2692

Environmental Toxicology and Chemistry, Vol. 21, No. 12, pp. 2692–2697, 2002q 2002 SETAC

Printed in the USA0730-7268/02 $9.00 1 .00

COPPER TOXICITY THRESHOLDS FOR IMPORTANT RESTORATION GRASS SPECIESOF THE WESTERN UNITED STATES

MARK W. PASCHKE* and EDWARD F. REDENTEColorado State University, Department of Rangeland Ecosystem Science, Fort Collins, Colorado 80523-1478, USA

(Received 17 October 2001; Accepted 25 May 2002)

Abstract—Copper toxicity thresholds for plant species that are used in restoration activities in western North America have notbeen established. As a result, ecological risk assessments must rely on toxicity thresholds established for agronomic species, whichusually differ from those of species used in restoration. Thus, risk assessors have the potential for classifying sites as phytotoxicto perennial, nonagronomic species and calling for intensive remediation activities that may not be necessary. The objective of thisstudy was to provide a better estimate of Cu toxicity thresholds for five grass species that are commonly used in restoration effortsin the western United States. We used a greenhouse screening study where seedlings of introduced redtop (Agrostis gigantea Roth.),the native species slender wheatgrass (Elymus trachycaulus [Link] Gould ex Shinners var. Pryor), tufted hairgrass (Deschampsiacaespitosa [L.] Beauvois), big bluegrass (Poa secunda J. Presl var. Sherman), and basin wildrye (Leymus cinereus [Scribner &Merrill] A. Love var. Magnar) and the agricultural species common wheat (Triticum aestivum L.) were grown in sand culture andexposed to supplemental concentrations of soluble Cu of 0 (control), 50, 100, 150, 200, 250, and 300 mg/L. We determined sixmeasures of toxicity: the 60-d mean lethal concentration (LC50), 60-d mean effective concentration (EC50)-plant, 60-d EC50-shoot,60-d EC50-root, phytotoxicity threshold (PT50)-shoot, and the PT50-root. Results suggest that these restoration grass speciesgenerally have higher Cu tolerance than agronomic species reported in the past. Of the species tested, redtop appeared to beespecially tolerant of high levels of substrate and tissue Cu. Values of EC50-plant for restoration grasses were between 283 and710 mg Cu/L compared to 120 mg Cu/L for common wheat. Measured PT50-shoot values were between 737 and 10,792 mg Cu/L. These reported thresholds should be more useful for risk assessors than those currently used, which are based largely on agronomiccrops.

Keywords—Ecological effects Phytotoxicity Pollution Restoration Risk assessment

INTRODUCTION

Copper is a natural constituent of soils in terrestrial eco-systems. Copper is a required element for plant growth, as itserves important biochemical functions [1,2]. However, humansources of Cu, such as smelting, fertilizers, pesticides, andagricultural and municipal wastes, can lead to elevated con-centrations of Cu in soils [1–3], which can lead to Cu toxicityin plants [2,4,5]. The detrimental effects of uptake of excessamounts of Cu on plants usually result from root tissue dam-age, increased permeability of root cell plasma membranes,inhibition of photosynthesis, and damage to DNA [2,6,7].

In establishing metal toxicity thresholds for plants, it isimportant to consider several characteristics of toxicity: thequantity and species of metal, the route of exposure, the dis-tribution of the metal both spatially and temporally, the typeand severity of injury, and the time needed to produce theinjury [8]. Several methods for describing metal toxicity inplants have been proposed. Most of these have been derivedfrom measures of human or animal health assessments. A dis-cussion of these methods is presented in Ross and Kaye [8].The lethal concentration (LC) is the concentration of a toxinthat kills a specified percentage of organisms. Effective con-centration (EC) is the concentration of a toxin that producesan observable negative effect in the organism. The phytotox-

* To whom correspondence may be addressed(mark.paschke@colostate.edu).

