Arbuscular mycorrhizal inoculation enhances Robinia pseudoacacia performance in zinc-contaminated soils via improved nutrient use and stress mitigation

Arbuscular mycorrhizal fungi (AMF) are recognized for their ability to mitigate the effects of zinc (Zn) stress in plants, though the underlying mechanisms are not fully understood. In this study, Robinia pseudoacacia seedlings were exposed to varying levels of Zn stress (0, 500, and 1000 mg kg^-1) with or without inoculation of Rhizophagus irregularis to explore how AMF symbiosis affects Zn uptake, phosphorus (P) acquisition, abscisic acid (ABA) distribution, and photosynthetic performance. Compared with the control, 500 mg kg^-1 Zn reduced shoot and root biomass by 35 % and 28 %, respectively, whereas 1000 mg kg^-1 Zn caused stronger inhibition, with reductions of 60.8 % and 56.0 %. Root P decreased by 25 % and 40 %, ABA distribution was altered, and photosynthetic rate and PSII efficiency declined. AMF inoculation mitigated these effects: Zn accumulation in roots and shoots decreased by 35 %, with higher root-to-shoot Zn ratios, which indicates enhanced Zn sequestration in roots. AMF-colonized plants also showed improved P uptake (40.4 %-75.1 %), increased ABA levels in roots (52.5 %-169.6 %), and alleviated reductions in biomass, net photosynthetic rate, and PSII efficiency under Zn stress. Additionally, the expression of photosynthesis-related genes (RprbcS, RppsbA, and RppsbD) was significantly upregulated in AMF-colonized plants. These findings suggest that AMF enhance tolerance to Zn stress in Robinia pseudoacacia by limiting Zn translocation to shoots, improving P acquisition, modulating ABA allocation, and maintaining photosynthetic efficiency.