The Role of Mycorrhizae Associated with

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O papel das Mycorrizas associadas a Vetiver em solos contaminados por Chumbo e Zinco 

The Role of Mycorrhizae Associated with
Vetiver Grown in Pb-/Zn-Contaminated Soils:
Greenhouse Study
Ching Chi Wong,1,2 Sheng Chun Wu,1,2 Clem Kuek,3 Abdul G. Khan,4 and
Ming Hung Wong1,2,5
Abstract
The effects of mycorrhizae on growth and uptake of N,
P, Zn, and Pb by plants were investigated in a greenhouse
trial using vetiver grass (Vetiveria zizanioides) as host.
Inoculation of the host plants with arbuscular mycorrhizal
fungi (AMF), Glomus mosseae and G. intraradices
spores, significantly increased the growth and P
uptake. Mycorrhizal colonization increased Pb and Zn
uptake by plants under low soil metal concentrations (at
0 and 10 mg/kg of Pb or Zn), whereas under higher concentrations
(at 100 and 1,000 mg/kg of Pb or Zn), it
decreased Pb and Zn uptake. P concentration in soil was
negatively correlated with mycorrhizal colonization
as well as Zn or Pb concentrations. The results showed
that inoculation of the host plants with AMF protects
them from the potential toxicity caused by increased
uptake of Pb and Zn, but the degree of protection varied
according to the fungus and host plant combination. The
potential of arbuscular mycorrhizae in phytoremediation
of the Zn- or the Pb-contaminated soils is discussed in
this article.
Key words: arbuscular mycorrhiza, lead, nitrogen, phosphorus,
vetiver grass, zinc.
Introduction
Phytoremediation technology is emerging as a promising
environmentally friendly method for large-scale cleanup
of contaminated waters and soil (Brooks 1998; Chaudhry
et al. 1998). It is important to select an appropriate pioneer
plant species for successful site reclamation and in
phytoremediation efforts to ensure a self-sustainable vegetative
cover. Recently, Shu et al. (2002) showed successful
establishment and colonization of vetiver grass as pioneering
plant species on Pb/Zn mine spoils in China and concluded
that this plant should be considered as one of the
plants to be used for mine site revegetation. The vetiver
plant (Vetiveria zizanioides L. Nansh), a common wetland
grass species from the tribe Andropogonaeae native to
tropical and subtropical areas, has been cultivated commercially
for many industrial applications and for the production
of the medicinally valued volatile essential oil that
can be distilled from its roots (Maffei 2002). Once established,
it is not affected by droughts or floods. It is also
highly tolerant to frost, heat, extreme soil pH, sodicity,
salinity, and alkalinity as well as to a range of potential
toxic elements such as As, Cd, Cu, Cr, Pb, Se, Zn, Ni, Al,
and Mn in the soil (Truong & Claridge 1996). Due to its
unique physiological and morphological characteristics
such as higher biomass; fast growth; higher metal tolerance,
uptake, and accumulation; and strong ecological
adaptability, the vetiver plant can play an important role
in all subsets of phytoremediation of heavy metal–contaminated
soils and water, that is, phytostabilization, phytoextraction,
and phytofiltration of heavy metal contaminants
(Mucciarelli et al. 1998; Khan et al. 2000; Lavania & Lavania
2000; Shu et al. 2002). Another benefit of using vetiver
grass for phytoremediation is its foliage, which can be
used as a mulch to improve soil physical properties.
Many plants growing on metal-contaminated soils possess
mycorrhizae (Chaudhry et al. 1998), indicating that
these fungi have evolved a tolerance to heavy metals and
that they play an important role in the phytoremediation of
contaminated soils (Khan et al. 2000; Khan 2001). Therefore,
metal-tolerant mycorrhizal inoculants are promising
for the phytoremediation of metal-contaminated soils.
Arbuscular mycorrhizal fungi (AMF) are known to
improve plant growth on nutrient-poor soils and enhance
their uptake of P, Cu, Ni, Pb, and Zn (Khan et al. 2000;
Zhu et al. 2001). Despite the importance of the role that
AMF play in plant interactions with the soil environment
in general, and heavy metals in particular, relatively few
studies have focused on their effect on metal-remediation
efforts. Previous phytoremediation studies have focused
on the predominantly nonmycorrhizal plant families, e.g.,
Brassicaceae or Caryophyllaceae, and arbuscular mycorrhizae
(AM) have not been considered as important
component of phytoremediation practices (Pawlowska et al.
