Water Air Soil Pollut (2013) 224:1450
DOI 10.1007/s11270-013-1450-3
Growth Responses, Metal Accumulation and Phytoremoval
Capability in Amaranthus Plants Exposed to Nickel
Under Hydroponics
Valentina Iori & Fabrizio Pietrini &
Alexandra Cheremisina & Nina I. Shevyakova &
Nataliya Radyukina & Vladimir V. Kuznetsov &
Massimo Zacchini
Received: 31 July 2012 / Accepted: 11 January 2013
# Springer Science+Business Media Dordrecht 2013
Abstract The characterisation of plant responses to
metal exposure represents a basic step to select a plant
species for phytoremediation. In the present work, 3week-old Amaranthus paniculatus L. plants were subjected to nickel chloride concentrations of 0 (control),
25, 50, 100 and 150 μM in hydroponic solution for
1 week to evaluate morphophysiological responses,
such as biomass production and partitioning, nickel
accumulation in plants and nickel removal ability from
the polluted solutions. The results showed a progressive
decrease in plant organ dry mass with the enhancement
of nickel (Ni) concentration in the solution, suggesting a
good metal tolerance at 25 μM Ni and a marked sensitivity at 150 μM Ni. The modification of biomass partitioning was particularly appreciated in leaves,
analysing the organ mass ratio, the total leaf area and
the specific leaf area. Amaranthus plants accumulated a
significant amount of Ni in roots exposed to the highest
Ni concentrations, while lower metal contents were
observed in the aerial organs. The Ni uptake ratio was
progressively reduced in plants exposed to increased Ni
concentrations. The metal translocation from root to
shoots, appreciated by the Ni translocation index,
showed a far lower value in Ni-exposed plants than in
controls. Moreover, by measuring the daily Ni content
of the solutions, a lower Ni removal ability was found in
Amaranthus plants at increasing Ni concentrations.
Remarkably, plants exposed to 25 μM Ni succeeded in
removing almost 60 % of the initial Ni content of the
solution showing no stress symptoms. The potential of
A. paniculatus for phytoremediation was discussed.
Keywords Biomass partitioning . Heavy metals .
Metal tolerance . Phytoremediation . Rhizofiltration
1 Introduction
V. Iori : F. Pietrini : M. Zacchini (*)
Institute of Agro-environmental and Forest Biology,
National Research Council of Italy, Via Salaria Km 29,300,
00015 Monterotondo Scalo, Rome, Italy
e-mail: [email protected]
A. Cheremisina : N. I. Shevyakova : N. Radyukina :
V. V. Kuznetsov
Timiryazev Institute of Plant Physiology, Russian Academy
of Sciences, Botanicheskaya ul., 35, 127276 Moskow, Russia
The increased level of nickel (Ni) in the environment,
due to anthropogenic sources mainly linked to mining
and smelting activities, represents a growing concern
for the food chain and terrestrial and aquatic ecosystems. Ni is extremely toxic for living organisms, and
severe damage induced by this metal on mammals,
fishes and plants has been reported (Pereira et al.
2002; Pyle et al. 2002; Seregin and Kozhevnikova
2006). To reduce the risks associated with elevated
1450, Page 2 of 10
Ni concentrations contaminating soils and waters,
phytoremediation, i.e. the use of plants to remove or
render less harmful heavy metals and other contaminants from polluted substrates, has emerged as a valuable technology (Padmavathiamma and Li 2007;
Hassan and Aarts 2011). Among the most interesting
characteristics of phytoremediation are ecological sustainability, economic feasibility, public acceptance,
low levels of technological demand and low levels of
energetic input.
Ni is an essential micronutrient for plants, as it is a
constituent of many enzymes, such as urease, hydrogenases, superoxide dismutase and glyoxalases (Seregin
and Kozhevnikova 2006; Küpper and Kroneck 2007;
Chen et al. 2009). In most plants species, concentrations
of Ni range from 0.05 to 10 ppm (on a dry weight basis)
and are commonly associated with normal growth and
development (Nieminen et al. 2007). A deficiency of
this metal can result in the disruption of the metabolism
of ureides, amino acids and organic acids at the leaf
level, with visible symptoms of stress (Bai et al. 2006).
