Alleviation of low phosphorus stress in Eucalyptus grandis by arbuscular mycorrhizal symbiosis and excess Mn

•E. grandis seedlings were greatly impacted by low P availability.•Positive effects from AMF were evident in soil without Mn additions.•Mn additions decreased mycorrhizal colonisation and its effects.•High Mn accumulation partially alleviated P-deficiency stress.•Nutrient relationship patterns suggest influence from the mycorrhizal pathway. Tropical soils are highly-weathered acidic soils and frequently present two intersecting problems that can be detrimental to plant development: low phosphorus (P) and high manganese (Mn) availability. Mn can be phytotoxic, damaging the photosynthetic apparatus and/or competing with other nutrients. Eucalyptus grandis is often cultivated in such soil types and, despite being a tree generally tolerant to Mn excess, P deficiency may exacerbate toxicity. Arbuscular mycorrhizal (AM) fungi are known for their ability to provide P to the host plant in exchange of photosynthates. This symbiosis may also decrease Mn accumulation and overall alleviate metal stress. In a glasshouse experiment, E. grandis plants were grown for seven months in soil with low P availability adjusted to three Mn dose levels (control, 75 and 150 mg kg–1), with (AM) or without (NM) inoculation with Rhizophagus irregularis. We measured biomass, height, flavonoids (anthocyanin/flavonol) and chlorophyll indexes, chlorophyll a fluorescence parameters, AM colonization and Mn, P, Mg, K and Fe concentrations in shoots and roots. All plants presented low biomass production, and shoot P concentrations ranged from 0.13 to 0.52 g kg–1. Mn additions decreased mycorrhizal colonization, but no other negative impacts were seen in E. grandis, indicating that this species is relatively tolerant to excess Mn. At dose 75, shoots accumulated up to 2,000 mg Mn kg–1; however, this amount was not detrimental to most parameters, especially in NM plants, which had higher growth and Mg concentrations while presenting much lower flavonol and anthocyanin contents. At dose 150, plant Mn concentrations were similar to those at 75 Mn, despite higher soil availability, indicating that shoot concentrations around 2,000 mg kg–1 were a threshold for E. grandis under P deficiency. AM colonization intensity and arbuscule abundance were overall low, resulting in no improvements in P nutrition nor reduction of Mn uptake in high Mn treatments. However, AM symbiosis led to higher chlorophyll contents and non-photochemical quenching values, which are important in withstanding abiotic stress. Eve