Concentration-dependent transfer and phytotoxicity of iodine and rhenium in lettuce (Lactuca sativa L.) with...

The long-term geological disposal of radioactive waste necessitates robust risk assessment models to accurately predict radionuclide transfer from contaminated growing media into the human food chain. Iodine-129 (¹²⁹I) and the rhenium isotopes ¹⁸⁶Re and ¹⁸⁸Re are of significant regulatory concern due to their anionic mobility, environmental persistence, and high bioavailability. However, current risk assessment frameworks typically rely on the assumption of constant, concentration-independent transfer factors (TFs), a foundational premise that remains largely untested across gradients spanning trace levels to acute phytotoxicity. Through controlled greenhouse experiments, we quantified the concentration-dependent uptake and physiological responses of iodine (0, 50, 100, 200 mg kg^-1) and rhenium (0, 5, 10, 20 mg kg^-1) in lettuce (Lactuca sativa L.). Iodine reached a lethal threshold at 200 mg kg^-1 (EC₅₀: 58.9 mg kg^-1), following a classical monotonic response. Conversely, rhenium exhibited significantly higher phytotoxic potency (EC₅₀: 19.7 mg kg^-1) and a distinct biphasic hormetic response. Concentrations of 5-10 mg kg^-1 stimulated shoot biomass and canopy density, whereas 20 mg kg^-1 induced severe growth suppression. Crucially, TFs for both elements were concentration-dependent, indicating that the constant-TF assumption does not hold under the present conditions. Rhenium demonstrated exceptional mobility with shoot-to-root ratios of 22-52, indicating rapid translocation to edible tissues. In addition, a scenario-based radiological assessment using literature-reported ¹²⁹I soil activities indicated low screening-level ingestion doses under the modeled conditions. These findings provide vital empirical parameters for refining predictive environmental models and establishing evidence-based soil screening levels supporting human health and radiological risk assessment of radionuclide transfer through crop systems.