Sorption of Volatile Organic Chemicals in Plant Surfaces

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Environ. Sci. Technol. 1998, 32, 1099-1104

Sorption of Volatile Organic Chemicals in Plant Surfaces BIRGIT WELKE, KARIN ETTLINGER, AND MARKUS RIEDERER* Lehrstuhl fu ¨ r Botanik II, O ¨ kophysiologie und Vegetationso¨kologie, Biozentrum der Universita¨t Wu ¨ rzburg, Julius-von-Sachs-Platz 3, D-97082 Wu ¨ rzburg, Germany

Sorption in the cuticles covering most of the aboveground surface of plants is the first step of the atmosphereto-vegetation transfer of volatile organic compounds (VOCs). The partitioning of 50 reference VOCs of varying physical-chemical properties between the vapor and the cuticular matrix (MX, obtained by dewaxing isolated cuticles) or aqueous phase, respectively, was studied at 25 °C using static headspace gas chromatography. Linear sorption isotherms were obtained over a wide range of concentrations, indicating that Henry’s law applies. Concentration-independent cuticular polymer matrix (MX)/ air (KMXa), air/water (Kaw), and MX/water (KMXw) partition coefficients were derived from the slopes of the sorption isotherms. The experimentally determined values of KMXa ranged from 39 (isoprene) to 33 000 (1-hexanol). KMXas were linearly related to the corresponding partition coefficients for the native cuticular membrane. The values of Kaw of the reference compounds varied from 1.94 × 10-4 (methanol) to 1.66 (limonene). MX/water (KMXw) partition coefficients ranging from 0.090 (methanol) to 18 094 (limonene) were estimated from KMXa and Kaw. For predictive purposes, a set of quantitative property/property and structure/property relationships between KMXa and simple physical-chemical properties and structural descriptors was established.

Introduction Technical and natural processes release huge amounts of volatile organic compounds (VOCs) into the atmosphere. These atmospheric contaminants are intercepted by terrestrial vegetation (1-7) leading to their accumulation and entry into food chains. The air-to-vegetation transfer of VOCs may substantially contribute to ecosystem and human exposure to organic chemicals since in most types of vegetation the total surface area of the above-ground parts by far exceeds the area plants are growing on. This fact has to be considered in environmental research and applied risk assessment (8). The predominant initial site of interception of atmospheric contaminants by vegetation is the plant cuticle, a lipophilic polymer membrane with associated cuticular waxes (9, 10). Depending on the physical-chemical properties of the VOCs and/or the physiological state of the leaf, volatiles may be taken up into or released from plant leaves either via the stomatal and/or the cuticular pathway. In addition to controlling leaf/atmosphere exchanges, the cuticle may also function as a compartment where lipophilic chemicals accumulate. * To whom correspondence should be addressed. Fax +49 931 888 6235; e-mail: [email protected]. S0013-936X(97)00763-3 CCC: $15.00 Published on Web 02/24/1998

 1998 American Chemical Society

The partitioning of organic compounds between the cuticle and an adjacent aqueous phase has been studied extensively (9-12) while only scarce information is available on the mechanisms ruling the interaction of airborne compounds and the plant cuticle. Most of the knowledge on the latter subject is limited to inorganic gases such as SO2, NO2, or O2 (13, 14) with only a few reports dealing with organic vapors (15-18). Field studies on the uptake of airborne organics into vegetation (19-22) have drawn attention to the decisive role of the cuticle during the processes involved. Dry and wet deposition of airborne VOCs onto the leaf surface, the aqueous cell wall phase in the interior of the leaf, and the lipoid cuticle are the four main compartments involved in the partitioning and transfer of VOCs at the leaf/ atmosphere interface (Figure 1). Their relative importance for the accumulation of organic volatiles can be assessed by the appropriate partition coefficients between the phases. Therefore, the objectives of the present work are (1) to characterize the sorptive properties of plant cuticles for a diversity of VOCs with varying physical-chemical properties, (2) to investigate the effect of chemical structure on the interaction with the plant cuticle, and (3) to derive quantitative property/property and structure/property relationships for estimating the accumulation of organics by the leaf/air interface in the absence of experimental data.

