Comparative study on the growth and Cd uptake of Suaeda salsa and Suaeda glauca under the stress of salt and Cd and their interaction

Remediation of heavy metal-contaminated saline-alkali soil by halophytes is a cost-effective and environmentally friendly method, but there is less information on the comparative study of different halophytic species under salt and heavy metal stresses. In this study, hydroponic experiments were conducted to compare the effects of salt and Cd interactions on the growth and Cd uptake between two typical leaf succulent halophytes, Suaeda salsa and Suaeda glauca, and to explore the regulatory effect of salt on Cd absorption. There were four treatments with two salt levels (0 and 10 g·L⁻¹ NaCl) and two Cd levels (0 and 5 mg·L⁻¹ Cd). The results showed that Cd treatment significantly inhibited the growth of the two plants and reduced leaf succulence and root-to-shoot ratio. Additionally, the inhibitory effect was greater on S. salsa than on S. glauca, especially with respect to root growth. Salt treatment promoted the shoot growth of both plants, and the promotion effect on shoot growth was significantly greater in S. salsa than in S. glauca. However, the root growth of S. glauca was significantly inhibited. The salt treatment under Cd stress significantly alleviated the inhibitory effect of Cd on the growth of both plants, and the alleviation effect on S. salsa was greater than that of S. glauca, especially on the root growth of S. salsa. There was no significant difference in shoot Cd content between S. salsa and S. glauca under Cd stress, but the Cd content in the shoots of the two plants was significantly reduced by salt treatment. The Na⁺ content and Na⁺/Cd ratio in the shoots of S. salsa were higher than those in S. glauca, but the K⁺ content, K⁺/Na⁺ ratio, and K⁺/Cd ratio were significantly lower than those in S. glauca. Salt treatment significantly promoted Na⁺ absorption in the shoots of both plants, and Cd treatment further promoted Na⁺ absorption in the shoots. Salt treatment reduced shoot K⁺ content and K⁺/Na⁺ ratio in both plants; these values were higher in S. glauca than in S. salsa. There was a tendency for shoot proline content to increase in both species under salt and Cd stresses, but there were no significant differences among species, salt, and Cd treatments. The shoot soluble sugar content of S. salsa was significantly higher than that of S. glauca under no stress and Cd treatment. Salt treatment significantly decreased the shoot soluble sugar content of S. salsa and increased that of S. glauca. Cd treatment increased the shoot soluble sugar content in both species, especially that in S. glauca. The above results show that the growth of S. salsa and S. glauca have different responses to salt, Cd, and their interactions. Salt can alleviate the inhibitory effect of Cd on growth, especially the inhibitory effect of Cd on root growth of S. salsa. S. salsa tends to accumulate inorganic osmotic regulators such as Na⁺, whereas S. glauca tends to accumulate inorganic osmotic regulators such as K⁺ and organic osmotic regulators such as soluble sugars to resist salt and Cd stress.