Application of modern spectroscopic techniques to study heavy metal accumulation and uptake mechanisms in tumbleweed (Salsola kali).

摘要

Environmental pollution has long been recognized as one of the major problems faced by modern society. Soil pollution by heavy metals and metalloids is especially important since they are not biodegradable and may remain in the environment even for thousands of years. Researchers are constantly trying to improve existing remediation methodologies and provide new options. Phytoremediation, the use of plants for the restoration of polluted soils and waters, has been proposed as a promising alternative to other remediation techniques. This research investigated the ability of tumbleweed (S. kali) plants to uptake and tolerate Cd(II), Cr(III), Cr(VI), Pb(II), As(III), As(V), Cu(II), Ni(II), and Zn(II). In addition, different experiments were conducted in order to identify the possible mechanisms of metal absorption, tolerance, and biotransformation. Results showed that tumbleweed plants exposed to 20 mg Cd L-1 concentrated about 2,000 mg Cd kg-1 d.wt. in the leaves, indicating that this plant is a potential Cd hyperaccumulator. X-ray absorption spectroscopy (XAS) studies indicated that in tumbleweed Cd is bound to organic acids, cell wall and phytochelatins or other thiol compounds. Biochemical experiments performed in hydroponically grown plants demonstrated that Cd concentration in tissues influenced thiol production. These experiments also allowed the initial identification of a new protein expressed in Cd-treated plants and of two proteins probably associated to Cd. Evidence suggested that oxalic acid present in plants may serve as a Cd-detoxifying agent by forming Cd-oxalate crystals. The use of degenerated primers of the Brassica family allowed the identification of a possible phytochelatin synthase gene. Tumbleweed plants grown in agar media accumulated between 10 to 20 times more Cr when supplied as Cr(VI) (2900, 800, and 600 mg kg-1 in roots, stems, and leaves, respectively), as compared to those cultivated in Cr(III). Experiments performed in agar and hydroponic media demonstrated that EDTA increased Pb translocation. Leaves of plants hydroponically grown in Pb and Pb-EDTA showed Pb concentrations of 50 and 1400 mg Pb kg-1 d.wt., respectively. A significant relationship was found between the production of thiols and the Pb content in roots. Microscopic studies suggested that in the presence of EDTA, Pb is transported in the complexed form. On the other hand, arsenic speciation affected arsenic/phosphorous uptake as more arsenic was uptaken when supplied as arsenite. However, plants exhibited less phytotoxicity when exposed to arsenate. Results suggested that arsenate was uptaken via a phosphate system. In As(III) and As(V) treated plants, arsenic was found as As(III) and coordinated to three sulfur ligands possibly indicating the presence of As-glutathione related compounds. Experiments performed with Cu(II), Ni(II), and Zn(II) showed the following order of tolerance Zn > Cu > Ni. Additionally, shoots of plants exposed to Cu(II) accumulated 1300 mg kg-1 d.wt., indicating that tumbleweed may hyperaccumulate this metal. In conclusion, the results of this research indicate that S. kali can be considered as an option to remove toxic elements from polluted soils located in desert areas. In addition, the information presented herein is useful to better understand the uptake mechanisms and hyperaccumulation of toxic elements in S. kali.