On the contrary, the exposure of plants to elevated Ni
concentrations can alter the uptake of nutrients provoking chlorosis, the inhibition of photosynthesis and respiration, impaired water uptake processes and the
induction of oxidative stress by the production of reactive oxygen species, although Ni is not a redox metal
(Seregin and Kozhevnikova 2006; Chen et al. 2009).
Plants have evolved different tolerance strategies to
withstand the elevated Ni concentrations often present
in soils due to natural causes, i.e. serpentine and volcanic soils (Baker 1981). Among these, metal exclusion,
hyperaccumulation and metal confinement in roots have
been thoroughly investigated. In the case of hyperaccumulation, some plants species, termed Ni hyperaccumulators, can concentrate at least 1,000 mg Ni kg−1 DW in
their shoots, as observed in plants belonging to the
genera Alyssum and Thlaspi (Baker and Brooks
1989; Gabbrielli et al. 1991; Kramer et al. 1997;
Galardi et al. 2007). Hyperaccumulator plants have
been proposed for phytoremediation of heavy
metal-contaminated sites (Robinson et al. 1997;
Singer et al. 2007). However, these plants species
are characterised by a small biomass and a limited
soil exploration by roots (Prasad 2003), which can
result in a lower metal removal on area basis
compared to non-hyperaccumulator plants, for instance, Salicaceae plants (Marmiroli et al. 2011).
As a result, even if their metal bioconcentration
Water Air Soil Pollut (2013) 224:1450
potential is far lower in comparison to hyperaccumulator plants, non-hyperaccumulator plants are
largely studied and characterised for their possible
application in phytoremediation of soils and waters
contaminated by heavy metals (Marmiroli et al.
2011). In particular, efforts were made to investigate the natural variability towards pollutant absorption and translocation in aboveground organs,
e.g. characterising some plant species (Prasad and
Freitas 2003; Zacchini et al. 2009). A limit for the
utilisation of non-hyperaccumulator plants is the
pollutant concentration in the substrate that these
plant species can bear. Therefore, studies addressing the evaluation of plant tolerance responses to
metals, in order to set the bounds in which an
effective pollutant removal can be exerted by
plants, represent a useful tool for the wider and
more successful application of phytoremediation.
To this scope, laboratory trials aimed at exploiting
the potential of a plant species to tolerate and
accumulate a particular pollutant, without the interference of other abiotic and biotic factors, have
been commonly reported in the literature (Kotyza
et al. 2010; Iori et al. 2012). Furthermore, the
suitability of hydroponics for testing the phytoremediation capability of plants, especially for metal
removal in aqueous matrices, was established (Dos
Santos Utmazian et al. 2007; Zacchini et al. 2009;
Pietrini et al. 2010a). In a previous study of naturally growing vegetation on a metal-contaminated
site in Russia (Madzhugina et al. 2008), it was
found that Amaranthus paniculatus plants were
able to survive and accumulate a considerable
amount of Ni. Amaranthus species have been
reported to produce high biomass, to be easily
cultivated and to have a great competitive capability and a wide geographical distribution (Zhang et
al. 2010). In fact, plants belonging to Amaranthus
sp. are characterised by C4 photosynthesis, resulting in greater water and nitrogen use efficiencies
and a higher productivity under stress conditions
compared to C3 plant types (Sage and Pearcy
1987; Long 1999). Therefore, these plant species
could potentially be very interesting for the phytoremediation of metal-polluted sites. Very few
studies are present in the literature regarding the
ability of Amaranthus species to tolerate and accumulate metals (Mellem et al. 2009; Li et al.
2009; Zhang et al. 2010; Shevyakova et al.
Water Air Soil Pollut (2013) 224:1450
2011). The aim of this work was to evaluate the
growth responses of A. paniculatus plants to increasing concentrations of Ni and the ability to
remove this metal from a nutrient solution and
accumulate it in the different plant organs under
hydroponics, in order to better assess the potential
of this species for phytoremediation.
