Foliar uptake, translocation and its contribution to Cadmium accumulation in rice

Rice may absorb Cadmium (Cd) from the air through its leaves. The process of Cd foliar absorption, accumulation, and redistribution is yet unknown, nevertheless. In this study, the process of Cd absorption from rice leaves and its accumulation and redistribution during all stages of the rice plant's growth were examined. Stable isotope (¹⁰⁸Cd) tracing was used to investigate the distribution of Cd absorbed by leaves in different organs, and a short-term targeted leaf feeding experiment was used to explore the transport pathways and distribution of leaf-absorbed Cd in rice. Leaf-absorbed Cd could be transported to other parts of rice over long distances. Most of it was retained in the leaves and husks, a small part in the grains, and it was rarely distributed to the roots. The contribution of leaf-absorbed Cd to total Cd accumulation was the maximum in the husks (48.96 %-88.24 %), followed by the shoots (23.13 %-44.11 %), grains (22.13 %-24.15 %), and roots (4.57 %-15.21 %). Though the Cd accumulated in grains was predominately derived from root uptake compared with foliar uptake, foliar uptake was not negligible and the contribution rate was >20 %. Additionally, with the short-term fluorescent labeling experiment of leaf-targeted feeding, strong Cd fluorescence signals were observed in the phloem of both labeled and upper leaves, indicating that the Cd retransport process between the above- and belowground parts of rice was very active. Our findings preliminarily revealed the pathway and physiological mechanism of Cd absorption and reuse in rice leaves and provided theoretical support for the formulation of field management policies and control of Cd accumulation in rice. ENVIRONMENTAL IMPLICATION: Previous research indicates that the absorption of soil Cd by rice roots is the main mechanism responsible for Cd accumulation in plants. Therefore, many scholars have proposed various deterrent measures for overcoming soil pollution but neglected the role of leaves in absorbing heavy metals, e.g., Cd, from the atmosphere. Moreover, Cd foliar uptake, accumulation, and redistribution processes are still unclear. Our findings preliminarily revealed the physiological mechanism and pathway of Cd absorption and reuse in rice leaves and provided theoretical support for the formulation of field management policies and control of Cd accumulation in rice.