Environ. Sci. Technol. 2008, 42, 1517–1523
Role of the Extractable Lipids and Polymeric Lipids in Sorption of Organic Contaminants onto Plant Cuticles B A O L I A N G C H E N , * ,† Y U N G U I L I , † YITING GUO,† LIZHONG ZHU,† AND JERALD L. SCHNOOR‡ Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, China, and Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242
Received September 22, 2007. Revised manuscript received December 8, 2007. Accepted December 14, 2007.
The distinct role of extractable and polymeric lipids in plant cuticle, precursors of SOM, has received scarce attention to elucidate plant uptake and soil affinity with organic contaminants. Sorption of naphthalene and 1-naphthol to fruit cuticular fractions isolated from two species were investigated. The polarity index, physical conformation, and glass transition temperature (Tg) of these cuticular fractions were characterized by elemental analysis, Fourier transform infrared spectroscopy, and differential scanning calorimetry, respectively. Cutin, a polymeric lipid, is the major sorption medium of the cuticle due to its large mass fraction and liquid-like nature (Tg ≈ -30 °C). Sorption of cutin is suppressed by the extractable lipids (wax, Tg ≈44 °C) acting as an antiplasticizer (enhance cutin’s Tg) over nonpolar contributor. Whereas polysaccharide, as a plasticizer (lower Tg value) and polar contributor, regulates affinity of polymeric lipids (cutin and cutan). The contribution of cutin to sorption by bulk cuticle overshadows the role of waxes, and the sorption capability (Koc) of cutin overwhelms the octanol–water partition coefficient (Kow). Therefore, uptake of organic contaminants by these plants would be seriously under-predicted by their extractable lipid content and compound’s Kow values. Along with the observed linear relationships of Koc with cutin content in these cuticular fractions, we suggest for the first time that the depolymerizable lipid fraction (cutin) is required to accurately predict plant accumulation of organic contaminants.
Introduction Uptake of organic pollutants by plant cuticle plays a key role in the fate of many organic pollutants (1, 2). Plant cuticles (above-ground parts) along with suberin (below-ground) and bark, precursors of soil organic matter (SOM), serve as an important sorptive medium for organic pollutants. Plant cuticle, a continuous extracellular membrane, basically consists of a wax fraction (lipids extractable by organic solvents) and an insoluble cuticular matrix (3, 4). This cuticular matrix includes cutin and cutan, termed polymeric * Corresponding author e-mail:
[email protected]; phone: 0086571-8827-3901; fax: 0086-571-8827-3693. † Zhejiang University. ‡ University of Iowa. 10.1021/es7023725 CCC: $40.75
Published on Web 01/31/2008
2008 American Chemical Society
lipids (3, 5, 6). Cutin, the insoluble ester-bound lipids, can be depolymerized. In addition, polar components (e.g., polysaccharides) may also be present (3). The extractable lipids have been widely used as tracers for vegetation (7), and thus plant uptake of organic compounds was predicted presumably based on the extractable lipids for environmental transport models (8–12). However, recent studies have emphasized that the extractable lipids are not sufficient to illustrate plant uptake and soil affinity (9, 13–15). For example, Li et al. (9) reported that the sorption of plant root predicted using the extractable lipids was notably lower than the measured values. Considerable interspecies variability of accumulation of volatile and semivolatile organic contaminants, with a factor of 10–30 at one site, was not related to the extractable lipid content or the cuticle volume fraction in the vegetation ( (14, 15) and references therein). The removal of lipids from soil via Soxhlet extraction increased the affinity of mineral soils with organic pollutants (13). Therefore, the inconsistent role of the extractable lipids in actual sorption and in general presumption needs to be illustrated. In most plant species, the major structural component of the plant cuticle is the polymeric lipids (30–80% by weight) (3, 16). Polymeric lipids along with extractable lipids are well preserved in acid (sandy) soil environments ( (7, 17) and references therein). Additionally, the polymeric lipids, a powerful sorptive medium (18, 19), significantly contribute to sorption of the cuticle residues during decomposition (20). Rationally, the role of polymeric lipids needs more attention in the study of plant accumulation and soil storage of organic contaminants. Chemical structures and physical conformations of natural organic matter (NOM) regulating sorption properties and mechanisms have been of increasing concern (21–27). Several studies have emphasized the contribution of aliphatic-rich plant biopolymer to the overall sorption and sequestration of organic pollutants because of its high sorption capacity and accumulation in the soil environment (18–21, 28, 29). Physical phase transition mechanisms were employed to elucidate sorption–desorption characteristics of aliphaticrich NOM (16, 30). Shechter et al. (16) reported that cutin biopolymer facilitated reversible and noncompetitive sorption due to its rubbery nature. Therefore, the conformational and sorptive differences of the extractable lipids and the polymeric lipids are quite worthy of further investigation. The objective of the current study is to elucidate the conformational and sorptive characteristics of the extractable lipids and the polymeric lipids due to their distinct presence in plant cuticle and then significant contribution to SOM. Sorption of naphthalene and 1-naphthol to fruit cuticular fractions isolated from different species is investigated. The cuticular fractions were characterized by elemental analysis, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The interesting inference from this study is that an accurate prediction model for plant uptake should be based on the depolymerizable lipids (cutin) rather than the extractable lipids as generally assumed.
