Efficacy of Arbuscular Mycorrhizal Fungi in Alleviating Manganese Stress in Trifoliate Orange

Manganese (Mn) toxicity, commonly triggered by soil acidification, poses a significant threat to citrus production. Arbuscular mycorrhizal (AM) fungi can alleviate heavy metal stress, while their specific function and quantitative effectiveness in conferring Mn tolerance to citrus remain unclear. This study investigated the physiological regulation conferred by four AM fungal species, Rhizophagus intraradices (Ri), Funneliformis mosseae (Fm), Paraglomus occultum (Po), and Diversispora epigaea (De), on trifoliate orange (Poncirus trifoliata L. Raf.) under Mn stress. Mn toxicity reduced root colonization in a species-dependent manner, significantly lowering colonization by all AM fungal isolates except Fm. It also severely inhibited plant growth and induced pronounced oxidative damage, accompanied by metabolic imbalance. Under Mn-stressed conditions, AM fungal inoculation, especially Ri, significantly enhanced plant biomass relative to the non-AM control, with respective increases of 148% in leaves, 33% in stems, and 64% in roots, demonstrating a marked species-specific efficacy. Furthermore, AM symbiosis effectively promoted chlorophyll index and limited Mn translocation to the leaves under both non-stress and Mn-stress conditions, with Ri being the most effective in reducing leaf Mn content. Symbiosis with AM fungi, particularly Ri, fine-tuned the antioxidant enzyme defense under Mn stress by selectively suppressing superoxide dismutase and peroxidase activities while further boosting catalase activity. Concurrently, AM fungi alleviated Mn-induced oxidative damage, with the magnitude of mitigation varying by species: Ri delivered the most comprehensive protection, most effectively reducing hydrogen peroxide and malondialdehyde levels in both leaves and roots, whereas Po was particularly effective in suppressing root superoxide anion radical and malondialdehyde levels in roots. Furthermore, AM fungi reversed Mn-induced shifts in organic osmolytes: they significantly reduced the excessive accumulation of soluble sugars and proline while mitigating the loss of soluble proteins, thereby assisting in restoring metabolic homeostasis. The alleviative effects varied significantly among AM fungal species, with Ri identified as the most efficient and Mn-tolerant strain. These findings highlight the potential of utilizing specific AM fungi, particularly Ri, as a sustainable biological strategy to enhance citrus productivity in acidified, Mn-contaminated soils.