Silicon attenuates cadmium toxicity in Solanum nigrum L. by reducing cadmium uptake and oxidative stress

Solanum nigrum L. is considered to be a potential plant for restoring Cd-contaminated soils. Si could enhance plants tolerance to heavy metal; however, the mechanism of Si-mediated alleviation of Cd toxicity in S. nigrum was not clear. Three-week-old S. nigrum seedlings were grown in Hoagland solution containing 0 or 100 μM Cd with or without 1 mM Si for 4 days. The results showed that the Cd concentration both in roots and shoots of Si-supplied plant was significantly reduced, especially in expanding and old leaves. The relative proportion of ethanol-extractable Cd, water-extractable Cd and NaCl-extractable Cd in roots was increased by adding Si, while the root-to-shoot Cd translocation was not decreased. Furthermore, in comparison with single Cd treatment, supplying Si could reduce H₂O₂ accumulation and cell death in roots, and the electrolyte leakage and H₂O₂ concentration in functional leaves. Moreover, the activity of superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7) and ascorbate peroxidase (APX, EC 1.11.1.11) in functional leaves was markedly increased by Cd exposure, while the antioxidative enzyme activities in Cd plus Si treatment seedlings were significantly lower than that in Cd treatment alone, this decrease might be attributed to the reduction of Cd concentration and Cd-induced oxidative damages. These results demonstrate that Si-enhanced Cd tolerance in S. nigrum is mainly due to the decrease of Cd uptake in roots and Cd distribution in expanding and old leaves, as well as lowering oxidative stress induced by Cd in plants.