Chemosphere 119 (2015) 217–223

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Screening biological traits and fluoride contents of native vegetations inarid environments to select efficiently fluoride-tolerant native plantspecies for in-situ phytoremediation

http://dx.doi.org/10.1016/j.chemosphere.2014.06.0070045-6535/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +33 4 13 55 12 30.E-mail address: isabelle.laffont-schwob@imbe.fr (I. Laffont-Schwob).

Asma Boukhris a,b, Isabelle Laffont-Schwob a,⇑, Imed Mezghani b, Lefi El Kadri b, Pascale Prudent c,Anca Pricop a,c, Thierry Tatoni a, Mohamed Chaieb b

a Aix Marseille Université, Avignon Université, CNRS, IRD, Institut Méditerranéen de Biodiversité et d’Ecologie marine et continentale (IMBE), 52 avenue Normandie-Niemen,13397 Marseille cedex 20, Franceb Plant diversity and Ecosystems in Dry Environment, Faculty of Science, University of Sfax, 3000 Sfax, Tunisiac Aix Marseille Université, CNRS, LCE FRE 3416, Laboratoire de Chimie de l’Environnement, case 29, 3 place Victor Hugo, 13331 Marseille cedex 3, France

h i g h l i g h t s

� Emphasing a gradient of fluoride pollution in soils of the arid East-coast of Tunisia.� Statement of a fluoride transfer to the aerial parts of steppic native vegetation.� Specific biological traits of the native plant species are useful for plant selection.� Proposal of an efficient approach for plant screening for in-situ phytoremediation.

a r t i c l e i n f o

Article history:Received 26 February 2014Received in revised form 19 May 2014Accepted 2 June 2014

Handling Editor: X. Cao

Keywords:Fluoride pollutionPollutant toleranceFunctional traitsPhytostabilization

a b s t r a c t

High fluoride pollution has been detected in the surrounding soils of the coastal superphosphate indus-tries in the Gulf of Gabes (Southeast of Tunisia). A study was conducted in vicinity of factories analysingplant functional traits combined with plant fluoride accumulation and soil metal concentrations aimingto screen more efficiently native plant species tolerant to this pollution. Aerial parts of 18 plant speciesout of the 10 most abundant species per site were harvested on two polluted sites of Gabes and Skhira atthe vicinity of the factories and on the less polluted site of Smara. Native plant species accumulatedfluoride following the gradient of soil pollution. Fluoride contents of plant aerial parts ranged from37 mg kg�1 to 360 mg kg�1 and five plant species were only found in the most polluted site. Howeverthese latter had low biomass and soil cover. Crossing biological traits and fluoride contents, a selectiongrid for potentially restorative plant species enabled the selection of three native perennials i.e.Rhanterium suaveolens, Atractylis serratuloides and, Erodium glaucophyllum as potential candidates foran in-situ phytoremediation program on arid fluoride-polluted sites. This approach may be used in otherfluoride-polluted Mediterranean environments.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Under arid conditions, plant perennials play a key role in thefunctioning and services of the ecosystem reducing soil erosion.Along the East-coast of Tunisia, the steppe shrub vegetation withbiological traits adapted to drought (Jauffret and Visser, 2003), issubmitted to stressful and disturbing environment. Since thefifties, the chemical industry of phosphogypsum has developed insome Tunisian cities along this coast. A main source of

environmental pollution is due to industries of phosphates rockmining and processing, with release of trace metals and fluoridein particular. Rain and storage water can seep through the stock-piled phosphogypsum, carrying the contaminants into the soiland groundwater; air pollution also occurs due to wind-dispersedwaste products. More precisely, in the manufacturing process andduring the phosphate attack by sulphuric and phosphoric acids,fluoride compounds such as HF, H2SiF6, and CaF2 release off bythe factory chimneys (Rouis and Ballivy, 1990; Tayibi et al.,2009). In addition, analysis of the air surrounding the factory ofSfax has shown that fluoride air contents ranged between 3 and12 lg dm�3 d�1 (Mezghani et al., 2005; Ben Abdallah et al.,

218 A. Boukhris et al. / Chemosphere 119 (2015) 217–223

2006a). Fluoride pollution is always concomitant with heavy metalcontamination (Rouis and Ballivy, 1990; Tayibi et al., 2009), caus-ing a mixed pollution of soils.

