Sorption to leaf blades Modeling Uptake and Translocation
Sorption to leaf blades Modeling Uptake and Translocation of DDE in Cucurbita Martin P. N. Gent, J. C. White, B. D. Eitzer, M. J. I. Mattina Connecticut Agricultural Experiment Station, P. O. Box 1106, New Haven, CT 06504 USA Abstract Uptake of organic chemicals into plants depends on the properties of the contaminant and the physiology of the plant. A mass balance model based on fugacity was developed to quantify the uptake and transport in plants of a very hydrophobic chemical, p, p'-dichlorophenyl-1, 1 -dichloroethylene (DDE). The model included processes for sorption or influx of chemical with water from hydroponic solution to root; and sorption or exchange of chemical between the shoot and air. Movement among compartments of the plant was governed by the transfer of water in xylem and phloem. The movement of water was entirely determined by transpiration, growth rate, and weight distribution among tissues. This model was used to predict the kinetics of uptake and movement of DDE from hydroponic solution by seedlings of two species of Cucurbitacea, cucumber and zucchini. These predictions were compared to the results of experiments in a companion paper. These experiments showed the translocation of DDE in zucchini was much greater than that in cucumber. The model correctly predicted the negligible uptake into the shoot of cucumber. The model predicted the greater uptake of DDE by zucchini only if the apparent partitioning of DDE in the xylem was 25 fold higher than that expected in pure water. Predictions using similar parameters were made for uptake and distribution of DDE for plants grown into fruit production in field soil contaminated with DDE. To match the observed concentration of DDE in fruit, the model coefficient for partitioning of DDE into water in phloem had to be increased to 200 times that in pure water. Model Development Fugacity is the escaping tendency of a chemical from its local environment. Fugacity, F, is related to concentration, C, by a factor Z: F = [C] / Z. For a hydrophobic chemical, the escaping tendency from an aqueous environment is high, and ZW is small. Because of lipid and protein components, ZT for plant tissues is about 10, 000 -fold higher than ZW. Figure 2. DDE supply in hydroponics solution Schematic diagram of the system used to supply DDE in hydroponics solution to plants. The chemical p, p'-dichlorodiphenyldichloroethylene was sorbed at 10 w/w% onto Tenax. TM (2, 6 -diphenyleneoxide polymer) porous beads of 80 to 100 mesh. These were placed in a 1 -L polyethylene reservoir equipped with inlet and outlet ports and a magnetic stirrer. Approximately 0. 5 L of water solution was maintained in the reservoir with 10 g of Tenax. Solution flowing through the reservoir percolated through the layer of Tenax before leaving through a steel frit filter at the bottom of the reservoir. Water was passed through the mixing chamber at approximately 20 ml min-1, and distributed to EIGHT troughs of plants through an 8 -channel peristaltic pump. A schematic of this system is shown in Figure 2. This configuration ensured that all troughs received equal flow rates of solution at equal inlet concentrations of DDE. This system supplied plants with solution containing 0. 7 ng m. L-1 DDE for a 14 day interval. Eluent from trough Aluminum trough Mixing chamber TENAX TM Solution reservoir Solution Stirrer Frit filter Peristaltic pump The uptake and distribution of p, p'-dichlorodiphenyldichloroethylene (DDE) was predicted for cucumber and zucchini grown for 14 d in hydroponic solution containing DDE. The simulations assumed constant concentrations of DDE in solution and in air, and a constant relative growth rate and weight distribution among plant parts. The relative growth rates and final fresh weights per plant were 0. 085 g g-1 day and 44 g for cucumber, and 0. 