Materials and Methods Isolation of Plant Cuticular Fractions. The commercially ripe fruits of tomato (Solanum lycopersicum) and apple (Malus domestica) were selected to isolate cuticle sheets, due to their distinctly different cuticular compositions (5, 18–20, 31). Cuticular fractions were isolated from the fruit cuticles by a modified version of an earlier method (19), detailed in the Supporting Information and flowchart in Figure S-1. The yield percentages of each cuticular fraction VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(i.e., TC1, TC2, TC3, TC5, and TC6 for tomato; AC1, AC2, AC3, AC4, AC5, AC6, and waxes for apple) were recorded. All isolated fractions were dried, ground, and sieved ( apple (0.11) > pepper (0.07), suggesting that extractable lipids would be viewed as nonpolar contributors. Interestingly, the elemental composition and polarity of cutin of the three plant species were almost the same. AC4 and PC4 (cutan) presented relative low carbon content contrast with wax and cutin, and resulted in relative high polarity (0.48 vs 0.69 for AC4 vs PC4). The polysaccharide exhibited the lowest carbon content and highest polarity in comparison with the other components, indicating it would function as a significant polar contributor. Selected FTIR spectra are shown in Figure 1. The intense IR absorption bands at ∼2919 and ∼2850 cm-1 are assigned to the antisymmetric (vas) and symmetric (vs) CH2 stretching modes. The bands at 1740 cm-1 are assigned to the C)O stretching vibrations of ester groups (18, 19), attributed to the cutin with high cross-linking. The bands at 1708 cm-1 are assigned to the carbonyl stretching of the -COOH group, ascribed to the extractable lipids. During the saponification treatment (AC2fAC3), ester bonds were broken, corresponding to the completely removal of hydroxyl-fatty acids. The AC3 sample was dominated primarily by cuticular polysaccharides (1054 cm-1) and ionized carboxyl groups -COO- (1635 cm-1). For cutan (AC4), the bands at 1647 and 1517 cm-1 are assigned to aromatic cores, which are attached by the carboxyl groups (1717 cm-1) (19). After hydrolyzing the AC2 sample, the resultant AC5 sample was dominated by CH2 bands and C)O stretching vibrations of ester groups (i.e., the cutin component) (19). It is desirable to evaluate the link between the different hydroxyl-fatty acids to form the cutin cross-linking by the ratio of the two FTIR peaks
FIGURE 1. Selected FTIR spectra of bulk cuticle (AC1), dewaxed cuticle (AC2), nonsaponifiable fraction (AC3), nonsaponifiablenonhydrolyzable residue (AC4), dewaxed-hydrolyzed residue (AC5), desugared cuticle (AC6), and cuticular wax of apple at 3000–2800 cm-1 (A) and 1800–800 cm-1 (B). featured near 2919 cm-1 and at 1740 cm-1: the lower the ratio, the higher the degree of cross-linking. The AC2 (1.05) sample is the most cross-linked, followed by AC5 (1.14), AC4 (1.37), AC6 (1.60), AC3 (2.33), and wax (5.38). It is well-established that the individual vas,CH2 and vs,CH2 of alkyl chains appear around 2919 and 2850 cm-1 if the chains are highly ordered (all-trans). If a conformational disorder occurs in the chains, their frequencies shift upward, depending upon the average content of the gauche conformers, eventually to 2930 and 2856 cm-1 for liquid-state (all gauche) (32). For AC1, the respective vas,CH2 and vs,CH2 are assigned at 2919 and 2850 cm-1, indicating that the alkylchain aggregated in solid-like state. In contrast, for PC1 with low wax (∼6%) and high cutin content (∼65%), the vas,CH2 and vs,CH2 were 2931 and 2854 