Selenium (Se) is an essential element for humans, whereas cadmium (Cd) is a highly toxic heavy metal to both plants and animals. Because of the different charges of Cd (Cd²⁺) and Se (SeO₃^2- / SeO₄^2-), they are taken up by plants through different channels or transporters, so they may not exhibit competition on root surface. However, numerous studies have indicated that Se can regulate Cd uptake into plants. Considering the tested plants, Se forms and different experimental conditions, the results are sometimes paradoxical. Therefore, in this study we investigated the effect of different forms of Se on Cd uptake, transportation and transformation by different plants grown in varied conditions.

The concentration-dependent uptake kinetics of Cd into rice and pak choi roots were affected by Se addition. Compared with that of no Se addition, selenite slightly promoted Cd influx into rice roots, while selenate had no effect. However, both selenite and selenate remarkably enhanced Cd uptake kinetics in pak choi; the maximum Cd uptake rates increased by more than 100%.

Although Se promoted the Cd influx into the roots of plants during short term exposure, Cd translocation from roots to shoots in plants was effectively reduced by Se addition with long time exposure. Se decreased Cd uptake and transportation from rice roots to shoots with prolonged exposure time, but the decreasing degree varied with both Se forms and doses. Selenite was more effective than selenate in decreasing Cd uptake and translocation. As the selenite levels in the treatment solutions increased, Cd uptake and translocation were both significantly reduced, while the inhibiting effect was more significant at lower levels of selenate. Different results were obtained from the vegetable. Regardless of the exposure time or Cd doses, selenite decreased both the translocation of Cd from pak choi roots to shoots, whereas selenate increased Cd translocation.

Cd subcellular compartmentalization (namely, cell walls, organelles, and the soluble fraction), as affected by Se, was also investigated. Selenite addition significantly decreased Cd content in different subcellular fractions and organelles in rice root, while it increased the proportion of Cd distributed to soluble cytosol. When selenate was added, only the Cd distribution ratio of cytosol was increased. As for pak choi, selenite and selenate enhanced Cd sequestration by the shoot cell walls by 20% and 59%, respectively. However, Se caused opposite effects on Cd sequestration on the root cell walls; selenite enhanced it by 9%, while selenate reduced it by 22%, compared to the non-Se treatment.

Generally, selenite and selenate affected Cd accumulation and translocation in plants by altering Cd subcellular distribution or other uncertain mechanisms; such effects depended on the Se forms and doses, and the tested plants.