Experimental Section Plant Cuticles. Cuticular membranes (CM) were obtained from mature tomato fruits (Lycopersicon esculentum Mill. cultivar Vendor) by digesting enzymatically interior fruit tissues (23). Isolated cuticles were subsequently extracted for 16 h with CHCl3 (Riedel de Hae¨n) in a Soxhlet apparatus yielding polymer matrix membranes (MX). MX membranes were recovered after each experiment by exhaustive Soxhlet extraction (16 h) and reused. Preliminary tests had shown that this treatment did not affect the sorptive properties of MX and that they do not drastically depend on the species or organ from which they have been obtained (24). Chemicals. A total of 50 volatile organic chemicals (VOCs) was chosen representing a wide diversity of structural and physical-chemical properties (properties taken from refs 2534 are listed in Supporting Information). For experimental reasons, only compounds with vapor pressures at 25 °C g 124 Pa could be tested. All compounds (purity g 97%) were used without any further purification. Partition Experiments. Partitioning of VOCs between the gas and the cuticular material or the aqueous phase was measured by a static headspace gas chromatographic method described in detail elsewhere (35). In short, experimental procedures were as follows: exactly determined amounts of MX (4-400 mg) were added to 20 mL glass vials (MachereyNagel, Du ¨ren, Germany) sealed with PTFE-lined septum caps. Precisely known liquid or vapor volumes of the test compounds (see Supporting Information) were injected into the vials. The amounts of MX in the sample vials were chosen as to reduce the vapor-phase concentration of VOCs by 2080% during equilibration. For each compound, eight sample vials containing varying amounts of MX and additionally three reference vials without MX were prepared. Subsequently, the vials were equilibrated for 3 h at 25 °C. Previous tests had shown that this interval was sufficient for complete equilibration. After equilibration, the resulting concentration in the gas phase was measured (see below). The equilibrium concentration of the VOC in the MX was estimated according to VOL. 32, NO. 8, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Main compartments involved in the atmosphere-leaf interchange of volatile organic compounds. The equilibrium partition coefficients between the phases are also given. 0

CMX )

0 Ca0Va - Ca(0Va - VMX) Va 0 ) ( C - Ca) + Ca (1) VMX VMX a

where 0Ca is the mean vapor-phase concentration in the reference vials, Ca the equilibrium vapor-phase concentration in the sample vial, and 0Va and VMX stand for the mean volumes of the vials and the MX sample, respectively. The volume of the MX sample was estimated from MX mass assuming a specific mass of 1120 kg/m3 (36). The mean 0Va of the vials used was 23.6 ( 0.1 mL as determined gravimetrically with H2O. For all compounds, sorption isotherms were determined over the concentration range accessible by the method chosen. The concentration in the MX (CMX) was plotted against the concentration in the air (Ca). Molal MX/air (KMXa) partition coefficients

KMXa ) CMX/Ca

(2)

were obtained from the slopes of the isotherms as all isotherms obeyed Henry’s law. In some experiments, the method was modified in order to test the influence of relative humidity on the MX/air partition coefficient. In these cases, air was saturated with water vapor at 4 and 20 °C and passed through the headspace vials kept at 25 °C with a flow rate of 30-70 mL/min. The resulting relative humidities in the vials were 26 and 76%, respectively. Static Headspace Gas Chromatography. Vapor-phase concentrations were measured by static headspace gas chromatography. A gas chromatograph (Dani 8610, Monza, Italy) equipped with a PTV injector and a FID detector was connected to an autosampler (Dani HSS 3950), which had been modified as to allow for the control of the temperature of the vial holder by means of an external thermostat. The sample taken from the headspace of a vial was preheated to 170 °C in the transfer line leading to the injector (at 200 °C). Separation was achieved using a fused silica wall-coated open tubular capillary column (25 m, 0.2 mm inner diameter, CPSil 19 CB, film thickness 0.2 µm, Chrompack, Middelburg, Holland) isothermally at 70 °C with He2 as carrier gas (inlet pressure 0.6 bar) and N2 as makeup gas (0.6 bar). Split flow was 15 mL/min. VOC headspace concentrations g0.001 mol/ m3 could be measured using this system. Molecular Descriptors. Substructure lipophilic (PL) and hydrophilic (PH) contributions to the 1-octanol/water (Kow) partition coefficient were obtained from additive fragment values (∑f) and constitutive corrections (∑F) used to estimate 1100

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FIGURE 2. Representative isotherms for the sorption of volatile organic compounds in plant cuticular matrix (MX) at 25 °C determined by static headspace gas chromatography. MX/air partition coefficients were derived from the slopes of the isotherms. 1-octanol/water partition coefficients (37, 38). Molar refraction (MR) was determined using the atomic refraction method of Eisenlohr (39). The H-bonding descriptors HBD (H-bond donors) and HBA (H-bond acceptors) were set to either 1 or 0 depending on the activity of the molecule in hydrogen bonding. Connectivity indices (χ) according to Kier and Hall (40, 41) were calculated with Molconn-Z, version 3.0. For numerical values of the descriptors, see Supporting Information.