Phytoremediation is an emerging approach for the remediationof polluted sites, using plants and their associated micro-organ-isms (Pilon-Smits, 2005). In the context of the fluoride pollutionof the East coast of Tunisia, two avenues are worth exploring:one using plants as a vegetation barrier aiming at mitigating pol-luted particle transfer to the surroundings, and another usingplants for pollutant containment in soils namely phytostabiliza-tion. For both, targeting potential native plant species to limit thedispersion of this phytotoxic pollutant (Fornasiero, 2001), andavoiding introduction of potentially invasive plant species thatmay result in decreasing regional plant diversity (Mendez andMaier, 2008), seem to be an innovative idea for in situ pollutedsites’ phytoremediation. However, as reported by Roccotielloet al. (2014), only a relatively small number of Mediterranean plantspecies is being studied for phytoremediation purposes. Moreover,although fluoride effects on crops such as vineyards (Ben Abdallahet al., 2006b) and, pomegranate and almond orchards (BenAbdallah et al., 2006a), have already been evaluated on some ofthese sites, there is a lack of knowledge about the impact of thispollution on natural vegetation in the vicinity of the factories,and a fortiori, on their fluoride tolerance.

The sites of Gabes and Skhira were selected and compared toSmara, the reference site. On all three sites, the functional traitsof vegetation differed related to environmental features, and anal-ysis of life history data showed the reduction of plant cover ofperennials to only 5%. A continuous process of degradation of thesesteppes has been demonstrated by Hanafi and Jauffret (2008). How-ever, this environmental deterioration has never been related toindustrial pollution. Thus, there is no information on potential tol-erance of the native species to this pollution. Therefore, as recentlyreported by Schellberg and Pontes (2012) and Pérez-Harguindeguyet al. (2013), gathering species functional traits is a recent promis-ing approach to better understand the ecological functioning ofecosystems. Consequently, in this study, we combined the levelof fluoride accumulation in native plant species with informationof their life form, dispersal and dissemination, as well as their pal-atability and ecological preferences, aiming at selecting the mostaccurate local plants species to support a phytoremediation processon fluoride-polluted soils in arid environments.

2. Materials and methods

2.1. Study area

The three studied sites are located along the East coast of Tunisia(Fig. 1). The sites of Gabes and Skhira correspond to a long plainalong the coast (30 m above sea level for both sites) and are nearbyindustrial phosphate factories. The site of Smara (15 m above sealevel) is 70 km and 20 km from Gabes and Skhira, respectivelyand, in similar geochemical, ecological and climatic conditions asboth former sites. All the sites are submitted to an arid climate withlow average rainfall (from 167 to 176 mm average annual pluviom-etry with average annual temperature from 18.8 to 19.3 �C). Theirdistance to the sea were 1.8, 2.8 and 4.3 km for Skhirra, Gabesand Smara, respectively, and they are thus not submitted toseasprays. Climatic parameters were derived from the Wordclimdatabase (Hijmans et al., 2005).

2.2. Floristic survey

First, we compared the spontaneous floristic structure and com-position of Gabes and Skhira with those in Smara in March 2011,on one ha surface area per site. The vegetation in the three sites

is typical of the arid chamaephytic steppes with Rantheriumsuaveolens (Le Houérou, 1969), however with different faciesdepending on the local disturbances and variability of the chemicalcomposition of soils. Therefore, for each site, the ten plant speciesoffering the most important plant cover were selected and at theend of the survey, the lists of these most abundant plant specieswere compared between the three sites to define which were incommon. The plant identification was confirmed at the laboratoryusing the Flora of arid and Saharan zones (Chaieb and Boukhris,1998) and voucher specimens are kept at the herbarium of theUniversity of Sfax.

