The objective of this study was to understand the colonization effects of dark septate endophytes on Pinus sylvestris var. mongolica under drought stress.

With the pot experiment in the greenhouse, the 2-year-old P. sylvestris var. mongolica seedlings were inoculated with J09, J32, J35, A065 and the sterile PD medium. Under normal watering (WW), moderate drought (MW) and severe drought (LW) conditions, the difference in the colonization morphology of DSE, the growth index of seedlings, physiological indexes of seedlings, physical and chemical properties of rhizosphere soil, and enzyme activities of rhizosphere soils were studied.

With the intensification of drought stress, the diameter of hyphae became thicker, the septum became shorter, and the number of microsclerotia increased. The inoculation of DSE enhanced the ground diameter, fresh weight of the aboveground part, fresh weight of the underground part, dry weight of the aboveground part and dry weight of the underground part of P. sylvestris var. mongolica by 67.72%, 36.13%, 41.58%, 36.13%, and 41.48% under the MW treatments compared with CK. The inoculation of DSE promoted the CAT, POD, SOD and GSH activities of the plant by 229.60%, 67.80%, 28.16% and 29.92%, respectively, under drought stress, in comparison with CK. The inoculation of DSE elevated the content of MDA, PSS and PRO of the P. sylvestris var. mongolica seedings by 74.16%, 105.91%, and 51.24%, respectively, in comparison with CK. DSE increased the contents of the soil OM, TN, AN, TP, AP, TK and AK by 19.85%, 40.64%, 35.25%, 57.92%, 48.11%, 55.62%, and 68.84%, respectively, in comparison with CK. DSE also increased the S-CAT, S-SC, and S-UE activities by 71.85%, 128.64%, and 65.05%, respectively, compared with CK. Specifically, the inoculation of J09 and A065 significantly increased the growth and physiological indexes of seedlings, the physical and chemical properties of rhizosphere soils, and the enzyme activity of rhizosphere soils under drought stress. However, the inoculation of J35 significantly inhibited the growth and physiological indexes of P. sylvestris var. mongolica seedlings.

The change of DSE colonization morphology may be one of the mechanisms to improve the drought tolerance of host plants, and the type of DSE colonization significantly affects the drought tolerance of host plants. Inoculation of J09 and A065 can alleviate the adverse effects of moderate and severe drought stresses on host plants by improving plant growth indicators, increasing physiological indicators, enhancing rhizosphere soil physicochemical properties, and maintaining higher rhizosphere soil enzyme activities. Therefore, J09 and A065 have the potential to be applied in the microbial fertilizer of P. sylvestris var. mongolica in drought areas.