Phytotoxicity and cytogenotoxicity screening of cerium in contrasting tropical soils using rapid Petri-dish assays

Cerium (Ce), a rare earth element, can accumulate in agricultural soils through inputs such as phosphate fertilisers, yet effect thresholds for tropical soils remain scarce. Here, we performed a rapid plant-based screening of Ce toxicity in two representative Brazilian tropical soils (Latosol and Cambisol) and an artificial tropical soil (ATS). Seeds of Pennisetum glaucum, Phaseolus vulgaris, Lactuca sativa, and Allium cepa were exposed for 96 h to increasing Ce doses (0-2051.7 mg kg⁻¹) in Petri-dish soil assays. These doses are reported as nominal (spiked) concentrations (post-equilibration concentrations were not measured). Macroscopic endpoints (germination, germination speed index, root and hypocotyl length, and fresh mass) were assessed for all species, and cytogenetic endpoints (mitotic index and nuclear/cytogenetic alterations) were evaluated in A. cepa root tips. Germination-related variables were generally less sensitive than seedling growth. Across endpoints, L. sativa showed the highest sensitivity, and effects were stronger in the Latosol compared with the Cambisol and ATS, indicating soil-dependent toxicity. EC₂₅/EC₅₀ values were obtained from concentration-response modeling, and a screening-level hazardous concentration for 5% of species (HC₅) was derived from a species sensitivity distribution using EC₅₀ values, resulting in HC₅ = 208.4 mg Ce kg^-1. This work provides a rapid screening framework and preliminary effect thresholds for Ce in tropical soils, while acknowledging limitations related to short-term exposure and the restricted set of test species/endpoints.