Nano-delivery of selenium and iodine improves spinach biofortification

Abstract Selenium (Se) and iodine (I) are essential trace elements whose deficiencies remain a major global health concern. Agronomic biofortification of leafy vegetables offers an effective strategy to address this issue, although conventional ionic fertilizers are limited by nutrient losses and inconsistent performance. This study evaluated whether chitosan nanoparticle (ChNP; chitosan-based nanoparticles used as a foliar carrier)-complexed Se and I improve biofortification efficiency and yield stability of spinach (Spinacia oleracea L. cv. ‘Pawnie F1’) compared with selenate and iodide. A field experiment in a randomized complete block design was conducted across three growing seasons in 2025 (spring, summer, autumn) in the North European Plain. Fresh biomass yield was highest in spring (1.62 kg·m⁻2) and lowest in summer (1.17 kg·m⁻2). Under less favourable summer and autumn conditions, ChNP treatments maintained more stable yields than ionic forms, indicating their biostimulant effect. Both ChNP and conventional treatments significantly increased Se and I concentrations in leaves; however, ChNPs showed higher fertilization efficiency, particularly under stress conditions. Dual Se-I application via ChNPs did not reduce the efficiency of either element. PCA identified growing season as the main source of variability (PC1, 36.5%), while PC2 (27.5%) reflected biofortification; ChNP treatments formed a distinct cluster associated with higher yield and nutrient content. Nitrate and nitrite levels remained within EU limits, and MSDI values confirmed safety for all consumer groups. These results provide the first field-based, three-season evidence supporting ChNPs as effective tools for simultaneous Se and I biofortification, highlighting their potential in functional food production. Averaged across seasons, ChNP-based formulations increased fertilization efficiency for Se by approximately 41% and for I by approximately 34% relative to ionic salt formulations. Key points • ChNPs outperform ionic salts in Se and I fertilization efficiency under seasonal stress. • Simultaneous Se-I biofortification via ChNPs is safe for all consumer age groups. • Spring cultivation maximizes spinach yield; ChNPs buffer yield under suboptimal seasons. Graphical Abstract