cm-1 (19), suggesting that the alkyl-chain self-assembled in the liquid-like state, in line with the results of solid-state 13C NMR (19). After removing cuticular waxes (44.7%) from AC1, the vas,CH2 and vs,CH2 shifted to frequencies of 2924 and 2854 cm-1 for AC2 with 63% cutin. Further removing the polysaccharide from AC2, these bands were further shifted to higher frequencies (i.e., 2931 and 2857 cm-1) for AC5 with 83% cutin. These observations suggest that the intrinsically amorphous conformation of cutin gradually exposed/relaxed after sequential removal of wax and polysaccharide, consistent with the rubbery nature derived from 13C NMR data (16). For cuticular waxes and cutan (AC4), the respective vas,CH2 and vs,CH2 are 2921 and 2850 cm-1, illustrating that they contain some gauche conformers supported by 13C NMR result (30). The DSC measurements for AC1 showed two distinct glass transition temperatures (Tg) at -44 and 44 °C (see Table S-2
in SI), assigned to the respective temperature of phase transition inside the cutin biopolymer and the cuticular waxes. The observed Tg for AC2 were at -42 and 12.8 °C but without the Tg of 44 °C. The Tg for pure cuticular waxes was observed at 45 °C. Therefore, the Tg at ∼44 °C was surely assigned to the epicuticular waxes. A continuous change of heat flow rather than a distinct glass transition was obtained with the AC3 sample. A weak phase transition was observed at -33 °C for AC4 (cutan). Only one Tg for AC5 (desugared AC2) was detected at -30 °C, higher than the corresponding Tg of AC2 (-42 °C). Simultaneously, cutin’s Tg in AC6 (desugared AC1, -25 °C) was also higher than the Tg of AC1 (-44 °C). These data demonstrate that the presence of polysaccharide decreases the Tg of cutin, indicating that the polysaccharide would play a role as a plasticizer in the phase transition of cutin. The presence of waxes increased the cutin’s Tg, such as Tg (AC6, -25 °C) > Tg (AC5, -30 °C), implying that the waxes would be viewed as an antiplasticizer to cutin. The waxes’ Tg in AC6 (41 °C) was lower than the Tg in AC1 (44 °C) and pure waxes (45 °C), suggesting that to waxes the polysaccharide acts as an antiplasticizer, and cutin plays as a plasticizer. The low Tg value also provides evidence that cutin should be present in an amorphous and flexible state (i.e., liquid-like) at room temperature, whereas the extractable lipids should exhibit a condensed state (i.e., solidlike) due to high Tg value. Sorption of Naphthalene and 1-Naphthol with Cuticular Fractions. Sorption of naphthalene and 1-naphthol to plant cuticular fractions (12 samples) containing different levels of extractable and polymeric lipids was investigated. The selected isotherms for naphthalene are presented in Figure VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Selected sorption isotherms of naphthalene to cuticular fractions of tomato (A) and apple (B). 2. Isotherms of naphthalene and 1-naphthol fit well to the Freundlich equation; the regression parameters are listed in Table 1 and Kd values are provided in Table S-3. Compared with naphthalene, isotherms of 1-naphthol exhibited higher nonlinear behavior. Obviously, the bulk cuticles have high affinity for organic pollutants, and the sorption coefficients (Koc) for each chemical vary only by a factor of 1.6. However, which component, the extractable lipids or polymeric lipids, dominates the sorption of plant cuticle is still unresolved. Due to their heterogeneous nature, sorption of plant cuticles was regulated by their chemical components and physical