Research
Mycorrhiza and root hairs in barley enhance acquisition of
phosphorus and uranium from phosphate rock but
mycorrhiza decreases root to shoot uranium transfer
Blackwell Publishing, Ltd.
Baodong Chen1,2, Per Roos3, Ole K. Borggaard4, Yong-Guan Zhu1 and Iver Jakobsen2
1
Department of Soil Environmental Science, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China;
2
Plant Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark; 3Radiation Research Department, Risø National Laboratory,
DK-4000 Roskilde, Denmark; 4Department of Natural Sciences, The Royal Veterinary & Agricultural University, DK-1871 Frederiksberg C, Denmark.
Summary
Author for correspondence:
Iver Jakobsen
Tel: +45 46774154
Fax: +45 46774202
Email: [email protected]
Received: 2 June 2004
Accepted: 23 August 2004
• Some phosphate rocks (PR) contain high concentrations of uranium (U), which
are potentially toxic via accumulation in soils and food chains, and plant uptake of
U is likely to be influenced by characteristics of roots and associated microorganisms.
The relative importance of root hairs and mycorrhiza in U uptake from PR was
studied using a root hairless barley (Hordeum vulgare) mutant (Brb) and its wild
type (WT).
• Both plant genotypes were grown in pots with Glomus intraradices BEG 87, or
in the absence of mycorrhiza, and three P treatments were included: nil P, 2% (w/w)
PR and 50 mg KH2PO4-P kg−1 soil.
• Mycorrhiza markedly increased d. wts and P contents of Brb amended with nil P
or PR, but generally depressed d. wts of WT plants, irrespective of P amendments.
Mycorrhiza had contrasting effects on U contents in roots and shoots, in particular in
Brb where mycorrhiza increased root U concentrations but decreased U translocation
from roots to shoots.
• The experiment supports our understanding of arbuscular mycorrhiza as being
multifunctional by not only improving the utilization of PR by the host plant but also
by contributing to the phytostabilization of uranium.
Key words: Glomus intraradices, Hordeum vulgare (barley), phosphate rock,
phosphorus, root hairless mutant, uranium (U) uptake.
New Phytologist (2005) 165: 591–598
© New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01244.x
Introduction
Direct application of phosphate rock (PR) can be an attractive
alternative to commercial P fertilizers, especially in developing
countries, where the poor farmers cannot afford to buy expensive,
processed fertilizers (Semoka et al., 1992; Rajan et al., 1996).
Depending on the origin, PRs contain various amounts of
potentially toxic elements such as cadmium (Cd) and uranium
(U), which accumulates in arable soil after continuous application
of Cd- and U-containing P fertilizers (Williams & David,
1976; Andersson & Siman, 1991; Van Kauwenbergh, 1997).
The PR deposit at Minjingu, north-east Tanzania has a proven
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high potential for direct application as P fertilizer with plant
growth responses similar to those of superphosphates on acidic
soils low in available Ca and P (Semoka et al., 1992, Msolla
et al. unpublished, data). However, the relatively high Ucontent of Minjingu PR (Van Kauwenbergh, 1997) may pose
threats to the health of mine workers and people in the
mine surroundings that are exposed to the radioactive PR dust
(Banzi et al., 2000). Moreover, the use of this PR as fertilizers
could possibly endanger animal and human health by
accumulation of U through food chains. Obviously effective
measures should be taken to control the translocation of U in
the cropping system and to manage the contaminated soils.
591
592 Research
The role of arbuscular mycorrhiza (AM) in plant uptake of
heavy metals has been extensively studied (Leyval et al., 1997).
