Polymer‐Coated Manganese Dioxide Nanoparticles for Foliar Mn Delivery: Mechanisms of Uptake and Metabolic Responses in Mn Deficient Barley

Efficient foliar delivery of manganese (Mn) remains a major challenge in crop nutrition due to the low phloem mobility of Mn² ⁺ ions and the risk of leaf scorching from conventional Mn salts. This study investigates the uptake and assimilation of foliar-applied ∼25 nm polyacrylic acid-coated manganese dioxide nanoparticles (nPAA-MnO₂) in 4-week-old Mn-deficient barley (Hordeum vulgare) and compares them to ionic Mn. Complementary imaging methods showed rapid (2 h) leaf penetration of nPAA-MnO₂ through stomata, facilitated by the formulation´s low surface tension and the addition of 3% glycerol as a humectant. Although ionic Mn displayed higher uptake efficiency than nPAA MnO₂ (∼90% vs ∼11%), the latter restored Mn-dependent photosynthetic functionality without inducing phytotoxicity, even at high Mn concentrations (4 g L^-1). Labeling experiments with tracer ions revealed nPAA-MnO₂ hotspots around vascular bundles and a small (1.9%), yet significant, basipetal translocation of Mn out of the foliar application zone after 4 days, compared to only 0.1% for ionic Mn. By delineating the distinct uptake pathways and metabolic fate of nPAA-MnO₂ versus ionic Mn, these findings provide the mechanistic foundation for developing a novel generation of nano-enabled strategies to address Mn and other micronutrient deficiencies in crops.