12 g g-1 day and 74 for zucchini. Table 1 gives the measured composition used to calculate Z values for various plant parts. The composition did not vary significantly between species. Zucchini had more weight in petiole and cucumber had more weight in roots. Exchange between plant compartments is assumed to occur via water flow in xylem and phloem. A chemical is partitioned between plant tissues and water in transit through that tissue according to fugacity. [CW] / ZW=[CT] / ZT Flux of chemical from one plant compartment to another is equal to the flux of water times the concentration in solution at fugacity F Water in tissue with DDE at fugacity = F Growth conditions and harvests Zucchini summer squash, Cucurbita pepo L. cv Black Beauty, and cucumber, Cucumis sativus L. cv Marketmore, seeds were germinated and transferred to aluminum troughs containing complete nutrient solution when cotyledons emerged [Gent et al 2007]. When plants had three true leaves, they were thinned to 5 zucchini or 9 cucumber per trough and moved to a greenhouse under ambient sunlight in October. Temperatures in the greenhouse varied from 15 o. C at night to 30 o. C during the day. HID lights supplemented sunlight in the greenhouse for a 14 -h photoperiod. Maximum light intensity was 1400 μmol m-2 s-1 PAR. DDE was supplied in solution for 14 days, as described above, then plants were harvested around noon. The roots were separated from the aerial parts, to prevent contamination of the shoot by nutrient solution, and partially dried on paper towels. The area of detached leaves was measured with a leaf area meter. A short segment at the base of the stem with adventitious roots was cut free of the roots and discarded. The shoot was cut into stem, petiole and leaf blade fractions. Cotyledons and any flowers were included with the stem. These parts were weighed and tissues stored in the freezer. A sub-sample of each tissue type was freeze-dried to determine dry weight and chemical tissue composition. Thawed tissue was extracted with propanol/petroleum ether. The extract washed with water and dried over sodium sulfate. These tissue extracts were analyzed by gas chromatography as described in Gent et al. (2007). The model was also used to predict the uptake of DDE in plants grown into fruit production in soil contaminated with weathered DDE. Comparisons were made to measurements taken 70 d after planting and after a substantial amount of fruit had been picked, as reported by Wang et al. (2004). We assumed a relative growth rate of 0. 06 d 1 for vegetation, and a 2 -fold faster growth rate for fruit, to delay fruit growth to the later part of the cropping cycle. The tissue composition and area to volume ratios were assumed to be similar to that for vegetative plants grown in hydroponics (Table 1). Stomata conductance and transpiration per unit area was assumed to be two-fold faster than in the greenhouse, due to the longer day length and warmer temperatures of summer. The Z values for DDE in air and water come from (Pontolillo and Eganhouse, 2001). For partitioning DDE to soil, Z was 415 times greater than that for water. Physiological parameters used to estimate Z values and transfer functions in the model. These parameters were the same for zucchini and cucumber, except where noted Root Water and air in leaf tissue with DDE at fugacity = F Figure 1. Schematic diagram showing the flux of water in xylem and phloem between compartments in a plant. The flux is proportional to the thickness of the xylem in black, and the phloem in white. Arrows indicate the direction of flow. Branch points indicate water from xylem used in volume expansion, and solution from phloem used for dry matter accumulation. Flux due to transpiration Exchange at leaf boundary layer For a given plant tissue or compartment, the increase in fugacity equals the flux of water from an adjacent compartment, j, times fugacity in the donor compartment j, divided by volume of receiving compartment. Flux over volume is a turnover rate, k. Leaf blade kj = fluxj / volume d. F / dt = kj * Fj These fluxes are needed to predict the movement of a chemical between compartments. There are six fluxes between compartments in the plant and other fluxes to air and solution. If we assume a constant relative growth rate and constant functionality of each organ in terms of rates of photosynthesis, transpiration, and metabolism per unit size, then all fluxes can be calculated in terms of whole plant transpiration, growth rate and the weight fraction in each tissue. The latter were measured in each trial and were not independent parameters. The water flows in the model, calculated from these measurements, are shown in Figure 1. Petiole Leaf Fruit Programming Fruit The model was programmed in EXCEL as finite difference equations with a one-hour time step. Each compartment of the plant is a column of the spreadsheet. Roots Solution Exchange at root boundary layer Table 2. Model predictions compared to concentrations observed after feeding DDE to plants for 14 d in hydroponics solution in October in a greenhouse. DDE concentration, ng g-1 fresh weight Species Cucumber observed a No exchange with cuticle With 1. 