Root colonization by AM fungi can alleviate deficiency
of micronutrients, such as copper (Li et al., 1991) and zinc
(George et al., 1994; Chen et al., 2003) while in the case
of metal contaminations, mycorrhiza reduced metal concentrations in shoots and increased plant growth (Leyval &
Joner, 2001). The proposed underlying mechanisms include
enhanced adsorption and immobilization of metals both in
roots and mycorrhizosphere, which could be caused by direct
binding of metals by fungal mycelium ( Joner et al., 2000)
and possible pH changes (Li & Christie, 2001). Mycorrhizal
fungi have also been observed to enhance acquisition of 137Cs
(Entry et al., 1999; Berreck & Haselwandter, 2001; Declerck
et al., 2003), and 90Sr (Entry et al., 1999). Therefore, AM
fungi could possibly play a role in the adaptation of their
host plants to radionuclide pollutions. It was recently shown
that the extraradical AM fungal mycelium took up and translocated U towards root cultures in vitro (Rufyikiri et al., 2002,
2003), but U transfer from root to shoot could not be quantified by this model system. The influence of AM fungi on U
uptake under natural conditions is still unknown although
such information is required to determine the importance
of mycorrhizal technology for remediation of uraniumcontaminated environments. In terms of nutrient transport from
soil to root, AM fungal hyphae and root hairs are functionally
equivalent (I. Jakobsen, unpublished data). Plant species with
fine and dense root system or plant genotypes with long root
hairs would depend less on mycorrhizal fungi for acquisition
of P from soil (Baylis, 1975; Baon et al., 1994; Schweiger
et al., 1995). However, it is still unknown whether such functional similarity is also valid for U transport from soil to root.
The objective of the present work was to study effects of
the AM fungus Glomus intraradices on uptake of U from a Ucontaining PR. The study was carried out with a root hairless
barley mutant and its wild type and we tested the hypothesis
that plant U uptake would be increased both by the presence
of root hairs and by the G. intraradices hyphae. Our study
contributes to understanding the potential role of AM fungi
in reducing environmental risks and in phytostabilization of
U- contaminated environments.
Materials and Methods
Host plants and AM fungus
Seeds of wild type (WT) and a rot hairless mutant (bald root
barley, Brb) of Hordeum vulgare L. cv. Pallas (Gahoonia et al.,
2001) were pregerminated on moist filter paper for c. 24 h
and were selected for uniformity before sowing. Both plant
types were grown without or with inoculum of the AM
fungus Glomus intraradices Schenck & Smith (BEG 87).
The fungus was propagated in pot culture on medic plants
(Medicago trunctula L.) grown in a soil-sand mixture for
New Phytologist (2005) 165: 591–598
10 wks. Inoculum from pot culture was a mixture of spores,
mycelium, sandy soil and root fragments.
Growth medium
The growth medium was a 1 : 1 (w/w) mixture of sand and
soil, which was partially sterilized (10 kGy, 10 MeV electron
beam). Basal nutrients without P (Pearson & Jakobsen, 1993)
were added to the mixture. The soil-sand mixture (subsequently
referred to as soil) had a pH of 6.70 (1 : 2.5 soil to water) and
final extractable P content of 5.9 mg kg−1 (Olsen et al., 1954).
Experimental procedure
The experiment had three P treatments: nil (P0), 2% (w/w)
PR and 50 mg KH2PO4-P kg−1soil (P50). The P0 and P50
treatments served as control treatments for U uptake from
the U-rich phosphate rock and as treatments to evaluate the
relative contribution of mycorrhizal fungi and root hairs to
plant P uptake from different P sources. The phosphate rock
obtained from Minjingu, Tanzania, had a particle size < 250 µm,
and contained 30.3% of P2O5 and 310 mg kg−1 of U (4367
Bq kg−1). The P sources were carefully mixed into the soil
together with either 70 g of the G. intraradices inoculum or
the equivalent amount of the soil-sand mixture, and the amended
growth media were filled into black ConeTainers® at 650 g
per pot. Each of the six combinations of P source and inoculation
were sown with three barley seeds of either the WT or the
mutant. The resulting 12 treatments had three replicates
and the 36 pots were set up in a randomized block design.
Seedlings were thinned, 3 d after emergence, to two per pot.
