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Uptake, distribution and transformation of CuO NPs in a floating plant Eichhornia crassipes and related stomatal responses Jian Zhao, Wenting Ren, Yanhui Dai, Lijiao Liu, Zhenyu Wang, Xiaoyu Yu, Junzhe Zhang, Xiangke Wang, and Baoshan Xing Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 10, 2017
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Environmental Science & Technology
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Uptake, distribution and transformation of CuO NPs in a floating
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plant Eichhornia crassipes and related stomatal responses
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Jian Zhao,a Wenting Ren,a Yanhui Dai,a Lijiao Liu,a Zhenyu Wang,*,b,c Xiaoyu Yu,a
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Junzhe Zhang,a Xiangke Wang,d and Baoshan Xing*,e
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a
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Laboratory of Marine Environment and Ecology, Ocean University of China, Qingdao 266100,
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China
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b
College of Environmental Science and Engineering, and Ministry of Education Key
Institute of Environmental Processes and Pollution Control, and School of Environmental
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and Civil Engineering, Jiangnan University, Wuxi 214122, China
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c
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Marine Science and Technology, Qingdao 266071, China
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d
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Beijing 102206, China
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e
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01003, USA
Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for
School of Environment and Chemical Engineering, North China Electric Power University,
Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts
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*Corresponding authors
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Tel.: +1 413 545 5212; fax: +1 413 577 0242
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E-mail address:
[email protected] (Dr. Baoshan Xing);
[email protected] (Dr. Zhenyu
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Wang)
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Abstract.
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Engineered nanoparticles (NPs) are releasing into aquatic environments with their
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increasing applications. In this work, we investigated the interaction of CuO NPs with a
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floating plant, water hyacinth (Eichhornia crassipes). CuO NPs (50 mg/L) showed significant
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growth inhibition on both roots and shoots of E. crassipes after 8-day exposure, much higher
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than the bulk CuO particles (50 mg/L) and corresponding dissolved Cu2+ ions (0.30 mg/L).
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Scanning electron and light microscopic observations showed that root caps and meristematic
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zone of E. Crassipes were severely damaged after CuO NP exposure, with disordered cell
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arrangement and destroyed elongation zone of root tips. It is confirmed that CuO NPs could be
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translocated to shoot from both roots and submerged leaves. As detected by X-ray absorption
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near-edge spectroscopy analysis (XANES), CuO NPs were observed in roots, submerged
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leaves, and emerged leaves. Cu2S and other Cu species were also detected in these tissues,
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providing solid evidence on the transformation of CuO NPs. In addition, stomatal closure was
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observed during CuO NPs-leaf contact, which was induced by the production of H2O2 and
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increased Ca level in leaf guard cells. These findings are helpful for better understanding the
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fate of NPs in aquatic plants and related biological responses.
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Environmental Science & Technology
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INTRODUCTION
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CuO nanoparticles (NPs) are commonly used in pesticides, paints, textiles, plastics,
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sensors and catalysts.1,2 Rapid growth in production and application of CuO NPs will
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inevitably result in the introduction of NPs into aqueous environments. As a type of high-toxic
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NPs, toxicological researches on CuO are widely conducted towards various species including
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terrestrial plants, algae and bacteria.3-6 However, little nanotoxicological information is
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available for aquatic plants in comparison with above species.7 Aquatic plants are critical
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components of aquatic ecosystems, functioning as sources of organic matters, food for aquatic
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animals, and sinks for nutrients.8 Currently, there are only three reports on the toxicity of CuO
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NPs towards aquatic plants. Perreault et al. found that growth and photosynthesis of Lemna
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gibba L. were inhibited after CuO NP exposure, and the release of Cu2+ ions was identified as
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a main reason for the observed toxicity.1 However, the nanoparticulate CuO rather than soluble
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Cu2+ ions caused reduction in growth of both duckweeds (Landoltia punctata)9 and soft-stem
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bulrush (Schoenoplectus tabernaemontani).10 These contradictory results suggest that the
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contribution of nanoparticulate CuO to the total toxicity is unclear for aquatic plants. The
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toxicity mechanism of CuO NPs and related biological responses of aquatic plants deserve
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further investigations.
