Substrate pH mediates growth promotion and resilience to water stress of Tilia tomentosa seedlings after Ectomycorrhizal inoculation

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anglais. Colonization by Ectomycorrhizal (EcM) fungi is key for the health and performance of plants under differentstress scenarios, such as those faced by trees in urban environments. Because urban environments can belacking EcM fungi, we here assessed the benefits of inoculating Tilia tomentosa seedlings in a pre-transplantationnursery context with the EcM fungi Lactarius deliciosus and Paxillus involutus, using substrates of different pHand facing water-stress. P. involutus had a more evident positive effect in T. tomentosa seedlings and had a goodperformance in both acidic and alkaline substrate. In acidic substrate the fungus increased the plant height by0.91-fold, increased the mycorrhization rate by 3.23-fold, expansion rate by 5.03-fold and formation of secondaryroots by 0.46-fold, compared to the non-inoculated control. This species also improved the phosphorus contentof leaves, which revealed a promotion of nutrient uptake. In alkaline substrate P. involutus increased root dryweight by 3.92-fold and the mycorrhization parameters. In contrast, L. deliciosus only had a positive effect in theimprovement of mycorrhization and expansion rates and phosphorus content in the root, effects visible only inalkaline substrate. When exposed to water-stress the increase of proline content was visible in acidic substrate forboth fungi, L. deliciosus and P. involutus, and in alkaline substrate for the fungus P. involutus, a response indicativeof the enhancement of defenses in stressing scenarios such as water scarcity. We conclude that fungal inoculationimproves the vigour and resilience of Tilia seedlings and that it is of utmost importance to select a suitable EcMfungus and to consider the soil pH of the transplanting site. The inoculation approach can be a valuable tool toproduce robust seedlings which may have a better performance when transplanted to the challenging urbanenvironment.