Results and Discussion Sorption Isotherms and Partition Coefficients. The sorption isotherms of the various reference compounds studied in the system Lycopersicon esculentum fruit cuticular matrix (MX)/air were strictly linear within the range of concentrations studied and the intercepts were not significantly (p ) 0.05) different from zero (Figure 2). For this reason, the slopes of plots of equilibrium concentrations in the cuticular matrix (CMX) versus the equilibrium concentrations in the gas phase (Ca) were equivalent to MX/air (KMXa) partition coefficients at 25 °C. They varied by approximately 3 orders of magnitude from 39 with isoprene to 33 000 with 1-hexanol (Table 1). Within a range from 0 to 76%, the relative humidity of the vapor phase had no significant effect on the sorptive properties of the cuticular material studied. For a subset of seven compounds, cuticular membrane (CM)/air (KCMa) partition coefficients were determined in addition to KMXa. In general, the removal of cuticular waxes led to an increase of the sorption capacity of the cuticular material for VOCs. The following linear free energy relationship exists between the partition coefficients determined for the two systems:

KCMa ) 0.77 ((0.05)KMXa (3) n ) 7; r2 ) 0.996; s ) 0.020; F ) 1554 where n is the sample size, r2 the coefficient of determination, s the standard error of the estimate, and F is the F-statistic for the goodness-of-fit. Equation 3 can be used for estimating KCMa from experimentally determined or estimated KMXa. Thus, the sorption of VOCs by plant cuticles in their natural state can be deduced. The isotherms for the sorption of VOCs from the gas into the cuticular matrix phase followed Henry’s law over a wide

TABLE 1. Experimentally Determined MX/Air (KMXa), Air/Water (Kaw), and MX/Water (KMXw) Partition Coefficientsa

a

compound

KMXab

1,1,1-trichloroethane 1,1-dichloroethylene 1,2-dibromoethane 1,2-dichloroethane 1,2-dichloropropane 1,4-dioxane 1-butanol 1-hexanol 1-nitropropane 1-pentanol 1-propanol 2-butanol 2-butanone 2-hexanol 2-methyl-2-butanol 2-pentanol 2-propanol 3-methyl-3-pentanol 4-methyl-2-pentanone acetone acetonitrile acrylonitrile allyl chloride benzene carbontetrachloride chlorobenzene chloroform cyclohexane cyclohexanone dichloromethane epichlorohydrin ethanol ethyl acetate ethylbenzene isobutanol isoprene limonene methanol n-butyl acetate n-heptane n-propyl acetate o-xylene pyridine styrene tert-butyl alcohol tetrachloroethylene tetrahydrofuran toluene trichloroethylene trichloronitromethane

380 ( 50 72 ( 9 2900 ( 200 590 ( 60 770 ( 30 1400 ( 0 5000 ( 0 33 000 ( 1000 2500 ( 0 13 000 ( 100 1800 ( 0 2000 ( 0 360 ( 10 12 000 ( 0 2300 ( 0 4800 ( 0 760 ( 0 4900 ( 0 1100 ( 0 250 ( 20 380 ( 30 250 ( 10 120 ( 20 430 ( 0 270 ( 20 3300 ( 0 430 ( 30 170 ( 10 8400 ( 230 240 ( 40 2200 ( 0 640 ( 10 340 ( 40 2500 ( 0 2600 ( 0 39 ( 12 11 000 ( 1000 450 ( 20 1900 ( 0 320 ( 30 710 ( 60 3600 ( 100 6700 ( 400 5500 ( 300 820 ( 0 960 ( 60 470 ( 10 1400 ( 0 760 ( 50 1900 ( 0

Given in parentheses are 95% confidence intervals.