Mention of trade names and companies is solely to inform thereader and does not constitute endorsement by Colorado State Uni-versity or criticism of similar products or companies not mentioned.

icity threshold (PT) is the tissue concentration of a plant thatcorresponds with a defined growth reduction.

Metal toxicity thresholds for plants can be used to estimatea plant’s ability to establish and survive on a contaminatedsite. Unfortunately, a paucity of data exists on toxicity thresh-olds for native plant species [8], and ironically information islacking for species that are used to restore heavy metal–con-taminated sites. Miles and Parker [9] have identified Cd tox-icity thresholds for seven plant species native to northwesternIndiana, and in previous work we have determined Zn toxicitythresholds for a variety of grass species [10]. Others haveattempted to establish toxicity thresholds for individual nativeplant species using a few metals (e.g., [11–14]). Most workon metal effects on native plant species has focused on relativetoxicity of species or ecotypes for selection and use in phy-toremediation efforts (e.g., [15–18]). The vast majority of plantmetal toxicity thresholds have been determined for agriculturalspecies (reviewed by Gough et al. [19]). This is especially thecase for Cu, which has been studied extensively in agriculturalcrops where Cu in pesticides, fertilizers, and organic amend-ments are of concern. As a result, ecological risk assessmentsand natural resource damage assessments under the Compre-hensive Environmental Response, Compensation, and LiabilityAct [20] must rely on toxicity thresholds established for ag-ronomic species. These crop plants may have very differentphysiological characteristics and sensitivity levels than speciesused in the restoration of sites contaminated with metals andmay therefore be inappropriate for these ecological assess-ments.

Many metal toxicity thresholds for plants are determined

Copper toxicity thresholds for restoration grasses Environ. Toxicol. Chem. 21, 2002 2693

Table 1. Composition of the nutrient solution provided to plants onTuesdays and Thursdays. Plants were given Cu treatment solutionson Monday, Wednesday, and Friday and water as needed on Saturday

and Sunday

Compound Concentration applied

K2SO4

MgSO4·7H2OKH2PO4

K2HPO4

CaSO4·2H2OCaCl2·2H2ONH4NO3

FeSO4·7H2OH3BO3

MnSO4·H2OZnSO4·7H2OCuSO4·5H2ONaMoO4·2H2OMES buffer

0.400 mM0.500 mM0.042 mM0.208 mM0.150 mM0.125 mM0.895 mM

22.50 mM5.78 mM1.15 mM0.20 mM0.08 mM0.05 mM0.250 mM

Table 2. Percentage survival (after 60 d) of grass species exposed to various Cu treatment levels. These survival values were used to estimateLC50s (Table 3). Values are raw scores for survival of all the seedlings in the experiment. Values for n are shown after each percentage survivalvalue. The number of seedlings (n) used in each species by treatment combination varied because of a lack of germination in some of the tubes.

The original number of tubes planted for each species by treatment combination was 49

Treatment

Cu (mg/L)

Species (n)

RedtopSlender

wheatgrassTufted

hairgrassBasin

wildryeBig

bluegrass Wheat

050

100150200250300

100 (42)98 (43)98 (46)91 (47)96 (46)76 (37)37 (48)

100 (48)100 (49)100 (49)100 (49)100 (49)

85 (48)18 (49)

100 (40)97 (39)97 (38)

100 (40)88 (41)82 (40)82 (40)

100 (42)98 (46)

100 (42)96 (47)

100 (46)61 (44)

2 (45)

95 (44)98 (44)95 (38)

100 (46)94 (49)50 (42)70 (43)

100 (49)100 (49)100 (49)

96 (49)92 (49)87 (48)90 (49)

in greenhouse or laboratory experiments by growing plants innutrient solutions containing known concentrations of metals(reviewed by Macnicol et al. [21]). While these conditions donot mimic field conditions, they may provide a conservativefirst estimate of toxicity thresholds. Many factors that are lack-ing in solution culture experiments would be expected to re-duce metal toxicity to plants growing in the field. These factorsinclude rhizosphere organisms such as mycorrhizae [22–26]and metal binding with soil organic matter [27–29] and clays[27]. Thus, toxicity thresholds determined from solution cul-ture experiment would likely be lower than actual field toxicitythresholds.

