 800 mg¡Pkg
-1
Al), however, slightly inhibited
leaf growth, and hence the lowest leaf area was at 800
mg¡Pkg
-1
Al. Similarly, RSA increased as Al
3+
concentration
increased from 0 to 200 mg kg
-1
, and decreased as Al
continued to enhance, especially for Huachun No.18
(Figure 1B). It is suggested that low concentration Al
(200 mg kg
-1
) may increase RSA of the soybean plants,
and higher concentration Al
in soil may decrease LA and
RSA. No clear difference in SLW emerged among the
Al treatments, except for a slight increase under the 200
mg¡Pkg
-1
Al
3+
treatment (Figure 2).
It is also suggested that low concentration Al in soil
can facilitate the synthesis of carbohydrate (increase of
SLW, LA and RSA) and high concentration Al
can affect
plant growth (decrease of LA and RSA). Variations in
plant growth (RSA) also occurs between different soybean
cultivars.
leaf chlorophyll content
Al in soil altered the leaf chlorophyll content of soy-
beans. As the Al concentration ascended, the content of
Chl a and Chl b in the soybean plants declined gradually
(Figure 3). Chl a content in Zhechun No.3 under R1, R2,
R3, R4 decreased 18.6%, 20.6%, 26.2%, 32.2% more
than in CK, respectively, and the decrease was a little
more significant for Huachun No.18 with increasing Al
concentration in soil (p<0.05). Chl b content changed in
the same pattern as Chl a content did under Al treatments
(Figure 3).
photosynthetic characteristics under Al
treatment
Net photosynthetic rates (P
N
) in the two treated culti-
vars were significantly reduced compared to CK (P<0.05)
when Al concentration in soil reached 400 mg¡Pkg
-1
(Figure
4). The P
N
of Huachun No.18 and Zhechun No.3 decreased
noticeably (44.5% and 50.7%, relative to their controls).
Further increases in Al concentration produced no further
Figure 1. Effect of Al on LA (A) and RSA (B) of the soybean plant in the 5-trifoliate period. The subscripts represent the difference
significant at p=0.05; each value is the mean ¡Ó S.E. of three leaves from three pots, based on three determinations for each sample.
Figure 2. Effect of Al on leaf SLW of the soybean plants in the
5-trifoliate period. Different subscript represents the difference
significant at p = 0.05. Each value is the mean ¡Ó S.E. of three
leaves from three pots, based on three determinations for each
sample.
pg_0004
438
Botanical Studies, Vol. 48, 2007
significant reductions. The transpiration rate (TR) in both
plants increased significantly when the Al concentration
ascended to and surpassed 400 mg¡Pkg
-1
(Figure 5). Higher
Al concentration treatments (600 or 800 mg¡Pkg
-1
) seemed
to produce no marked differences in plant response. For
instance, TR of R2, R3, R4 in Huachun No.18 increased
43.7%, 52.6%, 47.7%, respectively, compared with
control.
A slight decrease and a subsequent increase, though not
significant, were observed in the stomatal conductance (g
s
)
of the Al-treated soybean plants (Figure 6). The difference
between the two cultivars was that g
s
in Zhechun No.3
showed a more evident change, i.e., Zhechun No. 3 had
a lower g
s
at 400 mg¡Pkg
-1
than control. In contrast to sto-
matal conductance, stomatal resistance (r
s
) in soybeans
of both varieties was enhanced under the 400 mg¡Pkg
-1
Al
treatment and fell under higher Al treatments (Figure 7).
Figure 4. Net photosynthetic rate under different Al treatments.
Different subscript represents the difference significant at p =
0.05. Each point is the mean ¡Ó S.E. of 3~9 leaves from three
pots, based on ten determinations for each sample.
Figu re 5. Transpiration rate under different Al treatm ents .
Different subscript represents the difference significant at p =
0.05. Each point is the mean ¡Ó S.E. of 3~9 leaves from three
pots, based on ten determinations for each sample.
Figure 6. Stomata conductance under different Al treatments.
Different subscript represents the difference significant at p =
0.05. Each point is the mean ¡Ó S.E. of 3~9 leaves from three
pots, based on ten determinations for each sample.
Figure 7. Stomata resistance under different Al treatments .