conformations. After removal of extractable lipids, sorption coefficients of bulk cuticle increased (Table 1), consistent with previous reports (16, 18, 20). Furthermore, the TC5, AC5, or PC5 samples contained only polymeric lipids (a mixture of cutin and cutan), exhibited much higher sorption capacity than the pure waxes (e.g., isolated apple waxes), and showed greater affinity with organic pollutants than the corresponding bulk cuticles, i.e., Koc (TC5, AC5, or PC5) > Koc (TC1, AC1, or PC1) > Koc (apple waxes). These results demonstrated further that the organic pollutants were preferably sorbed by the polymeric lipids rather than extractable cuticular lipids, and the depolymerizable lipid (cutin) would serve as an ultimate sink for organic pollutants accumulated in plant leaves and fruits. The extractable lipid (wax) formed a high crystalline phase, thus reducing the effective accessibility of NOM for organic contaminants (30). Additionally, sorption of TC6 (or AC6) was lower than that of TC5 (or AC5), due to the sorption capability of cutin being suppressed by the presence of waxes, consistent with FTIR and DSC conclusions that the present waxes reduced 1520
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and even inhibited the rubbery nature of cutin. The inhibition effect was intensified by a high content of waxes in plant cuticle. For example, the Koc ratios of AC2 to AC1 (i.e., AC2/ AC1) are higher than those of TC2/TC1 (or PC2/PC1), and the ratio of AC5/AC6 is higher than that of TC5/TC6. Similar phenomena of the removal of lipids from soil increasing the affinity of mineral soils with PAHs have been reported, which suggested that the removal of the lipids opened up additional sorption sites and then mechanisms changed from partition to specific adsorption (13). The present study confirmed that the presence of fillers (e.g., waxes or polysaccharides) in the polymer matrix changed the conformation of the plant cuticle. Polymerized lipids have a unique properties of rheology, whereas extractable lipids lack this properties due to lesser polarity (oxygen-alkylate) (3). Therefore, the proposed mechanisms, involved in the enhanced sorption of cuticular fractions (e.g., bulk cuticle and desugared cuticle) after the removal of lipids, are transitioned from limitedpartition to full-partition, consistent with structural transition from solid- to liquid-like nature. Several studies have emphasized the importance of the cutin biopolymer as a sorbent due to its rubber-like nature and further softened by a plasticizer (e.g., water molecule) (16, 19, 20), but more studies on its quantitative contribution are still necessary. Koc values of dewaxed-fractions are 5–15 (naphthalene) and 1.5–6 times (1-naphthol) higher than their individual depolymerized fractions. This further proves that cutin is a major reservoir for organic pollutants. Because the cutan composition cannot be removed from cutin by enzymatic or chemical isolation methods, direct investigation of sorption of the pure cutin (at a polymeric state) of apple and pepper fruits is not possible. Hence, their sorption coefficients were indirectly estimated through mass balance, i.e., Kd,XC2×fXC2 ) Kd,XC3×fXC3 + Kd,cutin×fcutin, where Kd,XC2, Kd,XC3, and Kd,cutin are sorption coefficients of the dewaxed fraction, the depolymerized fraction, and cutin, respectively; and fXC2, fXC3, and fcutin are their corresponding contents in the bulk cuticle (Table S-1). The calculated Kd,cutin values for cutin of apple and pepper are comparable to the determined Kd,cutin