Elucidating the mechanisms for plant uptake and in-planta speciation of cerium in radish (Raphanus sativus L.) treated with cerium oxide nanoparticles
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2016 Elsevier Ltd. CeO 2 NPs dissolution was enhanced by the low molecular weight organic acids in root exudates and were taken up by radish as both nanoparticles and dissolved ions. Even though the plant uptake of cerium oxide nanoparticles (CeO 2 NPs) has been reported, the mechanisms remain unknown. This study aimed to provide new insights into CeO 2 NPs plant uptake through two objectives: (1) to investigate whether CeO 2 NPs dissolute before their plant uptake and (2) to determine the in-planta speciation of Ce. Bench scale experiments were conducted by growing radish in solutions containing 10mg elemental Ce/L of bulk CeO 2 particles, CeO 2 NPs or ionic Ce. Transmission electron microscope and inductively coupled plasma-mass spectrometry (ICP-MS) analysis suggested that one pathway for CeO 2 NPs uptake was through direct uptake of intact CeO 2 NPs. More importantly, our results confirmed that part of the particulate CeO 2 was transformed into ionic Ce on the root surface before they were taken up by plants. Ionic Ce uptake and transport was a primary mechanism for Ce accumulation in plant shoots. This study further demonstrated that enhanced CeO 2 dissolution on root surface was due to the organic acids with lower molecular weight (e.g. succinic acid) in radish root exudates. Large particles composed of high contents of P and Ce were detected in radish roots treated only with ionic Ce, suggesting the formation of CePO 4 particles. In summary, the results indicated that CeO 2 NPs was taken up by radish as both intact nanoparticles and dissolved ions. Inside plant tissues, Ce is present as a cocktail of CeO 2 NPs, dissolved Ce and Ce salt (e.g. CePO 4 ) and the specific combination of Ce species is tissue dependent.