0 ng m-3 b Root Stem Petiole Leaf 465 3. 3 1. 6 3. 0 25 0. 1 0. 0 589 2. 9 0. 0 DDE in air Stomata exchange only 589 3. 1 0. 4 1. 7 Air exchange with leaf cuticle c 585 3. 4 1. 3 5. 9 0. 040 0. 085 0. 065 0. 150 0. 050 Zucchini observed Protein ab 0. 215 0. 140 0. 070 0. 200 0. 070 Predicted with no DDE in air Carbohydrate ab 0. 720 0. 795 0. 880 0. 700 0. 800 No exchange with cuticle Lipid ab 0. 055 0. 050 0. 100 0. 050 Water exchange with root cuticle b Octanol c 343 a 47. 6 11. 7 3. 7 0. 4 0. 0 476 69. 1 8. 5 0. 6 0. 0054 0. 0051 0. 0062 0. 0201 . 0048 9010 8450 10300 33300 7890 Stomata exchange only Air exchange with leaf cuticle 473 63. 6 8. 4 a From (Gent et al. 2007) b predictions with apparent conductance = 6 10 -3 m h-1. c predictions with Cstomata = 3 m h-1 and Ccuticle = 10 m h-1. 476 69. 3 c 4000 400 400 Cboundary d 0. 006 10 10 Cstomatal d N. A. 3 3 Weight ratios a hydro ponics Zucchini 0. 35 0. 15 0. 25 Cucumber 0. 50 0. 18 0. 08 0. 24 Weight ratios e Field Zucchini 0. 06 0. 12 grown 0. 06 0. 64 Cucumber 0. 08 0. 12 0. 04 0. 12 0. 64 a From (Gent et al. 2007) b Carbohydrate, protein and lipid are expressed as a fraction of dry weight. c Octanol equivalent is expressed as a volume fraction. d Conductance for transfer of air or water in units of m h-1. e From (Wang et al. 2004) Roots accumulated most of the DDE that was supplied in hydroponics solution, and cucumber and zucchini did not differ substantially in the amount of contaminant accumulated by the roots (Table 2). Concentrations differed due to differences in root biomass and biomass dilution. Simulations based solely on transfer of fugacity in the transpiration stream (Table 2, No exchange with cuticle) predicted roots should only accumulate 3% of the DDE provided in flowing nutrient solution, because the turnover rate of water flowing through the root zone, 3 h-1, was much faster than that due to transpiration, 0. 1 h-1. The nearly complete sorption of DDE that was observed suggests that sorption was limited only by diffusion to the root surface. An apparent boundary layer conductance of 0. 006 m h-1 could predict the sorption of DDE to plant roots. This sorption process was not reversed when DDE-free solution flowed past the roots. Less than 10% of the DDE adsorbed on the roots was released during 9 d of contaminant-free flow (Gent et al 2007). Such rapid sorption was observed in another hydroponic study of POPs exposing Glycine max (soybean) or Zea mays (corn) to a nutrient solution containing 2, 3, 7, 8 -tetrachlorodibenzo-p-dioxin (TCDD) (Mc. Ready et al 1990). These comparisons indicate a rapid partitioning of hydrophobic chemicals between hydroponic solution and plant roots that is only limited by the solution boundary layer. 4. 0 32 With 1. 0 ng m-3 DDE in air Area/Volume Cucumber and zucchini grown into fruit production in the field accumulated similar amounts of DDE in root tissue, but, zucchini accumulated more than an order of magnitude more DDE in stem and other shoot tissues than did cucumber (Table 3). This phenomenon was seen in other field trials and under controlled conditions (White 2002, Wang et al 2004). Simulation of uptake of DDE only in the transpiration stream was not sufficient to predict the accumulation of DDE in the plant. A boundary layer conductance of 3 10 -3 m h 1 for diffusion of DDE in soil water to the root surface was required to predict the quantity of DDE that was found in the plants. The model predicted more DDE in roots of cucumber than zucchini because there was little movement of DDE to the aerial parts of cucumber. The model predicted more than enough DDE in the stem of cucumber with a Z value for xylem equal to that in pure water, but this model predicted very little DDE in leaves plus petioles of cucumber, even after 70 d of growth, unless it included sorption of DDE from the air (Table 3). Thus, the DDE detected in leaves of cucumber was likely due to aerial deposition. However, the concentration of DDE predicted in fruit of cucumber was increased only slightly by the contribution from aerial deposition. The efficient translocation of DDE into the shoot of zucchini could be predicted using a Z value for water in the xylem that was 25 fold greater than that in pure water. The model predicted most of the DDE to be in the petiole rather than the leaf blades. The experimental data did not separate these two tissues. 8. 