Different subscript represents the difference significant at p =
0.05. Each point is the mean ¡Ó S.E. of 3~9 leaves from three
pots, based on ten determinations for each sample.
Figure 3. Effect of Al on chl a and chl b content of soybean
leaf in the 5-trifoliate period. Different subscript represents the
difference significant at p = 0.05. Each value is the mean ¡Ó S.E.
of three leaves from three pots, based on three determinations
for each sample.
pg_0005
ZHANG et al. ¡X Effect of Al on photosynthesis and related characteristics of two soybean genotypes
439
No statistical differences emerged among the r
s
values of
Huachun No.18 under different treatments, but significant
decreases were observed in R3, R4 for Zhechun No.3
(P<0.05).
pUE under Al treatment
PUE represents the PAR utilization efficiency during
photosynthesis, and a high PUE guarantees a high crop
yield. Figure 8 depicts a sharp drop of PUE in aluminium-
treated plants, 45% and 48.8% on average for Huachun
No.18 and Zhechun No.3, respectively. The toxicity of Al
in soil is considered to exert a remarkable influence on a
plant¡¦s PAR utilization, which then affects the whole proc-
ess of photosynthesis.
WUE under Al treatment
WUE is one of the indexes that evaluates the degree of
damage to plants by water utilisation under ionic stresses.
Results show that WUE in both treated cultivars fell as the
Al concentration ascended (Figure 9). As Al concentration
increased from 0 to 400 mg¡Pkg
-1
, WUE plummeted.
However, further increases in Al concentration produced
no significant difference. Compared to Zhechun No.3,
WUE for Huachun No. 18 was much higher in CK and
dropped more obviously as Al stress was applied. WUE for
Huachun No.18 and Zhechun No.3 under other treatments
dropped 60.4% and 56.9% on average, respectively,
relative to their controls. This indicates that Zhechun No.3
is more tolerant than Huachun No.18, and shows that
keeping a stable WUE can relieve the side-effects of soil
aluminum stress.
lWp variations
Soybean plants were able to balance the internal water
potential even with high aluminium concentrations in their
leaves. As the data in Figure 10 make clear, no significant
difference existed in leaf water potential (LWP) among
different treatments (P>0.05). The two cultivars¡¦ response
F igu re 8. PAR utilization efficiency (PUE) under different
Al treatments . Different subs cript represents the difference
significant at p = 0.05. Each point is the mean ¡Ó S .E. of 3~9
leaves from three pots, based on ten determinations for each
sample.
Figure 9. Water utilization efficiency (WUE) under different
Al treatments. Different subscript repres ents the difference
significant at p = 0.05. Each point is the mean ¡Ó S.E. of 3~9
leaves from three pots, based on ten determinations for each
sample.
Figure 10. Leave water potential (LWP) under different Al treatments. Different subscript represents the difference significant at p =
0.05. Each point is the mean ¡Ó S.E. of the first, second, third leaves from three pots, based on ten determinations for each sample.
pg_0006
440
Botanical Studies, Vol. 48, 2007
differed little in the first leaf. There was a slight increase
at 200 mg¡Pkg
-1
before a continuous decrease at higher Al
concentrations in Zhechun No.3 while no increase was
observed in Huachun No.18. This effect may be magnified
in drought areas.
lipid peroxidation and antioxidant enzyme
activities
Figures 11and 12 show the responses of lipid peroxi-
dation and antioxidant enzyme activities to the various
aluminium treatments. Al in soil increased leaf membrane
peroxidation and SOD activities in the Huachun No.18
soybean leaves. As the concentration of Al increased, the
malondialdehyde (MDA) content in Huachun No.18 in-
creased progressively to 64.16% over control. The SOD
activities were also enhanced progressively under the Al
treatments (Figure 12). However, contrasting with the
MDA content trend, the highest SOD activity was at 600
mg¡Pkg
-1
, and some decline was observed at 800 mg¡Pkg
-1
for Zhechun No.18. For Zhechun No.3, the MDA content
declined by 3.74% under the 200 mg¡Pkg
-1
treatment and
then recovered 17.93%, 12.63%, 61.68% under the 400,
600, 800 mg¡Pkg
-1
Al treatments, respectively, compared
with control. Variations of the SOD activities were similar
to those of the MDA content in a larger margin. These re-
sults show that Al at middle or high concentrations causes
oxidative damage while Al at low concentrations helped
Zhechun No.3 soybean plants resist peroxidation.
