Cadmium found in peanut (Arachis hypogaea L.) kernels mainly originates from root uptake rather than shell absorption from soil

The roots and pod shells both located in the soil are two potential organs through which peanuts absorb Cd from soils; however, the relative contribution of the two uptake pathways to kernel Cd accumulation and their translocation characteristics are poorly understood. In this study, the relative contribution of the two pathways to Cd accumulation in two peanut cultivars (cv. Xianghua2008 and Yueyou43) was accurately assessed by labeling rooting and podding zone soils with 113Cd and 111Cd isotopes (0.3 mg kg-1 dry soil), respectively, in a split-pot unit. The results showed that approximately 96% of the Cd that accumulated in the peanut kernels was derived from root uptake, while only 4% originated from pod shell absorption. Only 1% of the Cd that accumulated in whole peanut plants was attributed to pod shell absorption from soil, of which 41–44% was retained in shells and 56–59% was translocated to kernels. In contrast, the Cd absorbed by roots was efficiently translocated into all plant organs, of which 80–84% was distributed in shoots. Although Yueyou43 accumulated 1.3 times more of Cd in whole plants than Xianghua2008, the relative contribution of the two pathways to Cd accumulation in each plant organ was barely affected. Due to the strong retention effect of pod shells, shell-derived Cd was approximately 2 times higher than root-derived Cd in shells. These results have improved the understanding of Cd accumulation processes in peanuts, revealing that the root uptake pathway contributes predominantly to the Cd content in peanuts (kernels), based on which strategies and technology for the reduction of Cd in peanuts should be designed and developed.