Structure and function of mycorrhizal rhizomorphs with special reference to their role in water transport

Anastomosing systems of fungal rhizomorphs are found in association with mycorrhizal roots of trees in arid zones¹, in soils of temperate deciduous², coniferous³ and humid tropical⁴ forests, and even in mine spoils⁵. Although it is recognized that the fungus depends on the host for its carbon supply^6,7, the functional relationship between the partners has not been investigated fully. Transport of ³²P through rhizomorphs has been demonstrated⁸, but their potential to absorb and transport water is not known. We have synthesized mycorrhizal rhizomorphs in association with pine, and preliminary experiments with tritiated water (³H₂O) showed that the cut distal ends could absorb water which was transported to mycorrhizal roots and to seedling needles⁹. We have since developed a method for feeding ³H₂O to the intact terminal portions of rhizomorphs in sealed vessels with negligible disturbance to the mycelial system, and we report here that mycorrhizal rhizomorphs can absorb water and facilitate its transport over ecologically significant distances. Electron microscopy shows that the rhizomorphs are differentiated structures with large central `vessel' hyphae.