Abstract
Cadmium (Cd) is present in soil and water at low concentrations that can be harmful to the biota. A Cd-tolerant plant species named ramie (Boehmeria nivea (L.) Gaudich) was applied to probe the precise Cd tolerance mechanism with different treatment duration and the up-regulation effect for antioxidant capacity by spermidine (Spd) pretreatment. The results showed that short-term (0-7 d) Cd stress resulted in the enhancement of pigment content, the accumulation of soluble sugar and the activation of partial antioxidants, meanwhile reducing the level of malondialdehyde (MDA), which is the indicator of oxidative stress. Spd pretreatment markedly increased the soluble protein, soluble sugar and reduced glutathione (GSH) content in ramie leaves under short-term Cd stress, whereas, it showed no significantly beneficial effects on photosynthesis and other antioxidants. Long-term (0-15 d) Cd stress may lead to growth inhibition which is associated with Cd accumulation, protein oxidation, sugar degradation, MDA surge and antioxidant inactivation. Spd application markedly alleviated long-term Cd toxicity by decreasing Cd accumulation, stabilizing cellular macromolecules such as pigments, soluble protein and sugar and inhibiting peroxidase (POD), catalase (CAT) and malate dehydrogenase (MDH) inactivation and GSH degradation. Our present work concluded that ramie could tolerate moderate Cd stress during a certain time span. Spd application could play a vital role in the promotion of Cd tolerance by adjusting different kinds of parameters.