2.3. Plant and soil collections

In March 2011, three composite samples of 1 kg of topsoil(0–20 cm depth) were collected at the vicinity of the plant specieson each site, to analyze concentration of major and trace elements,including fluoride. Each soil sample was composed of 6 subsamplesof equal weight, separated by a distance of 1 m according to a crosspattern on each site. Since the samples were composite samplesconstituted by equal quantity of 6 mixed subsamples, they couldbe considered representative of each site. Soil samples were storedin plastic bags until return to the laboratory, where they weredried at room temperature until analysis. Three individuals of the10 most abundant plant species were sampled by a random har-vest i.e. three replicates of 30 samples out of 18 plant species. Onlyleaves and stems of the middle of the shoots were taken and storedin paper bags until return to the laboratory. Plant samples werethen dried at 80 �C during 72 h in an oven (Memmert).

2.4. Fluoride analysis

Each composite sample of soil and each plant sample wasanalyzed in triplicates. Soil samples were sieved to 2 mm mesh.Five grams of soil subsamples were mixed with HCl:H2O (1:1)during 90 min. Then, HCl extract was mixed with total ionicstrength-adjustment buffer and analyzed using a fluoride-specificion electrode (inlab/Model WTW) coupled to a pH-meter (pHION R503) (Mezghani et al., 2005).

Dried plant samples were ground (<1.0 mm) in a stainless steelmill. For each species, 0.5 g of powdered sample was heated in anelectric oven with a mixture of carbonate potassium and carbonatesodium at 700 �C during 3 h. Then samples were mineralized byhydrochloric acid wet process. After filtration and adjustment withdistilled water, F concentrations were determined by potentiome-try (Mezghani et al., 2005).

Accumulation factors (AF) for fluoride were determined accord-ing to Mendez and Maier (2008) i.e. total F concentration in shoottissue/total F concentration in soil.

2.5. Analysis of other major and trace elements

Aliquots of the same composite soil samples were ground(RETSCH zm 1000 with tungsten blades) to pass through a0.2 mm titanium sieve before analyses. Soils were mineralized ina microwave mineralizer (Milestone Start D) using aqua regia (1/3HNO3 + 2/3HCl). The mineralization products were filtered witha 0.45 lm mesh and metal levels were determined by ICP-AES(Jobin Yvon Horiba, Spectra 2000) (Affholder et al., 2013). Qualitycontrols and accuracy were checked using standard soil referencematerials (CRM 049-050, from RTC-USA) with accuracies within100 ± 10%. Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Ti and Zn were analyzedbut results for As and Cd were not shown because most of theconcentrations of these elements were below the detection limitof ICP-AES (i.e. <0.1 mg kg�1 for Cd and <1.4 mg kg�1 for As).

Fig. 1. Map of the superphosphate factories and the three studied sites localization on the basis of the bioclimatic map by Emberger (1954) in arid bioclimate. GPScoordinates (Lambert II) were 33�5404900N, 10�303900E; 34�2001900N, 10�705200E; 34�2604300N, 10�1604200E for Gabes, Skhira and Smara, respectively.

A. Boukhris et al. / Chemosphere 119 (2015) 217–223 219

2.6. Soil pH and conductivity

Measurements of pH and conductivity were determined bypotentiometry in a 1:5 soil:water suspension using a portable pHmeter (Hanna Instruments�), and a WTW� conductivity meter on5 soil samples per site (Baize, 1988).

2.7. Statistical analysis

Statistical analyses were performed for all data using JMP 9 sta-tistical software (SAS Institute, Cary, North Carolina, USA). Differ-ences between fluoride concentrations and major and traceelements in soil on the three sites and between fluoride contentsin aerial parts of the plant species in the three sites were studiedwith the nonparametric Dunn’s test.

