Effect of selenium on the uptake kinetics and accumulation of and oxidative stress induced by cadmium in Brassica chinensis

Pak choi can readily accumulate cadmium (Cd) into its edible parts; this can pose a threat to human health. Although not essential for higher plants, selenium (Se) can be favorable for plant growth and antioxidative defense under heavy metal stress conditions. A pak choi hydroponic experiment was conducted to investigate the effect of two forms of Se on the Cd uptake kinetics and accumulation and oxidative stress. The results showed that selenite and selenate remarkably enhanced Cd uptake kinetics in pak choi. The maximum Cd uptake rate increased by more than 100% after treatment with 5 mu M of selenite and selenate, and it further increased after treatment with 20 mu M of both Se forms. The effects of Se on Cd content depended on the Se form, exposure time, and Cd dosage. Selenite reduced the Cd content in shoots by 41% after 3 days of treatment with 10 mu M Cd, whereas selenate increased this rate by 89%. Both forms of Se decreased Cd content in the shoots by 40% after 7 days of treatment with 10 mu M Cd, but they increased the Cd content by approximately 30% after treatment with 50 mu M Cd. Se enhanced Cd-induced oxidative stress in pak choi. Malondialdehyde (MDA) generation was promoted by more than 33% by selenite and selenate treatments in combination with 10 mu M Cd, and it was further enhanced by 106% and 185% at 50 mu M Cd, respectively. Selenite also increased the H2O2 content at both Cd doses, but selenate did not have significant effects on H2O2 production. The effects of Se on antioxidative enzyme activity also depended on the dose of Cd. Selenite and selenate inhibited catalase activity by 11% and 29%, respectively, at 10 mu M Cd, and by 13% and 42%, respectively, at 50 mu M Cd. Moreover, both forms of Se increased superoxide dismutase activity after treatment with 10 mu M Cd but inhibited its activity at 50 mu M Cd. Therefore, Se exhibits dual effects on Cd accumulation and oxidative stress in pak choi and might cause further stress when combined with higher doses of Cd.