Environ. Sci. Technol. 2002, 36, 4282-4287
Investigation into the Importance of the Stomatal Pathway in the Exchange of PCBs between Air and Plants
TABLE 1. Percentages of PCB Congeners Predicted to Enter the Leaf through the Stomatal Pathway Using the Model of Riederer (10) in Plants with Relatively Impermeable Cuticle (Citrus aurantium) and 100 Times More Permeable Cuticlesa Citrus aurantium cuticle
cuticle 100 times more permeable
stomatal density J O N A T H A N L . B A R B E R , * ,† PERIHAN B. KURT,‡ GARETH O. THOMAS,‡ GERHARD KERSTIENS,‡ AND KEVIN C. JONES‡ Departments of Environmental and Biological Sciences, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K.
The transfer of persistent organic pollutants (POPs) from air to vegetation is an important air-surface exchange process that affects global cycling and can result in human and wildlife exposure via the terrestrial food chain. To improve understanding of this process, the role of stomata in uptake of gas-phase polychlorinated biphenyls (PCBs) was investigated using Hemerocallis x hybrida “Black Eyed Stella”, a plant with a high stomatal density. Uptake of PCBs was monitored over a 72-h period in the presence and absence of light. Uptake rates were significantly greater in illuminated (stomata open) plants than unilluminated (stomata closed) plants for 18 of the 28 measured PCB congeners (p < 0.05). Depuration of PCBs was monitored in a subsequent experiment over a period of 3 weeks. Levels after 3 weeks of depuration time were still much higher than the concentration prior to contamination. Triand tetrachlorinated PCBs showed the greatest depuration, with less than 20% and 50% of accumulated PCBs respectively remaining, while ∼70% of higher chlorinated PCB congeners remained in the plants at the end of the experiment. Treatments with/without light (to control stomatal opening during uptake) and with/without abscisic acid (ABA) application (to control stomatal opening during depuration) were compared. After contamination indoors for 3 days, there was a significantly higher concentration of PCBs (p < 0.05) in the light contaminated plants than the dark-contaminated plants for 13 of the 28 measured PCB congeners. The ABA treatment affected depuration of PCB-18 only. “Light/ABA-treated” plants had a significantly slower depuration rate for PCB-18 than “light/ untreated”, “dark/ABA-treated”, and “dark/untreated” plants (p < 0.05). The results of the study indicate that there is a stomatal effect on the rate of exchange of PCBs between Hemerocallis leaves and air.
Introduction The transfer of persistent organic pollutants (POPs) from air to vegetation is an important process that determines the * Corresponding author phone: 01524 593 957; fax: 01524 593 985; e-mail:
[email protected]. † Department of Environmental Sciences. ‡ Department of Biological Sciences. 4282
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PCB high low high low congener (nas ) 0.02)b (nas ) 0.002)b (nas ) 0.02)b (nas ) 0.002)b 18 28 52 101 136 153 171 202
98 94 86 43 21 4 0 0
87 72 50 10 3 1 0 0
21 6 1 0 0 0 0 0
3 1 0 0 0 0 0 0
a See Supporting Information for details of how values were calculated. b nas is the fraction of the leaf surface taken up by stomata.