Table 1Concentrations of major and trace elements, pH and conductivity in soils of the three stud

Soillocation

Major elementconcentration (g kg�1)

Trace element concentration (mg kg�1)

Al Fe F Cr Cu Mn

Gabes 8.6 ± 0.1b 5.72 ± 0.07b 1431 ± 289a 66 ± 2b 3.0 ± 0.1b 82 ± 2a

Skhira 11. 9 ± 0.3a 7.6 ± 0.2a 144 ± 16b 120 ± 7a 3.5 ± 0.2a 87 ± 5a

Smara 9.5 ± 0.3b 5.6 ± 0.2b 35 ± 8c 41 ± 1c 2.1 ± 0.1c 61 ± 4b

Element concentrations, pH and conductivity are expressed as means ± SD (n = 9). Differ(Dunn test).

3. Results and discussion

3.1. Soil characterization

All three soils are gypsum–calcareous soils in agreement withMtimet (2001). Among the three locations, the values of F revealeda gradient of fluoride pollution, with the highest levels in Gabeswith an average of 1431 ± 289 mg kg�1 and the lowest in Smarawith only 35 ± 8 mg kg�1 (Table 1). With a worldwide average con-centration of fluoride in soils of 625 mg kg�1, concentrations>1000 g kg�1 can occur in soils affected by anthropogenic inputssuch as phosphate fertilizers (Kabata-Pendias, 2011). Gabes andSkhira sites are ca. 0.5 km East of superphosphate factories. Asreported by Tayibi et al. (2009), only 15% of phosphogypsumworldwide production is recycled, while 85% is stored in the vicin-ity of factories in coastal regions. Skhira is concerned by this type

ied sites.

pH Conductivity(mS cm�1)

Ni Pb Ti Zn

48 ± 2b 5.3 ± 0.3a 0.78 ± 0.01a 15.2 ± 0.5a 7.58 ± 0.03a 1.5 ± 0.7a

109 ± 7a 4.6 ± 0.3a 0.82 ± 0.01a 15.3 ± 0.5a 7.58 ± 0.09a 1.6 ± 0.6a

29.4 ± 0.4c 3.2 ± 0.1b 0.57 ± 0.02b 10.7 ± 0.5b 7.53 ± 0.01a 2.30 ± 0.01a

ent superscript letters in the same row indicate a significant difference at p 6 0.05

220 A. Boukhris et al. / Chemosphere 119 (2015) 217–223

of external non-covered storage and phosphogypsum managementseems an important issue faced by this phosphate factory. In Gabes– where is the oldest factory – the surrounding soils are affected bya long-term exposure to fluoride atmospheric deposits and corre-sponded to the highest concentrations of fluoride we analyzed.

On the three sites, the pH values (ca. 7.56) revealed neutral toslightly alkaline soils, with the conductivity values (1.5–2.3 mS cm�1), that confirmed that they were salt-affected (Table 1).In the same conditions of neutral soils, considering the sustainablerelationship between pH and release/uptake of metals by plants inpolluted area (Kirkham, 2006; Conesa et al., 2011), comparison ofmajor and trace elements showed that the highest concentrationsof Al, Fe, Cr, Cu, and Ni occurred in the site of Skhira with 11.9 g kg�1,7.6 g kg�1, 120 mg kg�1, 3.5 mg kg�1 and, 109 mg kg�1, respec-tively. The soil of Smara was the least polluted soils notably for Cr,Cu and Ni. Occurrence of these elements along with fluoride iswell-documented (Tayibi et al. 2009) and our works clearly confirmthese findings.