2 Materials and Methods
2.1 Plant Material and Growth Conditions
Seeds of A. paniculatus L. were soaked and germinated in the dark on wet filter paper in a growth chamber
at 26 °C. Plantlets were transferred to plastic pots
filled with one-sixth-strength Hoagland’ solution in a
controlled climate chamber at a photon flux density of
300 μmolm−2 s−1 for 14 h/day and at a temperature of
25/20 °Cday/night and a relative humidity of 70–
80 %. Air pumping was provided to avoid oxygen
deprival. After 3 weeks, plants were transferred to
single pots (six plants per pot) and subjected to concentrations of 0 (control), 25, 50, 100 and 150 μM
NiCl·6H20 in one-sixth-strength Hoagland’ solution
for 1 week. Solutions were sampled every day to
evaluate Ni removal. At the end of the experiment,
plants were harvested, carefully washed with distilled
water, separated in their organs, dried in an oven at
80 °C and finally weighed.
2.2 Biomass Partitioning
The calculation of organ mass ratio was performed as
the ratio of the leaf (LMR), shoot (SMR) and root
(RMR) biomass to the total plant biomass. Leaf area
was measured using the Leaf Area Meter Li 3000
(Licor, NE, USA), and the specific leaf area (SLA)
was calculated as the ratio of the leaf area relative to
the leaf mass.
2.3 Nickel Content Analysis
Ni determination was performed using an atomic absorption spectrophotometer (Varian SpectrAA model 220FS,
Mulgrave, Australia) on acidified samples of nutrient
solution and on digested samples of leaves, stems and
roots. The oven-dried material was finely ground
(Tecator Cemotec 1090 Sample Mill; Tecator, Hoganas,
Page 3 of 10, 1450
Sweden), weighed and mineralised. Mineralisation was
performed by treating 250 mg of powdered samples with
4 ml of concentrated HNO3, 3 ml of distilled water and
2 ml of H2O2 (30 %v/v in water), followed by heating
(EXCEL Microwave Chemistry Workstation, PreeKem
Scientific Instruments Co., Ltd., Shanghai, China) in a
four-step procedure: 100 °C for 1 min at 250 psi, 140 °C
for 1 min at 350 psi, 170 °C for 1 min at 450 psi and
200 °C for 12 min at 550 psi. Samples were then filtered
and analysed. For each Ni concentration, the plant uptake ratio was calculated as the ratio of the total Ni
content (in microgram) of the plants to the Ni content
(in microgram) of the corresponding growth solution at
the end of the experiment. The metal translocation index
was calculated as the ratio of the Ni content (in microgram) of aboveground organs to the Ni content (in
microgram) of the corresponding roots.
2.4 Statistical Analysis
The data reported refer to a single typical experiment
with six replicates. Normally distributed data were
processed with a one-way ANOVA. Statistical significance of the mean data was assessed by Duncan’s test
using the SPSS software tool.
3 Results
The exposure of A. paniculatus plants to Ni resulted in
a remarkable dry mass modification, according to the
increasing Ni concentrations of the growth solution
(Fig. 1). In fact, the leaf, stem and root dry mass of
plants was progressively reduced as the Ni concentration in the solution enhanced. The reduction in dry
mass was particularly evident in plants grown at
150 μM Ni. In these plants, the root and stem dry
mass was more than halved compared to the control
but leaf dry mass was also notably reduced. A decrease of the root to shoot ratio was also observed in
Amaranthus plants following the increase of the metal
concentration in the growth solution. Regarding biomass partitioning (Table 1), results showed that plants
allocated almost 50 % of the dry mass in the leaves,
followed by stems and roots, irrespective of the Ni
concentration in the nutrient solution. LMR was
higher in plants exposed to 150 μM Ni compared to
plants subjected to other Ni treatments, except for
control. In contrast, control and 150 μM Ni-treated
1450, Page 4 of 10
Water Air Soil Pollut (2013) 224:1450
ƒFig. 1
2.0
Leaf, stem and root dry mass (in gram per plant), and
root to shoot ratio in plants of A. paniculatus L. exposed to
different Ni concentrations for a week in hydroponics (mean ±
S.E., n=6). Different letters in the columns correspond to statistically different values (Duncan’s test, P≤0.05)
-1
Leaf dry mass (g plant )
a
1.5
ab
b
b
1.0
b
0.5
0.0
0
25
50
100
150
1.2
-1
Stem dry mass (g plant )
a
a
1.0
ab
ab
0.8
0.6
b
0.4
0.2
0.0
0
25
50
100
150
0.5
-1
Root dry mass (g plant )
a
0.4
ab
0.3
b
b
0.2
c
0.1
0.0
0
25
50
100
150
0.25
a
0.20
Root / Shoot
ab
b
0.15
b
b
0.10
0.05
0.00
0
25
50
100
Nickel concentration (µM)
150
plants showed a lower SMR than plants exposed to 25,
50 and 100 μM Ni. A reduction of RMR as a consequence of Ni exposure was observed in Amaranthus
plants, with the lowest significant value occurring at
150 μM Ni. The modification of leaf biomass was
evaluated also by assessing the total leaf area and
SLA. Both parameters showed a clear trend to decrease as the Ni concentration in the growth solutions
increased.
