Growth Responses, Metal Accumulation and PhytoremovalCapability in Amaranthus Plants Exposed to NickelUnder 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 tometal exposure represents a basic step to select a plantspecies for phytoremediation. In the present work, 3-week-old Amaranthus paniculatus L. plants were sub-jected to nickel chloride concentrations of 0 (control),25, 50, 100 and 150 μM in hydroponic solution for1 week to evaluate morphophysiological responses,such as biomass production and partitioning, nickelaccumulation in plants and nickel removal ability fromthe polluted solutions. The results showed a progressivedecrease in plant organ dry mass with the enhancementof nickel (Ni) concentration in the solution, suggesting agood metal tolerance at 25 μM Ni and a marked sensi-tivity at 150 μM Ni. The modification of biomass par-titioning was particularly appreciated in leaves,analysing the organ mass ratio, the total leaf area andthe specific leaf area. Amaranthus plants accumulated asignificant amount of Ni in roots exposed to the highest

Ni concentrations, while lower metal contents wereobserved in the aerial organs. The Ni uptake ratio wasprogressively reduced in plants exposed to increased Niconcentrations. The metal translocation from root toshoots, appreciated by the Ni translocation index,showed a far lower value in Ni-exposed plants than incontrols. Moreover, by measuring the daily Ni contentof the solutions, a lower Ni removal ability was found inAmaranthus plants at increasing Ni concentrations.Remarkably, plants exposed to 25 μM Ni succeeded inremoving almost 60 % of the initial Ni content of thesolution showing no stress symptoms. The potential ofA. paniculatus for phytoremediation was discussed.

Keywords Biomass partitioning . Heavy metals .

Metal tolerance . Phytoremediation . Rhizofiltration

1 Introduction

The increased level of nickel (Ni) in the environment,due to anthropogenic sources mainly linked to miningand smelting activities, represents a growing concernfor the food chain and terrestrial and aquatic ecosys-tems. Ni is extremely toxic for living organisms, andsevere damage induced by this metal on mammals,fishes and plants has been reported (Pereira et al.2002; Pyle et al. 2002; Seregin and Kozhevnikova2006). To reduce the risks associated with elevated

Water Air Soil Pollut (2013) 224:1450DOI 10.1007/s11270-013-1450-3

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, Italye-mail: massimo.zacchini@ibaf.cnr.it

A. Cheremisina :N. I. Shevyakova :N. Radyukina :V. V. KuznetsovTimiryazev Institute of Plant Physiology, Russian Academyof Sciences, Botanicheskaya ul., 35, 127276 Moskow, Russia

Ni concentrations contaminating soils and waters,phytoremediation, i.e. the use of plants to remove orrender less harmful heavy metals and other contami-nants from polluted substrates, has emerged as a valu-able technology (Padmavathiamma and Li 2007;Hassan and Aarts 2011). Among the most interestingcharacteristics of phytoremediation are ecological sus-tainability, economic feasibility, public acceptance,low levels of technological demand and low levels ofenergetic input.

Ni is an essential micronutrient for plants, as it is aconstituent of many enzymes, such as urease, hydro-genases, superoxide dismutase and glyoxalases (Sereginand Kozhevnikova 2006; Küpper and Kroneck 2007;Chen et al. 2009). In most plants species, concentrationsof Ni range from 0.05 to 10 ppm (on a dry weight basis)and are commonly associated with normal growth anddevelopment (Nieminen et al. 2007). A deficiency ofthis metal can result in the disruption of the metabolismof ureides, amino acids and organic acids at the leaflevel, with visible symptoms of stress (Bai et al. 2006).On the contrary, the exposure of plants to elevated Niconcentrations can alter the uptake of nutrients provok-ing chlorosis, the inhibition of photosynthesis and res-piration, impaired water uptake processes and theinduction of oxidative stress by the production of reac-tive oxygen species, although Ni is not a redox metal(Seregin and Kozhevnikova 2006; Chen et al. 2009).Plants have evolved different tolerance strategies towithstand the elevated Ni concentrations often presentin soils due to natural causes, i.e. serpentine and volca-nic soils (Baker 1981). Among these, metal exclusion,hyperaccumulation and metal confinement in roots havebeen thoroughly investigated. In the case of hyperaccu-mulation, some plants species, termed Ni hyperaccumu-lators, can concentrate at least 1,000 mgNi kg−1 DW intheir shoots, as observed in plants belonging to thegenera Alyssum and Thlaspi (Baker and Brooks1989; Gabbrielli et al. 1991; Kramer et al. 1997;Galardi et al. 2007). Hyperaccumulator plants havebeen proposed for phytoremediation of heavymetal-contaminated sites (Robinson et al. 1997;Singer et al. 2007). However, these plants speciesare characterised by a small biomass and a limitedsoil exploration by roots (Prasad 2003), which canresult in a lower metal removal on area basiscompared to non-hyperaccumulator plants, for in-stance, Salicaceae plants (Marmiroli et al. 2011).As a result, even if their metal bioconcentration

