Transport properties and accumulation patterns of trivalent chromium in rice: A hydroponic and modeling approach

Although rice is the staple food of the majority of the human population worldwide, rice consumption is a significant pathway of human exposure to chromium (Cr). However, the primary pathways and periods of accumulation of Cr(III), the main form of Cr in rice, remain unexplored. Hence, in this study, we conducted experiments involving foliar application of Cr(III) and hydroponic Cr(III) supplementation at various growth stages of rice. The results showed that the phloem translocation rate of Cr(III) was lower than that of strontium, which is a phloem-immobile element. The Cr(III) concentration in leaf phloem sap drastically decreased soon after Cr(III) supply was stopped in hydroponic culture solutions, indicating rapid sequestration and compartmentalization of Cr(III) in the leaves. The filling and heading stages were identified as the critical Cr(III) accumulation periods for rice grains and whole rice plants, respectively. According to our model calculations, 80 % of Cr(III) that accumulated in rice grains is transported via the xylem, while the remaining 20 % is remobilized from nutrient tissue. Moreover, Cr(III) and Fe(III) exhibited similar remobilization and accumulation patterns in various parts of rice plants. These findings highlight the importance of implementing soil Cr immobilization during the filling stage to ensure the safe production of rice.