Abstract: Drought stress restricts plant growth in the Karst area of Guizhou. Ectomycorrhizal fungi can form mutualistic symbionts with the roots of most woody species, which can improve their drought tolerance. In this study, a pot experiment was conducted to evaluate the effects of ectomycorrhizal fungi inoculation on growth and photosynthetic characteristics of 1-year-old Broussonetia papyrifera seedlings under drought stress. There were six treatments, including singleinoculated with Pisolithus tinctorius (Pt), single-inoculated with Scleroderma polyrizum (Sp), single-inoculated with Pseudomonas fluorescens (Pf), mixed-inoculated with P. tinctorius and Pseudomonas fluorescens (Pt+Pf), mixed-inoculated with S. polyrizum and P. fluorescens (Sp+Pf), and non-inoculated (CK). B. papyrifera seedlings were subjected to three water treatments: normal water, moderate drought, and severe drought. Six months later after incubation, the growth and photosynthetic indices (including root infection rate, net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, transpiration rate, aboveground and belowground biomass) of B. papyrifera seedlings were measured. Results showed that root infection rate and root/shoot ratio increased significantly with decreasing water supply level, while net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, transpiration rate and plant biomass decreased significantly (P<0.05). The inoculation significantly affected infection rate, photosynthetic parameters, aboveground and underground biomass of B. papyrifera seedlings (P<0.05), but there were obvious differences among different inoculation methods. Compared with CK, single and mixed-inoculation treatments significantly promoted the symbiotic relationship between B. papyrifera and ectomycorrhizal fungi. The root infection rates of Pt+Pf and Sp+Pf were 9.7 and 5.9 times higher than that of CK, respectively. Mycorrhizal helper bacteria (Pf) showed a significant promoting effect on the colonization of Pt at root tips ofB. papyrifera. Compared with CK, shoot and root biomass of Pt+Pf, Sp+Pf and Pt increased significantly, and the net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration and transpiration rate of Pt+Pf and Sp+Pf also increased significantly. Inoculation of ectomycorrhizal fungi could therefore alleviate the impacts of drought stress on the growth and photosynthetic parameters of B. papyrifera seedlings, and mixed inoculation of ectomycorrhizal fungi and mycorrhizal helper bacteria could achieve stronger promotion effect than singleinoculation of ectomycorrhizal fungi.

Key words: Broussonetia papyrifera, ectomycorrhizal fungi, mycorrhizal helper bacteria, drought stress, photosynthetic characteristics.