1 Croucher Institute for Environmental Sciences, Hong Kong Baptist University,
Kowloon Tong, Hong Kong, China.
2 Department of Biology, Hong Kong Baptist University, Kowloon Tong,
Hong Kong, China.
3 Meumax Consultants, Sydney, Australia.
4 Department of Microbiology, Faculty of Biological Sciences, Quaid-e-Azam
University, Islamabad, Pakistan.
5 Address correspondence to M. H. Wong, email mhwong@hkbu.edu.hk

2007 Society for Ecological Restoration International
60 Restoration Ecology Vol. 15, No. 1, pp. 60–67 MARCH 2007
2000). It is possible to improve the phytoremediation
capabilities of plants by inoculating them with appropriate
AMF. The term ‘‘mycorrhizo-remediation’’ (Jamal et al.
2002) means the use of mycorrhizal plants in the phytoremediation
of heavy metal–contaminated soils. The
interaction between AM and minerals other than P, particularly
heavy metals, has been the subject of a number
of recent studies because of the possibility of the beneficial
effect of mycorrhizae in improving the tolerance of
plants against toxicity (Karagiannidis & Nikolaou 2000;
Khan et al. 2000; Hayes et al. 2003). The uptake of heavy
metals by mycorrhizal plants depends on several factors
such as physicochemical properties of the soil, particularly
its fertility level and pH; the host plants and the fungi
involved; and, above all, the concentration of the metals
in the soil (Smith & Read 1997).
Although vetiver grass is regarded to be a suitable
pioneering candidate for phytoremediation of heavy
metal–contaminated sites and for rehabilitation of abandoned
metalliferous mine wastelands due to its high
growth rate and tolerance to various unfavorable soil conditions
(Truong & Baker 1998; Truong 1999, 2002; Shu
et al. 2002), no record of its mycorrhizal status exists in
literature. Vietmeyer (2002) suggested that areas that still
require investigations include the symbiosis of vetiver
grass roots with mycorrhizal fungi and N-fixing bacteria.
As far as we are aware, the first record of occurrence of
AM in vetiver grass was reported in plants growing in the
South China Botanical Gardens in Guangzhou, PRC, in
soil containing moderate amounts of basic nutrients and
trace elements (Wong 2003).
The objectives of this greenhouse investigation were to
(1) confirm the presence of AM associations in Vetiveria
zizanioides; (2) determine the extent of its dependency on
AMF when grown on heavy metal–contaminated soil; (3)
study the effect of soil Pb and Zn concentrations on the
growth and mycorrhization of plants inoculated with
Glomus mosseae or G. intraradices; and (4) evaluate the
influence of AM infection on the uptake of Pb and Zn by
mycorrhizal plants.
Methods
Soil Preparation
The garden soil (loamy sand) used in this study was
obtained from a commercial company in Hong Kong. The
soil was sieved (4 mm) and steam sterilized (100C for
1 hour for 3 consecutive days) to eliminate naturally
occurring AMF propagules. The different metal contents
at the rates of 10, 100, and 1,000 mg Zn or Pb/kg soil were
obtained by adding appropriate amounts of aqueous solution
of zinc sulphate or lead nitrate, respectively. The soil
without metal addition was maintained as a control. The
metal solutions were sterilized by passing through a
0.45-lm filter paper (GN-6 Metricel Grid 47 mm; Gelman
Laboratory, Pall Corporation, East Hills, NY, U.S.A.) to
get rid of unwanted microbes. After mixing the soil with the
added chemical solution, soil moisture was adjusted to a field
capacity (approximately 70% of water-holding capacity) by
adding deionized water. The soils were then stored in a plastic
box at 20 ± 4 for 15 days with frequent mixing (once every
3 days) to allow thorough equilibration (Diaz et al. 1996).