b

Kawb

KMXw

(3.1 × 10-2) ( 0 (5.4 × 10-2) ( (3 × 10-3) (7.4 × 10-2) ( (4 × 10-3) (2.0 × 10-4) ( (2 × 10-5) (3.8 × 10-4) ( 0 (6.9 × 10-4) ( 0 (2.5 × 10-3) ( 0 (5.2 × 10-4) ( 0 (2.8 × 10-4) ( 0 (4.9 × 10-4) ( 0 (2.6 × 10-3) ( (1 × 10-4) (8.5 × 10-4) ( 0 (5.8 × 10-4) ( 0 (6.1 × 10-4) ( 0 (3.2 × 10-4) ( 0 (8.4 × 10-4) ( 0 (8.6 × 10-3) ( (9 × 10-4) (4.2 × 10-3) ( (4 × 10-4) (1.2 × 10-3) ( 0 (1.3 × 10-2) ( 0 (1.1 × 10-1) ( (4 × 10-2) (2.5 × 10-1) ( (5 × 10-2)

110

(1.5 × 10-1) ( (2 × 10-2) (1.2 × 10-1) ( 0

500 54

91 32 57 0.28 1.90 23 6.2 6.6 0.48 0.95 0.92 10 1.3 2.9 0.24 4.1 9.4 1.1 0.44 3.2

(1.1 × 10-4) ( 0 (1.4 × 10-3) ( 0 (2.1 × 10-4) ( 0 (1.2 × 10-2) ( 0

26 3.0 0.14 4.0

(5.2 × 10-4) ( 0

1.3

1.7 ( (5 × 10-1) (1.9 × 10-4) ( 0 (1.5 × 10-2) ( 0

18 000 0.090 28

(1.1 × 10-2) ( 0 (1.8 × 10-1) ( 3 × 10-2) (4.1 × 10-4) ( (3 × 10-5) (1.6 × 10-1) ( (4 × 10-2) (5.0 × 10-4) ( 0 (6.3 × 10-4) ( (6.5 × 10-4) (2.8 × 10-1) ( (2 × 10-2)

7.5 640 2.7 890 0.41 680 0.30 400

(1.0 × 10-1) ( 0

190

Data for the alcohols are taken from ref 35.

range of concentrations (up to 0.6 mol/L). This observation suggests that the interaction of the organic vapor and the solid sorbent resembles that of an ideal dilute solution. When a solid phase is involved, isotherms showing the behavior described are called constant partitioning isotherms, suggesting that the number and accessibility of sorption sites remain constant over a wide range of concentrations (4244). This phenomenon has been attributed to the successive expansion of the sorbent with increasing coverage by the sorbate. It is assumed that each molecule sorbed generates another vacant sorption site (43). Conditions that favor this type of isotherm are an amorphous substrate consisting of flexible molecules and a solute having the ability to swell the polymer (42). Both conditions appear to be fulfilled when organic vapors interact with plant cuticular matrixes. Air/Water Partition Coefficients. The partitioning of the reference VOCs between air and water also followed Henry’s law over a wide range of concentrations. Air/water (Kaw) partition coefficients were again calculated as the slopes of

the corresponding equilibrium isotherms (Table 1). For experimental reasons, Kaw could only be determined in the range from 1.7 (limonene) to 1.9 × 10-4 (methanol). The variation of Kaw exceeded that of KMXa by about a factor of 10. Cuticular Polymer Matrix/Water Partition Coefficients. KMXa and Kaw can be used for estimating the corresponding MX/water (KMXa) partition coefficients according to (10)

KMXw ) KMXaKaw

(4)

Estimated values of KMXw ranged over 5 orders of magnitude from 0.090 (methanol) to 18 000 (limonene). Thus, a full quantitative description of equilibrium partitioning of VOCs at the plant/atmosphere interface (Figure 1) has been achieved. Both KMXa and KMXw can be expected to decrease with increasing temperature (experimentally shown for KMXa, see ref 45). VOL. 32, NO. 8, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Property/Property Relationships. The number and diversity of VOCs emitted from technical and biological sources and known to be present in the atmosphere steadily increases. Therefore, experimentally determining the partitioning parameters necessary for assessing the environmental fate of a representative number of compounds is not feasible on economic and practical grounds. Consequently, establishing quantitative relationships for predicting partition coefficients in the system leaf surface/atmosphere for any VOC from easily accessible properties is a necessity. Boiling points (bp) and saturation vapor pressures (p°) are physical-chemical properties known for a large number of organic compounds. The MX/air partition coefficients of the reference compounds are linearly related to both properties. The corresponding regression equations are

log KMXa ) 1.343 ((0.238) + 0.017 ((0.002)bp (5) n ) 50; r2 ) 0.829; s ) 0.260; F ) 232.8 for the relationship with the boiling point (bp, in degrees celcius) and

log KMXa ) 6.290 ((0.319) - 0.892 ((0.087) log p° (6) n ) 50; r2 ) 0.899; s ) 0.199; F ) 428.7 for the correlation with saturation vapor pressure (p°, in pascal). Cuticle/water partition coefficients of organic compounds have been related to 1-octanol/water (Kow) partition coefficients (9, 11). In analogy to this finding, the variation of MX/air partition coefficients should follow 1-octanol/air (Koa) partition coefficients. As experimental data for Koa were available only for a small number of compounds (46), estimates of Koa had to be obtained from Kow and Kaw, according to