In a previous study [10], we determined zinc toxicity thresh-olds for five grass species that are commonly used in resto-ration activities in western North America. In this paper, wedescribe a similar study of Cu toxicity thresholds for grassspecies used in restoration efforts. The objective of this studywas to provide a better estimate of Cu toxicity thresholds forfive grass species that are commonly used in restoration effortsin the western United States. Until now, this information hasbeen unavailable, and as a result, ecological risk assessmentshave relied on Cu toxicity thresholds established for agronomicspecies.

MATERIALS AND METHODS

Plant growth conditions

A greenhouse screening study was used to determine Cutoxicity thresholds for redtop (Agrostis gigantea Roth.), slen-

der wheatgrass (Elymus trachycaulus [Link] Gould ex Shin-ners var. Pryor), tufted hairgrass (Deschampsia caespitosa [L.]Beauvois), big bluegrass (Poa secunda J. Presl var. Sherman),basin wildrye (Leymus cinereus [Scribner & Merrill] A. Lovevar. Magnar), and common wheat (Triticum aestivum L. var.Oslo). Common wheat is an agricultural crop, and redtop wasintroduced to North America from Europe; the remaining spe-cies are native to the western United States. Wheat seed wasobtained from a local agricultural seed supplier and the res-toration species were obtained from Granite Seed (Lehi, UT,USA), a company that typically supplies the restoration in-dustry. Although previous studies have noted ecotypic metaltolerance variation in native plant species [11–13], we usedseed that would typically be used in the restoration of metal-contaminated sites as an approximation of species toxicitythresholds.

A sand culture technique was used to establish toxicitythresholds because many of these arid and semiarid grass spe-cies do not grow well in aerated solution culture. Preliminarytests of leachate from the sand-filled tubes found no detectablewater soluble Cu in this media. Approximately three seeds ofeach species were sown directly into 3.8- 3 21-cm plasticCone-tainery tubes (Stuewe & Sons, Corvallis, OR, USA).Each tube was filled with approximately 350 cm3 of washedquartz sand, and the sand was covered with approximately 1cm of perlite to retain moisture at the soil surface. A glass-wool plug was put in the bottom of each container to keep thesoil from escaping through drainage holes. After emergence,seedlings were thinned to one individual per tube.

Copper treatment began when the seedlings were approx-imately four weeks old, and all plants were provided with acomplete nutrient solution (Table 1) on alternate days prior toCu treatments. Forty-nine seedlings of each of the six specieswere exposed to one of seven supplemental Cu treatments: 0,50, 100, 150, 200, 250, or 300 mg Cu/L. Copper treatmentswere administered by application of CuSO4 solutions on al-ternate days (Monday, Wednesday, Friday) with nutrient so-lution being added separately (Tuesday, Thursday). Plants wereprovided with water as needed on weekends (during the first30 d of the experiment, the small seedlings rarely requiredweekend watering). The pH of the nutrient solution was main-tained at 6.0 with the addition of 0.25 mM 2-(4-morpholino)-ethanesulfonate (MES), which has been shown not to signif-icantly alter uptake of mineral nutrients or complex substan-tially with the alkaline earth or transition metals [30]. Nutrientsolution and Cu treatments were applied in amounts that sat-urated the media as evidenced by drainage of solution out of

2694 Environ. Toxicol. Chem. 21, 2002 M.W. Paschke and E.R. Redente

Table 3. Calculated copper toxicity thresholds for lethal concentration (LC50), effective concentrations (EC50-shoot, EC-50-root, and EC50-plant), and phytotoxicity thresholds (PT50-shoot and PT50-root)