Plant and AMF Inoculum
Uniform-sized young slips of Vetiveria zizanioides collected
from South China Botanical Gardens in Guangzhou,
China, were used in this study. The inocula of the two different
AMF species, Glomus mosseae and G. intraradices,
were purchased from Biorize Sarl Co., Dijon, France. They
were sand-based mycorrhizal inocula containing abundant
chopped mycorrhizal root pieces, spores, and hyphae (Wu
et al. 2005). About 70 g of the inoculum was added to each
pot containing 2 kg of soil 3-cm deep and mixed with adjacent
soil. The experiment consisted of mycorrhizal and
nonmycorrhizal treatments for all four different levels of
Zn/Pb concentrations. There were four replicates for each
treatment. The pots were organized in a greenhouse (14-
hour, light intensity 250 lmol m22 second21 photon flux
density day; 10-hour, dark photoperiod; and temperature
range of 25–30C) under a randomized block. After a
growth period of 4 months, plants were harvested.
Chemical Analyses
Soil and Plant. Soil samples were air-dried for 7 days and
grounded and sieved through a 200-lm mesh. The following
soil properties were tested: pH (pH meter; soil:
distilled water ¼ 1:2), electrical conductivity (EC) (conductivity
meter), total N and P (Kjeldahal method), watersoluble
P (molybdenum blue method), total Zn and Pb
(microwave digestion—digested with concentated hydrochloric
acid, concentrated nitric acid, and hydrofluoric
acid at the ratio of 3:9:2), and DTPA-extractable
(extracted with 1.9 g diethlenetriamine-pentaacetic acid
and 14.9 g triethanolamine in 1 L deionized water, pH 7.3)
Zn and Pb (tested by inductively coupled plasma–atomic
emission spectroscopy [ICP-AES]). Analytical procedures
were based on the methods described by Sparks et al.
(1996). A standard reference material, Montana soil (SRM
2711) from U.S. Department of Commerce National Bureau
of Standard, was used to verify the accuracy of metal determination,
and the recovery rates were within 90 ± 10%.
All plant samples were dried at 105C for 24 hours. Dry
weights separated into root and shoot portions were
recorded. About 100 1-cm long segments of fine lateral
root were removed from each harvested plant before
drying and were stored in 50% glycerol for mycorrhizal
infection assessment. The dried plant material was analyzed
for the following parameters: total N and P (Kjeldahal
method) and total Zn and Pb (perchloric acid
digestion—digested with concentrated nitric acid and
perchloric acid at the ratio of 4:1), which were determined
Mycorrhizae and Phytoremediation of Contaminated Soil
MARCH 2007 Restoration Ecology 61
by ICP-AES. All the procedures were based on the methods
described by Sparks et al. (1996). A standard
reference material, Tomato leaves (SRM 1573a) from
U.S. Department of Commerce National Bureau of Standard,
was used to verify the accuracy of metal determination,
and the recovery rates were within 90 ± 10%.
Mycorrhizae. The preserved root segments from each
treatment were washed with deionized water to remove all
particles adhering to the root surface, cleared for 20 minutes
at 100C in 10% KOH, and stained with lactophenol cotton
blue (Phillip & Hayman 1970). The stained root segments
were mounted on glass slides (five pieces of root per slide)
for examination under a compound microscope (3100–
3400) with an eyepiece equipped with a crosshair that could
be moved to randomly select positions. Mycorrhizal colonization
was estimated for each sample by examining the hundred
1-cm stained pieces of the roots (Brundrett et al. 1996).
Mycorrhizal Dependency. For each plant species used in
this study that formed association with mycorrhizae,
a mycorrhizal dependency value, ‘‘relative mycorrhizal
dependency,’’ was calculated. This referred to the difference
between dry shoot mass of mycorrhizal plants and
nonmycorrhizal plants, expressed as a percentage of dry
mass of the mycorrhizal plant (Plenchette et al. 1983).
Statistical Analyses
Data analyses were preformed using SPSS statistical program
8.0 version (SPSS, Inc., Chicago, IL, U.S.A.). Analysis
of variance was used to test whether treatment effects
existed, followed by Duncan’s multiple range test to identify
means which differed significantly (at the 5% level) in
mycorrhizal treatments and nonmycorrhizal control at
each metal concentration (Little & Hills 1978). The correlation
coefficient between mycorrhizal dependency and
metal concentration was also analyzed.
