Background Elm (Ulmus pumila) is a masculine tree species with well-developed root system, rapid growth and drought tolerance, and it is an ideal tree species for afforestation in arid and semi-arid sandy areas. However, in recent decades, frequent droughts due to global climate change have led to serious degradation of elm forest ecosystems. The ability of arbuscular mycorrhizal (AM) fungi to form a reciprocal community with host plant roots and improve the growth and development of host plants is of great significance to the improvement of terrestrial ecosystem functions. In this experiment, we investigated the physiological mechanism of AM fungi increasing the drought resistance of U. pumila by artificially simulating the drought environment, and screened the drought-tolerant strains, so as to provide preliminary theoretical preparation and technical support for mycorrhizal technology to restore vegetation and improve ecological environment in arid and semi-arid areas.

Methods Funneliformis mosseae, Claroideoglomus etunicatum, Rhizophagus intraradices and their mixtures of above three were selected as experimental agents, and potted elm seedlings used as experimental materials. Three water gradients were set by manual water control: normal water supply, moderate drought, and severe drought. The root shoot ratio, mycorrhizal infestation rate, mycorrhizal dependence, photosynthetic parameters, malondialdehyde content, peroxidase, catalase and peroxide dismutase activities of elm seedlings under different treatments were measured, and the data were statistically analyzed by Microsoft Excel 2012, and the data were analyzed by two-factor ANOVA with SPSS23 software, and the fuzzy integrated evaluation method of affiliation function was used to comprehensively evaluate the drought resistance of different strains.

Results 1) Under different moisture conditions, inoculation with AM fungi increased the plant height, root shoot ratio and mycorrhizal dependence of U. pumila seedlings to different degrees and decreased the mycorrhizal infestation rate compared with the uninoculated control plants. 2) Under three moisture conditions, inoculation with AM fungi decreased the malondialdehyde content of U. pumila seedlings to different degrees and increased the catalase, peroxidase and peroxidase activities of U. pumila seedlings compared with the control plants. 3) The net photosynthetic rate, transpiration rate and leaf stomatal conductance of U. pumila seedlings gradually decreased and the intercellular CO₂ concentration gradually increased as the water supply decreased.4) Mycorrhizal elm seedlings were all more drought tolerant than non-mycorrhizal seedlings, and their drought tolerance was from the strongest to the weakest in the order of mixed bacterial agent > C. etunicatum > R. intraradices > F. mosseae > CK.

Conclusions Under water stress, AM fungi could make U. pumila seedlings more adaptable to soil moisture changes, and the mixed mycorrhizal agent had the best drought tolerance.