Orchid Mycorrhizas

These are endomycorrhizas formed between plants of the Orchidaceae and basidiomycetous, or rarely ascomycetous, fungi often of the form-genus Rhizoctonia. Where the perfect state is known, these have been shown to belong to genera of several fungal families.

The fungi form intracellular coils or less regular hyphal aggregates within host tissue. These structures are known as pelotons and usually only persist for a limited period before collapsing and degenerating. The pelotons in superficial tissues persist for a longer time than those in deeper tissues. Orchid mycorrhizas have no sheath or comparable structure but hyphae penetrate the soil around the infected organ. Achlorophyllous orchids, presumed to be epiparasitic on their fungal partner, have the facultative pathogen Armillaria as their endophyte and the fungus may simultaneously be mycorrhizal with a chlorophyllous host.

For all or part of their life cycle, orchids are obligately dependent on their mycorrhizal partner. Under natural conditions, orchid seeds will not successfully germinate and the protocorm will not develop without infection by the mycorrhizal fungus. During the non-photosynthetic portion of its life cycle an orchid obtains carbon compounds from the fungus. Early in the photosynthesing phase this transfer of carbon ceases and, with respect to carbon, mature green plants appear to be independent of their fungal partner.

It is thought that in mature orchid plants the mycorrhiza may primarily provide mineral nutrients. Evidence for this was provided by Alexander and her colleagues (refs below)from their studies of mycorrhizas formed between the orchid Goodyera repens Br. and Rhizoctonia goodyerae-repentis Constantin & Dufour [Ceratobasidium cornigerum (Bourd.) Rogers]. They found that plant growth and the uptake of phosphorus and nitrogen were enhanced in mycorrhizal plants. The rate of 32P uptake was up to 100 times greater than that of non-mycorrhizal plants. They also found a reduction in root:shoot ratio in mycorrhizal plants and plantlets. Decrease in root:shoot ratio is often associated with increased nutrient availability and has been shown to be an effect of infection in other mycorrhizal systems.

References

• Alexander, C.E. (1987) Mycorrhizal infection in adult orchids. Pages 324-327: IN: Sylvia, D.M., Hung, L.L. and Graham, J.H., eds. Mycorrhizae in the Next Decade - Practical Applications and Research Priorities. IFAS, Univ. Florida, Gainesville, Florida, U.S.A.

• Alexander, C. and Hadley, G. (1983) Variation in symbiotic activity of Rhizoctonia isolates from Goodyera repens mycorrhizas. Trans. Brit. Mycol. Soc. 80:99-106.

• Alexander, C. and Hadley, G. (1984) The effect of mycorrhizal infection of Goodyera repens and its control by fungicide. New Phytol. 97:391-400.

• Alexander, C. and Hadley, G. (1985) Carbon movement between host and mycorrhizal endophyte during the development of the orchid Goodyera repens Br. New Phytol. 101:657-665.

• Alexander, C., Alexander, I.J. and Hadley, G. (1984) Phosphate uptake by Goodyera repens in relation to mycorrhizal infection. New Phytol. 97:401-411.