Species Threshold type r2 p ModelCalculated

threshold (x)a

Redtop LC50EC50-shootEC50-rootEC50-plantPT50-shootPT50-root

0.64

0.110.150.050.09

0.0247

,0.0001,0.0001

0.49860.5175

y 5 110.134 2 0.167x

y 5 97.459 2 0.105x 2 1.50204 x2

y 5 117.195 2 0.095xy 5 137.979 2 8.51203xy 5 80.105 2 2.71203x

360NDb (.300)

311710

10,79211,088

Slender wheatgrass LC50EC50-shootEC50-rootEC50-plantPT50-shootPT50-root

0.480.410.390.540.400.65

0.0739,0.0001,0.0001,0.0001

0.0034,0.0001

y 5 115.618 2 0.196xy 5 116.933 2 0.196xy 5 109.626 2 0.271xy 5 118.870 2 0.233xy 5 61.387 2 0.017x 1 2.076206x2 2 8.606211x3

y 5 109.731 2 0.028x 1 4.228206x2 2 2.652210x3

335341220266737

3,952Tufted hairgrass LC50

EC50-shootEC50-rootEC50-plantPT50-shootPT50-root

0.360.140.290.510.59

,0.0001,0.0001,0.0001,0.0001,0.0001

y 5 124.844 2 0.215xy 5 104.541 2 0.249x 1 2.83204x2

y 5 111.096 2 0.186xy 5 119.123 2 0.033x 1 3.765206x2 2 1.353210x3

y 5 39.138 1 4.904 203x

ND (.300)348423328

2,9781,215

Basin wildrye LC50EC50-shootEC50-rootEC50-plantPT50-shootPT50-root

0.590.530.470.550.380.77

0.0372,0.0001,0.0001,0.0001

0.0008,0.0001

y 5 118.837 2 0.262xy 5 99.567 1 0.310x 2 6.07204x2 2 5.637206x3

y 5 102.346 2 0.336x 1 1.268203x2 6 4.073206x3

y 5 101.130 2 0.053x 1 4.48204x2 6 4.757206x3

y 5 117.340 2 0017xy 5 100.224 2 0.065x 1 2.492205x2 2 3.107209x3

263251210238

4,0501,331

Big bluegrass LC50EC50-shootEC50-rootEC50-plantPT50-shootPT50-root

0.060.100.09

0.11

0.0018,0.0001,0.0001

0.4187

y 5 99.198 2 0.175x 2 1.658203x2 2 4.568206x3

y 5 94.235 2 0.039x 2 4.39204x2 1 2.844207x3

y 5 96.580 2 0.105x 1 0.569204x2 2 2.055206x3

y 5 70.711 2 0.011x 1 5.379206x2 2 6.854210x3

ND (.300)115303330

ND5,904

Wheat LC50EC50-shootEC50-rootEC50-plantPT50-shootPT50-root

0.630.510.640.710.57

,0.0001,0.0001,0.0001,0.0001,0.0001

y 5 111.870 2 0.384x 1 4.84204x2

y 5 101.227 2 0.977x 1 4.553203x2 2 6.791206x3

y 5 101.895 2 0.461x 1 4.26204x2 2 1.463206x3

y 5 93.397 2 0.017x 1 4.726208x2 1 1.158210x3

y 5 43.012 2 0.022x 1 4.71206x2 2 3.086210x3

ND (.300)225

77120

2,7611,173

a Values for EC50s are mg Cu/L, values for PT50s are mg Cu/kg.b ND 5 not detected; threshold cannot be estimated from the data.

the bottom of the tubes. This treatment regime was continuedfor 60 d. During the growth period, the greenhouse was main-tained at 23 6 88C, with an extended photoperiod of 16 husing 400-W Na vapor lamps that provided approximately 300mmol/m2/s of photosynthetically active radiation at a distanceof 1.5 m.

