Foliar application of citrate-stabilized iron oxide nanoparticles modulates physiological responses of Spinacia oleracea L. under cadmium stress.

Cadmium contamination in agricultural soils threatens plant growth, food safety and human health, highlighting the need for strategies that mitigate metal stress while maintaining crop productivity. In this study, we optimized and evaluated trisodium-citrate-stabilized maghemite iron oxide nanoparticles as a foliar nanofertilizer under agronomically relevant conditions. Nanoparticles were characterized in terms of size, colloidal stability, and Fe content, yielding a stable aqueous suspension with a hydrodynamic diameter of ~ 60 nm and 60 ppm Fe, compatible with typical field-scale foliar applications. Spinach plants were cultivated in uncontaminated soil or in soil contaminated with cadmium at the maximum concentration permitted by Italian legislation (5 mg kg-1) or at twice this level (10 mg kg-1). Foliar treatments included citrate-stabilized iron oxide nanoparticles, a citrate-only solution, distilled water and a non-sprayed control, allowing the effects of the nanoparticles formulation to be distinguished from those of the carrier and spraying procedure. Cadmium reduced total plant dry biomass by approximately 20% and 50% at the two contamination levels, respectively, while the root/shoot ratio declined by 64% and 88%. After 6 weeks of treatment, FeO@TCD markedly improved plant physiological performance under moderate Cd stress, increasing stomatal conductance to water vapor by up to 113% relative to non-sprayed plants and enhancing net photosynthetic rate by 61%, whereas biomass was not significantly affected. FeO@TCD also increased saturated water content and promoted a more negative turgor loss point under moderate Cd stress. Overall, these findings demonstrate that foliar application of citrate-stabilized iron oxide nanoparticles alleviated physiological impairment caused by cadmium, particularly under moderate stress, and provide a framework for evaluating nanofertilizer-based approaches under realistic agronomic conditions.