Contrasting Fate of Thallium and Cadmium in the Soil-Rice System: Convergent Rhizosphere Bioavailability Responses Yet Divergent Translocation and Grain Partitioning.

Combined contamination of soils with thallium (Tl) and cadmium (Cd) is widespread. Despite comparable accumulation of Tl and Cd by rice plants, the mechanisms underlying significantly lower Tl concentrations than Cd in grains remain unclear. Here, pot experiments, rhizotron observations and field sampling were combined to investigate the fate of Tl/Cd across the soil-rice system. Sequential extraction shows that flooding decreased Tl/Cd mobility in bulk soil, with Cd enriched in the oxidizable fraction and Tl in the residual fraction. In the rhizosphere, planar optode and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) reveals that flooding induced iron-plaque formation, restricting rice Tl/Cd uptake compared with continuous drainage. Tissue- and microscale-level (LA-ICP-MS) analyses show that Tl transfer from node-to-leaf was 3.00-8.85 times that of Cd which exhibited greater upward translocation. In brown rice, 66.0-86.0% of Cd transported to grains was allocated to the endosperm. In contrast, Tl was preferentially enriched in the embryo, which represented only 1.78-2.66% of the biomass but contained 42.9-60.8% of Tl, potentially associated with high K demand and K-related transport pathways. Thus, contrasting shoot translocation and brown rice partitioning patterns of Tl/Cd explain differential grain concentrations, offering mechanistic insights for safe rice production.