Mycorrhizas drive stabilization of leaf manganese speciation to restore photosynthesis and alleviate toxicity in trifoliate orange

• AMF alleviated Mn toxicity by converting active (ethanol/water-extractable) Mn to stable (NaCl/HAc-extractable) forms. • Detoxification involves synergistic root remodeling and photosynthetic protection. • AM fungi alleviate Mn toxicity via species-specific mechanisms. • Rhizophagus intraradices is the optimal strain for Mn stress alleviation. • Fungal-specific effects highlight the need for tailored AMF inoculant selection. Manganese (Mn) toxicity in acidic soils poses a serious threat to sustainable crop production, especially in sensitive crops like citrus. However, whether and how arbuscular mycorrhizal (AM) fungi enhance host plant Mn tolerance through species-specific mechanisms remain unclear. This study investigated the effects of four distinct AM fungal species ( Rhizophagus intraradices, Funneliformis mosseae, Paraglomus occultum , and Diversispora epigaea ) on the growth, root architecture, gas exchange, chlorophyll content, and six chemical forms of Mn in trifoliate orange under 12 weeks of Mn stress (20 mmol/L MnSO 4 ). All AM fungi promoted plant growth and biomass production, with R. intraradices exhibiting the most pronounced effect. Under Mn stress, root colonization rates ranged from 55.7% to 72.0% and hyphal length from 17.7 to 24.9 cm/g across fungal species, with R. intraradices exhibiting the highest root colonization rate and hyphal length. AM fungi mitigated Mn toxicity through a coordinated mechanism: (i) optimizing root architecture (total root length increased by 20–106%); (ii) shifting leaf Mn speciation from highly active, toxic forms (ethanol- and water-extractable) to stable, inert forms (NaCl- and HAc-extractable), along with stable Mn increasing by 78–172% and active Mn reducing by 45–80%; (iii) enhancing net photosynthetic rate by 37–282% and total chlorophyll content by 175–479%. Correlation analysis further confirmed strong links among high colonization, reduced active Mn forms, and improved photosynthesis. Therefore, AM fungi alleviate Mn toxicity in a species-specific manner via a multi-level pathway centered on Mn speciation shift, with R. intraradices identified as the optimal strain. These findings provide a mechanistic basis for precision AMF inoculation in Mn-contaminated citrus orchards.