The optimum Se application time for reducing Cd uptake by rice (Oryza sativa L.) and its mechanism

The inhibitory effect of selenium (Se) on cadmium accumulation in brown rice (BR-Cd) is controversial, which may be related to the Se application time. The optimum Se application time for reducing the BR-Cd is investigated. A pot experiment was conducted to examine the changes in the dynamics of iron plaque on root surface and Cd accumulation in rice as Se was applied at different growth stages to two Cd-contaminated soils, and explore its mechanisms. Field trials were conducted to verify the results. Iron plaque formed mainly at the middle stage (tillering to booting stage) of rice and then decreased at the mature stage. Se application at the seeding, tillering, and booting stages reduced the BR-Cd by 10.6-18.4%, 14.9-23.7%, and 10.8-20.9% in the neutral soil, 18.3-32.4%, 40.3-63.0%, and 22.7-40.6% in the acid soil, respectively. Two field trials demonstrated that tillering application of Se caused significant reductions (27.1-35.1%) of BR-Cd. Tillering stage is optimal for Se application to reduce BR-Cd, mainly because tillering application of Se causes the maximum amount iron plaque at the booting stage, maximumly inhibit Cd uptake by the root; meanwhile, the transport of Cd from the root to brown rice is inhibited.