Effects of Cd on growth and low molecular mass thiols of Salix matsudana Koidz seedlings

Contamination of soil and water with cadmium (Cd) is a widespread and serious environmental problem. Cd is a non-essential element which is the most toxic when dispersed in the environment. Plants have evolved many mechanisms to tolerate and detoxify metals to reduce their cellular toxicity, and plants can produce low molecular mass thiol compounds that involve in tolerance and detoxification to toxic metals.Currently, willows (Salix spp.) with large biomass are high-metal accumulators which have been widely used in phytoextraction of heavy metals such as Cd. However, compared to the hyperaccumulators and some herbaceous plants, it remains unknown that willows can tolerate and detoxify Cd. Salix matsudana Koidz is an economically pioneer tree which is native to China, commonly planted on riverbanks and flooding plains. The objectives of this study were: 1) To investigate Cd tolerance and uptake in S. matsudana; 2) To evaluate thiol compounds in responses to Cd.Seedlings of S. matsudana were grown in hydroponic culture exposed to various Cd levels (0, 5, 25, 50, and 100 μmol/L) in a greenhouse for 28 d. The growth parameters were measured including relative growth rate (RGR) and biomass production. The EC₅₀ (50% effective concentration, EC₅₀) based on root, shoot (including stem and leaf) and whole plant biomass was calculated, respectively. Thiol compounds were assayed, i. e., non-protein thiols (NPTs), protein thiols (PTs), total thiols (TTs), cysteine (Cys), reduced glutathione (GSH) and oxidized glutathione (GSSG).The results showed that RGR of height significantly decreased at Cd≥25 μmol/L. Biomasses of shoot, root and whole plant decreased with increasing Cd concentrations in the medium. Root biomass was significantly reduced at Cd≥ 5 μmol/L, while shoot biomass was reduced at Cd≥ 25 μmol/L compared with the control. Tolerance indexes (based on root, shoot and whole plant, respectively) were decreased with increasing Cd concentrations in the medium. EC50 based on root biomass reached 82 μmol/L. Cd concentrations in shoots and roots were elevated with increasing Cd concentrations in the medium, and Cd accumulated mainly in roots. Cd concentration in shoots ranged from 64.81 to 163.13 μg/g exposed to 5-25 μmol/L Cd, and plant growth was not severe disturbed. The contents of NPTs, PTs and TTs in roots and leaves were induced strongly by Cd exposure, and their contents increased with increasing Cd concentrations in the medium. Compared with the control, high Cd (100 μmol/L) enhanced an increase in Cys content in roots; in contrast, Cys contents in leaves were not markedly altered at 5-50 μmol/L Cd, but reduced significantly at 100 μmol/L Cd. GSH contents increased in roots but showed no significant alteration in leaves; GSSG contents of roots and leaves were reduced, and GSH/GSSG ratios were increased compared with the control.In conclusion, the present results show that the plants are not able to cope with Cd stress displaying phytotoxic effects exposed to high concentrations of Cd. Cd treatments induce markedly changes in thiol compounds, and the results further suggest that the thio-l based mechanism may contribute to Cd tolerance and detoxification in S. matsudana.