human exposure to POPs from the grass/cow/human food chain (1) and affects the global distribution of POPs (2). Plant leaves are covered by a nonliving lipid surface structure called the cuticle, which acts as a barrier against water loss and pathogen invasion (3). The cuticle consists of a biopolymer framework encrusted by and interspersed with soluble plant waxes, which as a unit is a formidable barrier to diffusion into the leaf (4). POPs are believed to enter the plant primarily via the cuticle because of their high solubility in lipids (5). Regulated pores called stomata penetrate through the cuticle to the intercellular air spaces below, to allow the necessary exchange of gases between cells in the leaf interior and the surrounding air (6). Stomatal aperture responds to light intensity, air humidity, CO2 concentration, internal concentration of ABA, and other factors (6). These stomatal pores are known to be a major uptake pathway for gaseous pollutants such as SO2, NOX, ozone, and toluene (7-9) which are less lipophilic than POPs. However, theoretical models have shown that the transfer of most gas-phase POPs through stomata is negligible compared to transfer through the cuticle (5, 10, 11), and therefore uptake through stomata has largely been ignored as an uptake pathway uptake for POPs into leaves. Riederer (10) showed how the relative importance of the stomatal pathway increases as the permeability of cuticle decreases. Riederer’s model predicts that, in plants with a relatively impermeable cuticle, low KOA POPs (e.g. PCBs 18, 28 + 52) may largely be taken up by stomata, while higher KOA PCBs are not (see Table 1). In the case of a plant with a very permeable cuticle, the importance of the stomatal pathway is predicted to be reduced almost to zero (see Table 1). Stomata may therefore play an important role in airplant transfer of some POPs, with the relative importance of the cuticular and stomatal pathways varying between POPs depending on their physicochemical properties, and between plant species depending on the permeability of the cuticle and number of stomata. To our knowledge, there is currently no experimental data to confirm or refute these modeled calculations. An observable stomatal effect on plant leaf concentrations was predicted by Deinum et al. (12). They suggested that leaf concentrations would increase in the daytime when stomatal resistances were lowest and would decrease at night when stomata are closed. Bakker et al. (13), using calculations from a pesticide study (14), estimated transfer rates of PCBs 10.1021/es025623m CCC: $22.00
2002 American Chemical Society Published on Web 08/16/2002
through the surface wax layer of beech and spruce leaves to be very slow, taking months and years, respectively, to reach equilibrium. However, when they investigated uptake of chlorobenzenes into plantains in a chamber experiment, they found uptake and attainment of steady state/equilibrium to be very rapid, taking only hours (15), a disparity which could perhaps be explained by stomatal uptake. A recent model by Tao and Hornbuckle predicted that the stomatal pathway is a dominant pathway or dry gaseous deposition of POPs on leaves (16). The authors have found no published studies carried out specifically to investigate the contribution of stomata to plant accumulation of POPs. This study was therefore carried out, using Hemerocallis x hybrida “Black Eyed Stella” (Liliaceae), a monocot species with grass-like leaves, with a high density of leaves at the foliage base, and an open canopy at the floral top of the plant. This species has leaves that are approximately 30 cm long and 3 cm wide and are therefore big enough to allow direct measurement of stomatal conductances with a porometer. PCBs have been used to study this process without complications caused by particle deposition because very little (400 mmol m-2 s-1, which is at the high end of typical conductances (32). Sampling and Chemical Analysis. Indoor air samples were collected using 2 low-volume air samplers each containing a pre-extracted polyurethane foam (PUF) plug. Outdoor air samples were collected using a Hi-vol air sampler containing a glass fiber filter and two pre-extracted PUF plugs. Vegetation samples were extracted according to previously published methods (33, 34). Leaves were frozen with liquid nitrogen, blended up with anhydrous sodium sulfate, spiked with labeled recovery standards (13C PCB-28, 52, 101, 153, 138, 180), and Soxhlet extracted for 16 h in DCM. Vegetation extracts were evaporated to dryness, resuspended in hexane, and cleaned up using silica/acid silica chromatography (eluted with hexane) followed by gel permeation chromatography (using Biobeads S-X3, eluted with 1:1 DCM:hexane). For quality control, sodium sulfate blanks made up 20% of samples analyzed, and an in-house reference material composed of dried cow faeces made up 10% of samples analyzed. Air samples were Soxhlet extracted for 8 h in hexane, and extracts were cleaned up using activated silica columns, according to previously published methods (17). Analysis was carried out using a Fisons MD800 GC/MS, and PCBs quantified were IUPAC numbers 18, 28, 31, 41/64, 44, 49, 52, 74, 70, 87, 95, 90/101, 99, 110, 118, 138, 141, 149, 151, 153, 158, 174, 180, 183, 187, 194, 200, and 203. Total PCBs quoted in this paper are a sum of these 28 congeners. Recoveries of the 13C labeled PCBs averaged 102%, ranging between 92 and 112%. The results were not corrected for recovery.