3.2. Occurrence of plant species and their functional traits

During the field investigations, a total of 66 plant species on aone ha surface area were identified (data not shown). Out of these,18 were investigated since they represented the most abundantplant species on the sites in agreement with reports on other aridsteppes of Tunisia by Chaieb and Boukhris (1998) as well as Jauffretand Visser (2003). To better understand ecological processes andecosystem functioning of these steppes to overcome fluoride toxic-ity, we gathered, in this study, information on life form, dispersal

Table 2Decisional traits for selection of potential plant species to prevent fluoride dispersal out o

Plant species (plant family) Life formtrait

Disseminationperiod

Dispers

Anarrhinum brevifolium [1](Scrophulariaceae)

Ch Late Spr Barocho

Artemisia Campestris [2] (Asteraceae) Ch Aut Anemo

Atractylis carduus [3] (Asteraceae) He/Ch Late Spr Anemo

Atractylis serratuloides [1] (Asteraceae) Ch Late Spr Anemo

Atriplex inflata [4] (Amaranthaceae) Th Late Aut-earlyWin

Anemo

Cynara cardunculus [5] (Asteraceae) Ge Early Sum Anemo

Cenchrus ciliaris [6] (Poaceae) He Spr, Sum-Aut Anemo

Diplotaxis harra [7] (Brassicaceae) Th Early Spr Anemo

Erodium glaucophyllum [1](Geraniaceae)

He Early Spr Anemo

Hammada scoparia [1] (Amaranthaceae) Ch Early Win Anemo

Helianthemum intricatum [1](Cistaceae)

Ch Late Spr Barocho

Paronychia chlorothyrsa(Caryophyllaceae)

Th Late Spr Barocho

Plantago coronopus [8] (Plantaginaceae) He Late Spr Barocho

Reaumuria vermiculata [1](Tamaricaceae)

Ch Late Spr Anemo

Rhanterium suaveolens [1] (Asteraceae) Ch Early Sum Anemo

Salsola brevifolia [1] (Amaranthaceae) Ch Late Aut Anemo

Teucrium polium [2] (Lamiaceae) Ch Late Spr Barocho

Thymelaea hirsuta [9] (Thymelaeaceae) Ch Late Spr Myrme

1–9 refers to Floret (1981), Vickerman et al. (2002), Sotek (2006), Tlig et al. (2008), GominJoubert et al. (2012), respectively, literature giving information about these under-docuCh: Chamaephyte; He: Hemicryptophyte; Th: Therophyte; Ge: Geophyte; Spr: Spring; ALack of values means no determined index for the considered plant species.* Values in brackets refers to palatability indices as defined by Le Houérou and Ionesco

and dissemination, palatability and ecological preference of thenative plant species growing on the three sites (Table 2).

The 18 plant species represent 12 plant families, and corre-spond to four life forms (Raunkiaer, 1937) i.e. 11 chamaephytes,3 hemicryptophytes, 3 therophytes and 1 geophyte.

Three of them, (Artemisia campestris L., Atractylis carduus (For-ssk.) C. Chr. (Asteraceae) and Cenchrus ciliaris L. (Poaceae)), wereonly found in Smara. Both former species are arable weeds andthe latter is a psammophyte. There is no information about theirfluoride tolerance but we assume that they may not be good can-didate for a field evaluation on fluoride tolerance because (i) theyare not found in the fluoride contaminated sites and (ii) two ofthem being arable weeds are more characteristic of organic distur-bance and the Poaceae is a hemicryptophyte with a reduced bio-mass and seasonal aboveground part.

Five plant species were only found in the most polluted site(Gabes) i.e. Cynara cardunculus L., Paronychia chlorothyrsa (L.) DC.,Plantago coronopus L., Diplotaxis harra (Forssk.) Boiss and Salsolabrevifolia Desf. Indeed, even if these 5 species appeared as the mosttolerant to high fluoride levels, their biological traits do not matchwith in-situ phytostabilization or vegetation barrier strategies. S.brevifolia is a halophyte and is mainly eaten by Camelidae andnot by sheep and goats (Gamoun et al., 2010). In the same way,even if its palatability is low, C. cardunculus has a culinary regionalimportance in Tunisia, since it is used in traditional cooking (Khaldiet al., 2012). These plant species have been discarded of our selec-tion to prevent fluoride transfer to the food chain and, for the lat-ter, human health risk. P. coronopus is a Mediterranean–Atlanticspecies found in various habitats preferably in slightly saline

f a pool of 18 native plant species.