Measures of toxicity

Numerous measures of metal toxicity thresholds in plantsexist [8]. In this study, we determined six commonly usedmeasures of toxicity: the 60-d LC50 (the concentration of met-al that kills 50% of the seedlings by 60 d), the 60-d EC50-plant (the concentration of metal that reduces seedling biomassby 50% after 60 d), the 60-d EC50-shoot (the concentrationof metal that reduces shoot biomass by 50% after 60 d), the60-d EC50-root (the concentration of metal that reduces rootbiomass by 50% after 60), the PT50-shoot (the shoot metalconcentration corresponding to a 50% seedling biomass re-duction), and the PT50-root (the root metal concentration cor-responding to a 50% seedling biomass reduction). The LC50was determined from observations of plant status, alive ordead, at the conclusion of the greenhouse experiment. Sixtydays after treatments began, seedlings were harvested, and thesand was separated from the roots by gently washing under astream of water. Roots were separated from shoots, and both

were dried to constant mass at 558C and weighed to determineEC50 values. Treatment effects on root, shoot, and plant masswere also evaluated directly using univariate analyses. Dif-ferences between control and treatment means were tested us-ing a Tukey’s Studentized range test (a 5 0.05) on SAStPROC GLM version 8.01 (SAS Institute, Cary, NC, USA). Asubset of root and shoot samples (five plants from each species3 Cu treatment combination) were then analyzed for Cu con-centrations by HNO3/HClO4 digestion and analysis by induc-tively coupled plasma emission spectroscopy at the Soil andPlant Analysis Laboratory at Colorado State University (FortCollins, CO, USA).

Toxicity thresholds were calculated from the data by fittingthem to linear and polynomial models using SAS version 8.01(SAS Institute). The model that resulted in the best fit to thedata, as determined by r2 and p values, was used to calculatetoxicity thresholds.

RESULTS

Mortality was relatively low during the 60-d study periodup through the 250-mg/L treatment level (Table 2). At the 300-mg/L Cu treatment, only three of the six species showed mor-tality greater than 50%. Therefore, we were not able to cal-culate LC50 values for three of the species, and those thatwere calculated were greater than 250 mg/L Cu (Table 3).

Copper toxicity thresholds for restoration grasses Environ. Toxicol. Chem. 21, 2002 2695

Fig. 1. Effect of Cu on plant biomass presented as a percentage ofthe control means. (A) Redtop, slender wheatgrass, and tufted hair-grass. (B) Basin wildrye, big bluegrass, and common wheat. Thinbars represent the standard error of the mean (n 5 between 49 and1, depending on mortality of test plants during the study period. Treat-ment means that are significantly different from the correspondingcontrol mean at a 5 0.05 by using a Tukey’s Studentized range testare indicated by an asterisk (*).

Despite the lack of mortality at high treatment levels for severalof the species, consistent and visually discernable differenceswere observed in plant size (Fig. 1) and vigor between high-Cu treatments and controls for all species. At lower Cu treat-ment levels, several of the species showed increased growth(especially of shoots), indicating a beneficial Cu fertilizer ef-fect at these levels (Fig. 1). It should be noted that the nutrient

solution that we provided to all plants, including controls,contained Cu intended to meet the plant’s basic nutritionalrequirements (Table 1).

Copper concentrations of 250 mg/L and greater generallycaused significant reductions in shoot mass. Calculated EC50-shoot values were between 115 and 348 mg Cu/L. The notableexception to this was redtop, which showed a marked increasein shoot biomass at Cu concentration up through 250 mg/L andonly a slight nonsignificant reduction in shoot growth at 300mg/L (Fig. 1). The EC50-shoot for redtop could not be calcu-lated because of this lack of response to Cu within the treatmentrange. Common wheat appeared to be the most sensitive specieswith respect to Cu affects on shoot growth (EC50-shoot 5 225),with significant reductions in shoot mass occurring at 100 mgCu/L (Fig. 1). Big bluegrass also appeared to be sensitive toCu with respect to shoot growth with a calculated EC50-shootof 115 mg Cu/L (Table 3), although this calculated value wasbased on a model with poor fit (r2 5 0.06). Roots of all thesegrass species appeared to be more sensitive than shoots to Cu-induced growth reductions (Fig. 1). Common wheat again wasthe most sensitive species to Cu, with root growth being sig-nificantly reduced at 50 mg/L (Fig. 1) and a calculated EC50-root of 77 mg Cu/L (Table 3). Significant reductions in whole-plant biomass over controls were observed for all species at250 mg Cu/L and higher (Fig. 1). The EC50-plant values rangedfrom 120 to 710 mg Cu/L. Common wheat showed the greatestsensitivity to Cu in terms of total plant biomass with significantgrowth reductions occurring at 50 mg/L and a calculated EC50-plant of 120 mg Cu/L. Redtop appeared to be the least sensitivespecies to Cu, with only modest reductions in total plant biomassat 250 and 300 mg/L Cu (Fig. 1) and a calculated EC50-plantof 710 mg Cu/L.