Koa ) Kow/Kaw

(7)

The double-logarithmic plot of experimental values of KMXa vs estimates of Koa is linear and can be described by

log KMXa ) 0.820 ((0.375) + 0.668 ((0.106)log Koa (8) n ) 38; r2 ) 0.819; s ) 0.237; F ) 163.3 The conclusion from eqs 5, 6, and 8 is that, for a wide range of compounds, boiling points, vapor pressures, and 1-octanol/air partition coefficients can be reliably used for predicting leaf surface/air partitioning of VOCs. These methods are applicable for the large number of organic compounds where basic physical-chemical properties are known. Structure/Property Relationships. There is, however, an at least equally large number of less extensively studied volatile organics which may also turn out to have environmental impact or biological activity. It is, therefore, desirable to establish tools for predicting the leaf surface/atmosphere partitioning of any VOC. This goal can be achieved by employing simple nonempirical descriptors which can be derived from molecular structure. During recent years, structure-based nonempirical descriptors such as molecular connectivity indices, molar refractivity, as well as substructure contributions to the 1-octanol/water partition coefficient have been successfully applied to property and activity predictions in the environmental sciences (37, 38, 47-51). Multiple regression analyses of experimental KMXa of the reference VOCs with various simple and valence molecular connectivity indices did not detect significant correlations. Only within the KMXa data for alcohols a relationship with the 1102

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FIGURE 3. Correlation between the experimentally determined MX/ air partition coefficient KMXa (exp) and values predicted according to eq 10. Regression line and 95% prediction intervals (dotted lines) are shown. first-order path index (1χp) was obtained:

log KMXa ) 1.758 ((0.432) + 0.717 ((0.176)1χp (9) n ) 14; r2 ) 0.868; s ) 0.206; F ) 79.1 Following an approach taken for predicting acute fish toxicity, bioconcentration factors in fish, and soil organic matter/ water partitioning (50), substructure lipophilic (PL) and hydrophobic (PH) contributions to log Kow, molar refraction (MR), and an ad-hoc hydrogen-bonding descriptor (for numerical values see Supporting Information) were included into the regression analyses. PL, PH, MR, the hydrogen-bond donor descriptor (HBD) and the third-order cluster index (3χc) turned out to be useful descriptors for the log KMXa values of the complete data set:

log KMXa ) 0.911 ((0.357) + 0.508 ((0.189)HBD 0.440 ((0.198)PL - 0.319 ((0.107)PH - (10) 0.263 ((0.147)3χc + 0.117 ((0.024)MR n ) 49; r2 ) 0.812; s ) 0.271; F ) 37.1 The hydrogen-bond acceptor descriptor (HBA) did not further improve the multiple correlation coefficient. Only the KMXa value of isoprene deviated more than two standard errors from the multiple regression line and, therefore, was excluded when eq 10 was calculated. The values of log KMXa predicted for the reference VOCs according to eq 10 agreed well with the experimental data (Figure 3). The experimental determination and prediction of KMXa are essential for understanding the air-to-vegetation transfer of airborne organic volatiles and their further fate in terrestrial ecosystems. The high affinities of cuticular material for organic substances and the extensive leaf areas present in most types of vegetation suggest that substantial amounts of VOCs will be removed from the atmosphere by this mechanism. Cuticle/air partition coefficients characterize the role of plant surfaces as lipophilic sinks which may also influence the atmospheric residence times of lipophilic organic pollutants. Additionally, air-to-plant surface partitioning is fundamental to the role of aerial parts of plants as biomonitors for atmospheric organic contaminants (1-6, 52, 53).

