High Selenate Doses Suppress Selenomethionine Formation in Chicory, Rocket, and Dandelion Leaves

Selenium (Se) biofortification of vegetables can improve dietary Se intake; however, the dose-dependent balance between inorganic Se retention and organic Se assimilation following foliar selenate application remains insufficiently resolved across species. Five leafy vegetable species (garden rocket, wild rocket, dandelion, and two chicory cultivars) were grown under controlled greenhouse conditions and treated twice with foliar sodium selenate at increasing application rates (1 + 1, 2 + 2, 5 + 5, 10 + 0, 10 + 10, and 10 + 50 mg Se L−1) across two experiments. Total Se and Se species were determined by HPLC-UV-HG-AFS following enzymatic extraction and cross-checked on selected extracts by HPLC-ICP-MS. Foliar selenate induced substantial Se accumulation in all species, reaching up to 102 µg g−1 DW in garden rocket. At moderate application rates (notably 2 + 2 and 5 + 5 mg Se L−1), a considerable proportion of extracted Se was converted into organic forms, with selenomethionine (SeMet) accounting for up to ~40% of total extracted Se. In contrast, at the highest application rate (10 + 50 mg Se L−1), inorganic Se(VI) became predominant (often >40%), while SeMet proportion declined sharply to ~2–4%, indicating a saturation of metabolic assimilation capacity under high Se exposure. Leaf biomass was promoted at intermediate treatments (e.g., 5 + 5 and 10 + 0/10 + 10 mg Se L−1), whereas the highest rate reduced growth. Overall, foliar selenate effectively biofortifies chicory, rocket, and dandelion leaves, but excessive application rates shift Se speciation toward inorganic storage and markedly suppress SeMet formation. These findings highlight the importance of dose optimization to maximize nutritional quality while avoiding metabolic overload.