The Ni content of plants exposed to different Ni
concentrations in the nutrient solution is reported in
Fig. 2. In leaves, a higher metal content was observed
in plants subjected to 150 μM Ni compared to other
Ni-exposed plants, with control plants showing the
lowest values. Stem Ni content analysis revealed a
progressive enhancement of metal accumulation in
this organ with the increase of the Ni concentration
in the growth solution, and the highest value occurring
in plants grown at 150 μM Ni. Concerning roots, a
higher Ni content was detected in plants exposed to
100 and 150 μM Ni in comparison to those growing at
25 and 50 μM Ni, with the lowest statistical value
observed in control plants. As a result of the different
distributions in the plant organs, the total Ni content of
Amaranthus plants was notably higher in plants growing at 100 and 150 μM Ni compared to those at 25 and
50 μM Ni, and control plants showed the lowest Ni
amount. In Fig. 3, the modification of the Ni uptake
ratio of plants to the increasing Ni concentrations in
the solution is reported. Control plants showed the
highest values for the Ni uptake ratio, and a progressive and significant reduction of this parameter with
the enhancement of metal content in the growth medium was observed. The metal translocation index,
which referred to Ni, is shown in Fig. 4. Also, in this
case, control plants showed the highest value, followed by plants exposed to 150 μM Ni, while the
lowest values were observed for plants subjected to
25, 50 and 100 μM Ni.
Nickel removal from the nutrient solution by
Amaranthus plants was monitored on a daily basis,
and trends referred to the different Ni concentrations
are reported in Fig. 5. Along the experimental time
interval, a different Ni removal ability was shown by
Water Air Soil Pollut (2013) 224:1450
Page 5 of 10, 1450
Table 1 Leaf, stem and root mass ratio (in gram per gram), total leaf area (in square centimetre) and specific leaf area (SLA, in square
centimetre per gram) in plants of A. paniculatus L. exposed to different Ni concentrations for a week in hydroponics (mean ± S.E., n=6)
Ni concentration
(μM)
Leaf mass ratio
Stem mass ratio
0
0.566 (0.017) ab
0.277 (0.023) b
25
0.479 (0.047) b
0.384 (0.041) a
50
0.478 (0.021) b
0.391 (0.022) a
100
0.512 (0.032) b
0.348 (0.041) ab
150
0.611 (0.026) a
0.262 (0.013) b
Root mass ratio
Total leaf area
SLA
0.156 (0.016) a
968 (95) a
638 (19) a
0.136 (0.012) ab
547 (52) b
561 (70) ab
0.114 (0.016) ab
364 (53) c
539 (10) ab
0.117 (0.009) ab
326 (24) c
520 (20) b
0.099 (0.012) b
335 (50) c
460 (12) b
Different letters within a column correspond to statistically different values (Duncan’s test, P≤0.05)
plants depending on the Ni concentration in the
growth solution. In fact, plants exposed to 25 μM Ni
highlighted a higher and more constant Ni removal
with time compared to plants subjected to 50, 100 and
150 μM Ni and succeeded in removing almost 60 % of
the initial Ni content of the growth solution at the end
of the experiment. Lower removal abilities were found
in plants treated with 50 and 100 μM Ni, resulting in
the decontamination of the initial Ni solution by
approx. 33 and 25 %, respectively. In this latter case,
Ni removal was exerted by plants during the first part
of the experimental time and was subsequently dramatically reduced. Plants exposed to 150 μM Ni
showed the lowest Ni removal ability, with the reduction of the Ni content of the growth solution at the end
of the treatment being close to 17 %. Moreover, these
plants exhibited a strong reduction of Ni removal
capability just 2 days after the start of the Ni exposure.