Results
Soil Properties
The chemical properties of the soil samples before mixing
with the different concentrations of Pb and Zn are listed
in Table 1. In general, the soil contained moderate
amounts of basic nutrients (N and P) and lower concentrations
of total and DTPA-extractable Pb and Zn. There
was no significant difference (p < 0.05) for soils added with
different concentrations of both Pb and Zn in terms of pH
and EC values between soils inoculated with the two
mycorrhizal inocula and the control. Total N concentrations
were lower (p < 0.05) in soils inoculated with Glomus
mosseae for both metal amendments. There were no
Table 1. Physicochemical properties of the artificially metal-contaminated soil after treatment with different mycorrhizal inoculations.
Metal Added (mg/kg) Mycorrhizal Fungus pH EC (ls/cm) Total N (g/kg) Total P (mg/kg) Available P (mg/kg)
Original soil 7.3 0.84 1.79 358 7.60
Pb (mg/kg)
0 Control 7.1a 0.79a 1.97a 459c 7.34b
Glomus mosseae 7.0a 0.80a 1.58b 654b 8.56a
G. intraradices 7.2a 0.85a 2.04a 753a 9.01a
10 Control 7.1a 0.82a 2.01a 393c 6.15c
G. mosseae 7.1a 0.85a 1.61b 681b 8.17b
G. intraradices 7.2a 0.87a 1.90a 703a 8.98a
100 Control 7.2a 0.83a 1.95a 315a 6.06b
G. mosseae 7.4a 0.91a 1.56b 329a 6.29b
G. intraradices 7.3a 0.87a 1.88a 414a 7.05a
1,000 Control 7.3a 0.85a 1.88a 299a 5.22b
G. mosseae 7.5a 0.91a 1.56b 218a 5.35b
G. intraradices 7.4a 0.90a 2.02a 308a 6.17a
Zn (mg/kg)
0 Control 7.2a 0.85a 1.97a 387a 7.21a
G. mosseae 7.2a 0.79a 1.67b 435a 7.34a
G. intraradices 7.2a 0.89a 2.01a 454a 7.87a
10 Control 7.4a 0.86a 2.02a 351b 6.96c
G. mosseae 7.2a 0.84a 1.67b 509a 8.85a
G. intraradices 7.3a 0.86a 1.78b 410b 7.64b
100 Control 7.3a 0.83a 1.85a 291c 6.05c
G. mosseae 7.4a 0.90a 1.59b 492a 7.77a
G. intraradices 7.3a 0.91a 1.81a 387b 6.83b
1,000 Control 7.3a 0.88a 1.90a 245b 5.88b
G. mosseae 7.4a 0.90a 1.57b 417a 6.95a
G. intraradices 7.3a 0.89a 1.85a 294b 6.10b
The different letters in the same column of a certain metal concentration indicate a significant difference between different mycorrhizal inoculation treatments at
p < 0.05 level according to Duncan’s multiple range test.
Mycorrhizae and Phytoremediation of Contaminated Soil
62 Restoration Ecology MARCH 2007
significant differences in terms of total K concentrations
in all soils with Zn amendments (ranging from 0.48 to
0.53%), whereas slightly lower (p < 0.05) concentrations
were observed in soils inoculated with G. mosseae in Pb
amendments. Total as well as water-soluble P decreased
according to the increase of metal additions in all three
sets of soils with both Pb and Zn amendments. For soils
with Pb amendments (Table 2), total as well as DTPAextractable
Pb increased according to the increase in Pb
additions. No significant difference was found in total
and DTPA-extractable Zn in soils with different Pb
amendments.
For soils with Zn amendments (Table 1), a similar trend
as for soils with Pb amendments was observed in terms of
total and water-soluble P. In general, both concentrations
decreased as Zn additions increased (except for soils inoculated
with G. mosseae). The total as well as DTPAextractable
Zn increased according to the increase of Zn
additions in all three sets of soils. The Pb concentrations
did not alter on Zn additions with or without inoculation
of either mycorrhizal fungus.
Mycorrhizal Colonization of the Roots
Figures 1 and 2 show the mycorrhizal colonization of vetiver
under different Pb and Zn concentrations, respectively.