potential is far lower in comparison to hyperaccu-mulator plants, non-hyperaccumulator plants arelargely studied and characterised for their possibleapplication in phytoremediation of soils and waterscontaminated by heavy metals (Marmiroli et al.2011). In particular, efforts were made to investi-gate the natural variability towards pollutant ab-sorption and translocation in aboveground organs,e.g. characterising some plant species (Prasad andFreitas 2003; Zacchini et al. 2009). A limit for theutilisation of non-hyperaccumulator plants is thepollutant concentration in the substrate that theseplant species can bear. Therefore, studies address-ing the evaluation of plant tolerance responses tometals, in order to set the bounds in which aneffective pollutant removal can be exerted byplants, represent a useful tool for the wider andmore successful application of phytoremediation.To this scope, laboratory trials aimed at exploitingthe potential of a plant species to tolerate andaccumulate a particular pollutant, without the in-terference of other abiotic and biotic factors, havebeen commonly reported in the literature (Kotyzaet al. 2010; Iori et al. 2012). Furthermore, thesuitability of hydroponics for testing the phytore-mediation capability of plants, especially for metalremoval in aqueous matrices, was established (DosSantos Utmazian et al. 2007; Zacchini et al. 2009;Pietrini et al. 2010a). In a previous study of nat-urally growing vegetation on a metal-contaminatedsite in Russia (Madzhugina et al. 2008), it wasfound that Amaranthus paniculatus plants wereable to survive and accumulate a considerableamount of Ni. Amaranthus species have beenreported to produce high biomass, to be easilycultivated and to have a great competitive capabil-ity and a wide geographical distribution (Zhang etal. 2010). In fact, plants belonging to Amaranthussp. are characterised by C4 photosynthesis, result-ing in greater water and nitrogen use efficienciesand a higher productivity under stress conditionscompared to C3 plant types (Sage and Pearcy1987; Long 1999). Therefore, these plant speciescould potentially be very interesting for the phy-toremediation of metal-polluted sites. Very fewstudies are present in the literature regarding theability of Amaranthus species to tolerate and ac-cumulate metals (Mellem et al. 2009; Li et al.2009; Zhang et al. 2010; Shevyakova et al.

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2011). The aim of this work was to evaluate thegrowth responses of A. paniculatus plants to in-creasing concentrations of Ni and the ability toremove this metal from a nutrient solution andaccumulate it in the different plant organs underhydroponics, in order to better assess the potentialof this species for phytoremediation.

2 Materials and Methods

2.1 Plant Material and Growth Conditions

Seeds of A. paniculatus L. were soaked and germinat-ed in the dark on wet filter paper in a growth chamberat 26 °C. Plantlets were transferred to plastic potsfilled with one-sixth-strength Hoagland’ solution in acontrolled climate chamber at a photon flux density of300 μmolm−2s−1 for 14 h/day and at a temperature of25/20 °Cday/night and a relative humidity of 70–80 %. Air pumping was provided to avoid oxygendeprival. After 3 weeks, plants were transferred tosingle pots (six plants per pot) and subjected to con-centrations of 0 (control), 25, 50, 100 and 150 μMNiCl·6H20 in one-sixth-strength Hoagland’ solutionfor 1 week. Solutions were sampled every day toevaluate Ni removal. At the end of the experiment,plants were harvested, carefully washed with distilledwater, separated in their organs, dried in an oven at80 °C and finally weighed.

2.2 Biomass Partitioning

The calculation of organ mass ratio was performed asthe ratio of the leaf (LMR), shoot (SMR) and root(RMR) biomass to the total plant biomass. Leaf areawas 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 tothe leaf mass.