DISCUSSION
Many studies (Kidd and Proctor, 2000; Yan et al.,
2003; Yang et al., 2005; Liu et al., 2006; Ying et al.,
2006) have reported that the low-level Al does not
affect or even accelerate plant growth. Though Al is a
nonessential element to plants, such studies are useful for
the management of Al contaminated soil and agricultural
development. In this study, the leaf area and specific leaf
weight of both cultivars increased under 200 mg kg
-1
Al
treatment. Neither chlorophyll content, photosynthetic rate,
lipid peroxidation, nor antioxidant enzyme activities were
significantly affected in the low-aluminum treated plants.
These results confirmed that Al at low soil concentrations
is helpful to soybean growth and that it is toxic only
when the concentration exceeds a certain threshold.
Therefore, Al toxicity to plants has a critical value in crop
management. The threshold value varies with the plant
involved, such as 40 mmol kg
-1
for Triticum aestivum and
Brassila campestris var. oleifera (soil cultured), 44 mmol
kg
-1
for Arachis hypogaea (soil cultured), 48 mmol kg
-1
for
Zea mays (Qin and Chin, 1999). This critical value also
varies with different growth phases (Li et al., 2000) and
crop genotypes (Pan et al., 1998). In our soil culture study,
the critical values for Zhechun No.3 and Huachun No.18
were above 200 mg L
-1
. Higher concentrations of Al,
however, affected plant growth, depressed photosynthesis,
enhanced transpiration, and induced lipid peroxidation.
Photosynthesis is probably the most important
metabolic event on earth and is certainly the most
important process to understand in attempting to maximize
crop productivity and minimize the side-effects of soil
contamination. However, it is a physiological process
affected by environmental factors, especially various
stresses, including aluminum toxicity stress. Since
photosynthesis runs a complicated course, the depression
of photosynthesis refers to several factors. Schnable and
Ziegler (1975) found that 1 mM Al
3+
inhibits stomata
opening in the illuminated epidermal strips of Vicia faba,
by preventing K
+
accumulation and starch mobilization
in the guard cells. In our study, however, the stomatal
conductance and stomatal resistance under Al treatments
changed so slightly that it made no statistical difference
when the photosynthetic rate declined. Hence, most atten-
tion was paid to non-stomata factors in this study. First,
the decrease of the chlorophyll content in leaves should
be considered. Several investigations have shown that
Figure 12. SOD activity under different Al treatments. Different
subscript represents the difference significant at p = 0.05. Each
point is the mean ¡Ó S.E. of 3~9 leaves from three pots, based on
three determinations for each sample.
Figure 11. MDA content under different Al treatments. Different
subscript represents the difference significant at p = 0.05. Each
point is the mean ¡Ó S.E. of 3~9 leaves from three pots, based on
three determinations for each sample.
pg_0007
ZHANG et al. ¡X Effect of Al on photosynthesis and related characteristics of two soybean genotypes
441
many tree species respond to Al exposure with various
amount of mineral uptake (Asp et al., 1988; Bengtsson et
al., 1988; Van Praag et al., 1991; Ericsson et al., 1995; Van
Praag et al., 1995; 1997). An Al-induced reduction in Mg
concentrations in beech roots (Bengtsson, 1992) was also
reported. Our work in buckwheat (Fagopyrum esculentum
Moench) also revealed that as the in vitro aluminum
concentration increased, the Al content in both root and
leaf surged, and the Mg contents in both root and leaf
decreased correspondingly (Chen et al., 2006). Therefore,
we speculated that the reduction in both Chl a and Chl b
contents (Figure 3) under Al toxicity stress may be due to
decreased Mg concentrations, which at least partly resulted
in a correspondingly decreased PAR utility efficiency
(Figure 8) and affected the photosynthetic capacity.
Secondly, decrease of water utility efficiency should be
partly responsible for photosynthesis reduction. Water
use efficiency is an important characteristic that provides
information on the potential of a species or variety to adapt
to contamination stress (Poschenrieder and Barcelo, 1999).