For vetiver growing in different Pb additions, on
inoculation with G. mosseae, the infection increased from
29.8 to 61.9% at 100 mg/kg Pb but dropped to 28.7% at
1,000 mg/kg Pb. On inoculation with G. intraradices, the
infection increased from 33.8 to 71.5% at 100 mg/kg Pb
addition but dropped to 45.5% at 1,000 mg/kg Pb (Fig. 1).
For vetiver growing in soils with different concentrations
of Zn, on inoculation with G. mosseae, mycorrhizal infection
increased in low Zn additions from 0 to 10 mg/kg,
reaching a maximum of 35.0% but dropped to 17.6% at
1,000 mg/kg Zn. When inoculated with G. intraradices,
a similar trend was obtained, in which root infection ratio
increased from 21.3 to 33.8% but dropped to 24.9% at
1,000 mg/kg Zn (Fig. 2).
Plant Analyses
Dry Weights. The shoot dry weights of vetiver are shown
in Figure 3. In general, shoot weights decreased as metal
concentrations increased. The shoot dry weights were significantly
lower (p < 0.05) in control than in mycorrhizal
plants at high Pb (100 and 1,000 mg/kg) and Zn (100 and
1,000 mg/kg) concentrations.
Mycorrhizal Dependency. In general, the dependency of
vetiver indicated by shoot dry weight on both AMF
increased as metal concentrations increased (p < 0.05)
(Fig. 3). Data analyses showed that there was a positive
Table 2. DTPA-extractable fraction in soil amended with Pb or Zn.
Metal Added
(mg/kg) Mycorrhizal Fungus Pb (mg/kg) Zn (mg/kg)
0 Control 40.3a 69.2a
Glomus mosseae 43.2a 69.9a
G. intraradices 50.1a 70.5a
10 Control 45.2a 76.1a
G. mosseae 50.3a 85.8a
G. intraradices 49.8a 77.8a
100 Control 88.2a 154a
G. mosseae 103a 97.5b
G. intraradices 99.2a 110a
1,000 Control 495b 498a
G. mosseae 608a 505a
G. intraradices 597a 551a
The different letters in the same column of a certain metal concentration
indicate a significant difference between different mycorrhizal inoculation
treatments at p < 0.05 level according to Duncan’s multiple range test.
Figure 1. Root infection ratio of Vetiveria zizanioides by different
mycorrhizal fungi under different Pb concentrations (X ± SD, n ¼ 4).
Different letters above the bars indicate a significant difference at
p < 0.05 level between different mycorrhizal inoculations under
same metal stress according to Duncan’s multiple range test.
Figure 2. Root infection ratio of Vetiveria zizanioides by different
mycorrhizal fungi under different Zn concentrations (X ± SD, n ¼ 4).
Different letters above the bars indicate a significant difference at
p < 0.05 level between different mycorrhizal inoculations under
same metal stress according to Duncan’s multiple range test.
Mycorrhizae and Phytoremediation of Contaminated Soil
MARCH 2007 Restoration Ecology 63
and linear correlation between mycorrhizal dependency
and metal concentrations, with r ¼ 0.8991 for Pb and r ¼ 0.9174 for Zn (both with p < 0.001) when inoculated
with G. mosseae and r ¼ 0.7469 for Pb and r ¼ 0.7558
for Zn (both with p < 0.05) when inoculated with
G. intraradices.
Metal Concentrations in Plant Tissues. Concentrations of
Pb and Zn in both root and shoot tissues of vetiver plants
mycorrhizal with G. mosseae were lower (p < 0.05) than in
the nonmycorrhizal controls (Table 3), except that under
low soil Zn (0 and 100 mg/kg) concentrations, higher
shoot Zn concentrations were also observed in mycorrhizal
plants.
When mycorrhizal with G. intraradices, root and shoot
Pb contents were significantly lower (p < 0.05) compared
with the nonmycorrhizal controls (Table 3). At low levels
of soil Zn, root Zn concentrations of mycorrhizal and the
control were more or less the same. At higher levels of soil
Zn (100 and 1,000 mg/kg), contents of Zn in mycorrhizal
roots were significantly lower (p < 0.05) than in the control.
