Tree Peony as an Efficient Organic Selenium Bioreactor: Selenium Uptake, Accumulation, Speciation, and Nutritional Enhancement via Foliar Sodium Selenite Application

Selenium (Se) is an essential micronutrient for human health, yet its deficiency remains prevalent worldwide. Biofortification through foliar Se application is an effective strategy to enhance Se levels in crops. Paeonia ostii ‘Fengdan’ is a multifunctional woody plant with potential for Se enrichment, though its Se uptake and transformation mechanisms remain unclear. This study systematically investigated the effects of foliar-applied Na2SeO3 (0–200 mg L−1) on Se uptake, accumulation, speciation, and nutritional quality in tree peony. Results showed that Se uptake increased with higher Na2SeO3 concentrations, displaying a clear dose-dependent pattern across all organs. Se accumulation significantly enhanced, with a pronounced shift in distribution towards above-ground organs under experimental conditions. Notably, tree peony exhibited strong biotransformation capacity, converting over 73% of Se in leaves and over 81% in seeds into organic forms, primarily SeCys2 and SeMet, with minor MeSeCys. Comprehensive evaluation indicated that 100 mg L−1 Na2SeO3 yielded optimal results, significantly enhancing leaf and seed biomass, increasing seed nutrient contents (soluble proteins, sugars, phenolics), and improving the unsaturated fatty acid profile of seed oil. These findings highlight tree peony’s potential as an efficient bioreactor for organic Se and provide a theoretical foundation for developing Se-enriched products from tree peony.