Effect of Hydroponically Supplied Selenium Forms on Cadmium and Nickel Accumulation in Rice

Rice (Oryza sativa L.) farmers face challenges with metal accumulation in grain, with nickel (Ni) recently emerging as a concern due to its potential to exceed legal limits, alongside cadmium (Cd). Information on Ni behaviour and its interaction with Cd remains limited. Selenium (Se) is commonly used for rice biofortification and can reduce the accumulation of toxic metals in plants. Therefore, this study investigates how Ni and Cd influence mutual accumulation in rice and examines the impact of different Se forms on their interactions. Plants were grown hydroponically with various combinations of Cd (5 or 20 μmol/L), Ni (20 μmol/L), and Se (5 μmol/L) as selenate (Se⁶⁺) or selenite (Se⁴⁺) for 7 d. Plant growth, lipid peroxidation, and element accumulation were measured, and the distribution of Se and Ni in tissues was assayed using synchrotron-based μXRF 2D imaging. Cd and Ni were toxic to rice, reducing leaf and root biomass by 40%‒50% and inducing oxidative stress. However, their combined presence did not further exacerbate leaf growth reduction. Cd reduced root Ni accumulation by approximately 50% at equimolar concentrations, likely due to competitive inhibition at shared transport sites. Se promoted root growth in the presence of Ni and low Cd, suggesting an antioxidant role in mitigating metal-induced stress. However, high doses of Ni and Cd together significantly reduced Se accumulation (by 60% and 77% for Se⁴⁺ in roots and Se⁶⁺ in leaves, respectively) and caused severe oxidative stress in the presence of Se⁴⁺. The effectiveness of Se biofortification varied depending on the Se form: Se⁶⁺ was more effective at reducing Ni accumulation, while Se⁴⁺ effectively reduced Cd accumulation (by 45%‒75%) at low concentrations and Ni accumulation in the absence of Cd (by 50%). In conclusion, this study demonstrates that Se can mitigate Cd and Ni accumulation in rice. However, the co-presence of Cd and Ni may compromise Se enrichment in rice, highlighting the complexity of their interactions.