Imhof, S. (1998):

Strategies for sustainable use of mycorrhiza in myco-heterotrophic plants

2nd International Conference on Mycorrhiza. Uppsala, Sweden, 5-10 July 1998.

Abstract:

Because achlorophyllous, mycotrophic plants are unable to assimilate carbon by themselves, they essentially depend on their mycorrhiza. For this reason, myco-heterotrophic plants must care for their root fungi to some degree, e.g. avoiding overuse, or through optimization of the fungal functions as symbionts. Here, three different patterns of mycorrhiza in achlorophyllous plants are compared with respect to their efficiency and sustainability.
The VAM in Voyria truncata (Gentianaceae) resembles the mycorrhizae of other Gentians, except for the missing arbuscules and the fast degeneration of the intracellular hyphal coils. There is no evidence of special maintanance mechanisms for the fungus, rather the large surface provided by the extensive root sytem allows sufficient fungal penetrations to nourish the plant.
Roots of Triuris hyalina (Triuridaceae) have three anatomically distinct cortex layers which also differ in their mode of fungal colonization. The outer layer hosts rather thin, very densely coiled hyphae. In the middle layer the hyphae are thicker, less densley coiled and quickly degenerate into amorphous clumps. Therefore, the middle cortex layer seems to serve as the digestive layer, whereas the outer cortex functions as a reservoir where the fungus is kept alive for some time.
Finally, in Voyria tenella, the innermost cortex carries straight, healthy looking hyphae which send hyphal branches to the rest of the cortex where they swell up enormously and soon become digested. This is referred to as an ‘intraradical fungus garden’, and it represents, perhaps, the most efficient sustainable use of a mycorrhizal partnership.