The experiment was conducted in a controlled environment
chamber with 16 h−21°C day and 8 h−16°C night. Osram
daylight lamps provided 550 µE m−2 s−1 PAR (400–700 nm).
The plants grew for 4 wks from 9 March to 6 April 2003. Deionized water was added as required to maintain moisture
content at 55% of water holding capacity by regular weighing.
A solution of NH4NO3 was added to the pots 14, 21 and 24
d after sowing to provide a total of 150 mg N per pot.
Harvest and chemical analysis
Shoots and roots were harvested separately. Soil samples
(c. 20 g f. wt) were collected for determination of hyphal length
density and roots were washed. All plant samples were
carefully washed with deionised water to remove adhering
soil particles. Sub-samples of fresh roots were collected for the
determination of AM colonization. D. wts of shoots and
roots were determined after oven drying at 70°C for 48 h.
Oven-dried subsamples were milled and digested in mixture
of HNO3 and HClO4 (4 : 1 v/v) at 225°C. The dissolved
samples were analysed for uranium (238U) on a HR-ICP-MS
(PlasmaTrace 2, Micromass) without any radiochemical
treatment. The uranium isotope 233U was used as an internal
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Research
standard. Phosphorus in plant samples was determined by
ICP-OES (Vista axial, Varian).
Sub-samples of fresh roots were cleared in 10% KOH and
stained with Trypan blue by a modification procedure of
Phillips & Hayman (1970), omitting phenol from solutions
and HCl from the rinse. Percentage root colonization and root
length were determined by the grid line intersect method
(Giovannetti & Mosse, 1980). External hyphae were extracted
from soil samples using a modified membrane filter technique
(Jakobsen et al., 1992). Duplicate 2 g soil samples were
blended with 250 ml water and hyphae in 3 ml aliquots were
collected on 25 mm membrane filters (1.2 µm) and stained
with Trypan blue. Hyphal length was recorded in 25 random
fields of view per filter. The length of stained hyphae on the
filters was determined by the grid line intercept method at
×200 magnification (Tennant, 1975).
Statistical analysis
Data were subjected to three-way  to compare mycorrhizal
status, barley genotypes and P application levels using GenStat
for PC/Windows 6.1 (GenStat Committee, 2002).
Results
Colonization of roots and soil by G. intraradices
Phosphate rock reduced the proportion of root length
colonized, but this effect coincided with an enhancement of
root length (Table 1). By contrast, mycorrhiza development
was markedly depressed by the soluble P treatment. Roots of
the Brb mutant given nil P or PR had higher mycorrhiza
colonisation than the corresponding WT roots. Roots of
non-inoculated plants remained uncolonized (Table 1).
Despite the decreased root colonization in response to P
applications, there were no significant differences in the hyphal
length density in soil among the different P treatments, while
G. intraradices in association with Brb produced more extraradical hyphae than WT (P < 0.05) (Table 1). Some hyphae
were also present in pots with non-inoculated plants, but P
application or genotype had no significant influence in this
case. These hyphae were probably dead or saprophytic and
were assumed to be present also in the inoculated treatments.
Plant growth
In general, the WT barley produced higher d. wts than Brb
(P < 0.001). The two barley genotypes responded differently
to mycorrhiza and P applications (Fig. 1). In WT barley,
shoot d. wt slightly increased with P applications, but was
suppressed by mycorrhiza. The corresponding root d. wts
were influenced neither by mycorrhiza nor P. On the other
hand, the Brb mutant grew poorly without P and both shoot
and root d. wts responded dramatically to the P supplies.
© New Phytologist (2005) www.newphytologist.org
Fig. 1 Shoot (a) and root (b) d. wt of wild type barley (Hordeum
vulgare) (WT) and a bald root mutant (Brb) affected by phosphorus
(P) application and arbuscular mycorrhizal fungal (AMF) inoculation.
