Manganese Phytoremediation Potential of Koelreuteria paniculata: Detoxification Mechanisms, Chemical Speciation, and Ultrastructural Adaptations

<i>Koelreuteria paniculata</i> demonstrates significant potential for remediating manganese (Mn)-contaminated soils, particularly in mining areas. This study investigated its tolerance and enrichment mechanisms through pot experiments under varying Mn stress (0-15 mmol·L^-1). The results revealed a typical "low-promotion and high-suppression" response, with optimal growth observed at 5 mmol·L^-1 Mn. The species exhibited a strong capacity for Mn accumulation, primarily in the roots (up to 2910.24 mg·kg^-1), though the enrichment factor decreased at higher concentrations. Physiological and subcellular distribution analyses indicated that low Mn levels enhanced chlorophyll content and antioxidant enzyme activities, while excessive stress induced membrane lipid peroxidation. Crucially, tolerance was attributed to effective Mn immobilization in root cell walls (46-76%) and vacuolar compartmentalization in leaves (46-52%), which prevented metal translocation to sensitive organelles. These findings clarify the physiological mechanisms behind Mn tolerance in <i>K. paniculata</i> and support its use in practical Mn phytoremediation.