Journl of Agriculturl Science; Vol. 5, No. 10; 2013 ISSN 1916-9752 E-ISSN 1916-9760 Pulished y Cndin Center of Science nd Eduction Screening Ornmentls for Their Potentil s As Accumultor Plnts Stewrt T. Reed 1, Toms Ayl-Silv 1, Christopher B. Dunn 1, Grry G. Gordon 2 & Aln Meerow 1 1 USDA, Agriculturl Reserch Service, Sutropicl Horticulture Reserch Sttion, 13601 Old Cutler Rod, Mimi, FL 33158, USA 2 Deprtment of Homelnd Security, U.S. Customs nd Border Protection, Mimi Crgo Clernce Center, 6601 NW 25 TH Street Room 272, Mimi, FL 33122, USA Correspondence: Stewrt T. Reed, USDA, Agriculturl Reserch Service, Sutropicl Horticulture Reserch Sttion, 13601 Old Cutler Rod, Mimi, FL 33158, USA. Tel: 1-786-573-7048. E-mil: stewrt.reed@rs.usd.gov Received: August 12, 2013 Accepted: August 28, 2013 Online Pulished: Septemer 15, 2013 doi:10.5539/js.v5n10p20 URL: http://dx.doi.org/10.5539/js.v5n10p20 Astrct Arsenic-sed pesticides, hericides nd insecticides re used in horticulturl opertions resulting in soil contmintion round greenhouse structures. Phytoremedition nd phytostiliztion re two techniques for treting rsenic (As) contminted soil. Severl ornmentl plnt species, Iris (Iris svnnrum), switchgrss (Pnicum virgtum), Tithoni rotundiflor, Coreopsis lnceolt, sunflower (Helinthus nnuus), nd mrigold (Tgetes erect), were evluted for their potentil use s ccumultor plnts. Bsed on dry weight, tithoni nd coreopsis were most sensitive to As. Tithoni hd n 85% reduction in dry weight t 0.75 mg As L -1 nd coreopsis 65% reduction t 2.25 mg As L -1 solution concentrtion. Iris dry weight incresed with incresing solution concentrtions ut As did not ccumulte in tissue. At the high As rte, mrigold nd sunflower hd uptke rtios of 7.4 nd 16.6, respectively, nd trnsloction fctors ner one. Both show little effect of As toxicity on dry weights production, therefore, re ppeling cndidtes for phytoremedition nd phytostiliztion. Switchgrss nd iris cn e hrvested multiple times yer, mking them cndidtes for phytostiliztion. Keywords: rsenic, iris, mrigold, sunflower, switchgrss, trnsloction fctor, uptke rtio Arevitions Trnsloction fctor: TF = shoot As / root As in mg As kg -1 plnt dry weight; uptke rtio: UR = mg As kg -1 plnt dry weight/ solution As concentrtion in mg L -1 1. Introduction Arsenic is metlloid tht cn ccumulte in soil s result of mining nd industril ctivities; ppliction of As sed hericides, insecticides, rodenticides nd wood preservtives; nd irrigtion with wter contminted with As (Zho, M, Mehrg, & McGrth, 2009). Nturlly occurring soil As cn rnge from 1-40 mg As kg -1 soil (Wlsh, Sumner, & Keeney, 1977), however, contminted levels cn rech s high s 2600 mg As kg -1 soil (Mehrg, Nylor, & Mcnir, 1994). In horticulturl crop production systems, rsenic-sed pesticides, hericides nd insecticides hve led to considerle contmintion on turf, orchrds, nd round greenhouses, shdehouses nd other structures (Woolson, Axley, & Kerney, 1971; Murphy & Aucott, 1998). Arsenic is not essentil for plnts (Mrin, Msscheleyn, & Ptrick, 1992) nd hs no known metolic function. Plnt species vry in their ility to tolerte As (Mehrg, 1994) with toxicity threshold levels rnging from 5 to 100 mg As kg -1 dry weight for most plnts (Kt-Pendis & Pendis, 1992). Brke fern (Pteris vittt) found growing on n As contminted site in Florid ws the first rsenic hyperccumultor descried in the literture (M et l., 2001). M reported tht rke fern fronds growing in 1500 mg kg -1 As contminted soil ccumulted up to 15861 ug As g -1 plnt dry weight over two week growing period. Since tht time severl other fern species hve een identified s As-hyperccumultors: Pteris cretic, Pteris longifoli, P. vittt, Pteris umros (Zho, Dunhm, & McGrth, 2002) nd Pityrogrmm clmelnos (Visoottiviseth, Frncesconi, & Sridokchn, 2002). However, not ll ferns hve the ility to hyperccumulte As. Mehrg (2003) reported tht Pteris strmine nd Pteris tremul did not hyperccumulte As in their fronds. Plnts tht ccumulte As trnslocte lrge portion of it to their shoot. The As hyperccumultor P. vittt shipped 8 x more As from root to shoot thn the nonhyperccumultor P. tremul (Cille, Zho, & McGrth, 20
