Zinc toxicity and phytoremediation potential of Oleander: How silicon can help?

Soil contamination by heavy metals, such as zinc, has significant environmental consequences. Phytoremediation, among various remediation techniques, has been developed and applied to restore contaminated soils. However, phytoremediation has limitations related to slow plant growth, influenced by contaminant toxicity. This study aims to investigate the effect of the interaction between zinc and silicon on growth, hydration, photosynthetic and biochemical behaviours, as well as the phytoremediation capacity in oleander ( Nerium oleander ), a promising species for phytoremediation. Oleander plants were exposed to four treatments, including two adequate zinc treatments (0.76 µM Zn) with two different silicon concentrations (0 mM and 0.5 mM Si) and two zinc toxic treatments (1800 µM Zn) with the same silicon concentrations (0 mM and 0.5 mM Si). The results revealed that zinc toxicity negatively affected most of the measured parameters. However, the depressive effects of zinc toxicity were significantly mitigated under silicon treatments. Adding silicon alleviated leaf chlorosis and improved biomass production, water status, photosynthetic pigment contents, photosynthetic gas exchange, oxidation state of photosystem I (PSI), membrane integrity, soluble protein content, and antioxidant enzyme activity (especially guaiacol peroxidase). Overall, silicon had beneficial effects on the phytoremediation capacity of N. oleander . Therefore, fertilization rich in silicon could represent an effective solution for enhancing the phytoremediation capacity of this species, minimizing one of the major disadvantages of phytoremediation, namely low biomass production influenced by toxicity.