2.3 Nickel Content Analysis

Ni determinationwas performed using an atomic absorp-tion spectrophotometer (Varian SpectrAA model 220FS,Mulgrave, Australia) on acidified samples of nutrientsolution and on digested samples of leaves, stems androots. The oven-dried material was finely ground(Tecator Cemotec 1090 Sample Mill; Tecator, Hoganas,

Sweden), weighed and mineralised. Mineralisation wasperformed by treating 250mg of powdered samples with4 ml of concentrated HNO3, 3 ml of distilled water and2 ml of H2O2 (30 %v/v in water), followed by heating(EXCEL Microwave Chemistry Workstation, PreeKemScientific Instruments Co., Ltd., Shanghai, China) in afour-step procedure: 100 °C for 1 min at 250 psi, 140 °Cfor 1 min at 350 psi, 170 °C for 1 min at 450 psi and200 °C for 12 min at 550 psi. Samples were then filteredand analysed. For each Ni concentration, the plant up-take ratio was calculated as the ratio of the total Nicontent (in microgram) of the plants to the Ni content(in microgram) of the corresponding growth solution atthe end of the experiment. The metal translocation indexwas calculated as the ratio of the Ni content (in micro-gram) of aboveground organs to the Ni content (inmicrogram) of the corresponding roots.

2.4 Statistical Analysis

The data reported refer to a single typical experimentwith six replicates. Normally distributed data wereprocessed with a one-way ANOVA. Statistical signif-icance of the mean data was assessed by Duncan’s testusing the SPSS software tool.

3 Results

The exposure of A. paniculatus plants to Ni resulted ina remarkable dry mass modification, according to theincreasing Ni concentrations of the growth solution(Fig. 1). In fact, the leaf, stem and root dry mass ofplants was progressively reduced as the Ni concentra-tion in the solution enhanced. The reduction in drymass was particularly evident in plants grown at150 μM Ni. In these plants, the root and stem drymass was more than halved compared to the controlbut leaf dry mass was also notably reduced. A de-crease of the root to shoot ratio was also observed inAmaranthus plants following the increase of the metalconcentration in the growth solution. Regarding bio-mass partitioning (Table 1), results showed that plantsallocated almost 50 % of the dry mass in the leaves,followed by stems and roots, irrespective of the Niconcentration in the nutrient solution. LMR washigher in plants exposed to 150 μM Ni compared toplants subjected to other Ni treatments, except forcontrol. In contrast, control and 150 μM Ni-treated

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plants showed a lower SMR than plants exposed to 25,50 and 100 μM Ni. A reduction of RMR as a conse-quence of Ni exposure was observed in Amaranthusplants, with the lowest significant value occurring at150 μM Ni. The modification of leaf biomass wasevaluated also by assessing the total leaf area andSLA. Both parameters showed a clear trend to de-crease as the Ni concentration in the growth solutionsincreased.

The Ni content of plants exposed to different Niconcentrations in the nutrient solution is reported inFig. 2. In leaves, a higher metal content was observedin plants subjected to 150 μM Ni compared to otherNi-exposed plants, with control plants showing thelowest values. Stem Ni content analysis revealed aprogressive enhancement of metal accumulation inthis organ with the increase of the Ni concentrationin the growth solution, and the highest value occurringin plants grown at 150 μM Ni. Concerning roots, ahigher Ni content was detected in plants exposed to100 and 150 μM Ni in comparison to those growing at25 and 50 μM Ni, with the lowest statistical valueobserved in control plants. As a result of the differentdistributions in the plant organs, the total Ni content ofAmaranthus plants was notably higher in plants grow-ing at 100 and 150 μMNi compared to those at 25 and50 μM Ni, and control plants showed the lowest Niamount. In Fig. 3, the modification of the Ni uptakeratio of plants to the increasing Ni concentrations inthe solution is reported. Control plants showed thehighest values for the Ni uptake ratio, and a progres-sive and significant reduction of this parameter withthe enhancement of metal content in the growth me-dium was observed. The metal translocation index,which referred to Ni, is shown in Fig. 4. Also, in thiscase, control plants showed the highest value, fol-lowed by plants exposed to 150 μM Ni, while thelowest values were observed for plants subjected to25, 50 and 100 μM Ni.