Results and Discussion PCB Air Concentrations. Air concentrations in the room where the uptake experiment was conducted were stable and elevatedsbetween 13 and 28 ng ∑PCB m-3 (average 20 ng ∑PCB m-3). Concentrations of individual congeners were between ca. 200-2000 times higher than ambient (data not shown), making it possible to clearly see the plant concentrations rapidly responding to the elevated exposures to a range of PCB congeners. The average PCB air concentration at Hazelrigg over the precontamination period was 0.16 ng ∑PCB m-3, which is a factor of nearly 100 lower than the air concentrations that the plants were exposed to during the uptake phase (average for experiment 2 was 12 ng ∑PCB m-3). Air concentrations over the depuration period averaged 74 pg ∑PCB m-3 (range 43-115 pg ∑PCB m-3). These concentrations are typical of other measurements at this site in previous years (22, 23). Temperatures of Laboratory and Hazelrigg Field Station during Experiments. The indoor laboratory in which the plants were contaminated had an average temperature of 22.1 °C under lights, with temperatures on average 1.1 °C higher than temperatures in the dark zone. The temperatures underwent diurnal fluctuations ranging from 19.0 °C in the day to 23.6 °C in the night. The Hazelrigg field station had an average temperature of 13 °C for the first day of the clearance experiment, and average daily temperatures of 13 °C (range 10-16 °C) over the following weeks. Experiment 1: Comparison of Uptake of PCBs by Plants with Stomata Open and Closed. Plant PCB concentrations for all three treatments increased with time of exposure to indoor air. Over the 2 days of the experiment, they increased from 0.63 ng to 8.4 ng ∑PCB g-1 fw in the light (day start) treatment plants, from 0.62 ng to 7.6 ng ∑PCB g-1 fw in the dark treatment plants, and from 0.34 ng to 7.9 ng ∑PCB g-1 fw in the light (night start) treatment plants. Figure 2 shows the uptake of PCB-18 and PCB-180 by Hemerocallis under light (day start), dark and light (night start) conditions. The graphical results on visual inspection show a great deal of scatter. The uptake rates, however, were significantly higher
FIGURE 2. Uptake of PCB-18 (a) and PCB-180 (b) over 48 h with stomata open (light) and stomata closed (dark).
FIGURE 3. Depuration of PCB-18 (a) and PCB-180 (b) over 3 weeks in ABA-treated and untreated plants after contamination in either light or dark conditions. (using a multivariate linear regression) in the light (day start) treatment than the dark treatment at p < 0.05 for 18 of the 28 measured PCB congeners and at p < 0.10 for 23 of the 28
congeners. For all other congeners uptake rates were also higher in light treated plants, but these differences were not statistically significant. There was no pattern in the size of this effect with increasing KOA or increasing degree of chlorination. There is therefore evidence that the stomatal uptake pathway can be important for many PCBs. However, in night-start illuminated plants only 5 of 28 PCB congeners at p < 0.05 and 11 of 28 at p < 0.10 have uptake rates significantly higher in light than in dark. There is no obvious explanation for differences between day- and night-start illuminated plants, since both experienced the same number of day and night periods. Visual inspection of the data indicated that some of the scatter around the regression line may have been caused by diurnal variations in the uptake rate (see Figure 2). The variation in light plants is in line with predictions of an increase in leaf concentration during the day when stomata are open (12). To test this, a constrained nonlinear regression was carried out incorporating a diurnal variation with a fixed period corresponding to that in the temperature and stomatal conductance fluctuations. This fluctuation was found to be significant at p < 0.05 for PCB-90/101 but not for all the other PCBs. Therefore, the statistical test does not confirm the visual impression. The degree of stomatal opening was reduced in indoor plants compared with outdoor plants, and therefore it is possible that a stomatal effect would be even greater in field conditions. In addition, it is possible that the low wind speed used in the uptake experiment compared with higher speeds found in the field may create an additional air-side resistance which will reduce the importance of any stomatal effect. It has been shown that under strong wind speed regimes (4 m s-1) aerodynamic resistance is much smaller than the canopy stomatal resistance for SO2 deposition in