–M and +M represent non-inoculation treatment and inoculation
with mycorrhizal fungus Glomus intraradices, and P0, PR and P50
represent application of nil, 2% (w/w) phosphate rock and 50 mg
KH2PO4-P kg−1 soil, respectively. By ANOVA, inoculation, genotype,
P application, the interaction between inoculation and genotype,
genotype and P application were all highly significant (P < 0.001).
The interaction between inoculation and P application was significant
for root (P < 0.05) but not for shoot d. wt.
Furthermore, mycorrhiza increased root growth of the Brb
amended with nil or PR (Fig. 1).
Root length of WT barley was unaffected by mycorrhizal
colonization, while mycorrhiza increased root to shoot ratio
(P < 0.001). There were significant interactions between P
supply and genotypes in root length density, as root length
and root to shoot ratio of Brb increased in response to both
mycorrhizal colonization and the supply of P (P < 0.001;
Table 1).
Plant phosphorus nutrition
Inoculation, P application and genotypes all had significant
influences on P concentrations in both shoots and roots of
barley plants (P < 0.001; Table 2). In general, the supply of
PR or soluble P led to increased plant P concentrations, which
were higher in WT barley than in the mutant. Mycorrhizal
colonization decreased P concentration of WT at all P treatments
and also for Brb given soluble P. Phosphorus concentrations
of Brb plants markedly responded to mycorrhiza when nil P
or PR was applied (Table 2).
Compared with nil P treatment, plant P content of both
barley genotypes increased dramatically with application of
PR or soluble P. Mycorrhizal colonization increased the P
New Phytologist (2005) 165: 591–598
593
Barley
genotype
WT
Brb
Significanceb of
Inoculation (I)
P application (P)
Genotype (G)
I×P
I×G
P×G
I×P×G
P
application
treatment
P0a
PR
P50
P0
PR
P50
Root colonization
(% of root length)
Root length
(m per pot)
Hyphal length density
(m g−1)
Root to shoot ratio
(g g−1)
Non-inoculated
Inoculated
Non-inoculated
Inoculated
Non-inoculated
Inoculated
Non-inoculated
Inoculated
0
0
0
0
0
0
24.3
18.9
1.8
88.2
69.1
1.5
1.5
1.2
2.0
1.1
2.0
1.9
4.8
5.5
6.6
7.9
7.8
6.2
542
575
557
188
231
536
545
573
548
235
340
625
0.96
0.95
0.91
0.43
0.50
0.83
1.17
1.01
1.02
0.83
0.88
0.89
***
***
***
***
***
***
***
***
ns
*
ns
ns
ns
*
ns
***
***
ns
ns
***
ns
***
***
***
***
***
***
***
P0, PR and P50 represent application of nil P, 2% (w/w) phosphate rock and 50 mg KH2PO4-P kg−1 soil, respectively.
By ANOVA; ***P < 0.001; *P < 0.05; ns, not significant.
a
b
Table 2 Phosphorus (P) and uranium (U) concentrations in wild type barley (Hordeum vulgare) (WT) and a root hair mutant (Brb) grown under different phosphorus applications
www.newphytologist.org © New Phytologist (2005)
Barley
genotype
WT
Brb
Significanceb of
Inoculation (I)
P application (P)
Genotype (G)
I×P
I×G
P×G
I×P×G
P
application
treatment
Shoot P concentration
(mg g−1)
Root P concentration
(mg g−1)
Non-inoculated
Inoculated
Non-inoculated
P0a
PR
P50
P0
PR
P50
1.28
2.68
4.47
1.03
1.04
3.03
1.19
2.64
4.10
1.41
2.45
2.91
1.11
2.30
2.09
1.47
1.41
2.05
***
***
***
***
***
***
***
Shoot U concentration
(ng g−1)
Root U concentration
(ng g−1)
Inoculated
Non-inoculated
Inoculated
Non-inoculated
Inoculated
1.13
2.23
1.87
2.87
3.63
1.99
1.71
11.5
0.95
4.40
21.1
1.76
0.51
24.8
0.52
2.86
9.91
0.56
214
2770
157
23
1056
74
220
2759
172
100
1927
81
***
***
***
***
***
***
***
P0, PR and P50 represent application of nil, 2% (w/w) phosphate rock and 50 mg KH2PO4-P kg−1 soil, respectively.