Nickel removal from the nutrient solution byAmaranthus plants was monitored on a daily basis,and trends referred to the different Ni concentrationsare reported in Fig. 5. Along the experimental timeinterval, a different Ni removal ability was shown by

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�Fig. 1 Leaf, stem and root dry mass (in gram per plant), androot to shoot ratio in plants of A. paniculatus L. exposed todifferent Ni concentrations for a week in hydroponics (mean ±S.E., n=6). Different letters in the columns correspond to sta-tistically different values (Duncan’s test, P≤0.05)

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plants depending on the Ni concentration in thegrowth solution. In fact, plants exposed to 25 μM Nihighlighted a higher and more constant Ni removalwith time compared to plants subjected to 50, 100 and150 μMNi and succeeded in removing almost 60 % ofthe initial Ni content of the growth solution at the endof the experiment. Lower removal abilities were foundin plants treated with 50 and 100 μM Ni, resulting inthe decontamination of the initial Ni solution byapprox. 33 and 25 %, respectively. In this latter case,Ni removal was exerted by plants during the first partof the experimental time and was subsequently dra-matically reduced. Plants exposed to 150 μM Nishowed the lowest Ni removal ability, with the reduc-tion of the Ni content of the growth solution at the endof the treatment being close to 17 %. Moreover, theseplants exhibited a strong reduction of Ni removalcapability just 2 days after the start of the Ni exposure.

4 Discussion

Reclamation of metal-polluted soils and waters is anissue that has been receiving increasing attention. Inparticular, the need to improve the quality of thewastewaters rises not only from the growing concernover the protection of the aquatic ecosystem but alsoas a result of the impact of water shortage on cropirrigation, leading many countries to reuse wastewater(Fatta-Kassinos et al. 2011). This practice, if not ap-propriately managed, could result in metal and otherpollutant accumulation in agricultural soils, with apossible contamination transfer into food plants, thusrepresenting a serious risk for animal and humanhealth. Phytoremediation has emerged as a suitable

technology to realise an economical and ecologicallysustainable approach to decontamination (Licht andIsebrands 2005; Schwitzguébel et al. 2009). The se-lection of plants with higher efficiencies for the re-moval of metals from polluted substrates, involvingtraits such as metal tolerance and the accumulation anddistribution into organs, is a basic issue for a wider andmore successful application of phytoremediation.

In this work, the capability ofA. paniculatus plants totolerate, accumulate into organs and remove Ni from anutrient solution was studied. Although the Ni concen-trations testedwere far higher than the legal Italian limitsfor subterranean waters and soils (ranging from 75- to400-fold and from 10- to 70-fold above the thresholdallowed, respectively), they were representative of Niconcentrations in polluted soils (Marchiol et al. 2004)and waters (Farkas et al. 2007). Moreover, these Niconcentrations were chosen to induce morphophysio-logical responses in Amaranthus plants and to appreci-ate the potential for this species to decontaminate ametal-polluted substrate (Shevyakova et al. 2011). Thetime scale of plant exposure to Ni was typical of a short-term experiment, allowing the evaluation of metal tol-erance and the accumulation ability of plants in hydro-ponics (Leblebici and Aksoy 2011; Shevyakova et al.2011). Plant adaptation to cultivation under hydroponicswas satisfactory, confirming previous investigations(Zhang et al. 2010; Shevyakova et al. 2011), and controlplants showed a biomass production similar to plantsgrown in pots filled with uncontaminated soils (data notshown). The exposure of Amaranthus plants to increas-ing Ni concentrations caused different growth reduc-tions depending on the organ studied. Roots showed aparticular sensitivity to Ni concentrations above 25 μMNi, while leaves and especially stems exhibited a greater

Table 1 Leaf, stem and root mass ratio (in gram per gram), total leaf area (in square centimetre) and specific leaf area (SLA, in squarecentimetre 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 Root mass ratio Total leaf area SLA

0 0.566 (0.017) ab 0.277 (0.023) b 0.156 (0.016) a 968 (95) a 638 (19) a

25 0.479 (0.047) b 0.384 (0.041) a 0.136 (0.012) ab 547 (52) b 561 (70) ab

50 0.478 (0.021) b 0.391 (0.022) a 0.114 (0.016) ab 364 (53) c 539 (10) ab

100 0.512 (0.032) b 0.348 (0.041) ab 0.117 (0.009) ab 326 (24) c 520 (20) b

150 0.611 (0.026) a 0.262 (0.013) 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)