value of TC5 (pure cutin, listed in Table 2), and all are much higher than the determined Kd,wax value of apple cuticular wax. Sorption capability of cutin was higher than cutan, i.e., Kd,cutin > Kd,cutan, except for the apple cutan interaction with naphthalene and 1-naphthol at low concentrations (e.g., Ce/ Cs ) 0.005). The affinity of polysaccharide components with organic pollutants was the lowest. Based on Table 2, the reconstituted Kd values of bulk cuticle are higher than the measured Kd values, supporting that bulk cuticle is in a physical and chemical mixture rather than in a physical mixture (18). For TC1 and PC1 containing ∼65% of cutin and ∼6.5% of wax, the contributions of cutin to the sorption of naphthalene and 1-naphthol approached 90%, and the contribution of wax was less than 10% (Table 2). For AC1 with ∼35% of cutin and 45% of wax, the contribution of cutin reached 50%, and the contribution of the extractable lipid was 35% for naphthalene and less than 20% for 1-naphthol. Obviously, the sorption capacity of polymeric lipids overshadows the extractable lipids, attributable to the higher mass fraction and stronger sorption of the former over the latter. Presumably, plant uptake and soil affinity predicted by the extractable lipids generally assumed would be significantly underestimated. Some studies found that the extractable lipids failed to predict plant uptake during phytoremediation practices (9), and it was also difficult to account for the interspecies variability of organic pollutants at one site (14, 15). From earlier and present studies, the polymeric lipids rather than the extractable lipids were the plant storage compartment, and exhibited stronger sorptive capability than the octanol medium. Additionally, organic pollutants may not have reached the internal extractable
TABLE 1. Regression Parameters of Isotherms of Naphthalene and 1-Naphthol with Tomato (Solanum lycopersicum), Apple (Malus domestica), and Pepper (Capsicum annuum) Cuticular Fractions and Corresponding Sorption Coefficients (Koc) naphthalene
1-naphthol Koc, mL/g carbon
plant
a
fraction
tomato TC1 TC2 TC3 TC5 TC6 apple AC1 AC2 AC3 AC4 (cutan) AC5 AC6 wax pepper PC1 PC2 PC3 PC4 (cutan) PC5
Freundlich
r2
0.989 0.999 0.988 0.993 0.994 0.999 0.987 0.983 0.948 0.999 0.999 0.999 0.999 1.000 0.999 0.997 1.000
b
N
Kf
0.929 0.933 1.009 0.849 0.844 0.822 1.024 0.885 0.513 1.010 1.014 0.993 1.001 1.009 0.982 0.860 1.018
1887 2127 588 3710 3538 2716 2736 162 2644 3049 2136 2172 2160 2400 243 683 3390
Koc, mL/g carbon
Ce/Cs ) 0.005
0.05
0.5
3395 3805 600 7082 6744 5440 4278 471 11337 4463 2880 2827 3360 3745 592 1817 4837
2883 3261 613 5002 4709 3611 4521 361 3694 4567 2974 2782 3368 3823 568 1316 5042
2448 2795 722 3533 3288 2397 4778 277 1204 4673 3072 2738 3375 3903 545 953 5255
Freundlich,
r2
0.991 0.991 0.990 0.995 0.996 0.986 0.978 0.957 0.948 0.995 0.991 0.996 0.998 0.998 0.996 0.969 0.994
b
N
Kf
0.899 0.879 0.822 0.884 0.851 0.868 0.823 0.551 0.623 0.918 0.814 0.834 0.898 0.892 0.727 0.761 0.929
817 961 403 1219 1344 692 1413 1047 1622 912 955 419 926 1000 300 1040 883
Ce/Cs ) 0.005
0.05
0.5
1112 1273 698 1473 1538 820 1782 1272 1615 1206 1006 422 1243 1355 474 1501 1174
883 965 461 1117 1089 605 1186 453 678 998 656 288 983 1056 253 866 997
702 732 305 848 771 446 789 161 285 827 427 196 777 824 135 499 847
a The mean of each cuticular fraction is indicated in Figure 1. b Freundlich equation: Q ) Kf CeN, Kf [(mg/kg)/(mg/L)N] is the Freundlich capacity coefficient, and N (dimensionless) describes the isotherm curvature.
