Objectives Selenium contributes to improve resistance ability of plant under environmental stress, and can alleviate the abiotic stresses. Effect of different valences selenium on reducing Cd stress in rapeseed plants was investigated, which could provide a practical reference for cadmium polluted soils.

Methods A pot experiment was carried out using three valences of selenium, they were Selenomethionine Se (–2), selenium Se (+4) and sodium selenite Se (+6), the concentration of selenium was 1 mg/kg soil. Three stress levels of cadmium were designed as 0, 2 and 5 mg/kg. The cadmium in plants was measured by atomic absorption spectroscopy, and the growth and physiological index of rapeseed were determined.

Results Without Cd stress, compare to the CK₀, all the tested Se could significantly promote the rape growth, and the Se (–2) and Se (+4) were superior than Se (+6) in the promoting the biomass, plant height, root length of rape. Under stress of Cd 2 mg/kg, the Se (–2) and Se (+4) significantly reduced the Cd accumulation in shoots of rapeseed by 13% and 5%, while Se (+6) increased that in roots by 25%. Under stress of Cd 5 mg/kg, and compared to the CK₅, the Se (–2) and Se(+4) reduced the Cd contents in the rapeseed roots and shoots by 13%～41%, while the Se (+6) enhanced the Cd accumulation in rapeseed roots and shoots by 38% and 17% respectively. Under Cd stress, application of Se (–2) and Se (+4) increased the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in plants, slightly raised the contents of ascorbic acid (AsA) and glutathione (GSH) in rapeseed leaves, while the Se (+6) reduced antioxidant enzymes activities, and significantly reduced the contents of AsA and GSH in rapeseed leaves.

Conclusions Under cadmium stress, selenium Se (–2) is effective in promoting the growth of rape through inhibiting the cadmium accumulation in the shoots of rapeseed, and improving the physiological activities of the related pretesting enzymes and compounds. Se (+4) shows the same but less significant effect than Se (–2), but Se (+6) shows adverse effect. Therefore, Se (–2) has the potential to reduce the Cd stress in rapeseed and should be considered in the practice use.