Systematic study on the selenium enrichment mechanism and regulatory strategies of selenium-enriched navel oranges

Selenium (Se) biofortification is a key strategy to enhance the nutritional value of agricultural products. This study systematically investigated the mechanisms and strategies for Se enrichment in Gannan navel oranges. Despite total soil Se (0.402–0.905 mg/kg) meeting the Se-rich standard, the low bioavailability of soil Se (available Se accounted for only 10–19%) resulted in negligible natural Se accumulation in pulp (mean: 0.00201 mg/kg). Foliar application of selenium nanoparticles (SeNPs) effectively overcame this limitation. A SeNPs spray during fruit enlargement significantly increased pulp Se content to 0.0165 mg/kg, a 9.5-fold increase, meeting the local standard for Se-enriched citrus. The organic Se proportion in pulp remained high and stable (78.1–79.8%). Se distribution followed a distinct gradient: leaves > stems > peel > pulp. Furthermore, significant correlations were observed between Se and 12 mineral elements. This study provides a theoretical basis and practical strategy for the quality production of Se-enriched navel oranges.