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Dependence of Plant Uptake and Diffusion of Polycyclic Aromatic Hydrocarbons on the Leaf Surface Morphology and Micro-structures of Cuticular Waxes

View Article: PubMed Central - PubMed

ABSTRACT

The uptake of organic chemicals by plants is considered of great significance as it impacts their environmental transport and fate and threatens crop growth and food safety. Herein, the dependence of the uptake, penetration, and distribution of sixteen polycyclic aromatic hydrocarbons (PAHs) on the morphology and micro-structures of cuticular waxes on leaf surfaces was investigated. Plant surface morphologies and wax micro-structures were examined by scanning emission microscopy, and hydrophobicities of plant surfaces were monitored through contact angle measurements. PAHs in the cuticles and inner tissues were distinguished by sequential extraction, and the cuticle was verified to be the dominant reservoir for the accumulation of lipophilic pollutants. The interspecies differences in PAH concentrations cannot be explained by normalizing them to the plant lipid content. PAHs in the inner tissues became concentrated with the increase of tissue lipid content, while a generally negative correlation between the PAH concentration in cuticles and the epicuticular wax content was found. PAHs on the adaxial and abaxial sides of a leaf were differentiated for the first time, and the divergence between these two sides can be ascribed to the variations in surface morphologies. The role of leaf lipids was redefined and differentiated.

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ΣPAH concentrations in the leaves and extractable lipids of selected plant species (a), PAH relative contents sorted by rings in the leaves (b), and correlation between ΣPAH concentrations and the contents of extractable lipids (c).
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f2: ΣPAH concentrations in the leaves and extractable lipids of selected plant species (a), PAH relative contents sorted by rings in the leaves (b), and correlation between ΣPAH concentrations and the contents of extractable lipids (c).

Mentions: The concentrations and distributions of ΣPAHs in selected leaf species are presented in Fig. 2. Even though the sampling site was very small in area, which suggested a similar air pollution level, ΣPAHs uptake by the plant leaves showed significant differences among the species (Fig. 2a). Among the selected leaf species, Hypericum with the densest wax structure and highest lipid content accumulated the most PAHs in terms of concentration (1.80 μg/g). However, from our hypothesis mentioned above, the most “clean” species was Photinia (0.31 μg/g), rather than Mahonia (1.19 μg/g) with the least lipid content, which, in contrast, even excelled at accumulating pollutants in terms of the normalized-to-lipid concentration (151 μg/g). The concentrations of 16 separated PAHs in the selected plant species are presented in Fig. S-1. Each plant species showed a different ability for the uptake of PAHs. The measured concentrations of PAHs were similar to those reviewed in the literature (1~4000 ng/g dry weight)3. The distributions of PAHs sorted by rings are presented in Fig. 2b, and 3,4-ring PAHs composed the highest uptake amount of ΣPAHs. Gaseous 2-ring PAHs usually undergo frequent exchanges at plant surfaces, so it had a very small fraction of ΣPAHs. Meanwhile, the mobility of organic substances within the plant cuticle was believed to be size dependent, so an increase in the molecular weight resulted in a decrease in mobility44. Hence, heavy PAHs with 5 and 6 rings had more difficulty entering the plant organism and as a consequence had negligible fractions. The correlation between ΣPAH concentrations and lipid content is demonstrated in Fig. 2c. Except for that of Photinia, there was a slight elevation in the ΣPAHs accumulation capacities of selected leaves with the increase of lipid content. Although it contained a relatively medium content of extractable lipids (2.15%), ΣPAH deposition was unfavorable on the leaf surface of Photinia. This result suggested that only considering the content of foliar lipids was not sufficient to explain the uptake and distribution characteristics of atmospheric PAHs into these leaves.


Dependence of Plant Uptake and Diffusion of Polycyclic Aromatic Hydrocarbons on the Leaf Surface Morphology and Micro-structures of Cuticular Waxes
ΣPAH concentrations in the leaves and extractable lipids of selected plant species (a), PAH relative contents sorted by rings in the leaves (b), and correlation between ΣPAH concentrations and the contents of extractable lipids (c).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5385540&req=5

f2: ΣPAH concentrations in the leaves and extractable lipids of selected plant species (a), PAH relative contents sorted by rings in the leaves (b), and correlation between ΣPAH concentrations and the contents of extractable lipids (c).
Mentions: The concentrations and distributions of ΣPAHs in selected leaf species are presented in Fig. 2. Even though the sampling site was very small in area, which suggested a similar air pollution level, ΣPAHs uptake by the plant leaves showed significant differences among the species (Fig. 2a). Among the selected leaf species, Hypericum with the densest wax structure and highest lipid content accumulated the most PAHs in terms of concentration (1.80 μg/g). However, from our hypothesis mentioned above, the most “clean” species was Photinia (0.31 μg/g), rather than Mahonia (1.19 μg/g) with the least lipid content, which, in contrast, even excelled at accumulating pollutants in terms of the normalized-to-lipid concentration (151 μg/g). The concentrations of 16 separated PAHs in the selected plant species are presented in Fig. S-1. Each plant species showed a different ability for the uptake of PAHs. The measured concentrations of PAHs were similar to those reviewed in the literature (1~4000 ng/g dry weight)3. The distributions of PAHs sorted by rings are presented in Fig. 2b, and 3,4-ring PAHs composed the highest uptake amount of ΣPAHs. Gaseous 2-ring PAHs usually undergo frequent exchanges at plant surfaces, so it had a very small fraction of ΣPAHs. Meanwhile, the mobility of organic substances within the plant cuticle was believed to be size dependent, so an increase in the molecular weight resulted in a decrease in mobility44. Hence, heavy PAHs with 5 and 6 rings had more difficulty entering the plant organism and as a consequence had negligible fractions. The correlation between ΣPAH concentrations and lipid content is demonstrated in Fig. 2c. Except for that of Photinia, there was a slight elevation in the ΣPAHs accumulation capacities of selected leaves with the increase of lipid content. Although it contained a relatively medium content of extractable lipids (2.15%), ΣPAH deposition was unfavorable on the leaf surface of Photinia. This result suggested that only considering the content of foliar lipids was not sufficient to explain the uptake and distribution characteristics of atmospheric PAHs into these leaves.

View Article: PubMed Central - PubMed

ABSTRACT

The uptake of organic chemicals by plants is considered of great significance as it impacts their environmental transport and fate and threatens crop growth and food safety. Herein, the dependence of the uptake, penetration, and distribution of sixteen polycyclic aromatic hydrocarbons (PAHs) on the morphology and micro-structures of cuticular waxes on leaf surfaces was investigated. Plant surface morphologies and wax micro-structures were examined by scanning emission microscopy, and hydrophobicities of plant surfaces were monitored through contact angle measurements. PAHs in the cuticles and inner tissues were distinguished by sequential extraction, and the cuticle was verified to be the dominant reservoir for the accumulation of lipophilic pollutants. The interspecies differences in PAH concentrations cannot be explained by normalizing them to the plant lipid content. PAHs in the inner tissues became concentrated with the increase of tissue lipid content, while a generally negative correlation between the PAH concentration in cuticles and the epicuticular wax content was found. PAHs on the adaxial and abaxial sides of a leaf were differentiated for the first time, and the divergence between these two sides can be ascribed to the variations in surface morphologies. The role of leaf lipids was redefined and differentiated.

No MeSH data available.


Related in: MedlinePlus