By ANOVA; ***P < 0.001; *P < 0.05; ns, not significant.
a
b
ns
***
ns
ns
ns
ns
*
***
***
***
***
***
***
***
594 Research
New Phytologist (2005) 165: 591–598
Table 1 Mycorrhizal colonization rates, hyphal length density, root length and root to shoot ratio of wild type barley (Hordeum vulgare) (WT) and a root hair mutant (Brb) grown under different
phosphorus applications
Research
Fig. 2 Phosphorus (P) content of wild type barley (Hordeum vulgare)
(WT) and a bald root mutant (Brb) affected by P application and
arbuscular mycorrhizal fungal (AMF) inoculation. –M and +M
represent non-inoculation treatment and inoculation with
mycorrhizal fungus Glomus intraradices, and P0, PR and P50
represent application of nil, 2% (w/w) phosphate rock and 50 mg
KH2PO4-P kg−1 soil, respectively. By ANOVA, inoculation, genotype,
P application and all interactions were highly significant (P < 0.001).
content of Brb plants receiving nil P or PR, whereas in other
treatments, mycorrhizal plants contained less (or similar,
only for WT under nil P application) P as compared to corresponding non-inoculated plants (Fig. 2).
Fig. 3 238U content of wild type barley (Hordeum vulgare) (WT)
and a bald root mutant (Brb) affected by phosphorus (P) application
and arbuscular mycorrhizal fungal (AMF) inoculation. –M and +M
represent non-inoculation treatment and inoculation with
mycorrhizal fungus Glomus intraradices, and P0, PR and P50
represent application of nil, 2% (w/w) phosphate rock and 50 mg
KH2PO4-P kg−1soil, respectively. By ANOVA, genotype was highly
significant for root but not for shoot U content. P application was
highly significant for both root and shoot U content. The interaction
between inoculation and genotype was highly significant (P < 0.001)
for root and significant (P < 0.05) for shoot U content. The
interaction between genotype and P application was highly
significant for root but not for shoot U content.
Uranium uptake and partitioning
Shoot U concentrations in barley plants were significantly
increased by the application of PR, but hardly affected by
genotypes (P = 0.394) or mycorrhizal colonization (P = 0.359).
In contrast, root U concentrations varied with genotype,
inoculation and P application (P < 0.001). Root U concentrations were higher in the WT than in the mutant, and the
mutant had higher root U concentrations in the presence than
in the absence of mycorrhiza when nil P or PR was supplied
(Table 2).
The U contents of barley shoots were affected by P supplies
only (P < 0.001) (Fig. 3), but genotype and inoculation interacted significantly (P < 0.05). This was caused by presumably
contrasting mycorrhiza effects on U content in shoots of
Brb and WT barley supplied with PR. Root U contents were
influenced by both P application and genotype (P < 0.001).
Wild type barley accumulated much more U in its roots than
Brb. Significant interactions between genotype and inoculation (P < 0.001) indicated that mycorrhizal colonization
increased root U content of Brb but decreased that of WT
barley. The U uptake efficiency, estimated as uptake per unit
root length, was clearly highest for the PR treatment and was
© New Phytologist (2005) www.newphytologist.org
likewise higher for WT than for Brb barley (Table 3). The
presence of mycorrhiza also enhanced U uptake efficiency, but
only in the Brb mutant.
Generally only a small fraction (< 15%), of U had been
translocated from roots to shoots of barley plant. Noninoculated Brb had much higher shoot to root ratios than
the corresponding inoculated treatments (Fig. 4). This was
also true for the WT barley except for the PR treatment.
