Eucalyptus Tolerance Mechanisms To Lanthanum And Cerium

In recent years, high demand for rare earth elements (REEs) in industry and agriculture has led to the accumulation of REEs in soils. Phytoremediation is a green remediation approach that uses green plants to decontaminate or detoxify pollutants. Guanglin 9(Eucalyptus grandis×Eucalyptus urophlla) and Eucalyptus grandis 5 are two eucalyptus species which have been found to grow normally in soils contaminated with lanthanum and cerium, but the tolerance mechanisms are not clear yet. In this study, a pot experiment was conducted to investigate the tolerance mechanisms of the eucalyptus to lanthanum and cerium. The major results showed as follows:(1) Both species of eucalyptus grew normally under La and Ce stress. Plant heights of Gn9 and Eg5 did not differ significantly under different levels of La and Ce stress, but root lengths did increase with increasing soil La and Ce concentrations. Biomasses of Gn9 did not decrease with increasing soil La concentrations, and biomasses in all La treatments were significantly higher than that of the control. Biomasses of the two eucalyptus species did not differ significantly between different Ce treatment levels.(2) Concentrations of the two metals in tissues of both species of eucalyptus increased with increasing soil treatment levels. The concentrations of La and Ce in the two species occurred in the following order:root concentrations> leaf concentrations> stem concentrations. The highest La and Ce concentrations in each organ of Gn9 and Ce were at the treatments of 500 mg kg-Accumulations of the two metals in tissues of both species of eucalyptus increased with increasing soil treatment levels. The accumulations of La and Ce in the Gn9 occurred in the following order:root accumulations>shoot accumulations. The accumulations of La and Ce in the Eg5 occurred in the following order:root accumulations<shoot accumulations. La and Ce accumulations in the roots of Gn9 reached 31.39 and 24.03 mg pot-1, respectively; in shoots were 24.28 and 18.46 mg pot-1, respectively; the highest accumulations in shoots of Eg5 were 33.97 and 30.50 mg pot-1, respectively; in roots were 26.75 and 29.56 mg pot-1, respectively.(3) Concentrations and proportions of the two metals in subcellular compartments of eucalyptus. The La and Ce in roots and leaves of the two species occurred in the following order:cell wall fraction> soluble fraction> cell organelle fraction> cell membrane fraction.The proportion of both La and Ce in cell wall fraction and soluble fraction were increased with increasing soil treatment levels. The proportion of both La and Ce in cell organelle fraction and cell membrane fraction were decreased with increasing soil treatment levels. As the first barrier preventing the passage of La and Ce into the cell, cell walls of leaves of Gn9 and Eg5 stored 45.40-58.87%,45.40-51.79% and 49.40-57.19%, 47.34-55.98% of the metals under conditions of exposure to the two metals, respectively; roots cell walls stored 45.40-58.87%,45.40-51.79% and 49.40-57.19%,47.34-55.98% of the metals under conditions of exposure to the two metals, respectively.(4) Enzyme activity increased significantly with increasing soil La and Ce concentrations. The activities of POD and CAT were significantly higher in leaves than in roots. POD activity was higher in Eg5 than in Gn9, while CAT activity was higher in Gn9 than in Eg5. POD activities in the two species of eucalyptus increased with increasing soil La concentrations up to 200 mg kg-1, then decreased, while POD activity in the leaves of Gn9 continued to raise with increasing soil Ce concentrations. CAT activities in two eucalyptus species showed different trend under Ce stress.Concentrations of GSH in two species of eucalyptus exhibited same trends under La and Ce stress, except for Gn9 with increasing Ce treatment levels. The highest GSH concentrations in the two eucalyptus species were at La treatment level of 200 mg kg-1. AsA concentrations in the leaves of both species did not vary significantly between different La and Ce treatment levels. AsA concentrations in the roots of Gn9 and Eg5 under La stress show the opposite trend:at the Ce treatment level of 30 mg kg-1, the highest AsA concentrations in the roots in Eg5 and Gn.9 were 23.67,24.83 μmol g-1, respectively.(5) In general, concentrations of all thiol compounds in the two species increased with increasing soil metal concentrations, and were higher than those of the control. Concentrations of NPT and PCs were higher in Eg5 than in Gn9. NPT concentrations in both species increased with increasing soil La and Ce concentrations up to 200 mg kg-1, then decreased in all parts of the plants in both species, except for in the leaves of Gn9 in which Ce concentrations continued to rise with increasing soil Ce concentrations; the highest concentration of (6.61μmol g-1) occurred in soil with a Ce concentration of 500 mg kg-1. Concentrations of PCs in the roots of Gn9 and Eg5 significantly increased with increasing soil La concentrations, and the highest PCs concentrations in the roots of Gn9 and Eg5 were 0.68 and 1.29 μmol g-1, respectively. The roots of Eg5 were more sensitive to Ce in soil than were the roots of Gn9.Therefore, the two eucalyptus species are La and Ce tolerant plants, and the tolerance mechanisms include cell wall deposition, antioxidant system response, and thiol compound synthesis.