Exogenous Application of Silicon and Spermidine Alleviate Cadmium Toxicity by Modulating Endogenous Polyamines Level and Glutathione Metabolism in Salix matsudana Koidz. Seedlings

Cadmium (Cd) is a pervasive phytotoxic metal which deteriorates soil quality, affecting crops and creating adverse effects on the environment, food safety, and human health. Cd in soil poses negative effects on plants at the physiological, structural, and molecular level. Application of silicon (Si) can reduce Cd accumulation by suppressing Cd uptake in plants, while spermidine (Spd) alleviates Cd toxicity through improved antioxidant capacity. However, their combined effects on antioxidant system and endogenous polyamines (PAs) level in Cd-stressed plants and the underlying antioxidative defense mechanism are poorly understood. Salix matsudana Koidz. is a fast-growing tree species with high Cd tolerance, making it potentially suitable for phytoremediation. Here, the S. matsudana seedings were subjected to 50 μM Cd stress with or without addition of 1.5 mM sodium silicate and 0.1 mM Spd. Following that, the non-enzymatic/enzymatic antioxidants, stressed-related genes and endogenous PAs levels were determined. The results showed that Cd stress suppressed the growth traits of S. matsudana while increasing reactive oxygen species (ROS) and malondialdehyde (MDA) accumulation in the leaves, which also showed heightened Cd levels. However, exogenous application of Si and Spd increased activities of antioxidative enzymes and ameliorated the Cd-induced oxidative damage. Moreover, combined treatment with Si and Spd showed higher glutathione (GSH) and GSH/GSSH (oxidized glutathione) ratio compared to their individual applications. The results provided sufficient evidence regarding the synergistic effect of Si and Spd in the amelioration of Cd-induced oxidative stress in S. matsudana seedlings.