Cadmium uptake, chemical forms, subcellular distribution, and accumulation in Echinodorus osiris Rataj

Phytoremediation is a technology for extracting or inactivating pollutants in soil. Echinodorus osiris (E. osiris) is a fast growing perennial wetland plant that is common in tropical and subtropical areas and has a high tolerance to cadmium (Cd). However, the absorption dynamics, subcellular distribution and accumulation of Cd by E. osiris had not been investigated. In this paper, hydroponic experiments with different levels of Cd2+ (0, 5.0, 15.0 mg L−1) were carried out to determine these characteristics of E. osiris. The results indicated that the Cd absorption rate of Echinodorus osiris decreased over time, and the absorption rate within 0.5–1.0 h was faster than after 1.0 h. In a 6.0 hour time period, the rate of Cd uptake fit a quadratic polynomial curve when E. osiris was grown under the 5 mg L−1 Cd treatment. However, the rate of Cd uptake by E. osiris fit a cubic polynomial model with the 15 mg L−1 Cd treatment. In the roots, the ethanol-extractable Cd, water-extractable Cd, and NaCl-extractable Cd were the largest proportions of the total Cd. The HAc-extractable Cd, HCl-extractable Cd, and residual-Cd represented a larger proportion of the total Cd in the leaves which was combined with phosphate including CdHPO4, Cd3 (PO4)2, and oxalic acid. When analyzing the subcellular distribution of Cd in the plant, the soluble fraction containing Cd accounted for the largest part (69.49–88.39%) followed by the Cd bound to the cell wall (8.44–25.62%). Both the lower and the higher Cd treatments demonstrated that compartmentation by the vacuole and cell wall binding were two effective defense mechanisms of the plant. However, the vacuole became the main site for Cd accumulation in the leaves under the 15 mg L−1 Cd treatment. E. osiris was able to accumulate high concentrations of Cd in both the roots and the leaves. The Cd concentration reached 502.97 mg kg−1 and 2742.95 mg kg−1 in the shoots and roots, respectively, after 27 days of cultivation. It was concluded that E. osiris is a potential hyperaccumulator of Cd.