A Study On The Uptake And Toxicity Of Nickel Added In The Form Of Different Salts To Maize Seedlings

Nickel (Ni) is used in a growing number of products in the modern world, including in stainless steel, electroplating and batteries. With the rapid development of these industries, Ni pollution is becoming an increasing problem. Nickel is an essential trace element for plants growth but excessive Ni levels in the soil can result in toxicity to plants. In soil ecotoxicological studies, a toxic metal is usually added in the form of either inorganic or organic salts of relatively high solubility. Nitrate, chloride, acetate or sulphate is commonly considered as valid options for that aim. However, it has recently been reported that different salts of the same metal at the same cationic concentration may exhibit different toxicities to plants and soil organisms. Therefore, the purpose of this study is to compare the different toxicity of five nickel compounds on plant and the influence of nickel enrichment through the pot experiments, and provide theoretical basis for the study of the soil ecological toxicology.Maize (Zea mays) was used in the experiment. Pot experiments were carried out in a greenhouse. By adding inorganic nickel salts (NiCl2·6H2O, NiSO4·6H2O), organic nickel salt (Ni(CH3COO)2·4H2O) and two kinds of nickel chelate (Ni(Ⅱ)-citrate, and Ni(Ⅱ)-EDTA)) manually, we carried out the comparative study of nickel toxicity on maize seedlings. There were two types of pot experiments. NiCl2-6H2O, NiSO4-6H2O and Ni(Ⅱ)-EDTA were used to generate concentration-response curves in the first pot experiment. The second pot experiment was conducted to assess the uptake and phytotoxicity of five nickel salts in maize. Five Ni compounds in the form of different salts (NiCl2·6H2O, NiSO4·6H2O, Ni(CH3COO)2·4H2O, Ni(Ⅱ)-citrate, andNi(Ⅱ)-EDTA) were similarly spiked into the soil to give 560 and 1000 mg Ni·kg-1 soil dry weight. The plants were harvested 28 days after emergence. We determined the physiological and biochemical parameters of plant and soil characteristics, then reveal the the different toxicity of five nickel compounds on plant and the influence of nickel enrichment and the reasons.The outcome of the study demonstrates that different salts of the same metal have quite different ecotoxicity. The results showed that when Ni was added to the soil, toxicity varied with the anionic partner selected, with toxicity ranking NiSO4< Ni(CH3COO)2< Ni(Ⅱ)-citrate< NiCl2< Ni(ll)-EDTA. Taking the plant-height metric as an example, the effective concentration at 50% inhibition (EC50) was 3148 mg kg-1 for NiSO4,1315 mg kg-1 for NiCl2, and 89 mg kg-1 for Ni(lI)-EDTA. Among the treatments using the various Ni compounds, Ni accumulation was observed in the following descending order:Ni(II)-EDTA> NiSO4≥NiCl2> Ni(Ⅱ)-citrate> Ni(CH3COO)2. These results indicated that the accumulation of Ni by maize plants occurred mainly in the roots, with only a small amount of Ni being transported to the shoots. The mean Ni concentrations in the three organs decreased in the order:root> stem> leaves.We found that at low Ni concentrations (such as NiCl2,0-320 mg·kg-1), the biomass of maize increased compared to the control treatement.This result indicated that low Ni levels stimulate maize growth. As the Ni concentrations increasing (>320 mg/kg), seedling growth and development were significantly inhibited. High soil Ni concentrations (1000 mg/kg) resulted in dwarfing compared with the control, and the leaves the leaves exhibited chlorosis and yellow spottingThe sucrose and starch content of maize leaves significantly increases at the Ni treatment of 180 mg·kg-1 and then decrease back to the same level as the control. This finding indicates that low Ni concentrations stimulate leaf sucrose and starch synthesis, and high concentrations of Ni inhibit the synthesis of soluble sugar, but not inhibit the synthesis of leaf sucrose and starch.