Root anatomical adaptations of contrasting ectomycorrhizal exploration types in Pinus sylvestris and Quercus petraea across soil horizons

The anatomical characteristics of ectomycorrhizal exploration types in response to soil variability remain insufficiently understood. We examined the root anatomy of contact and long-distance exploration types in Pinus sylvestris and Quercus petraea, species with distinct ecological needs, across different soil horizons.

The diameter of ectomycorrhizal roots, the root absorptive traits i.e. proportion of cortex and mantle area, the percentage stele in the diameter, and the weighted average diameter of vessels (Ra) in the ectomycorrhizas were measured within ectomycorrhizas collected from organic and mineral soils across the soil profile.

The absorptive traits varied along soil horizons, in which water and nutrient availability changed inversely. The proportion of cortex was associated with exploration type, but was not specific to tree species. However, the ectomycorrhizal diameter and the percentage of mantle within the root forming contact exploration type of P. sylvestris showed no variation among soil horizons. In contrast, the soil horizon significantly influenced all root anatomical traits in the contact exploration type of Q. petraea by enhancing the contribution of the absorption area of the root area, mainly in the illuvial horizon, but reaching the smallest value in the organic horizon. The Ra and the cell wall thickness of the vessels were strongly dependent on tree species. With increasing soil depth, Ra in Q. petraea increased, and stele proportion in root diameter decreased.

The results suggest that water acquisition traits differ among tree species, but traits associated with nutrient absorption (proportion of cortex and mantle area) within specific soil horizons are closely related to the ectomycorrhizal exploration type.