an oak forest (35). Under low wind speeds aerodynamic resistances are more important, which leads to the stomatal resistance having a smaller effect on the deposition velocity. Experiment 2: Comparison of Depuration of PCBs from Plants with Stomata Open and Closed. Precontamination Hemerocallis PCB concentrations were relatively stable with an average of 0.17 ng ∑PCB g-1 fw (range 0.14-0.19 ng ∑PCB g-1 fw). As concentrations did not significantly alter over the 2 week precontamination period, this concentration was used as the equilibrium concentration to which plants were expected to depurate in the clearance experiment. After contamination indoors for 3 days, light-contaminated plants contained on average 11 ng ∑PCB g-1 fw, and dark-contaminated plants contained on average 7.6 ng ∑PCB g-1 fw. There was a significantly higher concentration of PCBs (at the p < 0.05 level) in the light contaminated plants than the dark-contaminated plants for 13 of the 28 measured PCB congeners (PCBs 18, 44, 74, 87, 90/101, 110, 138, 141, 149, 151, 153, 183, and 187). For all other congeners, concentrations were also higher in light treated plants, but these differences were not significant. This confirms that plants contaminated in the light have significantly higher uptake rates than plants contaminated in the dark and is evidence for a stomatal contribution to uptake. Over the first 8 h of the depuration period, Hemerocallis PCB concentrations did not show a significant decrease for any of the treatments. Concentrations did, however, decrease over the rest of the 3 week depuration period, with PCB levels in light-contaminated plants decreasing from 10 to 5.9 ng ∑PCB gn-1 fw in ABA treated plants and from 12 to 6.2 ng ∑PCB g-1 fw in untreated plants, and in dark-contaminated plants PCB concentrations decreased from 7.6 to 3.9 ng ∑PCB g-1 fw in ABA treated plants and from 7.6 to 1.9 ng ∑PCB g-1 fw in untreated plants. It is important to note that the levels after 3 weeks were still much higher than the 0.17 ng ∑PCB g-1 fw found in the plants prior to contamination, and VOL. 36, NO. 20, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Percentage of PCB Contamination Remaining in the Leaf after 3 Weeks of Depuration for Selected Congeners light contaminated
dark contaminated
PCB congener
ABA-treated
untreated
ABA-treated
untreated
18 28 52 101 138 153 180 av
12 ( 3 17 ( 4 63 ( 16 86 ( 21 86 ( 21 86 ( 21 68 ( 17 60 ( 15
6.8 ( 2 6.3 ( 2 41 ( 10 83 ( 21 83 ( 21 85 ( 22 64 ( 16 54 ( 14
12 ( 3 12 ( 3 64 ( 16 83 ( 21 75 ( 19 73 ( 19 64 ( 16 54 ( 14
29 ( 7 25 ( 6 71 ( 18 86 ( 21 86 ( 21 77 ( 20 76 ( 19 63 ( 16
therefore the plants were not close to reaching the background concentration. Given that the leaf PCB concentrations did not fall greatly over a period of 3 weeks, it is likely that decreases over the initial hourly sampling period were so low as to be undetectable above the inherent analytical variability. There was considerable variability in the degree to which individual congeners had depurated after 3 weeks (see Table 2). Trichlorinated PCBs showed the largest reduction, with less than 20% of accumulated PCBs remaining after the 3 week depuration period. Tetrachlorinated PCBs also showed a large decrease in concentration, with typically less than 50% of accumulated PCBs remaining after the 3 week depuration period. However, higher chlorinated PCBs did not show much of a reduction in concentration, with more than 70% of accumulated PCBs remaining in the plants at the end of the experiment. These results are similar to those found by Ko¨mp and McLachlan in ryegrass (36), where partitioning of the lower chlorinated PCB congeners was largely reversible in the time scale of their depuration study, but not most other congeners, which remained “trapped” within the plant. There were differences in clearance rates between treatments for individual PCB congeners. However, only PCB 18 showed a significant difference between any of the treatments for any of the 28 different PCB congeners, although the “Light/ ABA” treatment generally contained more trapped PCBs at the end of the experiment than the other three treatments (see Table 2). Light/ABA plants had a significantly slower depuration rate than the other three treatments at p < 0.05 for PCB 18. If stomata are important for uptake and subsequent clearance, the Light/ABA treatment is expected to have the slowest release rate for PCB congeners, since compounds that had entered through stomatal pores during exposure will have been prevented from leaving the leaf