8 Table 3. Model predictions compared to concentrations observed in plants grown for 70 d in to fruit production in DDE contaminated soil in the field. Observed values are average ± standard deviation for ‘Marketmore’ cucumber and ‘Black Beauty’ zucchini grown in soil contaminated with 140 ppm DDE concentration, ng g-1 dry weight Factor Root Stem Leaf blade and petiole Fruit 1900 ± 160 99 ± 16 33 ± 6 120 ± 28 No exchange with cuticle 3640 24 1 5 Water exchange with root cuticle a 27800 197 4 39 Stomata exchange only 27800 203 20 45 Air exchange with leaf cuticle b 27600 172 32 47 Zphloem = Zwater c 27600 217 34 14 1800 ± 230 2100 ± 72 560 ± 300 510 ± 48 No exchange with cuticle 2100 365 104 86 Water exchange with root cuticle a 12300 2230 659 545 Stomata exchange only 12300 2230 675 550 Air exchange with leaf cuticle b 12300 1940 494 486 Zphloem = Zwater 12300 2430 736 103 Cucumber observed Predicted with no DDE in air Dry matter a DDE in seedlings grown in hydroponics Sorption to roots. Stem For each compartment: • Fluxes of water and air are calculated from current size and intrinsic constants, such as growth rate, weight and area ratios, etc. • Transfer in of a chemical is calculated from turnover rates and current values of fugacity in other compartments. • Transfer out and clearance of a chemical is calculated from turnover or clearance rates. • New values of fugacity and fresh weight or volume are calculated. Petiole Flux in xylem Flux in phloem Other studies imply that the equilibrium between air and leaf is weighted toward the air, and hydrophobic compounds in leaves are likely to be volatilized to the air. When PCBs were applied to a young soybean leaf, only 7% was recovered after 7 d, and only 2% was recovered in the lower part of plant (Weber and Mrozek 1979). Due to evaporation, azalea leaves lost 40% of sorbed PCBs within 2 h (Barber et al 2003). After short-term exposure of roots to radio-labeled dioxins in a hydroponics system with isolated root and shoot compartments, more of the label was in air in the container enclosing the shoot than in leaf tissue (Mc. Ready et al 1990). Component Z / ZW Movement into or from air is determined by area and boundary layer conductivity. Air is saturated with water at 100% humidity, and DDE at fugacity = F Stem Accumulation in root and shoot The path from air to leaf should predominate over transport in xylem for large hydrophobic compounds, because solubility in water decreases faster with molecular weight than does the vapor pressure in air (Trapp and Mathies 1995, Bacci and Gaggi 1986). Some observations support this. However, among poly aromatic hydrocarbons varying in molecular weight or volatility, the lighter compounds more easily moved to leaves through air (Fismes et al 2002). A comparative study of uptake of chlordane in zucchini either from contaminated soil or air showed there was far more uptake into the shoot from contaminated soil than from contaminated air (Lee et al 2003). Water exchange with root cuticle Table 1. DDE in fruiting field-grown plants Leaf blades accumulated less DDE than any other part of zucchini. The concentration in all shoot parts of cucumber was similar. The movement of DDE to leaf blades was simulated for three scenarios. The model predicted no DDE in leaf blades for cucumber, and very little DDE for zucchini, if there was no DDE in the air (Table 2). If the movement of DDE in air was limited to conductance through stomata, then the predicted concentration in leaf blades was still less than that observed. The predicted DDE in leaf blades was as great as that observed only when exchange of DDE in air was limited by an apparent boundary layer conductance of 10 m h-1. The model predicted these values for DDE in leaf blades would be achieved within 5 days. 2. 3 6. 4 Movement of DDE to stem and petioles The DDE concentration in the stem of cucumber was less than 1% of that in the roots (Table 2). The concentration in petioles was even less. The model predicted this concentration gradient if DDE partitioned to water in the transpiration stream as it were pure water. Most of the DDE found in stem tissue was predicted to be due to this relatively slow movement of DDE in the xylem from the roots. The stem and petioles of zucchini had a much higher concentration of DDE, than did those of cucumber, although the chemical composition and Z value of stem tissues was similar for the two species. A Z value of 25 times that in pure water was required to move sufficient DDE in the transpiration stream of zucchini. Exchange of DDE in air at the stem surface had relatively little effect on the concentration of DDE in stems of cucumber. This exchange actually decreased the amount predicted to be in stem and petiole tissue of zucchini. In these cases, the concentration of DDE in plant tissue was greater than the equilibrium concentration expected for partitioning DDE between air and plant tissue. Thus, net movement was predicted to be from the plant tissue to the air. Movement from air to leaves was a poor mechanism for transfer of DDE to other plant tissues of zucchini, compared to movement in the transpiration stream. Predicted with no DDE in air With 1. 