TABLE 2. Sorption Coefficients (Kd) of Organic Pollutants with Cuticular Components and Their Relative Contribution to Bulk Cuticlesa naphthalene b
Kd, mL/g
tomato
apple
pepper
TC1 waxes cutin sugar AC1 waxes cutin cutan sugar PC1 waxes cutin cutan sugar
1-naphthol c
reconst-Kd, mL/g
Ce/Cs ) 0.005
0.5
0.005
0.5
2154 2201 3146 313 3784 2201 4061 6553 315 2156 2201 3304 887 339
1553 2131 2249 326 1667 2131 4600 696 185 2166 2131 3457 465 312
2404 143 2186 75 2922 984 1405 491 42 2419 135 2141 70 73
1780 139 1563 78 2621 953 1591 52 24 2475 130 2241 37 67
d
contribution,% 0.005
0.5
100 100 5.95 7.78 90.93 87.82 3.12 4.40 100 100 33.67 36.35 48.09 60.73 16.82 1.99 1.42 0.93 100 100 5.57 5.27 88.51 90.53 2.91 1.49 3.01 2.71
Kd, mL/g
b
reconst-Kd, mL/gc
Ce/Cs ) 0.005
0.5
0.005
0.5
706 328 956 310 570 328 1418 6553 320 797 328 1146 887 271
445 153 568 135 311 153 730 696 14 499 153 726 465 77
760 21 664 74 1171 147 491 491 42 891 20 743 58 70
437 10 395 32 632 68 253 52 2 533 9 470 17 37
contribution,%d 0.005
0.5
100 100 2.81 2.27 87.42 90.31 9.77 7.42 100 100 12.53 18.20 41.89 67.39 41.97 13.92 3.61 0.49 100 100 2.25 1.75 83.31 88.21 6.54 3.11 7.90 6.93
a The mean of each cuticular fraction is indicated in Figure S-1. b Sorption coefficient: Kd ) Q/Ce. For TC1, AC1, and PC1 samples, Kd values were calculated from isotherms. Kd values of wax samples were calculated based on the isotherm of apple cuticular wax. Cutin’s Kd values were estimated according to mass balance: Kd,XC2×fXC2 ) Kd,XC3×fXC3 + Kd,cutin×fcutin, see text. For cutin, Kd values were corrected from TC3, AC3, and PC3. c Reconst-Kd is the reconstructed Kd value. Reconst-Kd, wax, -Kd, cutin, -Kd, sugar, and -Kd, cutan values were calculated as f wax Kd, wax, f cutin Kd, cutin, fsugar Kd, sugar, and fcutan Kd, cutan, respectively. Then, the sum of these is Reconst-Kd,total. d Contributions were calculated as the percentage of individual reconstructed-Kd in reconst-Kd,total.
lipids due to their crystalline nature (14). However, most popular plant accumulation models for organic contaminants assume that the lipophilic storage compartment is the extractable lipids and behaves like octanol ((8) and references therein (9, 12, 15)). The gap between the modeling presumption and the actual situation is derived from omitting the significant role of polymeric lipids, which are nonextractable by conventional solvent extraction. In the current study, we propose for the first time that the depolymerizable lipids (cutin) should be a more reasonable component, rather than the extractable lipid, to accurately predict plant uptake, which is consistent with the known role of cutin as the major reservoir for organic pollutants. In addition, for more practical significance, we suggest that the determination of lipid content by depolymerization of cutin in methanolic KOMe
should replace the conventional extraction of extractable lipid in solvent. Hence, more studies of the role of polymeric lipids in accumulation are urged in the future. The linear relationship of Koc with the cutin contents in the isolated cuticular fractions (Figure 3) further indicates that the cutin component is an important consideration in sorption. This observation is consistent in a practical sense with earlier results showing the good linear relationship of Koc values with amorphous polymethylene carbon in SOM (19, 29). As precursors of SOM, polymeric lipids of plant cuticle should contribute significantly to the dominant amorphous paraffinic domain in SOM (18, 21, 29). Note that the increase of Koc values with increasing cutin content is dependent on plant species. The trends are similar for both apple and VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Linear relationships of sorption coefficients (Koc) of naphthalene (A) and 1-naphthol (B) with cutin component contents in tomato, apple, and pepper cuticular fractions. Note that the Koc values for naphthalene and 1-naphthol were calculated at the half-concentration of respective aqueous solubility (i.e., Ce/Cs ) 0.5). pepper cuticular fractions, but less sensitive for tomato cuticular fractions (Figure 3). Sorption isotherms of AC4 and PC4 (cutan) were quite nonlinear. Isotherms of AC4 were much more pronouncedly nonlinear than those of PC4 (n ) 0.512 vs 0.860 for naphthalene, and 0.623 vs 0.761 for 1-naphthol), a fact that can be attributed to AC4’s exhibiting high aromaticity compared to PC4 (H/C ratio 1.12 vs 1.53). At a given concentration, naphthalene’s Koc