Comparative Effects of Iron Nanoparticles, Chelates, and Iron Sulfate on Biomass, Yield, and Nitrogen Assimilation in Spinach

Foliar application with iron is a promising strategy for improving nitrogen nutrition and productivity in horticultural crops. In this study, the effect of the foliar application of iron oxide nanoparticles (IONPs) compared to conventional iron sources on physiological, biochemical, and productive parameters of Spinacia oleracea L. was evaluated. Plants were treated with different concentrations (0, 25, 50, and 100 ppm) of IONPs, ferric sulfate (FS), and iron chelate (IC). Biomass, yield, nitrate reductase enzyme activity, soluble protein and amino acid contents, SPAD values, and photosynthetic pigments were analyzed. The results showed that IONPs, particularly at 50–100 ppm, promoted significant increases in biomass (50% more than the control), yield (47%), and nitrate reductase enzyme activity (NRmax) (246%) compared to the control (0 ppm) without negatively affecting pigment levels or leaf physiological condition. Likewise, increases in soluble protein and photosynthetic pigment levels were observed, reflecting improved nitrogen assimilation and photosynthetic efficiency. These findings suggest that IONPs represent an efficient and safe alternative to traditional Fe sources, contributing to the development of sustainable agricultural systems aimed at improving the nutritional value and productivity of leafy crops.