4 Discussion
Reclamation of metal-polluted soils and waters is an
issue that has been receiving increasing attention. In
particular, the need to improve the quality of the
wastewaters rises not only from the growing concern
over the protection of the aquatic ecosystem but also
as a result of the impact of water shortage on crop
irrigation, leading many countries to reuse wastewater
(Fatta-Kassinos et al. 2011). This practice, if not appropriately managed, could result in metal and other
pollutant accumulation in agricultural soils, with a
possible contamination transfer into food plants, thus
representing a serious risk for animal and human
health. Phytoremediation has emerged as a suitable
technology to realise an economical and ecologically
sustainable approach to decontamination (Licht and
Isebrands 2005; Schwitzguébel et al. 2009). The selection of plants with higher efficiencies for the removal of metals from polluted substrates, involving
traits such as metal tolerance and the accumulation and
distribution into organs, is a basic issue for a wider and
more successful application of phytoremediation.
In this work, the capability of A. paniculatus plants to
tolerate, accumulate into organs and remove Ni from a
nutrient solution was studied. Although the Ni concentrations tested were far higher than the legal Italian limits
for subterranean waters and soils (ranging from 75- to
400-fold and from 10- to 70-fold above the threshold
allowed, respectively), they were representative of Ni
concentrations in polluted soils (Marchiol et al. 2004)
and waters (Farkas et al. 2007). Moreover, these Ni
concentrations were chosen to induce morphophysiological responses in Amaranthus plants and to appreciate the potential for this species to decontaminate a
metal-polluted substrate (Shevyakova et al. 2011). The
time scale of plant exposure to Ni was typical of a shortterm experiment, allowing the evaluation of metal tolerance and the accumulation ability of plants in hydroponics (Leblebici and Aksoy 2011; Shevyakova et al.
2011). Plant adaptation to cultivation under hydroponics
was satisfactory, confirming previous investigations
(Zhang et al. 2010; Shevyakova et al. 2011), and control
plants showed a biomass production similar to plants
grown in pots filled with uncontaminated soils (data not
shown). The exposure of Amaranthus plants to increasing Ni concentrations caused different growth reductions depending on the organ studied. Roots showed a
particular sensitivity to Ni concentrations above 25 μM
Ni, while leaves and especially stems exhibited a greater
1450, Page 6 of 10
Water Air Soil Pollut (2013) 224:1450
a
80
b
60
40
c
0
0
25
50
100
-1
Stem Ni content (µg plant )
300
150
a
250
200
ab
150
bc
100
bc
50
c
0
0
25
50
100
150
500
a
-1
Root Ni content (µg plant )
600
400
a
300
b
b
200
15
0.12
10
c
0.10
5
800
700
600
500
400
300
200
25
50
100
150
a
a
b
b
b
0.08
bc
0.06
c
0.04
c
0.02
0.00
50
0
40
30
a
-1
0
-1
ability to tolerate Ni concentrations up to 100 μM
(Fig. 1). At 150 μM Ni, plants showed visible symptoms
of stress damage such as a diffuse chlorosis (data not
shown), and the biomass production was severely affected. Damage at the leaf level due to increasing Ni
concentrations in the solution was also proven by the
marked reduction in total leaf area and SLA (Table 1),
which are the two parameters that are usually measured
to evaluate tolerance to metals at the leaf level
(Sebastiani et al. 2004; Fernandez et al. 2012). A reduction of leaf area as a consequence of the exposition to
metals is a commonly reported plant response (Zacchini
et al. 2009; Di Baccio et al. 2010). A higher root biomass reduction in comparison to shoots was also
reported by Zhang et al. (2010) in Amaranthus plants
treated with a different non-redox metal, such as Cd, in
hydroponics. Regarding Ni-hyperaccumulator plants, a
higher sensitivity of roots to Ni toxic concentrations
compared to shoots was reported in Alyssum bertolonii
by Galardi et al. (2007), while Assunção et al. (2003)
showed a different strategy for Ni tolerance in Thlaspi
caerulescens, involving a higher resistance of roots with
respect to shoots. Biomass partitioning was also evaluated by calculating the organ mass ratio, which is a
suitable parameter to investigate the growth response
20
0
Plant Ni content (µg plant )
Leaf, stem, root and total nickel content (in microgram
per plant) in plants of A. paniculatus L. exposed to different Ni
concentrations for a week in hydroponics (mean ± S.E., n=6).