Discussion
This work is the first to demonstrate mycorrhizal effects on
U uptake from U-contaminated soil by a crop plant and
subsequently on U translocation from roots to shoots. The
root hairless barley used in present work also represented a
powerful tool to investigate the relative contribution of root
hairs and mycorrhizal fungi to U uptake.
Mycorrhiza formation is influenced by the P nutrition
status of the host plant (Thomson et al., 1991) and root colonization in the present study was accordingly reduced in P
fed barley plants. Root colonization by G. intraradices did not
New Phytologist (2005) 165: 591–598
595
596 Research
Root U uptake efficiency (ng m−1)
Barley genotype
WT
Brb
P application
treatment
Inoculated
Non-inoculated
P0a
PR
P50
P0
PR
P50
1.00
12.6
0.73
0.12
2.59
0.33
1.00
11.5
0.84
0.28
5.51
0.29
Significanceb of
Inoculation (I)
P application (P)
Genotype (G)
I×P
I×G
P×G
I×P×G
Table 3 Root uranium (U) uptake efficiency
of wild type barley (Hordeum vulgare) (WT)
and a root hair mutant (Brb) affected by
phosphorus (P) application and inoculation
treatments
*
***
***
*
***
***
***
a
P0, PR and P50 represent application of nil, 2% (w/w) phosphate rock and 50 mg KH2PO4P kg−1 soil, respectively.
b
By ANOVA; ***P < 0.001; *P < 0.05.
Fig. 4 Shoot to root ratio of total 238U uptake by wild type barley
(Hordeum vulgare) (WT) and a bald root mutant (Brb) affected by
phosphorus (P) application and arbuscular mycorrhizal fungal (AMF)
inoculation. –M and +M represent non-inoculation treatment and
inoculation with mycorrhizal fungus Glomus intraradices, and P0, PR
and P50 represent application of nil, 2% (w/w) phosphate rock and
50 mg KH2PO4-P kg−1soil, respectively. By ANOVA, genotype,
inoculation and P application were all highly significant (P < 0.01).
Significant interactions were found between genotype and
inoculation, genotype and P application (P < 0.01), but not between
inoculation and P application.
enhance P uptake and growth of WT barley at any P treatment
and this confirms that growth of barley with well-developed
root hairs is generally not aided by mycorrhiza (Fay et al.,
1996; Plenchette & Morel, 1996; Khaliq & Sanders, 2000)
except under extreme P deficiency ( Jakobsen, 1983; Baon
et al., 1993). On the other hand, the mycorrhiza responsiveness in plant P uptake in the root hairless Brb mutant is in
accordance with our previous characterization of the mutant
with respect to mycorrhiza (I. Jakobsen, unpublished data).
Although the enhanced P uptake in mycorrhizal Brb plants
(nil P and PR treatments) did not result in increased shoot
New Phytologist (2005) 165: 591–598
growth, we conclude that mycorrhiza can be essential to support growth of plants root with none or poorly developed root
hairs. The mycorrhiza status is therefore essential if such plant
species are selected in, e.g. revegetation and phytoremediation
programmes.
For non-mycorrhizal plants uptake of U was much higher
in WT barley with root hairs than in Brb, irrespective of P
treatments. On the other hand, Brb had a higher U uptake
in the presence than in the absence of G. intraradices. The
important role of AM fungus and root hairs in U uptake
become more obvious from their impact on specific U uptake
efficiencies. The higher U uptake efficiency of normal barley
roots compared with that of hairless roots of Brb and the
improved U uptake efficiency of Brb by G. intraradices, could
explain the differences in U uptake among treatments. In
general, an enlarged absorbing area provided by root hairs or
by mycorrhiza will be of key importance in plant uptake of U.
Less than 15% of the total U uptake was transferred to the
shoots suggesting the presence of a specific mechanism in
barley for retaining U in roots. This is consistent with other
studies showing that plant species differ in U accumulation,
and that this accumulation predominately occurs in the roots
(Sheppard & Thibault, 1984; Zafrir et al., 1992; Saric et al.,
1995). Non-inoculated Brb plants had markedly higher shoot
to root ratios of total U uptake than the corresponding inoculated treatments, and Brb had general higher ratios than WT.