In the case of shoots, both Pb and Zn concentrations
in mycorrhizal plants were lower (p < 0.05) than in nonmycorrhizal
plants at all levels of metal additions to soil.
Nutrient Contents (N and P). There was no significant difference
regarding N concentrations among different treatments
(data not shown). Figures 4 and 5 show the P
concentrations in the shoot of vetiver under different
metal treatments with or without AM inoculation. Additions
of both metals resulted in substantial decreases of
P concentrations in vetiver in all treatments, especially
in nonmycorrhizal plants (the control). However, significantly
higher (p < 0.05) P concentrations were noted in
mycorrhizal plants added with Pb and Zn compared with
the control.
Discussion
Vegetative slips of vetiver grass were able to produce
young plantlets in Pb-/Zn-amended soils in the greenhouse.
These observations are consistent with those of
previous workers (Truong & Baker 1998; Zheng et al.
1998) who reported that vetiver is able to tolerate a variety
of pollutants in soils and water.
Table 3. Metal (Pb, Zn) distribution in roots and shoots of Vetiveria zizanioides when treated with different mycorrhizal inoculations (mg/kg).
Metal Concentration (mg/kg) Mycorrhizal Fungus
Pb Zn
Root Shoot Root Shoot
0 Control 19.4 ± 1.82b 6.98 ± 1.22a 107 ± 44.5ab 64.7 ± 12.2a
Glomus mosseae 23.8 ± 4.07b 4.84 ± 0.16b 76.6 ± 29.b 69.2 ± 23.1a
G. intraradices 55.1 ± 7.82a 1.21 ± 0.29c 178 ± 57.6a 48.5 ± 0.87b
10 Control 48.8 ± 4.87b 7.53 ± 1.08a 110 ± 32.4a 27.9 ± 6.05b
G. mosseae 29.5 ± 15.5b 5.84 ± 0.33a 91.4 ± 5.98a 39.2 ± 12.9b
G. intraradices 78.8 ± 8.46a 2.73 ± 0.71b 128 ± 36.9a 77.8 ± 24.1a
100 Control 66.3 ± 20.8a 11.1 ± 0.88a 256 ± 24.3a 50.3 ± 8.15b
G. mosseae 56.2 ± 20.8a 5.35 ± 0.32b 113 ± 24.3b 48.5 ± 8.15b
G. intraradices 79.4 ± 8.14a 9.10 ± 3.48a 270 ± 14.1a 80.3 ± 10.9a
1,000 Control 449 ± 4.93a 17.4 ± 0.89b 477 ± 0.73a 154 ± 11.3a
G. mosseae 117 ± 53.0c 12.9 ± 0.71c 310 ± 35.4b 72.2 ± 23.7b
G. intraradices 213 ± 8.24b 29.4 ± 1.10a 495 ± 62.4a 85.5 ± 8.73b
The different letters in the same column of a certain metal concentration indicate a significant difference between different mycorrhizal inoculation treatments at
p < 0.05 level according to Duncan’s multiple range test.
Figure 3. Effects of mycorrhizal inoculation on shoot biomass of Vetiveria
zizanioides under different metal concentrations. Different letters
above the bars indicate a significant difference at p < 0.05 level
between different mycorrhizal inoculations under same metal stress
according to Duncan’s multiple range test.
Mycorrhizae and Phytoremediation of Contaminated Soil
64 Restoration Ecology MARCH 2007
The occurrence of AM endophyte was confirmed in vetiver
roots growing in heavy metal–contaminated soils as
first reported by Wong (2003). Vetiver grass is yet another
plant that has been found to be mycorrhizal with AMF in
heavy metal–contaminated soils (see Chaudhry et al. 1998
for references). Vetiveria zizanioides showed dependency
on mycorrhizae when soil is contaminated with Pb or Zn.
One of the questions posed in this study is whether
mycorrhizae of different AMF would behave differently
in the presence of heavy metals in soil. The sensitivity of
AM endophytes to high amounts of heavy metals,
expressed as a reduction or delay in its colonizing ability,
has been observed (Karagiannidis & Nikolaou 2000). This
does not seem to be the case for the fungi used in this
study because their ability to colonize increased when soil
Pb and Zn concentrations were increased.