Different letters in the columns correspond to statistically different values (Duncan’s test, P≤0.05)
b
b
20
ƒFig. 2
Ni uptake ratio (mg mg )
-1
Leaf Ni content (µg plant )
100
25
50
100
150
Nickel concentration (µM)
c
20
10
0
0
25
50
100
Nickel concentration (µM)
150
Fig. 3 Nickel uptake ratio (in milligram per milligram) in plants
of A. paniculatus L. exposed to different Ni concentrations for a
week in hydroponics (mean ± S.E., n=6). Different letters in the
columns correspond to statistically different values (Duncan’s
test, P≤0.05)
Water Air Soil Pollut (2013) 224:1450
Page 7 of 10, 1450
6
a
Ni translocation index
5
4
3
b
1
c
c
c
0
25
0
100
50
150
Nickel concentration (µM)
Fig. 4 Nickel translocation index in plants of A. paniculatus L.
exposed to different Ni concentrations for a week in hydroponics (mean ± S.E., n=6). Different letters in the columns correspond to statistically different values (Duncan’s test, P≤0.05)
in plants subjected to metal treatments (Sebastiani et al.
2004). A biomass partitioning plasticity in plants exposed to metals was reviewed by Audet and Charest
(2008) as a trait to be considered for phytoremediation
purposes. In the present work, the allocation of biomass
was predominantly in the shoot and especially in leaves,
regardless of the Ni concentration in the solution. An
increase in LMR and a decrease in RMR characterised
the plants exposed to 150 μM Ni (Table 1). This feature
was associated with significant damage, in terms of
biomass reduction, exerted by Ni on Amaranthus plants.
A reduction of RMR in barley plants treated with Cd
27.0
24.0
Nickel content (mg)
21.0
18.0
15.0
12.0
7.5
6.0
4.5
3.0
1.5
0.0
0
1
2
3
4
5
6
7
Days of treatment
Fig. 5 Nickel removal ability of A. paniculatus L. plants exposed to different Ni concentrations (closed circle, 25 μM
NiCl2; open circle, 50 μM NiCl2; closed triangle, 100 μM
NiCl2; open triangle, 150 μM NiCl2) for a week in hydroponics
(mean ± S.E., n=6)
was observed by Vassilev et al. (1998) as part of the
toxicity effect caused by this metal on plant growth.
Amaranthus plants showed higher Ni accumulation
when exposed to 100 and 150 μM Ni compared to
lower Ni concentrations (Fig. 2). This trend differed
slightly among organs, even though the exposure of
plants to 150 μM Ni always resulted in the highest Ni
accumulation in the organs studied. Except for the
control plants, whose Ni content was likely due to a
physiological metal content in seeds and impurities of
the growth solution, and irrespective of the Ni concentrations supplied, roots accumulated the largest
amount of metal, followed by the stem and leaves. A
higher Ni content in roots compared to leaves was also
observed in Ni-treated Matricaria chamomilla plants,
a non-hyperaccumulator plant species, by Kováčik et
al. (2009) in a similar experiment. As expected, the
Amaranthus plants assayed in this work exhibited a
markedly lower Ni accumulation capability compared
to a Ni-hyperaccumulator plant, such as Berkheya
coddii (Robinson et al. 2003), and the distribution
among organs also differed notably, with the leaves
of Berkheya being the primary sink for the metal, as
found in other Ni-hyperaccumulator plants (Wenzel et
al. 2003; Galardi et al. 2007). In order to evaluate the
ability of plants to accumulate Ni depending on the Ni
content of the solution, the uptake ratio was calculated
(Fig. 3). Amaranthus plants showed a progressive
decrease of Ni uptake capability with increasing Ni
availability in the solution, with the highest performance occurring in control plants. This feature can
be associated with the damage observed at the root
level in terms of a reduction in biomass production as
the Ni concentration in the growth medium increased.
A reduction of root cell membrane functionality in
plants treated with enhanced Ni concentrations in the
solution was also reported (Llamas et al. 2008; Sanz et
al. 2009), possibly occurring as a consequence of
oxidative stress induction (Chen et al. 2009).