when these pores remained closed during the clearance period (provided the molecules taken up via stomata did not accumulate in the leaf at locations from where they could depurate while encountering equal or less cuticular resistance than those molecules that entered the leaf through the cuticle). If we hypothesize that the PCBs that enter the cuticle could not leave the leaf through the stomata, we would expect that there was no difference between the two “dark” treatments. This is because PCBs would not have gone into leaves through the stomata when they were closed, and therefore the rate of depuration should not depend on whether the stomata are open. Depuration rates for “Dark/ ABA” plants were slower, but were not significantly different at p < 0.05 to those for “Dark/untreated” plants for any PCB congener. Discussion of Observed Differences in Importance of Stomata for Uptake and Depuration. It is interesting to consider why stomata had a significant effect on the uptake of PCBs, but with the exception of PCB 18, no significant effect on their depuration. It is possible, though unlikely, 4286
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that the repeated application of ABA did not cause the expected reduction of stomatal conductances, but unfortunately, due to technical problems we were unable to measure stomatal conductances in the field. The uptake effect is not likely to be the result of the small differences in temperature between light and dark plants, since a 1 °C temperature difference should not have a large effect on kinetics, and the higher temperatures in plants under lights should actually reduce the partitioning of PCBs to the leaves from the air (25). Since all other conditions were the same for light and dark plants, the obvious conclusion from these results is that stomatal opening did have a significant effect on uptake of PCBs by Hemerocallis plants in this experiment. With regard to the second experiment, it seems counterintuitive that compounds that have entered the leaf more rapidly through the stomata than the cuticle will depurate from the leaf at the same rate, whether stomata are open. It is possible that a stomatal effect does exist, but that the low clearance on day 1 for all four treatments is obscuring this effect. In addition, the lack of depuration observed for many PCB congeners does not allow the detection of any significant differences between clearance rates. General Remarks on the Importance of Stomatal Transfer for Air/Plant Exchange Processes for POPs. This experiment has shown that there is a stomatal effect on the rate of uptake of PCBs into Hemerocallis leaves. This occurred when the degree of stomatal opening in the laboratory (conductances of 20-40 mmol m-2 s-1) was small compared with that which we measured in the field (> 400 mmol m-2 s-1). This plant species has mesophytic leaves with a large number of stomata per leaf surface area. Table 1 shows that, according to Riederer (10), this tends to increase the relative importance of the stomatal route. It is possible that in species with more xerophytic leaves, which often are associated with lower cuticular permeability and stomatal conductance, the relative importance of the stomatal pathway for uptake of POPs is quite different and probably greater, as the range of typical stomatal conductances found in different species (32) is rather smaller than that for cuticular permeabilities (37). Stomatal conductances are affected by both environmental and leaf morphological characteristics. Therefore, the importance of the stomatal pathway is likely to vary both temporally and spatially in the natural environment. For instance in corn, spruce, and oak, stomatal resistances to water diffusion decrease by a factor of 2-4 as light intensity increases from low levels (50 W m-2) to high levels (400 W m-2) (38). Air humidity, light, leaf temperature, soil water availability, leaf age, and other factors all play a role in determining stomatal conductances, whereas with the exception of high leaf temperatures, cuticular permeability is insensitive to short-term changes in the environment. Further studies are clearly warranted, testing other plant species under more natural conditions and ideally compounds with a bigger range of properties. It is difficult to produce an accurate model of stomatal uptake of PCBs with our current level of understanding, because of the lack of rate measurements for many of the processes involved. Currently, cuticular and stomatal conductances of PCBs can only be estimated from analogies with other chemicals (10). Much experimental work is required before a model based upon measured data can be achieved.