It seems that the soil P concentration and its availability
are affected by metal concentrations. This in turn affects
P uptake by plants as reflected by shoot P concentration.
The present results showed that available P decreased
according to the increase in soil metal concentrations,
which could be attributed to the possible P precipitation
with added metals (Ma et al. 1997). When comparing the
control without mycorrhizal inoculation, two mycorrhizal
endophytes, Glomus mosseae and G. intraradices, both
stimulated P uptake by vetiver. However, the former had
a better performance under Pb stress, whereas the latter
under Zn stress. This implies that G. mosseae could
endure higher Pb but more sensitive to Zn; on the contrary,
G. intraradices seems to be more sensitive to Pb but
tolerant to Zn. When mycorrhizal with either AMF, shoot
P concentrations were significantly higher than in nonmycorrhizal
controls. It is generally believed that high metal
concentrations would lower the availability of P to host
plant (McGnigle et al. 1999; Clark & Zeto 2000; Tang
et al. 2001), thus favoring the colonization of the roots.
The present study indicated that Pb and Zn concentrations
in shoots of vetiver can be modulated by mycorrhizae
when growing in soil contaminated with these metals.
Mycorrhizae of vetiver appear to be protective by way of
reducing the amount of the heavy metals accumulating in
the plant. It is also apparent that G. mosseae is different
from G. intraradices in modulating movement of these
heavy metals into the plant, with the former being better
at excluding both Pb and Zn. These observations are in
agreement with those of Joner and Leyval (2001), who
concluded that mycorrhizae tend most often to lower
heavy metal concentrations in the shoots of nonhyperaccumulator
plants, and also with those of Diaz et al. (1996),
who showed that metal uptake by mycorrhizal plants
increases in soils with low metal concentrations but
decreases in soils with high metal levels. Although the
mechanisms of protection against heavy metals provided
by mycorrhizae to their host plants are not clear, a possible
retention of heavy metals by the fungal mycelium involving
adsorption to cell wall and fixation by polyphosphate
granules (Galli et al. 1994) could occur. The abundance of
external mycelium produced by the AMF can be important
for heavy metal–fixing ability of the fungi and consequently
for their plant–protecting action. Thus, differences
in the extrametrical development of the two fungal
species, translocation of mineral elements, or their symbiosis
efficiency under stress conditions could explain the
situations observed in the present study.
Conclusions
It is concluded that inoculation with AMF protects host
plant from the potential toxicity caused by Pb and Zn, but
the degree of protection varies according to the fungus
and host plant combination. It appears that the choice
of AMF is a factor in using mycorrhiza in rehabilitation
of heavy metal–contaminated sites. Little evidence was
Figure 4. Effects of mycorrhizal inoculation on the P assimilation
by Vetiveria zizanioides under different Pb concentrations (n ¼ 4).
Different letters above the bars indicate a significant difference at
p < 0.05 level between different mycorrhizal inoculations under
same metal stress according to Duncan’s multiple range test.
Figure 5. Effects of mycorrhizal inoculation on the P assimilation
by Vetiveria zizanioides under different Zn concentrations (n ¼ 4).
Different letters above the bars indicate a significant difference at
p < 0.05 level between different mycorrhizal inoculations under
same metal stress according to Duncan’s multiple range test.
Mycorrhizae and Phytoremediation of Contaminated Soil
MARCH 2007 Restoration Ecology 65
found for mycorrhizal vetiver in terms of phytoremediation,
that is, mycorrhiza on vetiver does not increase accumulation
of either Pb or Zn. However, it appears that
mycorrhiza will be important nevertheless because it
appears to reduce the accumulation of Pb and Zn in
mycorrhizal plants, thus offering a protective effect.
Implications for Practice
d Mycorrhizal inoculation can improve the growth
of host plant due to a better nutrient supply and effective
alleviation of metal toxicity.
d Thus, it would be a promising approach for revegetation
of phytostabilization. However, the role of
mycorrhizae in phytoextraction cases remains uncertain,
as mycorrhizae associated with vetiver did not
increase the accumulation of either Pb or Zn in the
aboveground plant tissues.
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