2005) nd 2.8 x more thn Pteris ensiformis (Singh & M, 2006). Severl investigtors hve identified vriety of species other thn ferns tht hyperccumulte or re tolernt to high levels of soil As. Ansri et l. (2013) screened ten Indin mustrd (Brssic junce L.) genotypes n reported tht t 50 um As, mustrd shoots contined from 1.84 to 3.65 mg As g -1 dry weight. As ccumultion ws reported in Chilopsis lineris (desert willow) y Cstillo-Michel et l. (2009), in the flowering plnt Cytisus stritus y Bleeker, Scht, Vooijs, Verkleij, nd Ernst (2003), nd in Istis cpdocic, rssic, y Krimi, Ghderin, R, Feldmnn, nd Mehrg (2009). Phytoremedition employs plnts to remove toxic sustnces from the soil. An idel cndidte for phytoremedition would hve vigorous growth to help prevent trnsporttion of the sustnce off site y erosion. The plnt should ccumulte the toxic sustnce in lrge mounts nd sequester it in esily hrvestle plnt prts. Plnts defined s As hyperccumultors hve Bioccumultion Fctor (BF) > 1000 ug As g -1 plnt dry weight (Mchdo-Estrd, Clderón, Moreno-Sánchez, & Rodríguez-Zvl, 2012). They hve shoot-to-root As concentrtion rtio (Trnsloction Fctor (TF)) higher thn 1. However, plnts with BF > 1 cn e useful in phytoremedition (McGrth & Zho, 2003), especilly if they hve TF >1. Ornmentl plnts generlly, hve not een investigted s potentil source for phytoremedition nd stiliztion. Ornmentls cn provide mrketle commodity, for exmple cut flowers, to somewht offset the cost of lnd tken out of production. In ddition, these plnts cn provide n esthetic qulity to uildings locted on contminted sites. The purpose of this study ws to screen severl ornmentls for their potentil use s As ccumultor plnts. 2. Mterils nd Methods 2.1 Plnt Species Six plnt species were used in this study: Iris (Iris svnnrum), switchgrss (Pnicum virgtum), Tithoni rotundiflor, Coreopsis lnceolt, sunflower (Helinthus nnuus), nd mrigold (Tgetes erect). Iris rhizomes, pproximtely 10 cm in length were collected from single clonl plnt. To stilize rhizomes until root system egn to develop, iris plnts were set in rockwool. All other species were plnted in 25% perlite, 37% pine rk, 8% snd, 30% coir potting mixture. Switchgrss seed ws evenly sowed in 28x53 cm trys. Once plnts reched 10 cm in height, 30 18-cm sections of turf were cutout nd plced into 26-cm dimeter pots (3.8 L). Switchgrss ws trimmed to uniform 15 cm height efore initition of tretments. Tithoni, coreopsis, sunflower nd mrigold were plnted four seeds to pot nd thinned to one plnt per pot shortly fter emergence. The study ws conducted over three different time periods with two plnt species growing during ech period. Dtes for ech time period re given in Tle 1. Tithoni, sunflower nd mrigold hd t lest two fully developed leves efore pots were ttched to hydroponic system. Iris, switchgrss nd coreopsis were t lest 10 cm tll nd iris nd coreopsis hd 3 leves when hydroponic systems were ctivted. Tle 1. Plnting, initition of rsenic tretments nd hrvest dtes for six plnt species: Iris (Iris svnnrum), switchgrss (Pnicum virgtum), Tithoni rotundiflor, Coreopsis lnceolt, sunflower (Helinthus nnuus) nd Mrigold mrigold (Tgetes