values with AC4 were all higher than those with PC4. However, the order of magnitude of 1-naphthol’s Koc value was reversed, due to higher polarity of PC4 over AC4 (0.69 vs 0.48) (Table S-1). These observations indicate that aromatic cores with low polarity (e.g., AC4) were more accessible to nonpolar aromatic compounds, while the aromatic cores with high polarity (e.g., PC4) were more compatible with polar aromatic compounds (19). Interestingly, polysaccharide components displayed distinct effects on the interaction of cutin (amorphous medium) and cutan (condensed medium) with organic pollutants. The significantly low sorption obtained by PC3 and AC3 was due to the high content of pectin polysaccharides in the samples. Removal of polysaccharides resulted in a significant increase in Koc values (e.g., PC4 and AC4). The Koc values of AC3 and PC3 with the presence of cutan were similar to that of TC3 without cutan. Additionally, the contributions of cutan to 1522
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sorption of AC3 (36% cutan and 64% pectin) and PC3 (27% cutan and 73% pectin) were negligible for both naphthalene and 1-napthol. These indicate that the pectin components shield the cutan residue almost completely from interaction with the introduced sorbate. For naphthalene, removal of pectin from dewaxed-fraction (containing cutin) resulted in the increasing sorption of the residues (i.e., TC5 and PC5) and hardly affected the AC5 residues. By contrast, the coexistence of pectin with cutin did not reduce the sorption of 1-naphthol (Table 2). The correlations of Koc values with polarities of three species’ cuticular fractions are demonstrated in Figure S-2. Sorption capability (Koc) of naphthalene was much higher than that of 1-naphthol to a sorbent at a given polarity. The Koc values decreased with increases of polarity, but the reduced intensity was stronger for naphthalene than for 1-naphthol. Almost all points were located in the region between two parallel lines of a and b for naphthalene, and of c and d for 1-napthol. Interestingly, the point dropped at the b (or d) line usually was the sorbent with more condensed domains, in comparison the sorbent at a (or c) line with more amorphous domains; additionally, the y-intercept difference between a and b lines (i.e., log(Koc(a)/Koc(b)) ) 0.3) was identical to that of the difference of c and d lines (i.e., log(Koc(c)/Koc(d)) ) 0.3). This demonstrated that the Koc values predicted from a (or c) line should be two times (i.e., 100.3 ≈ 2) as high as b (or d) line at a given (N + O)/C ratio, suggesting that the sorption capability of amorphous sorbents could be two times as strong as that of condensed sorbents. In summary, polymeric lipids and extractable lipids play a distinct role in conformational nature and sorptive character of plant cuticle. Depolymerizable lipid (cutin), a dominant cuticular component, is the major sorption reservoir of organic pollutants due to its liquid-like nature with a viscoelastic network. The extractable lipids (wax), serving as an antiplasticizer over nonpolar contributor, suppress the sorption of cutin. The polysaccharide component, a plasticizer and polar contributor, regulates the sorption properties of the polymeric lipids (cutin and cutan). The contributions of cutin to sorption by bulk cuticle overshadow the role of waxes. Therefore, the role of polymeric lipids along with extractable lipids needs more attention to determine actual plant lipid content and accurately predict plant accumulation.
Acknowledgments We are highly grateful to the three anonymous reviewers for their valuable comments. This project was supported by National Natural Science Foundation of China (20737002, 20577041, 40671168), and by the program for New Century Excellent Talents in University (NCET-05-0525).
Supporting Information Available Two methods of isolation of plant cuticular fractions and sample preparation for FTIR analysis; tables presenting relative mass fractions and elemental composition of cuticular cractions (Table S-1), the differential scanning calorimeter data (Table S-2), and sorption coefficients (Kd) (Table S-3); figures showing the flowchart of the isolation processes of plant cuticular fractions (Figure S-1) and relationship of sorption coefficients (logKoc) with polarity indexes ((N + O)/ C) (Figure S-2). This material is available free of charge via the Internet at http://pubs.acs.org.
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