Rhizophagus irregularis combined with biochar enhances Cr tolerance in Ricinus communis by altering host plant's antioxidant and photosynthetic characteristics

Arbuscular mycorrhizal fungi (AMF) and biochar (BC) have potential to enhance plant growth and tolerance under heavy metal stress. This study aimed to reveal the impact of BC application on AMF-crop symbiosis under heavy metal stress. Specifically, it investigated the effects of Rhizophagus irregularis (Ri) inoculation on Ricinus communis biomass, antioxidant levels, photosynthetic gas exchange parameters, and chlorophyll fluorescence characteristics under 150 mg/kg Cr stress. The results were as follows: (1) The AB Cr treatment (Ri inoculum + 20 g/kg BC + 150 mg/kg Cr) significantly increased mycorrhizal colonization rate by 36.67 % compared to the AM Cr treatment (Ri inoculum + 0 g/kg BC + 150 mg/kg Cr) (P < 0.05). Under AB Cr treatment, the concentrations of plant growth-promoting hormones were higher than those in the single treatments of AMF or BC alone. (2) The AB Cr treatment significantly improved plant leaf relative chlorophyll content, antioxidant enzyme activity, and photosynthetic parameters. Photosynthesis-related indicators, such as transpiration rate, stomatal conductance, photosynthetic rate, and PSII efficiency, were significantly higher than in other Cr stress treatments, with increases ranging from 76.82 % to 130.77 %. Additionally, light response curves and CO 2 response curves for different treatments were fitted to assess the variations in photosynthetic efficiency. (3) The combined application of AMF and BC enhanced Ricinus communis mycorrhizal symbiosis tolerance to Cr stress by improving growth physiology and resistance indicators. In conclusion, the combined application of AMF and BC enhances plant tolerance to Cr stress by promoting plant growth and improving photosynthesis. • Biochar improved the infective ability of Ri to the root system. • Ri combined with biochar increased plant biomass and antioxidant enzyme activity. • Ri combined with biochar improved the photosynthetic capacity of Ricinus communis . • Ri combined with biochar enhanced Cr tolerance in Ricinus communis .