An anatomical and physiological investigation of ectomycorrhizal Pinus banksiana (Lamb.) and Eucalyptus grandis (W. Hill ex Maiden) roots

Abstract

Tree roots have a very unique anatomy. It was only recently discovered that woody roots possess three distinct anatomical regions, the white, condensed tannin and cork zones. Additionally, many tree roots naturally form symbiotic associations with soil fungi. The resulting ectomycorrhizal root tips that are formed represent a fourth root region that must be considered as well. The distinct anatomy of each root region results in very different capacities for ion acquisition. In the present work, various aspects of ectomycorrhizal Pimus banksiana (Lamb.) and Eucalyptus grandis (W. Hill ex Maiden) root anatomy were examined, and the impacts of the results on potential for nutrient acquisition were considered. Firstly, the vitality of the extramatrical hyphae of growth-pouch grown P. banksiana ectomycorrhizae were investigated. It was found that while a large fraction of the hyphae are alive, most are encased in impermeable walls. Secondly, the permeability of P. banksiana and E. grandis ectomycorrhizal mantles to physiologically significant ions was tested, and the mantles were found to be impermeable to sulphate ions. Similarly, the permeability of the outer surface of the cork zone of both these tree species was tested. The phellem proved to be permeable to sulphate ions, though sparingly so. Lastly, the anatomy of ectomycorrhizal growth-chamber grown E. grandis seedlings and growth-chamber and field-grown P. banksiana seedlings were investigated to assess each root regions contribution to nutrient uptake. For both species, a considerable amount of the root tissue most suited for ion uptake is located within the ectomycorrhizal mantle. Because the remainder of the root system is rather poorly suited and situated for soil mineral uptake, it was concluded that the vast majority of the ions absorbed by the root system are supplied by the fungal partner and fed to the root tips within the fungal mantle. The remainder of the root system serves only as conduits from the root tips to the above ground portions of the tree.