Accumulation and fractionation of rare earth elements (REEs) in wheat: controlled by phosphate precipitation, cell wall absorption and solution complexation.

Previous studies on rare earth element (REE) bioaccumulation have focused on their accumulation rate and fractionation, but the processes involved remain unclear. In this study, the accumulation and fractionation of REEs in wheat (Triticum aestivum L.) were investigated using solution culture with exogenous mixed REEs. A decrease in REE contents was observed from the roots to the tops of wheat. Significant fractionations of REEs were found in wheat organs as compared to the exogenous mixed REEs. Middle REE (MREE, the elements from Sm to Gd) enrichment and an M-type tetrad effect (an effect that can cause a split of REE patterns into four consecutive convex segments) were observed in the roots, which were probably caused by phosphate precipitation of REEs in/on the roots and absorption of REEs to root cell walls. Light REE (LREE, the elements from La to Eu) and heavy REE (HREE, the elements from Gd to Lu) enrichments were observed in the stems and leaves, respectively, accompanied by conspicuous W-type tetrad effects (an opposite effect to the M-type tetrad effect) in the REE patterns. HREE enrichment decreased from the older to the younger leaves and increased upwards within a single leaf. It is suggested that the solution complexation that occurred in the xylem vessels plays an important role in REE fractionations in the above-ground parts of wheat.