The decrease in the photosynthetic activity of soybean
plants and the preservation of high transpiration induced
a reduction in water use efficiency. Besides, no significant
drop was found in the leaf water potential (Figure 10),
from which can be deduced that transpiration competes
with photosynthesis for water under Al toxicity stress, and
the water deficiency subsequently restrains photosynthesis.
The reduction of photosynthesis by aluminum stress is
also related to inactivation of many chloroplast enzymes,
such as ribulose 1, 5-bisphosphate carboxylase/oxygenase
(Rubisco) and fructose 1, 6-bisphosphate aldolase
(FBPase) (Bengtsson et al., 1988), which may be induced
by oxidative stresses. Oxidative stress can cause lipid
peroxidation and consequently membrane injury, protein
degradation, and enzyme inactivation (Meriga et al., 2004).
Therefore, the abilities to maintain cell membrane integrity
and diminish oxidative stress have been proposed as good
indicators of tolerance in plants. Our results suggest that
high Al stress significantly increased membrane lipid
peroxidation in both cultivars, especially in Huachun
No.18 (Figure 11). Along with the occurrence of oxidative
damage during Al stress, these two plants responded by
activating antioxidant enzymes like SOD,
POD or CAT
for detoxification and other antioxidative substances like
free proline. In our study, variations of SOD activities in
Zhechun No.3 and Huachun No.18 under stress conditions
(Figure 12) were observed. In comparison to the control
plants, the SOD activities decreased slightly in Zhechun
No.3 as Al concentration increased from 200 to 400 mg
kg
-1
while Huachun No.18 increased relatively less. This
result, along with responses of the corresponding MDA
level, indicates that the cell membranes in Huachun
No.18 were more susceptible to aluminum stress than
in Zhechun No.3 and that the ROS scavenging ability in
Huachun No.18 was lower than in Zhechun No.3. Under
high concentration Al treatment, MDA content increased
significantly in both cultivars while SOD activities were
only enhanced in Zhechun No.3. This suggests that the
stressed soybean plants also had an effective system for
detoxifying active oxygen species at low and intermediate
concentration Al treatments, but this system may have
collapsed and peroxidation may have gotten out of control
under high concentration Al treatments, e.g. above 800
mg kg
-1
. In addition, Yang et al. (2000, 2003) reported that
photosynthesis plays an important role in the exudation
of citrate exudation, which is an important Al tolerance
mechanism in aluminum-tolerant soybean plant, but the
contribution of shoots combined with light to Al-induced
secretion of citrate still needs confirmation. Further
research is planned.
Many studies have reported on large genotypic
variations in plant growth, physiology, and quality in
response to Al (Tang et al., 2001; Liu et al., 2004). Two
soybean varieties in our experiment exhibited some of
these variations. In addition to the SOD ability mentioned
above, the root surface area, PAR utilization efficiency,
and water utilization efficiency of the Zhechun No.3
soybean plants were somewhat less adversely affected than
those of Huachun No.18 under high Al treatment in soil.
These results suggest that Zhechun No.3 is slightly more
tolerant than Huachun No.18 to aluminum. This agrees
well with our previous studies on the root physiological
characteristics of soybean Huachun No.18 and Zhechun
No.3 under Al toxicity (Liu et al., 2004). In those studies,
decreases in the main length, system volume, dry weight,
root activity, and membrane permeability of root cells
and increases in the proline content, MDA, and soluble
carbohydrate were observed more in soybean Huachun
No.18 than in soybean Zhechun No.3 under increased of
aluminum contamination. Therefore, these indices tested
in our experiment are useful for screening Al-tolerant
soybean cultivars. As a light metal in the soil occurring
naturally or by contamination, Aluminium has influenced
crop production. The use of lime, organic fertilizer and
silicon can relieve the toxicity of the soil, but these are
not permanent solutions. Comparing aluminum-tolerance
characteristics in soybean cultivars like the ones this study
to screen for aluminum-tolerant cultivars, can provide a
foundation for transforming aluminum-tolerant genes to
the soybean cultivars of high quality and productivity thus
resolve the problem of high aluminum stress in acid soil.
This research can ultimately be useful to develop a strat-
egy of sustainable agricultural development and promote
environmental and ecological health.
Acknowledgments. Fundation item: The Natural Science
Foundation of China (30540056) and Zhejiang Province
(303461).
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