0 ng m-3 DDE in air Zucchini observed Predicted with no DDE in air With 1. 0 ng m-3 DDE in air b predictions with apparent conductance = 3 10 -3 m h-1. b predictions with stomata conductance = 6 m h-1 and cuticle conductance = 10 m h-1. c Except where noted Z phloem = 200 ZW, and for zucchini Zxylem = 25 ZW. a Movement of DDE to fruit Partitioning of DDE to water in phloem had to be increased to 200 times that in pure water, to predict the DDE observed in zucchini fruit, because fruit is supplied primarily by the phloem rather than by the xylem. Fruits have a low transpiration rate, and half of the requirement of water for volume growth can be provided by the phloem. This high Z value for the phloem decreased the DDE predicted in other tissues, because DDE was extracted from the leaf blade, petiole and stem, and translocated in the phloem to the fruit. Deposition of DDE from air to leaf blades was not required to predict an accumulation of DDE in zucchini fruit. In fact, an increase in the rate of air exchange decreased the DDE predicted in all shoot tissues of zucchini. For reasons similar to those given above, the Z value for partitioning DDE to water in the phloem of cucumber also had to be increased to 200 times that in pure water, in order to predict the amounts of DDE actually found in fruit. The sorption of DDE from air to fruit was too slow, primarily due to the low area to volume ratio of fruit. In addition, the rapid volume expansion of fruit tended to dilute any DDE accumulated by this route. Increasing the translocation of DDE in the phloem decreased the DDE predicted in the stem of cucumber and zucchini (Table 3). References Bacci E, Gaggi C. 1986. Chlorinated pesticides and plant foliage: translocation experiments. Bull Environ Contam Toxicol 37: 850 -857. Barber JL, Thomas GO, Kerstiens G, Jones KC. 2003. Study of plant-air transfer of PCBs from an evergreen shrub: Implications for mechanisms and modeling. Envir Sci Technol 37: 38383844. Fismes J, Perrin-Ganier C, Empereur-Bissonnet P, Morel JL. 2002. Soil-to-root and translocation of polycyclic aromatic hydrocarbons by vegetables grown on industrial contaminated soils. J Environ Qual 31: 1649 -1656. Gent MPN, White JC, Parrish ZD, Eitzer BD, Iseleyen M, Mattina MJI. 2007. Uptake and translocation of p, p’-dichlorodiphenyldichloroethylene supplied in hydroponics solution to Cucurbita. Environ Toxicol Chem (in press). Lee WY, Iannucci-Berger WA, Eitzer BD, White JC, Mattina MJI. 2003. Plant uptake and translocation of air-borne and comparison with the soil-to-plant route. Chemosphere 53: 111 -121. Mc. Cready JK, Mc. Farlane C, Gander LK. 1990. The transport and fate of 2378 TCDD in soybean and corn. Chemosphere 21: 359 -376. Pontolillo J, Eganhouse RP. 2001. The search for reliable aqueous solubility and octanol water partition coefficient data for hydrophobic organic compounds: DDT and DDE as a case study. USGS Water resources Investigations Report 01 -4201. Washington DC. Schroll R, Bierling B, Cao G, Dorfler U, Lahaniati M, Langenbach T, Scheunert I, Winkler R. 1994. Uptake pathways of organic chemicals from soil by agricultural plants. Chemosphere 28: 297 -303. Trapp S, Matthies M. 1995. Generic one-compartment model for uptake of organic chemicals by foliar vegetation. Environ Sci Technol 29: 2333 - 2338. Weber JB, Mrozek E. 1979. Polychlorinated biphenyls: Phytotoxicity, absorption and translocation by plants and inactivation by activated carbon. Bull Envir Contam Toxicol 23: 412 -417. White JC. 2002. Differential bioavailability of field-weathered p, p'-DDE to plants of the Cucurbita and Cucumis genera. Chemosphere 49: 143 -152. Wang 1919. Wang X, White JC, Gent MPN, Iannucci-Berger W, Eitzer BD, Mattina MI. 2004. Phytoextraction of weathered p, p'-DDE by zucchini (Cucurbita pepo) and cucumber (Cucumis sativus) under different cultivation conditions. Int J Phytoremed 6: 363 -385.
- Slides: 1