Apoplasmic barriers and their significance in the exodermis and sheath of Eucalyptus pilularis-Pisolithus tinctorius ectomycorrhizas

The apoplasmic permeability of ectomycorrhizal roots of intact Eucalyptus p ilularis seedlings infected with Pisolithus tinctorius oa aseptic agar plat es was examined using the nonbinding fluorochrome 8-hydroxypyrene-1,3,6-tri sulphonate and lanthanum ions in conjunction with anhydrous freeze substitu tion and dry sectioning. Most mycorrhizas formed in the air above the agar surface, and in these the sheath rapidly became nonwettable and impermeable to the fluorochrome but was nevertheless permeable to lanthanum ions. In a few mycorrhizas which developed in contact with the agar the sheath remain ed permeable to both tracers when fully developed. This increased hydrophob icity of the sheath in mycorrhizas in the air above the agar surface might be explained by deposition of hydrophobins, but nevertheless it still allow s an apoplasmic pathway for radial movement of ions. Regardless of their sh eath permeation both apoplasmic tracers were always found throughout the Ha rtig net and were arrested at the Casparian bands and suberin lamellae of t he exodermis. It is concluded that the fluorochrome must have moved longitu dinally along the Hartig net which is a region of higher permeability than the sheath. Casparian bands in the exodermis of ectomycorrhizal roots have similar properties to those in nonmycorrhizal roots in excluding solutes an d their exclusion of lanthanum ions indicates that they are not permeable t o ions. The data do not support the concept of a totally sealed apoplasmic exchange compartment, but the differential permeability suggests that the s heath might allow radial transfer of ions but block loss of sugars and orga nic molecules of similar size.