Mechanisms of Cadmium Tolerance and Detoxification in Two Ornamental Plants

Cadmium (Cd) is an important environmental heavy metal and one of the main soil pollutants in southwest China and even the Yangtze River Basin because of its toxicity to plants and humans. To clarify the potential of Euryops pectinatus L. and Gardenia jasminoides J. and the mechanism they use to remediate Cd-contaminated soil, a soil pot experiment with 0, 5, 10, 20, and 40 mg kg−1 of Cd was used to investigate the accumulation characteristics, subcellular distribution, chemical forms, and the antioxidative defense systems of the two ornamental plants. When the concentration of Cd was below 40 mg kg−1, it promoted the growth of E. pectinatus shoots, and the tolerance index (TI) was >1. However, 20–40 mg kg−1 Cd significantly inhibited the growth of G. jasminoides, and the TI was <1. The shoots of both varieties accumulated more Cd than the roots, and the E. pectinatus shoots accumulated more Cd (1.45 mg plant−1) than those of G. jasminoides (0.71 mg plant−1). The Cd in E. pectinatus and G. jasminoides was primarily distributed in the soluble fraction (52.83–68.97%) and cell walls (44.62–54.98%), respectively. Higher proportions of Cd bound to NaCl and acetic acid (HAc) in E. pectinatus (55.32–73.44%) than in G. jasminoides (42.94–61.58%), while the inorganic and water-soluble proportions of Cd bound in the opposite manner. E. pectinatus maintained high activities of antioxidant enzymes under Cd treatments, and its levels of malondialdehyde (MDA) and relative electrical conductivity (REC) were comparable to those of the control. Nonetheless, G. jasminoides had low levels of activity of antioxidant enzymes, but its levels of MDA and REC were significantly higher than those of the control under the 20–40 mg kg−1 Cd treatment. Therefore, both types of plants have a strong ability to tolerate and accumulate Cd, which makes them suitable for the remediation of Cd-polluted soil. However, E. pectinatus is more effective at remediating Cd and tolerant to it than G. jasminoides. These plants utilize different mechanisms to detoxify Cd.