erect) Species Plnting Tretment Hrvest Iris (Iris svnnrum) 21-Dec-09 10-Mr-10 13-My-10 Switchgrss (Pnicum virgtum) 21-Dec-09 10-Mr-10 13-My-10 Tithoni rotundiflor 6-Oct-10 20-Oct-10 16-Nov-10 Coreopsis lnceolt 6-Oct-10 20-Oct-10 23-Nov-10 Sunflower (Helinthus nnuus) 1-Fe-11 16-Fe-11 22-Mr-11 Mrigold (Tgetes erect) 1-Fe-11 16-Fe-11 22-Mr-11 2.2 Hydroponic System Six e-nd-flow type hydroponic plnt mintennce systems were setup. Ech system contined 132 L reservoir tnk connected to 12, 3.8-L pots. A timer llowed the system to cycle etween 30 min. wet nd 4 hr. drin periods, eginning t 8:00 A.M., ending t 4:00 P.M. followed y 12 hr. drin period. Ech 132 L tnk contined modified Hoglnd solution with 2.0 mm C(NO 3 ) 2, 3 mm KNO 3, 1.0 mm MgSO 4, 0.25 mm C(H 2 PO 4 ) 2, 12.5 um H 3 BO 3, 1.0 um MnSO 4, 1.0 um ZnSO 4, 0.25 um CuSO 4, 0.2 um (NH 4 ) 6 Mo 7 O 24, nd 10 21
um Fe-EDDHA. Tp wter used to mix nutrient solutions verged 0.0028 mg L -1 As (0.448 mg per reservoir tnk). Plnts were llowed to cclimte to hydroponic feeding for minimum of one week efore eginning As tretments. Enough N 2 HAsO 4 ws dded to different reservoirs to mke 0, 10, 20, 30, 40, 50 or 70 um As s finl solution concentrtion. Iris, switchgrss, sunflower nd mrigold were grown in 0, 10, 20, 40, 50 or 70 um As (0.0, 0.75, 1.5, 3.0, 3.75, 5.25 mg L -1 As, respectively) while coreopsis nd tithoni were grown in 0, 10, 30, 50, or 70 um As. Reservoir ph ws djusted dily to ph 6.5 with either NOH or H 2 SO 4. Nutrient nd As solutions were replced weekly. Arsenic wste wter ws collected, filtered to remove As, nd filters properly disposed of s hzrdous wste. Plnts were mintined in hydroponic solution until flowering, 40 to 121 dys depending on the species. A portion of iris rhizome from the 0.0 nd 5.25 As tretments were mintined in 0.0 nd 133 um (10 mg L -1 ) As solution, respectively, for 366 dys efore plnts flowered. 2.3 Smpling Plnt height ws mesured from the soil surfce long the min stem to the upper most node for tithoni, sunflower nd mrigold or with coreopsis to the uppermost point when leves were held upright. For iris the comined length of ech lef sustituted for plnt height. Prior to initil As tretment switchgrss ws clipped to 10 cm height. Cnopy height ws mesured t 76, 91 nd 121 dys nd following ech mesurement the cnopy ws clipped ck to 15 cm height. Plnt growth ws clculted s height t hrvest (ccumulted height for switchgrss) minus initil height. Prior to pplying tretments, nd gin t hrvest the numer of leves ws recorded for ll species except switchgrss. The initil lef numer ws sutrcted from the finl count to determine new lef growth. Shoot nd root tissue were hrvested seprtely, oven dried t 45 C until there ws no longer weight chnge with dditionl drying nd the dry weights recorded. Dried tissue ws stored for nlysis. An uptke rtio UR = mg As kg -1 plnt dry weight/ solution As concentrtion in mg L -1 (1) nd trnsloction fctor TF = shoot As / root As in mg As kg-1 plnt dry weight (2) were clculted for ech species nd ech tretment. 2.4 Smple Anlysis Approximtely 0.25 g of oven dried plnt tissue ws plced in 100 ml digestion tues. Ten ml HNO 3 ws dded nd smples digested in Strt D microwve digestion system (Milestone Inc., Shelton, Connecticut) for 15 min to rech 200 C then kept t this temperture for n dditionl 15 min. Digests were diluted to 100 ml nd stored t 4 C prior to nlysis. Arsenic concentrtion ws determined y inductively coupled plsm opticl emission spectrometry with n icap 6300 Duo View (ThermoFisher Scientific, West Plm Bech, Florid). Dt were nlyzed nd concentrtions determined using ThermoFisher Scientific icap 6300 iteva softwre. Arsenic ws determined t the most intense tomic emission. 