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

ABSTRACT Efficient foliar delivery of manganese (Mn) remains a major challenge in crop nutrition due to the low phloem mobility of Mn ^{2 +} 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‐MnO ₂ ) 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‐MnO ₂ 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 MnO ₂ (∼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‐MnO ₂ 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‐MnO ₂ 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.