The copper was readily taken up in large amounts by allspecies (Fig. 2). Large amounts of Cu were retained in rootswith lesser amounts translocated to shoots (Fig. 2). This wasespecially true for big bluegrass, which appeared to translocaterelatively little Cu to shoots (Fig. 2). Calculated PT50-shootvalues ranged from 10,792 mg Cu/kg for redtop to 2,761 mgCu/kg for common wheat. The PT50-root values were gen-erally lower than shoot values and ranged from 11,008 mg Cu/kg for redtop to 1,173 mg Cu/kg for common wheat.

DISCUSSION

Metal toxicity thresholds in plants can be difficult to de-termine because of complex interactions between the toxicmetal and other nutrient elements as well as other complexbiological and physical factors [31]. Here we have identifiedCu phytotoxicity thresholds for several important restorationgrass species and common wheat using a simplified approachthat circumvents many of these experimental pitfalls.

Lethal concentrations of Cu were determined for redtop (360mg/L), slender wheatgrass (335 mg/L), and basin wildrye (263mg Cu/L). The remaining species did not experience highenough mortality, within the range of Cu concentrations used(Table 2), to allow for calculation of LC50s; they are thereforereported as .300 mg Cu/L. Two of these species, big bluegrassand tufted hairgrass, also had EC50-plant values greater than300 mg Cu/L. However, common wheat (LC50 5 .300) hadan EC50-plant of 120 mg Cu/L. This discrepancy was due tothe high survival of stunted wheat seedlings in the higher treat-ment levels. Most of these common wheat seedlings were smalland had very little green tissue remaining at the time of harvest.

Mean Cu concentrations for plants in the control treatment

2696 Environ. Toxicol. Chem. 21, 2002 M.W. Paschke and E.R. Redente

Fig. 2. Relationships between plant tissue Cu concentrations andgrowth reduction in shoots (right column) and roots (left column) ofvarious grass species growing in sand culture and exposed to sup-plemental Cu treatments ranging from 0 (control) to 300 mg Cu/L.Error bars represent the standard error of the mean. Note that axesfor shoots and roots are not scaled uniformly.

Table 4. Some published Cu phytotoxicity thresholds (PT) andeffective concentrations (EC)

Plant Threshold ValueaRefer-ence

Agricultural speciesGrasses

Barley (Hordeum vulgare)BarleyBarleyBarleyCorn (Zea mays)CornCornRape (Brassica napus)RapeRye (Lolium perenne)RyeRyeWheat (Triticum aestivum)WheatWheatWheat

PT10PT50EC10EC10PT10PT10PT50PT10EC10PT10PT20EC10PT10PT10EC10EC50

14–24100

42–18

51040

15–220.3–3

21.30

218111.3

;300

[33][34][34][33][35][36][37][33][33][33][38][33][33][39][33][39]

ForbsCassava (Manihot esculenta) PT10 15 [40]Bushbean (Phaseolus vulgaris)BushbeanLettuce (Lactuca sativa)LettuceLettuceLettuceLettuceLettuceLettuceLettuceCabbage (Brassica oleracea)Carrot (Daucus carota)Cauliflower (Brassica oleracea)Spinach (Spinacia oleracea)Spinach