The metal translocation from roots to shoots is a
suitable trait to select a plant for phytoremediation
(Prasad and Freitas 2003; Zacchini et al. 2009). In
fact, the preferential accumulation of metals in the
aboveground organs results in higher metal removal
efficiencies in plants, as these organs are easily and
commonly harvested. In this regard, hyperaccumulator
plants are characterised by a large metal accumulation
in their shoots. Ni-hyperaccumulators plants, for instance, accumulate this metal beyond 1,000 μgg−1 in
1450, Page 8 of 10
shoots as a tolerance response by safely storing the
metal at the tissue and cellular level (Kramer 2010).
On the contrary, the restriction of metal movement to
shoots is a defence response carried out by nonhyperaccumulator plants (Seregin and Kozhevnikova
2006), in order to protect the photosynthetic processes
occurring in leaves by the oxidative attack exerted by
metals (Pietrini et al. 2010b). In the present work
(Fig. 4), Amaranthus plants exposed to different Ni
concentrations in the growth solution showed a lower
capability to translocate Ni in the shoots compared to
control plants, where the Ni transport along the plant
axis can be associated to the limited amount requested
for the physiological processes of leaves (Seregin and
Kozhevnikova 2006; Chen et al. 2009). Therefore, the
limitation of metal movement to leaves in plants exposed to 25, 50 and 100 μM Ni can be ascribed to a
defence response aimed at avoiding the onset of oxidative damage in the photosynthetic machinery in
leaves. The higher Ni translocation observed in plants
exposed to 150 μM Ni could be due to an impairment
of metal transport restriction processes, resulting in the
aboveground biomass reduction found at this Ni concentration. Accordingly, a low Ni translocation ability
in Amaranthus species was also found by Mellem et
al. (2009).
Nickel removal ability by A. paniculatus plants was
followed on a daily basis by analysing the metal
concentration in the solution (Fig. 5). Different trends
of Ni removal along the experimental time intervals
were observed in plants depending on the Ni concentration in the growth solution. In particular, a significant amount of Ni removal was shown by plants
exposed to 25 μM Ni, as they succeeded in removing
almost 60 % of the initial Ni amount of the solution.
On the contrary, a limited Ni removal ability was
observed in plants exposed to Ni concentrations higher
than 25 μM, which highlighted a notable metal removal capability in the first few days of exposure but
probably suffered the toxicity effects exerted on plants
by the high metal concentrations in the second part of
the experimental interval.
Very few studies have been reported in the literature
regarding the evaluation of the metal phytoremediation ability of plants of the Amaranthus genus, with
contrasting indications about the suitability of this
plant species for the decontamination of metalpolluted substrates (Mellem et al. 2009; Li et al.
2009; Zhang et al. 2010; Shevyakova et al. 2011). In
Water Air Soil Pollut (2013) 224:1450
this work, A. paniculatus plants showed a notable Ni
accumulation ability, mostly confined to the roots, and
a good tolerance to metal exposure at concentrations
that are far higher than those allowed by Italian laws
for public soils and waters. The low metal translocation from roots to aerial parts could indicate a possible
limitation for the utilisation of this plant species for Ni
phytoextraction in metal-polluted soils, even if the
large biomass produced by Amaranthus species could
compensate the lower Ni accumulation in shoots compared to other plants species. On the contrary, the
confinement of Ni accumulation in the roots that
avoided damages at the leaf level, as evidenced by
the lack of biomass reduction at a concentration of
25 μM Ni, could open interesting perspectives for the
utilisation of the Amaranthus plant species for rhizofiltration of metal-polluted waters, either alone or in
association with conventional wastewater treatment
processes.
In conclusion, the good adaptation to hydroponics
and the valuable Ni removal observed for A. paniculatus plants in this study, especially at environmentally
relevant metal concentrations, indicate the remarkable
potential of this plant species for the decontamination
of polluted substrates in water-based systems (wetlands or mesocosms). Further evaluations over a longer time scale and by the utilisation of actual metalpolluted wastewater are planned to better ascertain the
suitability of this plant species for metal phytoremoval
purposes.
Acknowledgments This work was realised within the joint
project between National Research Council of Italy and Russian
Academy of Sciences “Mechanisms of plant adaptation to stress
action of heavy metals: possible implications for the phytoremediation technology”. Authors wish to thank Mr. Ermenegildo
Magnani for his expert technical assistance in metal content
analysis.
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