Arbuscular mycorrhizal fungi can ameliorate salt stress in Elaeagnus angustifolia by improving leaf photosynthetic function and ultrastructure.

(1) Arbuscular mycorrhizal fungi (AMF) can form symbiosis with Elaeagnus angustifolia, allowing this species to tolerate salt stress. However, the physiological mechanism through which AMF improves E. angustifolia tolerance is still unclear. (2) Hence, in this study, we carried out an experiment on E. angustifolia inoculated with AMF Rhizophagus irregularis (M), and inactivated inoculum (NM) under 0 and 300 mM NaCl stress for the determination of photosynthetic gas exchange, pigments, chlorophyll fluorescence, antioxidant capacity and chloroplast ultrastructural observations of leaves. (3) net photosynthesis (Pn), transpiration rate (Tr), stomatal conductance (Gs), and intercellular CO2 concentration (Ci) in the leaves of M and NM decreased significantly under salt stress, while the M treatment significantly reduced the effect of salt stress compared with NM. Various chlorophyll components in the M treatment were 2-3-fold higher than those in NM, together with a much more complete chloroplast structure and higher number of plastoglobules. The content of total flavonoids and proline in leaves of E. angustifolia seedlings inoculated with R. irregularis increased significantly, and the concentration of malondialdehyde (MDA) decreased significantly under salt stress. Chlorophyll fluorescence data also showed good PSII function in the M treatment, together with salt stress reduction of photochemical reactions (Maximum fluorescence efficiency (Fv/Fm), the actual PSII efficiency (ФPSII), the photochemical quenching coefficient (qP), the potential activity of the light reaction centre of PSII (Fv/Fo) and sharp enhancements in NPQ (non-photosynthetic quenching). AMF inoculation ameliorated the inhibition on ФPSII and qP by 10-15%. (4) Our results clearly demonstrated that R. irregularis can improve the salt tolerance of plants by improving leaf photosynthetic performance, PSII function, antioxidant capacity and leaf chloroplast ultrastructure, and halophytes together with AMF inoculations will favour saline soil rehabilitation.