al type Ecological preference Palatabilityindex*

Occurrence on the threesites

ry Gypsophyte High (4) 2/3

chory Arable weed Low (1) 1/3

chory Arable weed Low 1/3

chory Gypsophyte Low (2) 3/3

chory Gypso-halophyte Not (0) 1/3

chory Arable weed Low (1) 1/3

chory Psammophyte High (5) 1/3

chory Arable weed Low (2) 1/3

chory Common Low (1) 3/3

chory Limnophyte Low 2/3

ry Gypsophyte High 3/3

ry Arable weed High (1) 1/3

ry Halophyte Low (2) 1/3

chory Gypso-halophyte Low (1) 1/3

chory Steppe pioneer Low (2) 3/3

chory Psammophyte High (3) 1/3

ry Psammophyte Low (2) 2/3

cochory Land degradationindicator

Not (0) 2/3

ho et al. (2009), Gamoun et al. (2010), and El-Bana et al. (2010), Brown et al. (2012),mented arid plant species.ut: Automn; Win: Winter; Sum: Summer.

(1987).

A. Boukhris et al. / Chemosphere 119 (2015) 217–223 221

conditions (Khaldi et al., 2012). Like P. chlorothyrsa and D. harra,this species has a low biomass. These 3 species are thus not suit-able for a phytostabilization program because of their low soilcover.

Only 4 plant species i.e. Helianthemum intricatum Murb., R.suaveolens Desf., Erodium glaucophyllum L. and Atractylis serratulo-ides (Cass.) D.C. were found in all three sites that may signify theirgreat ecological amplitude. All of them are perennials. The threelatter have low index of palatability compared to H. intricatum witha higher index: this latter species may be excluded from ourselection grid in the perspective of fluoride containment.

3.3. Fluoride content in plant aerial parts

Plant species accumulation of fluoride differed significantly(p 6 0.05) in the three sites (Table 3, Fig. 2), in agreement withthe gradient of pollution showed by soil analyses (Table 1). InGabes, plant F accumulation was ca. three times higher than inSkhira. Average fluoride concentration of the vegetation based onthe ten selected plant species in the site of Gabes was about272 ± 103 mg kg�1 compared with 99 ± 40 mg kg�1 in Skhira andonly 46 ± 11 mg kg�1 in Smara. These results clearly showed a gra-dient of fluoride contamination of the vegetation linked to the soilfluoride contamination. The background level of fluoride in vegeta-tion is usually under 10 mg kg�1 and the maximum recommendedlimit for F content in hay and pasture grass is 30 mg kg�1 (Davisonet al., 1983) in agreement with the results by Arnesen (1997). A.serratuloides and Hammada scoparia from Smara were the less con-taminated of all samples with ca. 37 mg kg�1 fluoride howeverabove the threshold proposed by Davison et al. (1983) and thehighest mean values (about tenfold higher) were obtained for H.

Table 3Fluoride concentration in shoots of the common species of the three studied sites and acc

Plant species Shoot F concentration (mg kg�1)

Gabes Skhira

Atractylis serratuloides 252 ± 107aA 89 ± 13abAB

Erodium glaucophyllum 311 ± 9aA 70 ± 7bAB

Helianthemum intricatum 366 ± 28aA 102 ± 16abAB

Rhanterium suaveolens 360 ± 132aA 159 ± 37aAB

Results of shoot F concentration are expressed as mean ± SD (n = 3). For shoot F concentradifference at p 6 0.05 and different upper case superscript letters in a same line indicat

Fig. 2. Fluoride contents in aerial parts (mg kg�1, dry weight) of the ten most abundant pof all samples in all sites i.e. 135 mg kg�1.

intricatum, Paronychia chlorothyrsa and, R. suaveolens in Gabes(ca. 360 mg kg�1 fluoride). Amongst the five species onlyfound in Gabes, two groups were distinguished i.e. one with highfluoride content (mean = 325 ± 37 mg kg�1) corresponding to C.cardunculus, P. chlorothyrsa and P. coronopus and, another withaverage fluoride content (mean = 178 ± 46 mg kg�1) correspondingto D. harra and S. brevifolia (Fig. 2). These results suggest that strat-egies of fluoride storage may differ from one species to another.