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ability to tolerate Ni concentrations up to 100 μM(Fig. 1). At 150μMNi, plants showed visible symptomsof stress damage such as a diffuse chlorosis (data notshown), and the biomass production was severely af-fected. Damage at the leaf level due to increasing Niconcentrations in the solution was also proven by themarked reduction in total leaf area and SLA (Table 1),which are the two parameters that are usually measuredto evaluate tolerance to metals at the leaf level(Sebastiani et al. 2004; Fernandez et al. 2012). A reduc-tion of leaf area as a consequence of the exposition tometals is a commonly reported plant response (Zacchiniet al. 2009; Di Baccio et al. 2010). A higher root bio-mass reduction in comparison to shoots was alsoreported by Zhang et al. (2010) in Amaranthus plantstreated with a different non-redox metal, such as Cd, inhydroponics. Regarding Ni-hyperaccumulator plants, ahigher sensitivity of roots to Ni toxic concentrationscompared to shoots was reported in Alyssum bertoloniiby Galardi et al. (2007), while Assunção et al. (2003)showed a different strategy for Ni tolerance in Thlaspicaerulescens, involving a higher resistance of roots withrespect to shoots. Biomass partitioning was also evalu-ated by calculating the organ mass ratio, which is asuitable parameter to investigate the growth response

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Fig. 3 Nickel uptake ratio (in milligram per milligram) in plantsof A. paniculatus L. exposed to different Ni concentrations for aweek in hydroponics (mean ± S.E., n=6). Different letters in thecolumns correspond to statistically different values (Duncan’stest, P≤0.05)

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�Fig. 2 Leaf, stem, root and total nickel content (in microgramper plant) in plants of A. paniculatus L. exposed to different Niconcentrations for a week in hydroponics (mean ± S.E., n=6).Different letters in the columns correspond to statistically dif-ferent values (Duncan’s test, P≤0.05)

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in plants subjected to metal treatments (Sebastiani et al.2004). A biomass partitioning plasticity in plants ex-posed to metals was reviewed by Audet and Charest(2008) as a trait to be considered for phytoremediationpurposes. In the present work, the allocation of biomasswas predominantly in the shoot and especially in leaves,regardless of the Ni concentration in the solution. Anincrease in LMR and a decrease in RMR characterisedthe plants exposed to 150 μMNi (Table 1). This featurewas associated with significant damage, in terms ofbiomass reduction, exerted by Ni on Amaranthus plants.A reduction of RMR in barley plants treated with Cd

was observed by Vassilev et al. (1998) as part of thetoxicity effect caused by this metal on plant growth.

Amaranthus plants showed higher Ni accumulationwhen exposed to 100 and 150 μM Ni compared tolower Ni concentrations (Fig. 2). This trend differedslightly among organs, even though the exposure ofplants to 150 μM Ni always resulted in the highest Niaccumulation in the organs studied. Except for thecontrol plants, whose Ni content was likely due to aphysiological metal content in seeds and impurities ofthe growth solution, and irrespective of the Ni con-centrations supplied, roots accumulated the largestamount of metal, followed by the stem and leaves. Ahigher Ni content in roots compared to leaves was alsoobserved in Ni-treated Matricaria chamomilla plants,a non-hyperaccumulator plant species, by Kováčik etal. (2009) in a similar experiment. As expected, theAmaranthus plants assayed in this work exhibited amarkedly lower Ni accumulation capability comparedto a Ni-hyperaccumulator plant, such as Berkheyacoddii (Robinson et al. 2003), and the distributionamong organs also differed notably, with the leavesof Berkheya being the primary sink for the metal, asfound in other Ni-hyperaccumulator plants (Wenzel etal. 2003; Galardi et al. 2007). In order to evaluate theability of plants to accumulate Ni depending on the Nicontent of the solution, the uptake ratio was calculated(Fig. 3). Amaranthus plants showed a progressivedecrease of Ni uptake capability with increasing Niavailability in the solution, with the highest perfor-mance occurring in control plants. This feature canbe associated with the damage observed at the rootlevel in terms of a reduction in biomass production asthe Ni concentration in the growth medium increased.A reduction of root cell membrane functionality inplants treated with enhanced Ni concentrations in thesolution was also reported (Llamas et al. 2008; Sanz etal. 2009), possibly occurring as a consequence ofoxidative stress induction (Chen et al. 2009).

