| Summary: | Studies on the dynamics of non-structural carbohydrates (NSCs) play an important role in understanding the mechanisms of plant responses to drought stress. The objective of this study was to assess the influence of ectomycorrhizal fungi (ECMF) on the content and distribution of NSCs in <i>Pinus massoniana</i> seedlings under different drought intensities and to further explore the possible mechanism by which ECMF enhances the stress resistance of host plants. We conducted a pot experiment using <i>P. massoniana</i> seedlings that were inoculated (M) or non-inoculated (NM) with <i>Suillus luteus</i> (<i>Sl</i>) under well-watered, moderate, and severe drought stress conditions. The results showed that drought significantly reduced the photosynthetic capacity of <i>P. massoniana</i> seedlings and inhibited their growth rate. <i>P. massoniana</i> could respond to different degrees of drought stress by increasing the accumulation of NSCs and increasing WUE. However, compared with well-watered treatment, NSCs consumption began to appear in the roots of NM due to the decrease in starch content under severe drought, whereas NSCs content in M seedlings was higher than that in the well-watered treatment, showing that the ability to maintain C balance was higher in M seedlings. Compared with NM, inoculation with <i>Sl</i> increased the growth rate and biomass of roots, stems, and leaves under moderate and severe drought. In addition, <i>Sl</i> can also improve the gas exchange parameters (net photosynthetic rate, transpiration rate, intercellular CO<sub>2</sub> concentration and stomatal conductance) of <i>P. massoniana</i> seedlings compared with NM seedlings, which was conducive to the hydraulic regulation of seedlings and improved their C fixation capacity. Meanwhile, the content of NSCs in M seedlings was higher. Moreover, the soluble sugar content and SS/St ratio of leaves, roots, and whole plants were higher under drought stress after <i>Sl</i> inoculation, indicating that <i>Sl</i> could also change the C distribution mode, regulate more soluble sugar to respond to drought stress, which was conducive to improving the osmotic adjustment ability of seedlings, and providing more available C sources for plant growth and defense. Overall, inoculation with <i>Sl</i> could enhance the drought resistance of seedlings and promote their growth under drought stress by improving NSCs storage, increasing soluble sugar distribution, and improving the plant water balance of <i>P. massoniana</i> seedlings.
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