Mechanisms of lead uptake and accumulation in wheat grains based on atmospheric deposition-soil sources

Lead (Pb) accumulation in wheat grains depends on two aspects: i) Pb uptake by the roots and shoots, and ii) the translocation of organ Pb into the grain. However, the underlying mechanism of the uptake and transport of Pb in wheat remains unclear. This study explored this mechanism by establishing field leaf-cutting comparison treatments. Interestingly, as the organ with the highest Pb concentration, only 20.40 % of the root's relative contribution to grain Pb. The relative contributions of the spike, flag leaf, second leaf, and third leaf to grain Pb were 33.13 %, 23.57 %, 13.21 %, and 9.69 %, respectively, which was opposite to their Pb concentration distribution trends. According to Pb isotope analysis, it was found leaf-cutting treatments reduced the proportion of atmospheric Pb in grain, and grain Pb predominantly comes from atmospheric deposition (79.60 %). Furthermore, from the bottom to the top, the concentration of Pb in internodes decreased gradually, and the proportions of Pb originating from soil in the nodes also decreased, revealing that wheat nodes hindered the translocation of Pb from roots and leaves to the grain. Therefore, the hindering effect of nodes on the migration of soil Pb in wheat resulted in atmospheric Pb having a more convenient pathway to the grain than soil Pb, and further leading grain Pb accumulation primarily depended on the contribution of the flag leaf and spike.