Migration and transformation of α-Fe₂O₃ in plant and the associated physiological response mechanism

Understanding the size-dependent behavior of iron oxide nanoparticles in crop systems is critical for assessing their environmental risks and application potential. In this study, α-Fe₂O₃ materials with particle sizes of 25 nm, 200 nm, and 400 nm were successfully synthesized and systematically evaluated for their transport, transformation, and physiological effects on rice. Among these, the 25 nm α-Fe₂O₃ exhibited a higher zeta potential, facilitating electrostatic interaction with the negatively charged rice surface. Exposure to α-Fe₂O₃ increased the formation of iron plaque on root surfaces, with the most pronounced effect observed for the 25 nm particles. While iron plaque enhanced the immobilization of metal elements on the root surface, it concurrently hindered the uptake of both metal and non-metal elements by rice roots. Root exudates secretion was stimulated by α-Fe₂O₃ exposure, with larger particles inducing a more pronounced secretory response at equivalent concentrations (TOC increased by 12.91-23.82%). Electron transfer interactions between α-Fe₂O₃ and root exudates occurred in the rhizosphere, leading to the reduction of Fe(III) to Fe(II), large particle size α-Fe₂O₃ induces more root exudates and more pronounced Fe (III)/Fe (II) transformation. Rice growth was promoted by exposure to 50 mg/L of 25 nm α-Fe₂O₃, but inhibited at 500 mg/L. In contrast, larger particles consistently promoted growth, albeit with diminished efficacy at higher concentrations. Exposure to α-Fe₂O₃, particularly the 25 nm particles, triggered significant increases in reactive oxygen species (ROS) levels, plasma membrane permeability, antioxidant enzyme activities, and lipid peroxide content in rice roots. These findings provide a mechanistic basis for understanding the size-dependent phytotoxicity and transformation behavior, and offering insights for its application in controlling pollutant migration in agricultural systems.