This means that U partitioning to shoots was inhibited by the
presence of root hairs or AM fungus. Such results support
previous findings that mycorrhizal fungi significantly contributed to the U immobilization by mycorrhizal roots (Rufyikiri
et al., 2003). Hyphae of G. intraradices had higher U flux rates
than host roots (Rufyikiri et al., 2003) and U accumulation
was observed in fungal vesicles of mycorrhizal Cynodon dactylon
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Research
roots (Weiersbye et al., 1999). Similarly, using a compartmented pot system, Joner & Leyval (1997) demonstrated that
109Cd added to the hyphal compartment was adsorbed by
extraradical hyphae and subsequently transported to plant
roots, but transfer from fungus to plants was restricted by
fungal immobilization. The extensive fungal colonization
of Brb would have contributed to U immobilization in the
fungal tissue and lowered U concentration in shoots.
Moreover, the lower shoot to root ratios in U content of noninoculated WT than non-inoculated Brb provided evidence
for the similar impacts of root hairs and AM fungi in U partitioning. The negative impact of the PR and P50 treatments
on partitioning of U to shoots in non-mycorrhizal Brb plants
in particular corresponded to a higher root-shoot ratios in
P treated plants.
Additionally, U speciation is known to be highly pHdependent (Langmuir, 1978; Mortvedt, 1994; Ebbs et al.,
1998). It was established that soluble uranyl cations or uranylsulfate species that are stable under acidic conditions were
translocated to a higher extent to roots through fungal tissues,
while phosphate and hydroxyl species dominating under
acidic to near neutral conditions or carbonate species dominating under alkaline conditions were rather immobilized by
hyphal structures (Rufyikiri et al., 2002). The results obtained
under soil pH 6.7 in present work confirmed this immobilization processes. Under solution culture conditions Ebbs
et al. (1998) demonstrated that U was presumably taken up
predominantly as the free uranyl cations at pH 5.0, as at this
pH the U content and concentration were the highest and
lowest in shoots and roots, respectively. It deserves further in
vivo studies to investigate the role of mycorrhizal fungi in U
uptake and redistribution under acid conditions.
Phytoremediation received much research interests in
recent years (Salt et al., 1998; McGrath & Zhao, 2003) and
attention was also paid to phytoremediation of radionuclides
(Zhu & Shaw, 2000; Dushenkov, 2003). Obviously, the
potential importance of arbuscular mycorrhiza in phytoremediation of metal-contaminated soils should be taken into
account due to its ubiquity in natural ecosystems (Smith &
Read, 1997). It is generally agreed that at high concentrations
of any nonessential metals AM fungi often improve plant
resistance against toxic effects by enhancing metal retention
in the roots, thereby reducing metal concentrations in shoots
(Leyval & Joner, 2001). Consequently, arbuscular mycorrhiza
could possibly help phytoremediation of metal contaminated
sites (Leyval et al., 2002). Phytostabilization is proposed as an
alternative management practice when it is considered impossible to clean up heavily contaminated environments or waste
deposits. The present study demonstrates a potential role of
arbuscular mycorrhiza in such phytostabilization of U contaminated environments, possibly by reducing U translocation in the plant-soil continuum and food chains, thereby
reducing environmental risks. However, there is still a need to
carry out systematic studies to screen suitable plant species/
© New Phytologist (2005) www.newphytologist.org
genotypes and compatible symbionts, and to understand the
function of critical factors influencing U acquisition.
Acknowledgements
This work was financially supported by the EU-MYRRH
project contract-CT-2000-00014 ‘Use of mycorrhizal fungi for
the phytostabilization of radio-contaminated environments’.
B D Chen also gratefully acknowledges the continuous help
and technical support from Anne Olsen and Annette Olsen
throughout the experiments.
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