2.5 Sttisticl Anlysis Ech plnt species ws nlyzed seprtely. The dt represent mens clculted from six replicted pots for ech As tretment. Anlysis of vrince ws preformed using the Proc Mixed procedure of Sttisticl Anlysis System (SAS Inst., 1999). Tukey djusted lsmens were used for comprison t P < 0.05 unless stted otherwise. Arithmetic mens were used to clculte uptke rtio nd trnsloction fctor. 3. Results 3.1 Generl Comments Sttisticl nlysis reveled significnt differences in As concentrtion t P 0.05 etween tretments for ll plnts species used in this study. Significnt differences in As concentrtion t P 0.05 lso were found etween root nd shoot tissue for ll species. An increse in solution As led to n increse in As tissue concentrtion, however, this ws not the cse with iris lef nd flower tissues. There were no differences for As etween tretments. Iris nd mrigold dry weights tended to increse with incresing solution As up to 5.25 mg L -1. In Pnicum virgtum As uptke incresed with successive hrvests (dt not shown). 3.2 Plnt Physicl Chrcteristics Compred to the control, significnt tretment dry weight differences in root nd shoot tissue for coreopsis nd tithoni first ppered t 2.25 nd 0.75 mg L -1 solution As concentrtion, respectively (Tle 2). At 5.25 mg L -1 coreopsis hd n 84% reduction in totl plnt dry weight producing 29 fewer leves nd 15 cm less growth thn the 0.0 mg As L -1 control. The overll inhiition of growth seen in coreopsis ws even more striking in tithoni. 22
At 5.25 mg As L -1 there ws 98% reduction in dry weight, virtully no new leves produced nd 30 cm less growth thn the control. Coreopsis nd tithoni hd URs > 20 nd TF of 0.07 nd 1.13, respectively. Tle 2. Selected physicl chrcteristics, uptke rtio (UR) nd Trnsloction Fctor (TF) of plnts grown in vrying As solution concentrtions Solution concentrtion (ppm) As Root dry wt (g) shoot dry wt (g) New lef growth Plnt growth (cm) Coreopsis lnceolt 0.00 1.97 4.96 43.8 20.4 0.0 c 0.44 0.75 1.64 3.91 48.5 13.9 85.4 0.17 2.25 0.70 c 1.75 c 27.3 c 11.0 79.9 0.16 3.75 0.48 c 1.39 c 24.8 c 8.8 52.5 0.13 5.25 0.44 c 0.69 c 15.0 c 5.5 73.0 0.07 Tithoni rotundiflor 0.00 16.19 26.72 101.8 36.8 0.0 c 0.00 0.75 3.44 4.10 33.8 16.3 102.3 1.27 2.25 1.27 1.55 9.8 c 10.3 c 47.3 2.38 3.75 2.01 1.31 8.1 c 7.0 c 20.2 c 1.06 5.25 0.61 0.46-0.5 c 6.2 c 33.4 c 1.13 Iris svnnrum 0.00 0.75 2.29 29.0 1467.2 0.0 2.93 0.75 0.42 1.15 28.3 1354.8 5.0 1.23 1.50 0.41 1.10 20.7 874.7 5.9 0.82 3.00 1.40 3.95 28.5 1703.4 5.5 0.63 3.75 1.06 3.69 24.8 1333.0 3.3 1.54 5.25 1.62 5.31 17.3 1060.1 6.7 9.21 Tgetes erect 0.00 15.91 54.12 58.3 59.9 0.0 c 0.23 0.75 13.42 65.13 59.0 61.3 20.3 0.50 3.00 16.45 58.31 74.0 59.2 7.3 c 0.34 3.75 13.60 59.05 65.7 52.9 10.6 0.27 5.25 14.58 68.13 81.3 63.4 7.4 c 1.20 Helinthus nnuus 0.00 2.55 16.75 16.2 16.4 0.0 c 0.89 0.75 2.10 14.57 20.8 15.2 32.1 0.84 3.00 2.47 17.23 23.0 18.2 25.7 1.23 3.75 1.60 14.18 24.8 15.6 18.1 0.84 5.25 1.98 12.89 21.2 16.2 16.6 0.88 Pnicum virgtum 0.00 39.55 71.54-79.1 0.0 0.20 0.75 37.03 44.74 c - 59.5 c 3.5 0.24 1.50 36.74 57.26-69.3 2.7 0.10 3.00 29.45 30.24 cd - 55.8 cd 1.8 0.56 3.75 43.05 46.73 c - 69.7 2.5 1.65 5.25 28.16 22.16 d - 49.8 d 1.5 0.16 mens followed y the sme letter re sttisticlly similr t P 0.05. Lsmens estimtes not djusted y Tukey's. UR TF 23