Fluoride pollutants have effects both via root uptake and/or viadirect foliar absorption (Fornasiero, 2001). Though, it is generallyconcluded that in situations where plants are growing in the vicin-ity of a source of atmospheric fluoride, foliar fluoride concentra-tions will be dominated by direct uptake from the air and thecontribution from the soil will be minimal (Braen and Weinstein,1985). However, there is no evidence to confirm this in our fieldsituation where the total or ‘‘available’’ soil fluoride has been sig-nificantly increased by deposition (Davison and Weinstein, 2006)and may be considered as a historical pollution notably in Gabes.

The 4 common species on the three sites showed high fluoridecontents in their aerial parts in the Gabes site i.e. 366 ± 28,360 ± 132, 311 ± 9 and 252 ± 107 mg kg�1 of fluoride, forH. intricatum, R. suaveolens, E. glaucophyllum and A. serratuloides,respectively (Table 3). Mezghani et al. (2005) with similar condi-tions of pollution in Sfax (Tunisia) showed lower levels of fluoridein leaves of Prunus harmonica (50 mg kg�1), Ficus carica(190 mg kg�1) and Rosa agrestis (70 mg kg�1) but higher levels inleaves of Olea europaea (420 mg kg�1). The highest fluoride contentin olive trees may be linked to the occurrence of hairs, thick layersof epicuticular waxes and other leaf surface patterns. It is thusinteresting to investigate more on this morphological trait in thenative flora in link with fluoride deposition.

umulation factor in shoot (concentration in plant/concentration in soil).

Accumulation factor

Smara Gabes Skhira Smara

37 ± 8aB 0.176 0.618 1.05744 ± 10aB 0.217 0.486 1.25768 ± 11aB 0.256 0.708 1.94351 ± 8aB 0.252 1.104 1.457

tions, different lower case superscript letters in a same column indicate a significante a significant difference at p 6 0.05 (Dunn test).

lant species for each site. The horizontal line corresponds to the mean concentration

222 A. Boukhris et al. / Chemosphere 119 (2015) 217–223

The difference of fluoride contents between the native plantspecies and tree crops could also be at least partly explained by dif-ference between shrubs and trees in size and in exposure of theleaves to atmospheric pollution (Sæbø et al., 2012). Concerningthe four common native plant species in this study, leaves fromchamaephytes and hemicryptophytes may presumably be moreexposed to polluted particles from soil. This hypothesis seemsparticularly accurate for E. glaucophyllum, with flat leaves, withan average leaf area of 6 cm2, arranged in rosettes at the soil levelmainly submitted to processes of saltation and reptation of pol-luted particles that may occur on the native vegetation, accordingto the review on wind-blown sand and dust by Kok et al. (2012).However, no macroscopic symptoms of F toxicity on plants weredetected on the field even in Gabes. As emphasized by Álvarez-Ayuso et al. (2011), plant F uptake from areas receiving F inputshas focused increasing attention due to the risk of plant F accumu-lation and further transfer to the higher trophic levels of foodchain. However, less attention has been paid to fluoride toleranceby plants.

3.4. Crossed criteria for an efficient plant species selection for in-situphytoremediation

Six ecological criteria were chosen for the grid (Table 2). Thefirst one corresponds to the Raunkiaer biological traits that catego-rize life-form of the plant species. For a strategy of containment,chamaephyte type seems to be of interest since the therophyteshave to be discarded with short life-cycle and non-permanent rootsystems. This criterion alone will restrict the list of the potentialcandidates only to 11 species. The dissemination trait has to betaken into account for in-situ use since using plants with concom-itant period of dissemination may simplify the phytoremediationcompletion. This criterion has to be grouped with seed dispersaltrait giving information on the competitive vigour and the abilityto exploit patches rich in key resources to species (Pérez-Harguindeguy et al., 2013) to better understand the processes ofvegetation development and to accompany it in an ecological res-toration purpose. Both criteria are however not determining for theplant selection.

