| Ionic liquids(ILs), also named room temperature ILs or room temperature molten salt, are a class of solvents entirely composed of ions at room temperature. ILs are usually made up of large asymmetric organic cations and small inorganic or organic anions. Moreover, the structure of ILs can be easily tuned, so changing the combinations of different cations and anions can be used to the synthesis of different ILs. The unique structures of ILs signify that they have many unique physicochemical properties, such as low vapor pressure, high polarity, non-volatility, good conductivity and recyclability. Moreover, ILs were considered as “green solvents” due to their unique properties and showed a very broad application prospects in many fields as alternatives to organic solvents, such as electrochemistry, organic synthesis, catalysis, biosensors, and separation processes.However, when ILs are used as industrial solvents or catalysts, they will eventually be released into the environment. Although ILs are considered to be “green” solvents, some of their unique properties such as their resistance to photodegradation and water stability enhance their risk to the environment. Moreover, ILs are difficult for microorganisms to degrade. Therefore, ILs may be potential soil and water contaminants due to accidental spills or via effluents. Moreover, ILs may be harmful to humans, and their toxicity may be even higher than that of traditional organic solvents. Therefore, we must consider the potential risks of this new chemical to people and the environment. In recent years, the toxicity of ILs caused widespread concern among scientists, and a large amount of research on the toxic effects of ILs towards aquatic and terrestrial environments has been evaluated at different trophic levels and different biological levels.However, it is worth noting that previous studies examining the toxic effects of ILs on plants were primarily performed in nutrient solutions. In the natural environment, plants are mainly grown in soil. Moreover, soil represents a more complicated environment in which to study the toxic effects of ILs on plants, especially the influence of ILs on crops. Although there are reports on the toxic effects of ILs in soil on plants, these reports have all focused on the inhibitory effects of ILs on the growth of plants. Nonetheless, the physiological and biochemical properties of plants may be more sensitive to ILs than growth indicators. In some cases, although the growth of plants was not significantly inhibited by pollutants, the physiological and biochemical properties were significantly influenced. If the toxic effects of ILs in soil on plants were studied at both molecular and cellular levels, it will be better to evaluate the toxicity of ILs. However, there are currently no comprehensive reports on the toxic effects of ILs in soil on the growth and physiologies of plants. In addition, it is worth noting that the toxicity of ILs varies because the structures of the cations and anions are different, and studying the influence of structure on IL toxicity is essential for designing nontoxic or low toxicity ILs.Vicia faba(V. faba) is a major crop both in China and worldwide, which has a very high economic value. In addition, V. faba is considered as a good environmental indicator organism and an ecotoxicological test organism, and widely used to detect the toxicity of chemicals. Therefore, the toxic effects of two groups of imidazolium- based ILs with different anions and one group of imidazolium-based ILs with different carbon chain lengths in soil on the growth and physiological indicators of V. faba seedlings were studied at the whole plant, cellular and molecular levels in the present study. In addition, the residual dynamics of these ILs in soil were determined using high performance liquid chromatography(HPLC) method. Moreover, the toxicity of these ILs was evaluated using the 50% effective concentration(EC50) values for growth indicators of V. faba seedlings. The purpose of the present study was to evaluate the toxic effects of ILs in soil on plants and to ascertain the influence of structure on the toxicity of ILs. The present study provides a theoretical basis for the comprehensive assessment of the toxicity of ILs and the design of non-toxic or low toxicity ILs. The main conclusions of the present study were as follows:(1) In the present study, the extraction recoveries of [C8mim]Cl, [C8mim]Br and [C8mim]BF4 from the soil were 80.1%-85.2%, 78.4%-84.4 and 81.4%-86.3%, respectively. The extraction recoveries of [C10mim]Cl、[C10mim]Br and [C10mim]NO3 from the soil were 71.0%-81.1%, 72.4%-80.3% and 70.2%-79.4%, respectively. The extraction recoveries of [C4mim]Cl, [C6mim]Cl and [C12mim]Cl from the soil were 85.5%-89.1%, 80.3-85.4% and 69.7%-79.0%, respectively. Additionally, the concentrations of these ILs in different treatments on the 10 th day showed a downward trend compared to that on the 0th day. Nonetheless, the range of concentrations measured across the different ILs treatments was no more than 10%.(2) At 100mg/kg, [C8mim]Cl, [C8mim]Br and [C8mim]BF4 had little influence on the growth of V. faba seedlings. At 25mg/kg, [C10mim]Cl, [C10mim]Br and [C10mim]NO3 had little influence on the growth of V. faba seedlings. At 1000mg/kg, 500mg/kg and 10mg/kg, [C4mim]Cl, [C6mim]Cl and [C12mim]Cl had little influence on the growth of V. faba seedlings, respectively. However, the V. faba seedlings of [C8mim]Cl-, [C8mim]Br- and [C8mim]BF4-exposed treatments almost stopped growing at 800mg/kg. The V. faba seedlings of [C10mim]Cl-, [C10mim]Br- and [C10mim]NO3-exposed treatments almost stopped growing at 400mg/kg. The V. faba seedlings of [C4mim]Cl-, [C6mim]Cl- and [C12mim]Cl-exposed treatments almost stopped growing at 8000mg/kg, 3000mg/kg and 400mg/kg, respectively.(3) At 200mg/kg, the pigment contents in V. faba seedling leaves decreased significantly after exposure to [C8mim]Cl, [C8mim]Br and [C8mim]BF4. At 50mg/kg, the pigment contents in V. faba seedling leaves decreased significantly after exposure to [C10mim]Cl, [C10mim]Br and [C10mim]NO3. The pigment contents in V. faba seedling leaves of [C4mim]Cl- and [C6mim]Cl-exposed treatments also decreased significantly at 2500mg/kg and 1000mg/kg, respectively. In addition, the hill reaction activity of V. faba seedlings was significantly inhibited by [C12mim]Cl at 40mg/kg.(4) At 1500mg/kg and 20mg/kg, the proline content was significantly inhibited by [C4mim]Cl and [C12mim]Cl, respectively. Moreover, the proline content increased with increasing the concentrations of the two ILs, which is a phenomenon that V. faba seedlings resistance to stress induced by ILs.(5) In the present study, the ROS level was enhanced in different degree after exposure to the nine ILs. The structure and function of V. faba seedling cells was destroyed by the excess ROS, resulting in lipid peroxidation, protein carbonylation and DNA damage. In addition, the activity and gene expression of antioxidant enzymes and the content of antioxidant compounds changed significantly. Moreover, oxidative damage may be one of the main toxic mechanisms of ILs.(6) Comet assay was the most sensitive indicators in the present study. Histochemical staining method was more intuitive to reflect the changes of ROS level in plant leaves. Additionally, studying the toxicity of ILs from the molecular level will be able to better expound the toxic mechanism of ILs.(7) The ILs with the same carbon chain length but different anions had similar toxic effects on V. faba seedlings, which means anions have little influence on the toxicity of ILs. The ILs with the same anion but different carbon chain lengths had increasing toxic effects on the V. faba seedlings with increasing the carbon chain lengths, which means the carbon chain lengths have a great influence on the toxicity of ILs. In the present study, [C4mim]Cl had the minimal toxic effects on V. faba seedlings, and [C12mim]Cl had the maximum toxic effects on V. faba seedlings.(8) ILs may cause harm to agricultural production at high concentrations, therefore, the applied concentrations of ILs should be reduced. Additionally, a new type of ILs with short carbon chain, multi-function, low toxicity should be desigened by changing the anions. |
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