Springer Online Journal Archives 1860-2000
Summary The tracer Cellufluor has been used to test the apoplastic permeability of the fungal sheath inPisonia grandis R. Br. mycorrhizas. In the tip region in the immediate vicinity of the root cap, where the sheath is not yet fully differentiated, Celluflor penetrates as far as the root epidermal cells. Behind this (i.e. just proximal to it) in differentiated regions, where the ultrastructure of both the root and fungal cells indicates that the mycorrhiza is likely to be functionally active, the sheath is impermeable to Cellufluor. During the development and differentiation of the sheath, the interhyphal spaces become filled with extracellular material. In the outer and middle regions this becomes electron opaque after fixation and staining. It is proposed that the dramatic decrease in apoplastic permeability over a short distance back from the root apex as the fungal sheath differentiates results from secretion of extracellular material by the fungus and its modification by deposition of phenolic substances. The symplastic pathway within the fungus may be very important for radial transfer of materials across the sheath. Blockage of the sheath apoplast could provide a sealed apoplastic compartment at the fungus-root interface, with resulting increase in efficiency of transfer between partners. The implications of these observations are discussed in relation to radial transfer across the sheath and transfer between partners in sheathing mycorrhizas in general.
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