Large-scale screening and characterization of Cd accumulation and ultrastructural deformation in duckweed

Cadmium (Cd) pollution in soil, rivers and lakes is a serious problem due to the current industrialization and urbanization in China. Duckweeds are recognized as promising species for Cd phytoremediation. However, intraspecific variations in Cd accumulation in duckweeds remain largely unknown. In this study, 16 accessions selected from 39 geographically isolated duckweed strains were chosen to investigate their Cd remediation abilities. The optimal accession Landoltia punctata named 07SGZP01 (L. punctata 0701) was identified and shown to accumulate maximal Cd in the body while maintaining the highest biomass. The dominant variety treated with different Cd concentrations showed that the biomass of L. punctata 0701 was significantly lower than that of the control group (CK). Cd contents in L. punctata 0701 were substantially increased from 2511.1 to 30,641.01 mg kg^-1 with an increase in Cd treatment levels from 0.3 to 20 mg L^-1. The transport coefficient (TF) increased as Cd levels increased from 0.3 to 2 mg L^-1. In addition, the Cd content in leaves was greater than that in roots (TF > 1) within this Cd concentration range, whereas the Cd content in roots was greater than that in leaves (TF < 1) when the concentration of the Cd treatment was greater than 5 mg L^-1. The bioaccumulation factor (BCF) decreased significantly with increasing Cd levels (P < 0.05). The rate of Cd removal in the solution gradually decreased with increasing Cd concentrations, and the removal rate achieved the highest value (75%) when the Cd concentration was 0.5 mg L^-1. In addition, Cd treatment (2 mg L^-1) not only damaged the ultrastructure of L. punctata 0701, as characterized by chloroplast deformation and cell vacuolation but also caused most of the stomata to close, and the leaf epidermal cells were damaged and ruptured.