PT10PT50PT10PT10PT10PT10PT10PT20EC10EC50PT10PT10PT10PT10PT20

3029–3417–21

85–10

1410

.231–4

;25025

.1416

.6425

[41][42][33][39][43][35][44][38][33][39][45][44][44][44][38]

Nonagricultural speciesBindweed

(Polygonum convolvulus) EC50-shoot 259 [14]Bindweed

(Polygonum convolvulus) EC50-root 291 [14]

a Values for phytotoxicity thresholds are mg/kg plant tissue; valuesfor effective concentrations are mg/L.

were between 19 and 73 mg/kg for shoots and between 22 and239 mg/kg for roots. These are within the normal range oftissue Cu concentrations reported for these and similar species[32]. The phytotoxicity thresholds that we have determinedfor these species (Table 3) are considerably higher than thosevalues that have been reported for agronomic species (Table4). Most studies have reported Cu PT values well below thosereported here (Tables 3 and 4). Our values for PT50-shoot forrestoration grass species are between 737 mg Cu/kg for slenderwheatgrass and 10,792 mg Cu/kg for redtop (Table 3). Theone agronomic species that we tested, common wheat, had aPT50-shoot of 2,761 mg Cu/kg. This value is considerablyhigher than values reported for common wheat in the literature(Table 4). However, common wheat did appear to be the mostCu-sensitive species that we tested, as it had the lowest thresh-olds for PT50-root, EC50-root, and EC50-plant.

Many of the species tested here had wide discrepanciesbetween effective concentrations calculated for roots versusshoots. This apparent differential effect of Cu on roots versusshoots for various species indicates that a more robust measureof effective concentrations is the EC50-plant. On sites withno existing vegetation, where PT measures are not possible,EC measures could be useful for selecting species and under-standing site limitations in restoration planning where they canbe related to levels of soil solution Cu. Monitoring Cu inlysimeter solutions could accomplish this. Our measures ofEC50-plant for these restoration grasses were between 238 and710 mg Cu/L. These Cu phytotoxicity concentrations should

be applicable to those obtained from lysimeter solutions. Basedon EC50-plant values, species tolerance to Cu for these grassescan be categorized as follows: redtop . big bluegrass . tuftedhairgrass . slender wheatgrass . basin wildrye . commonwheat. From our data, it appears that redtop would be a goodspecies for restoration of Cu-contaminated sites. Big bluegrassmight also be a useful restoration species on Cu-contaminatedsites where grazing is to occur, as this species appears to ex-clude uptake of Cu to shoots relative to the other species tested(Fig. 2). Our observation that the agronomic species used inthis experiment (common wheat) was the least Cu tolerantspecies based on EC50-plant is important because it indicatesthat risk assessments conducted using EC50s for agronomicspecies may call for remediation efforts that might not bejustifiable where restoration grass species are to be used.

The Cu toxicity thresholds reported here for restorationgrass species are generally high relative to common wheat andto values for other agronomic species reported in the literature(Table 4). Under field conditions, Cu stress would act syner-gistically with other environmental factors (e.g., competition,

Copper toxicity thresholds for restoration grasses Environ. Toxicol. Chem. 21, 2002 2697

disease, herbivory) and would result in greater mortality thanwas observed in this greenhouse study. We recognize that tox-icity thresholds reported here are only approximations of whatmight be observed in the field because of the assumptionsimplicit in the experimental design. Nevertheless, these thresh-olds might be more useful for risk assessors than those basedon crop plants that are currently available and widely used.

Acknowledgement—This research was made possible through fundingfrom the Colorado Agriculture Experiment Station, ProjectCOL006600. The authors are grateful for the valuable insights of KenBarbarick and David Levy during the planning of this research. Wewould also like to thank Daniel LeCain and the U.S. Department ofAgriculture, Agricultural Research Service Rangeland Resources Re-search Unit, for the generous use of their greenhouse facilities toconduct this research.

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