Mrigold nd sunflower hd no significnt differences in root nd shoot dry weights nd plnt growth etween tretments (Tle 2). Averged cross As tretments, the UR for mrigold ws 11.4 nd the TF 0.5. The UR in sunflower declined slightly with incresing As solution. Averged cross tretments the UR nd TF were 23.1 nd 0.9, respectively. Iris root nd shoot dry weights incresed with incresing solution concentrtions ove 3.00 mg As L -1 (Tle 2). Although not significntly different, totl dry weight from the 5.25 mg L -1 tretment ws 2 x tht found in the control. Although there were no differences in the numer of new leves developed nd totl lef length etween the control nd ll As tretments, with the 5.25 mg L -1 tretment 12 fewer new leves were produced, nd the new leves tht were produced were longer thn new leves produced y the control. An increse in solution As led to n increse in individul lef lengths (dt not shown). Iris produced 5.3 verge UR tht vried little with increses in solution As concentrtion. There were no significnt differences in iris TF, lthough the vlues vried from 0.82 to 9.21. High TF vlues tended to occur in plnts tht produced lrge numer of leves. Switchgrss control hd significntly greter root dry weight thn tht found in 3.0 nd 5.25 mg L -1 As tretments nd greter totl shoot dry weight thn ll ut the 1.5 mg L -1 tretment (Tle 2). Shoot dry weights incresed with successive cuttings in ll tretments (dt not shown). An increse in greenhouse temperture occurred s spring pproched nd likely contriuted to the increse in growth. The control hd greter totl growth, comined over three cuttings, thn ny As tretment. Growth showed wek trend to decrese with incresing solution As concentrtion. Switchgrss hd low UR, 2.4 verged over tretments nd low TF, 0.5. 3.3 Plnt Arsenic During the course of this study, severl tnks filed nd therefore dt reported in Figure 1 represent only tretments common to ll plnt species (0.0, 0.75, 3.75 nd 5.25 mg As L -1 ). Sttisticl groupings were djusted ccordingly. In this study As uptke ptterns y plnts could e seprted into three groups: 1) sensitive plnts tht t low As concentrtions hd reduced growth (coreopsis nd tithoni), plnts tht tolerted As in the growing medium y preventing its ccumultion in tissue (iris), nd plnts tht took-up As ut could moderte As toxic effects (mrigold, sunflower nd switchgrss). The verge concentrtion in control tnks ws 0.0028 mg As L -1 nd coreopsis redily ccumulted As t this low level. Both coreopsis nd tithoni hd mximum As ccumultion t low solution levels 2.25 (not shown) nd 0.75 mg As L -1, respectively (Figure 1). This ws ccompnied y 65% reduction in dry weight for coreopsis nd n 85% reduction for tithoni t mximum plnt As levels (Tle 2). Plnt As (mg) 5 4 3 2 1 0 0.0 0.75 3.75 5.25 coreopsis iris mrigold sunflower switchgrss tithoni Plnt species Figure 1. Whole plnt rsenic content (mg) from plnts grown in 0.0, 0.75, 3.75, or 5.25 mg As L -1 solution concentrtion c c 24
Iris ccumulted very little As in its tissue t solution levels up to 5 mg As L -1 with no negtive effects on growth (Figure 1). Mximum As ccumultion in iris ws ove the high level used in this study. It is worth mentioning tht s long s the rhizome remins helthy iris leves cn e removed nd will e replced y new growth. A portion of the rhizome remins ove the surfce nd cn e redily hrvested incresing totl As removl from soil. Switchgrss hd mximum plnt As content t 3.00 mg L -1 of 3.8 mg As kg -1 dry weight, the highest mount removed y ny species t ny solution concentrtion. Switchgrss cn e hrvested multiple times throughout growing seson. Mrigold nd sunflower continued to ccumulte As in plnt tissue through the highest solution concentrtion used in this study with no significnt effect on shoot or root dry weights (Tle 2). The solution concentrtion where mximum As ccumultion occurred ws ove the highest concentrtion used in the study. Given no pprent ffects on plnt dry weights these plnts my remove As from soils considerly more contminted. 