Acknowledgments We would like to thank the Food Contaminants Division of the UK Food Standards Agency (FSA) for funding this work. Special thanks to Dr. Mick Green from the Applied Statistics Department of Lancaster University for his help with the statistical analysis of the data.
Supporting Information Available Method for prediction of percentage of PCB congener that enters the leaf through the stomatal pathway, using the model of Riederer (10). This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) McLachlan, M. S. Environ. Sci. Technol. 1996, 30, 252-259. (2) Bennett, D. H.; McKone, T. E.; Matthies, M.; Kastenberg, W. E. Environ. Sci. Technol. 1998, 32, 4023-4030. (3) Kerstiens, G. Trends Plant Sci. 1996, 1, 125-129. (4) In Waxes: Chemistry. Molecular Biology and Functions; Riederer, M., Schreiber, L., Hamilton, R. J., Eds.; The Oily Press: Dundee, 1995; pp 131-156. (5) Riederer, M. Environ. Sci. Technol. 1990, 24, 829-837. (6) Wilmer, C.; Fricker, M. Stomata, 2nd ed.; Chapman & Hall: London, 1996. (7) Slovik, S.; Siegmund, A.; Fuhrer, H. W.; Heber, U. New Phytol. 1996, 132, 661-676. (8) Draaijers, G. P. J.; Erisman, J. W.; VanLeeuwen, N. F. M.; Romer, F. G.; teWinkel, B. H.; Veltkamp, A. C.; Vermeulen, A. T.; Wyers, G. P. Atmos. Environ. 1997, 31, 387-397. (9) Jen, M. S.; Hoylman, A. M.; Edwards, N. T.; Walton, B. T. Environ. Exp. Bot. 1995, 35, 389-398. (10) In Plant contamination: Modeling and simulation of organic chemical processes; Riederer, M., Trapp, S., McFarlane, J. C., Eds.; Lewis Publishers: London, 1995; pp 153-190. (11) McLachlan, M. S.; Welsch-Pausch, K.; Tolls, J. Environ. Sci. Technol. 1995, 29, 1998-2004. (12) Deinum, G.; Baart, A. C.; Bakker, D. J.; Duyzer, J. H.; Van Den Hout, K. D. Atmos. Environ. 1995, 9, 997-1005. (13) In Persistent, bioaccumulative and toxic chemicals I: Fate and exposure; Bakker, M. I., Tolls, J., Kollo¨ffel, C., Lipnick, R. L., Hermens, J. L. M., Jones, K. C., Muir, D. C. G., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000; pp 218-236. (14) Schreiber, L.; Kirsch, T.; Riederer, M. Planta 1996, 198, 104109. (15) Bakker, M. I. Doctoral Dissertation, University of Utrecht, 2000. (16) In Air-surface exchange of gases and particles 2000; Tao, Z., Hornbuckle, K. C., Fowler, D., Pitcairn, C., Erisman, J.-W., Eds.; Kluwer Academic Publishers: 2001; pp 275-283.
(17) Thomas, G. O.; Sweetman, A. J.; Ockenden, W. A.; Mackay, D.; Jones, K. C. Environ. Sci. Technol. 1998, 32, 936-942. (18) Barber, J. L.; Thomas, G. O.; Kerstiens, G.; Jones, K. C. Environ. Sci. Technol. 2002, 36, 3224-3229. (19) Davies, W. J.; Rodriguez, J. L.; Fiscus, E. L. Plant Cell Environ. 1982, 5, 485-493. (20) Franks, P. J.; Farquhar, G. D. Plant Physiol. 2001, 125, 935-942. (21) Wan, X.; Zwiazek, J. J. Planta 2001, 213, 741-747. (22) Lee, R. G. M.; Hung, H.; Mackay, D.; Jones, K. C. Environ. Sci. Technol. 1998, 32, 2172-2179. (23) Lee, R. G. M.; Jones, K. C. Environ. Sci. Technol. 1999, 33, 705712. (24) Thomas, G. O.; Smith, K. E. C.; Sweetman, A. J.; Jones, K. C. Environ. Pollut. 1998, 102, 119-128. (25) Simonich, S. L.; Hites, A. Environ. Sci. Technol. 1995, 29, 29052914. (26) Jones, H. G. Plants and microclimate, 2nd ed.; Cambridge University Press: Cambridge, 1992. (27) Holloway, P. J.; Baker, E. A. Plant Physiol. 1968, 43, 1878-1879. (28) Kerler, F.; Riederer, M.; Scho¨nherr, J. Physiol. Plant. 1984, 62, 599-602. (29) Lawson, T.; James, W.; Weyers, J. J. Exp. Bot. 1998, 49, 13971403. (30) Hultine, K. R.; Marshall, J. D. J. Exp. Bot. 2001, 52, 369-373. (31) In The plant cuticle; Price, C. E., Cutler, D. F., Alvin, K. L., Price, C. E., Eds.; Academic Press: New York, 1982; pp 237-251. (32) In Ecophysiology of photosynthesis; Ko¨rner, C., Schulze, E.-D., Caldwell, M. M., Eds.; Springer: Berlin, 1995; pp 463-490. (33) Thomas, G. O.; Sweetman, A. J.; Parker, C. A.; Kreibich, H.; Jones, K. C. Chemosphere 1998, 36, 2447-2459. (34) Hung, H.; Thomas, G. O.; Jones, K. C.; Mackay, D. Environ. Sci. Technol. 2001, 35, 4066-4073. (35) Baldocchi, D. Atmos. Environ. 1988, 20, 869-884. (36) Ko¨mp, P.; McLachlan, M. S. Sci. Total Environ. 2000, 250, 6371. (37) Kerstiens, G. J. Exp. Bot. 1996, 47, 1813-1832. (38) Baldocchi, D.; Hicks, B. B.; Camara, P. Atmos. Environ. 1987, 21, 91-101.
Received for review March 5, 2002. Revised manuscript received July 2, 2002. Accepted July 10, 2002. ES025623M
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