>For all their diversity, orchids share one common trait: they rely >solely on other organisms, particularly fungi, to survive b a >characteristic which Jutta says is quite distinctive and serves as a >source of confusion to scientists.B
I am not even close to being an orchid mycorrhizae authority, I am merely a hobbyist and my comments must be seen as such. I stand to be corrected by the mycologists, scientists, among us. It is my understanding that Mycorrhizae( root fungi, )do supply orchids with energy. It is however wrong to say that all orchids solely rely on fungi, only those incapable of photosynthesis do. Dr. H.N Rasmussen believes that orchids may use mycotrophic or phototrophic nutrition; they may use them alternately, or simultaneously where one supplements the other.. >"Fungi play a crucial role in the biology of many plants because they >convert oxygen to compounds that plants can use to grow, but orchids >depend on fungi well into maturity." Beneath their good looks, orchids >are parasites, says Jutta. "This conclusion was established only at >the beginning of the last century, and after much debate, it has >finally been accepted and scientifically proven.B As I understand it, Orchid Mycorrhizae obtain nutrients by breaking down (using enzymes) and absorbing chemical substances from their environment; then carry the nutrients into the roots of the orchid where they are being consumed. These include substances that the root itself could not extract nutrients from. One may consider the orchid parasitic on the fungus, yet at the same time it maintains a symbiotic relationship with the fungus, keeping part of it alive. This is not the same kind of parasitism people talked about before orchid mycorrhizae was seriously studied. Many believed then that orchids were parasitic on their hosts. >Once the fungus penetrates a certain point within the plant's cells, >says Jutta, the cells will secrete an enzyme that breaks down the >fungal tissues, keeping the fungus in check and preventing it from >invading the plant further.B When a symbiotic relationship exists between fungus and orchid, the plant produces phytoalexins, an induced chemical resistance to fungal infection, which may be explained as a static fungicide that keeps other non symbiotic and aggressive fungi from entering the roots, and the symbiotic one from becoming too aggressive and parasitic on the orchid. When the symbiotic fungus becomes more vigorous, the production of phytoalexins increases; like a feedback control system, keeping a balance and thus preventing the fungus from becoming parasitic on the orchid.. >"This cycle is repeated over and over, and each cell is re-colonised >several times." This process, she adds, begins as soon as an orchid >seed germinates.B Only certain cells within the roots are infected by the symbiotic fungus. The hypha go around and around within the cell wall and fuse together into a dense ball called a peloton. As the peloton is being formed, certain hypha penetrate the cell walls and infect adjacent cells to form more pelotons.Soon after the peloton is fully formed it is being consumed by the orchid and its size diminishes.i.e. fungal hypha collapse and being they are made of a hard substance called Chitin, they take longer to be consumed and can thus be seen in the cells as yellowish blobs that slowly get smaller. These blobs allows us to see how often the same root cell has become infected by the symbiotic fungus. In my "hobby experience," I have never seen a cell with more than three such blobs, indicating it was infected at least three times. Not seeing more "blobs", may be due to some pelotons having been totally consumed or due to the roots at some age becoming inactive. >Once this happens, the fungus is not killed but it simply becomes a >vessel through which the orchid derives nutrients for its own >development,...snip This is true for the fungal hypha that go in and out the roots, most often through the root hairs However, the fungal hypha in the fully formed pelotons are "dead" , either when consumption starts, or soon after. Unable to obtain fungal cultures from them, one obtains live fungal hypha from pelotons in the making; not yet fully formed.
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