Abstract
Perennial ryegrass (Lolium perenne L.) was planted in uranium-contaminated soil mixtures supplemented with different amounts of citric acid to investigate the defense strategies of perennial ryegrass against U and the enhanced mechanism of citric acid on the remediation efficiency in the laboratory. The uranium content in the plant tissues showed that the roots were the predominant tissue for uranium accumulation. In both root and shoot cells, the majority of U was located in the cell wall fraction. Furthermore, antioxidant enzymes were also stimulated when exposed to U stress. These results suggested that perennial ryegrass had evolved defense strategies, such as U sequestration in root tissue, compartmentalization in the cell wall, and antioxidant enzyme systems, to minimize uranium stress. For an enhanced mechanism, the optimal concentration of citric acid was 5mmol/kg, and the removal efficiency of U in the shoots and roots increased by 47.37% and 30.10%, respectively. The treatment with 5mmol/kg citric acid had the highest contents of photosynthetic pigment and soluble protein, the highest activity of antioxidant enzymes, and the lowest content of MDA (malondialdehyde) and relative electrical conductivity. Moreover, the TEM (transmission electron microscope) results revealed that after 5mmol/kg citric acid was added, the cell structure of plant branches partially returned to normal, the number of mitochondria increased, chloroplast surfaces seemed normal, and the cell wall became visible. The damage to the cell ultrastructure of perennial ryegrass was significantly alleviated by treatment with 5mmol/kg citric acid. All the results above indicated that perennial ryegrass could accumulate uranium with elevated uranium tolerance and enrichment ability with 5mmol/kg citric acid.
Highlights
Uranium pollution in soil commonly results from uranium mining and milling activities in the nuclear industry (Sha et al 2019)
These results suggest that citric acid has significant effects on improving the uranium phytoremediation potential of perennial ryegrass
The toxic effects of citric acid may damage the physiological structure of perennial ryegrass and cause a decrease in plant biomass when the citric acid concentration reaches 10 mmol/kg(Duquène et al 2008, Monroy-Figueroa et al 2015). These results suggest that 5 mmol/kg citric acid is the most effective concentration of citric acid to increase the biomass production and uranium enrichment in perennial ryegrass
Summary
Uranium pollution in soil commonly results from uranium mining and milling activities in the nuclear industry (Sha et al 2019). Phytoremediation, which uses the fixation and extraction functions of plants, can effectively remove uranium from soil and has several advantages, such as environmental friendliness, easy implementation and cost effectiveness (Burges et al 2017, Hu et al 2019, Li et al 2019). Some studies have demonstrated that some plants can remediate uraniumpolluted soil with high uptake capacity, such as Indian mustard and sunflower (Laurette et al 2012, Qi et al 2014). Most of these species are not suitable for commercial phytoremediation mainly because of their low annual harvestable biomass and low growth rate(Nascimento et al 2020)
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