Fungal ectomycorrhizal community and drought affect root hydraulic properties and soil adherence to roots of Pinus pinaster seedlings

Pinus pinaster seedlings were grown in a sandy dune soil either inoculated with Hebeloma cylindrosporum or let to natural colonisation. Six months later, half of the seedlings of both treatments were subjected to a 3-week moderate drought. Root colonisation analysis showed that root tips were colonised to almost 100%. independent of the inoculation. DNA determination of the ectomycorrhizal morphotypes showed that inoculated seedlings were extensively mycorrhized by H. cylindrosporum (more than 75%) whereas non-inoculated seedlings were mycorrhized by the exotic species Thelephora terrestris (50%) and Laccaria bicolor (30%) and to a lesser extent by H. cylindrosporum (20%). Drought did not affect these frequencies. Total plant biomass was not affected by the mycorrhizal status or by drought but the root/shoot biomass ratio as well as the root/leaf surface area ratio were much lower in seedlings extensively colonised by H. cylindrosporum. Root hydraulic conductivity was higher in plants mainly mycorrhized by H. cylindrosporum, showing that this fungus improved the water uptake capacity of the root system as compared to T terrestris and/or L. bicolor. This positive effect was also found under drought but to a lesser extent. H. cylindrosporum also increased the amount of root-adhering soil as compared to the other fungal symbionts, illustrating the performance of this association in aggregating sandy soil particles and developing the rhizosheath. The origin of the reduced root hydraulic resistance by H. cylindrosporum mycorrhization is discussed for the whole path including soil. soil-root interface and root cortex.