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The roots and submerged leaves of aquatic plants will directly interact with CuO NPs in
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water columns during exposure. CuO NPs (23-50 nm, 50 mg/L) were evidenced first to be
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accumulated in root tissues of the aquatic plant Schoenoplectus tabernaemontani,10 in which
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CuO NPs were aggregated around organelle after penetrating cell wall and plasma membrane.
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But the limited root-shoot translocation potential was suggested in this study. For rice which is
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commonly recognized as a semi-aquatic or terrestrial plant, CuO NP (43 ± 9 nm, 100 mg/L)
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transport from its roots to leaves was confirmed based on X-ray absorption near edge structure
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(XANES) analysis.2 Maize could also root-shoot transport CuO NPs (20-40 nm, 100 mg/L) via
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xylem.11 One possible reason for this difference on NPs transport between aquatic and
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terrestrial plants is the more extensive roots in terrestrial plants. In addition, terrestrial plants
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have highly-developed xylem which has been confirmed as a main pathway for NP transport.
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It is still unknown whether the poorly-developed xylem vessels in roots of aquatic plants could
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effectively transport NPs. For submerged leaves of most aquatic plants, the cuticle and its
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waxes are much thinner or absent as compared to terrestrial plants to facilitate foliar uptake of
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water and nutrients.12 Therefore, submerged leaf of aquatic plants could be an excellent model
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to investigate foliar uptake mechanism of water-exposed NPs. Aquatic plants are thus
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hypothesized to accumulate NPs from water column through both roots and submerged leaves,
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and the latter is expected to have a higher contribution on CuO NP accumulation in shoots. To
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confirm this hypothesis, a new device is designed in an emergent treatment with the separation
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of leaves from water column in this work. In addition, dissolution and sulphidation could be
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two transformation processes of CuO NPs in aqueous phase, especially in the presence of
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natural organic matter.13 After internalized by freshwater algae (Chlorella pyrenoidosa), CuO
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NPs were partially transformed to Cu2O.5 Cu2S was detected in the plants of both maize
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(xylem sap)11 and wheat (shoot)3 while it was not found in either roots or shoots of rice.2 The
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specific processes of CuO NP transformation in aquatic plants needs to be explored. Moreover,
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the stomata on submerged leaf is a key organ for acquirement of CO2 and nutrients, and uptake
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of exogenous matters such as heavy metals,14 mineral particles15 and nanopesticides.16 It is
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hypothesized that CuO NPs could be internalized through stomata during foliar uptake, which
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is a possible mechanism for CuO NP toxicity. The responses of stomata when interacting with
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CuO NPs and the relevant signaling pathway will be investigated in the present work.
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Overall, the main objectives are thus to investigate (1) the toxicity of CuO NPs to aquatic
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plants and related damages on roots and leaves; (2) root uptake, root-shoot translocation and
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foliar uptake of CuO NPs; (3) transformation of CuO NPs in aquatic plants; and (4) stomatal
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responses of leaves upon CuO NPs and related mechanism for CuO NP-induced stomatal
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closure. Water hyacinth is selected in this study because it is a representative free-floating
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plant that can float on the surface of water column. Leaves of water hyacinth could be
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submerged in water or emerged atop the water column, which is helpful for investigating the
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contribution of foliar uptake of NPs. The newly generated information on accumulation,
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distribution and transformation of CuO NPs in water hyacinth will be helpful for
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understanding the behavior and ultimate fate of NPs in aquatic plants and potential risk of NPs
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transfer through aquatic food chains because water hyacinths are sources of food for aquatic
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animals.
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MATERIALS AND METHODS
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Materials.
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CuO NPs (No. 544868,