4. Discussion All plnts used in this study were As non-hyperccumultors, however, there were different ptterns in As ccumultion mong the plnt species. Only smll mount of As ws found in iris root nd shoot tissue. Bsed on reduction in dry weight, tithoni nd coreopsis were most sensitive to As exposure nd did not produce sufficient dry weight for remedition purposes. Iris nd mrigold dry weights t the 5.25 mg L -1 tretments incresed over controls y 2.3 nd 1.3 x, respectively. Similr increses in dry weight t low As concentrtions were reported y Cronell-Brrchin, Ari, DeLune, Gmrell, nd Ptrick (1998) with Sprtin ptens nd Sprtin lterniflor, Shiur nd Kwi (2009) with Brssic rp, Srivstv et l. (2007) with Hydrill verticillt, Woolson et l. (1971) with mize, Mrin et l. (1992) with rice nd Jcos, Keeney, nd Wlsh (1970) with pes, whet nd pottoes. Jcos et l. (1970) ttriuted this to improved plnt P sttus s result of rsente displcing phosphte from soil incresing P vilility. However, in the present study plnts were grown hydroponiclly nd this mechnism would not e in effect. Improved minerl nutrition t low As concentrtion could ccount for the higher dry weights. Phytoremedition of As usully requires phytoextrction nd/or phytostiliztion. Phytoextrction involves As uptke from soil y plnt roots nd susequent trnsport from root to shoot tissue where it cn e hrvested. Phytostiliztion requires production of enough plnt iomss to prevent soil As movement from the contminted site y erosion nd As sequestrtion in plnt roots to limit leching. An pproprite cndidte for phytoremedition should ccumulte As t rte equl to or greter thn tht found in the soil, i.e., it should hve n uptke rtio (UR) 1. In the present study coreopsis nd tithoni hd the highest UR ut even t the lowest solution As concentrtion iomss production ws too low for use in As removl or soil stiliztion. Mrigold nd sunflower hve UR vlues t 5.25 mg L -1 solution As concentrtion of 7.4 nd 16.6, respectively. Their TFs (trnsloction fctor) re ner one, nd t the high As rte oth produce dry weights similr to their controls. Therefore, mrigold nd sunflower re ppeling cndidtes for phytoremedition nd phytostiliztion. Switchgrss nd iris re plnts tht cn e hrvested multiple times yer. This cn prtilly offset the low UR nd TF in switchgrss mking it cndidte for phytostiliztion. Iris hs UR of 6.7 nd TF of 9.2. The TF of iris nd mrigold continued to increse with incresing solution As up to the high rte used in this study. The mximum tissue As concentrtion for these species ws not determined. Iris is n ttrctive cndidtes for phytostiliztion nd perhps even phytoremedition. The fct tht oth mrigold nd iris produced flowers t ll As rtes my provide n dded dvntge in tht cut flowers my provide smll mounts of revenue to help offset the cost of remedition. References Ansri, M. K. A., Sho, H. B., Umr, S., Ahmd, A., Ansri, S. H., Iql, M., & Owens, G. (2013). Screening Indin mustrd genotypes for phytoremediting rsenic-contminted soils. Clen Soil, Air, Wter, 41, 195-201. http://dx.doi.org/10.1002/clen.201100752 Bleeker, P. M., Scht, H., Vooijs, R., Verkleij, J. A. C., & Ernst, W. H. O. (2003). Mechnisms of rsente tolernce in Cytisus stritus. New Phytologist, 157, 33-38. http://dx.doi.org/10.1046/j.1469-8137.2003.00542.x Cille, N., Zho, F. J., & McGrth, S. P. (2005). Comprison of root sorption, trnsloction nd tolernce of rsenic in the hyperccumultor Pteris vittt nd the nonhyperccumultor Pteris tremul. New Phytologist, 165, 755-761. http://dx.doi.org/10.1111/j.1469-8137.2004.01239.x 25
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