Spatial partitioning and threshold responses of cadmium accumulation and translocation in Populus × euramericana

Cadmium (Cd) contamination has become one of the most critical soil pollution issues worldwide, with China experiencing particularly severe impacts. Yet, the potential of widely planted trees such as Populus for Cd phytoremediation remains insufficiently understood. Here, an 18-month greenhouse pot experiment was conducted using seedlings of Populus × euramericana exposed to five soil Cd levels (0–100 mg Cd kg−1). Plants were separated into organs (leaf, branch, stem, root), and stems were further partitioned both longitudinally and radially to assess the spatial distribution and translocation of Cd. Cadmium concentrations in plants increased with soil Cd but followed a distinct organ hierarchy, with leaves containing the highest levels, followed by stems, roots, and branches. Within stems, Cd concentrations declined from the base to the top and from bark to heartwood. The bioaccumulation factor (BAF) increased with moderate Cd exposure (up to ~ 10 mg Cd kg−1 soil) but sharply decreased at higher concentrations, revealing a physiological threshold beyond which Cd uptake was strongly inhibited. The translocation factor (TF), defined as the ratio of cadmium concentrations in shoots to roots, showed a similar non-linear response, peaking under moderate Cd levels and stabilizing thereafter, suggesting partial suppression of long-distance Cd transport under severe stress. These findings reveal a clear trade-off between Cd uptake efficiency and ecological stability in Populus, a deciduous tree species, which acts as an efficient Cd removing agent but stores most of the metal in foliage, posing both remediation potential and risk. The findings provide mechanistic insight into Cd uptake and partitioning in trees and inform the potential use of Populus in sustainable soil-remediation programs across Cd-polluted regions.