Pak choi (Brassica chinensis) is a vastly consumed leafy vegetable in China, yet it is also a potential accumulator for cadmium (Cd). Therefore, it is of great urgency to reduce the Cd content in the edible part of the pak choi. There have been numerous reports of selenium (Se) regulating Cd accumulation in plants; however, considering the different experimental conditions, the results are now paradoxical, and the underling mechanisms remain unclear. Some researchers attributed such Se effects to the complexation between selenocompounds and Cd because of the chemical similarity between Se and sulfur (S). This study investigated Se transformation and how Se species affected Cd accumulation in pak choi exposed to either of selenite and selenate for different times under hydroponic conditions.

The addition of both Se forms insignificantly increased the Cd content in shoots with 1 h of exposure before a gradually enhanced reducing effect of selenite was observed. The selenite-induced reduction was maintained comparable after 72 h, and became significant after 96 h; the differences were 4% and 30% at 24 and 168 h, respectively, compared to the Cd content in shoots treated with only Cd. On the other hand, there was no significant in the Cd content between the selenate and Cd alone treatments at any time. Nonetheless, selenate enhanced Cd accumulation in shoots by approximately 1/4 at both 72 and 96 h, and began to dismiss it by no more than 1/5 after 120 h of exposure, which was 2 d later than selenite did.

Five Se species were discovered in pak choi; because of the distinct accumulation capacity of selenite and selenate, the total Se content and that of every Se species in selenate-treated plants were much higher than those in selenite-treated ones. However, after 24 h, organic Se such as selenomethionine (SeMet) and Se-methylselenocysteine (SeMeCys) accounted for roughly 3/5 in shoots and 9/10 in roots of selenite-treated plants, which were much higher than those in selenate-treated plants where SeO₄^2- was of 70% in shoots and 30% in roots. A substantial increase in proportions of organic Se species and a sharp decline in that of SeO₄^2- were found under both Se treatments, and Se transformation into seleno amino acids was in accordance with the dynamics of Se affecting Cd accumulation in the shoot.

In conclusion, Se could reduce Cd accumulation in the edible part of pak choi after certain times; the selenate-induced reduction was exhibited much later and less significant than the selenite-induced one. The different effects of the two Se forms applied should be attributed to their transformation in the plant. Most of the selenate taken up remained as SeO₄^2-, whereas selenite in the plant was readily assimilated to seleno amino acids; SeMet was invariably the dominant organic species whose content showed the greatest increase among those of the five species detected over the exposure times. Hence, Se effects on Cd accumulation could depend on the exposure times, where SeMet and SeO₄^2­- contents, their relative proportions, and Se transformation were also involved in the Se-mediated regulation.