TABLE 2. Cuticular Matrix (MX)/air (KMXa) Partition Coefficients, Measured Atmospheric Concentrations (Ca), Calculated Concentrations in the Cuticular Matrix (CMX), and Estimated Amounts of VOCs Sorbed in Plant Surfaces per Squared Kilometer of Vegetation compound benzene toluene ethylbenzene m/p-xylene o-xylene freon-12 freon-113 chloroform 1,1,1-trichloroethane tetrachloromethane trichloroethylene tetrachloroethylene total a

KMXa Caa (µg/m3) CMX (µg/kg) amount (g/km2) 430 1400 2500 3400 3600 22 50 430 350 270 760 960 41

3.4 13 2.1 7.5 3.7 3.6 0.95 0.25 2.3 0.89 0.60 2.30 62

1.3 17 4.8 23 12 0.07 0.04 0.10 0.73 0.22 0.41 2.0 1.6

0.034 0.42 0.12 0.59 0.31 0.002 0.001 0.002 0.019 0.006 0.010 0.051

Data from ref 54.

Cuticular matrix/air partition coefficients may also be applied to estimate the accumulation of VOCs in plant surfaces when their atmospheric concentrations are known. This can be demonstrated with an arbitrarily selected set of data on the concentrations of 12 aromatic and halogenated compounds in the ambient air of a densely populated region (54). The atmospheric concentrations in this example range from 0.25 to 13 µg/m3 for chloroform and toluene, respectively (Table 2). The corresponding equilibrium concentrations in cuticular material obtained from the ambient concentrations and measured or estimated values of KMXa vary from 0.04 to 23 µg/kg for Freon-113 and m/p-xylene, respectively. For converting volume- to mass-based concentrations, a specific mass of 1100 kg/m3 was assumed for MX (36). At equilibrium, the 12 organic compounds would make up a total VOC concentration in the cuticle of 62 mg/kg. The weighted cuticle/air partition coefficient of this mixture of VOCs amounts to about 1700. Assuming 1 µm and 2.3 × 107 m2/km2 as representative values for cuticular thickness and total leaf surface area of many plant stands, respectively (10), it can be concluded that a total of 1.6 g of the 12 volatile compounds determined will be sorbed by the cuticular matrix of 1 km2 of vegetation. Thus, the input of VOCs into the food chains of terrestrial ecosystems covered by vegetation may be estimated from analytical data on ambient concentrations and leaf surface/ air partitioning parameters determined experimentally or estimated according to eqs 5, 6, 8, and 10. However, it should be kept in mind that equilibrium partitioning is assumed here. Thus, the estimates will give only a first approximation of the real situation which may be further complicated by various disturbances. The most important factors leading to deviations from equilibrium in air-to-cuticle partitioning are slow equilibration kinetics for highly lipophilic compounds as well as metabolism and translocation within the plant (10, 55). The experimental results and predictive tools presented in this work may, nevertheless, be useful in assessing the tendency of various anthropogenous and biogenous organic volatiles to partition into the large lipophilic compartment making up the plant/atmosphere interface.

Acknowledgments This work was supported by a grant by the Bundesminister fu ¨ r Forschung und Technologie and by the Fonds der Chemischen Industrie. B.W. obtained additional support by the Graduiertenkolleg “Pflanze im Spannungsfeld zwischen Na¨hrstoffangebot, Klimastresse und Schadstoffbelastung”,

Universita¨t Wu ¨rzburg. Part of the experimental work has been performed at the Institut fu ¨ r Botanik und Mikrobiologie, Technische Universita¨t Mu ¨ nchen. The authors are indebted to Prof. J. Scho¨nherr, Institut fu ¨ r Gemu ¨ se-und Obstbau, Universita¨t Hannover, for valuable suggestions and stimulating discussions.

Supporting Information Available One table (6 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the Supporting Information from this paper or microfiche (105 × 148 mm, 24× reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St. NW, Washington, DC 20036. Full bibliographic citation (journal, title of article, and issue number) and prepayment, check or money order for $12.50 for photocopy ($14.50 foreign) or $12.00 for microfiche ($13.00 foreign), are required. Canadian residents should add 7% GST. Supporting Information is also available via the World Wide Web at URL http://www.chemcenter.org. Users should select Electronic Publications and then Environmental Science and Technology under Electronic Editions. Detailed instructions for using this service, along with a description of the file formats, are available at this site. To download the Supporting Information, enter the journal subscription number from your mailing label. For additional information on electronic access, send electronic mail to [email protected] or phone (202) 872-6333.

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Received for review August 27, 1997. Revised manuscript received January 5, 1998. Accepted January 14, 1998. ES970763V