The metal translocation from roots to shoots is asuitable trait to select a plant for phytoremediation(Prasad and Freitas 2003; Zacchini et al. 2009). Infact, the preferential accumulation of metals in theaboveground organs results in higher metal removalefficiencies in plants, as these organs are easily andcommonly harvested. In this regard, hyperaccumulatorplants are characterised by a large metal accumulationin their shoots. Ni-hyperaccumulators plants, for in-stance, accumulate this metal beyond 1,000 μgg−1 in

Days of treatment

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Fig. 4 Nickel translocation index in plants of A. paniculatus L.exposed to different Ni concentrations for a week in hydropon-ics (mean ± S.E., n=6). Different letters in the columns corre-spond to statistically different values (Duncan’s test, P≤0.05)

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shoots as a tolerance response by safely storing themetal at the tissue and cellular level (Kramer 2010).On the contrary, the restriction of metal movement toshoots is a defence response carried out by non-hyperaccumulator plants (Seregin and Kozhevnikova2006), in order to protect the photosynthetic processesoccurring in leaves by the oxidative attack exerted bymetals (Pietrini et al. 2010b). In the present work(Fig. 4), Amaranthus plants exposed to different Niconcentrations in the growth solution showed a lowercapability to translocate Ni in the shoots compared tocontrol plants, where the Ni transport along the plantaxis can be associated to the limited amount requestedfor the physiological processes of leaves (Seregin andKozhevnikova 2006; Chen et al. 2009). Therefore, thelimitation of metal movement to leaves in plants ex-posed to 25, 50 and 100 μM Ni can be ascribed to adefence response aimed at avoiding the onset of oxi-dative damage in the photosynthetic machinery inleaves. The higher Ni translocation observed in plantsexposed to 150 μM Ni could be due to an impairmentof metal transport restriction processes, resulting in theaboveground biomass reduction found at this Ni con-centration. Accordingly, a low Ni translocation abilityin Amaranthus species was also found by Mellem etal. (2009).

Nickel removal ability by A. paniculatus plants wasfollowed on a daily basis by analysing the metalconcentration in the solution (Fig. 5). Different trendsof Ni removal along the experimental time intervalswere observed in plants depending on the Ni concen-tration in the growth solution. In particular, a signifi-cant amount of Ni removal was shown by plantsexposed to 25 μM Ni, as they succeeded in removingalmost 60 % of the initial Ni amount of the solution.On the contrary, a limited Ni removal ability wasobserved in plants exposed to Ni concentrations higherthan 25 μM, which highlighted a notable metal re-moval capability in the first few days of exposure butprobably suffered the toxicity effects exerted on plantsby the high metal concentrations in the second part ofthe experimental interval.

Very few studies have been reported in the literatureregarding the evaluation of the metal phytoremedia-tion ability of plants of the Amaranthus genus, withcontrasting indications about the suitability of thisplant species for the decontamination of metal-polluted substrates (Mellem et al. 2009; Li et al.2009; Zhang et al. 2010; Shevyakova et al. 2011). In

this work, A. paniculatus plants showed a notable Niaccumulation ability, mostly confined to the roots, anda good tolerance to metal exposure at concentrationsthat are far higher than those allowed by Italian lawsfor public soils and waters. The low metal transloca-tion from roots to aerial parts could indicate a possiblelimitation for the utilisation of this plant species for Niphytoextraction in metal-polluted soils, even if thelarge biomass produced by Amaranthus species couldcompensate the lower Ni accumulation in shoots com-pared to other plants species. On the contrary, theconfinement of Ni accumulation in the roots thatavoided damages at the leaf level, as evidenced bythe lack of biomass reduction at a concentration of25 μM Ni, could open interesting perspectives for theutilisation of the Amaranthus plant species for rhizo-filtration of metal-polluted waters, either alone or inassociation with conventional wastewater treatmentprocesses.

In conclusion, the good adaptation to hydroponicsand the valuable Ni removal observed for A. panicu-latus plants in this study, especially at environmentallyrelevant metal concentrations, indicate the remarkablepotential of this plant species for the decontaminationof polluted substrates in water-based systems (wet-lands or mesocosms). Further evaluations over a lon-ger time scale and by the utilisation of actual metal-polluted wastewater are planned to better ascertain thesuitability of this plant species for metal phytoremovalpurposes.

Acknowledgments This work was realised within the jointproject between National Research Council of Italy and RussianAcademy of Sciences “Mechanisms of plant adaptation to stressaction of heavy metals: possible implications for the phytore-mediation technology”. Authors wish to thank Mr. ErmenegildoMagnani for his expert technical assistance in metal contentanalysis.

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