Plant tolerance to fluoride may not be dissociated from the soilcharacteristics. Plant ecological preference to gypsum in relationwith calcium soil availability (gypsum crust) may explain toleranceprocesses in plants to F� ion toxicity by trapping them in CaF2

insoluble complex form less disturbing to the metabolism ofgypsophytes. This physiological mechanism has been reported asan explanation to the fluoride resistance of pomegranate trees insimilar polluted conditions in the study by Mezghani et al.(2005). However, in our conditions of neutral to alkaline pH, Fmay exit predominantly as free F ion (Wenzel and Blum, 1992)and may be transferred from soil to plants. Other detoxificationmechanism may occur in these native gypsophytes. Otherwise,according to the ecological preference, arable weeds may not bethe best candidates if a population reinforcement of these speciesis envisaged that may generate other ecological disturbance inthe ecosystems.

Palatability trait seemed to us a founding criterion since all thesites are grazed and there is a need to lower the health risk tolivestock; the 5 plant species identified as highly palatable hadthus to be removed of the selection.

Evaluating fluoride tolerance in laboratory conditions is some-times not representative of what occurs spontaneously on field.However, for a field experiment of fluoride tolerance, ubiquitousplant species need to be selected for the evaluation. Only 4 plantspecies were found in the three sites. Their low index of palatabil-ity can also guarantee to these species the opportunity to achievetheir entire life cycle. As 3 of them have seed dispersal by

anemochory (Table 2), this enhances their probability to colonizemore polluted areas in these sites for future phytoremediation use.

Those ecological traits have not to be dissociated from the fluo-ride contents. Therefore, fluoride accumulation factors in aerialparts (Table 3) of the 4 native plant species common in Gabes werelow i.e. <1. This is in agreement with the criteria i.e. native,drought-, salt-, and element-tolerant, and low shoot elementaccumulation required for a phytostabilization strategy in aridenvironment (Mendez and Maier, 2008). Hence, with our approachcrossing biological traits and fluoride contents of 18 under-investigated arid native plant species adapted to fluoride-pollutedenvironments, we offer a new rationalized method for efficientselection of potential candidates for in situ phytostabilization offluoride.

4. Conclusion

Containment of soil pollution in arid environments is challeng-ing, notably dealing with wind erosion and dispersion and, scarcevegetation. Phytoremediation processes appear as potential lowcost strategies for fluoride containment and, more preciselyphytostabilization. In this context, an evaluation grid to selectMediterranean native shrubs for phytostabilization use on fluo-ride-polluted sites may be useful since there is scarce data in theliterature about the ecophysiology of these plant species. The pres-ent study is the first investigation about fluoride pollution on wildplant species in Tunisia. The major conclusion is that native plantspecies accumulate fluoride following the gradient of soil pollu-tion. Our results enabled to propose a grid of selection crossingchemical and biological traits and, in fine, to select three fluoridetolerant plant species i.e. R. suaveolens, E. glaucophyllum and A. ser-ratuloides as potential efficient shrub species for a in-situ phyto-remediation program. However only few information areavailable on the physiological traits and further studies have tofocus on their physiological mechanisms of fluoride tolerance.

Acknowledgements

This study was granted by the Action Intégrée Franco-Tunisi-enne of Ministère des Affaires Etrangères et Européennes françaisand of Ministère de l’Enseignement Supérieur, de la RechercheScientifique tunisien. The authors thank Laurent Vassalo andCarine Demelas for their analytical assistance for trace and majorelements measurements. We thank the Groupe Chimique Tunisien,in particular the Direction Centrale de la Recherche, for hisassistance and logistical support to the realization of this study.

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