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Mycorrhiza and Germination

The term "mycorrhiza" refers to the symbiotic relationship between a fungus and the roots of a vascular plant. Most plants show this relationship and many, including most forest trees, require it for survival. In the absence of fungi, the seeds of such plants usually germinate and grow well for a few weeks or months but then languish or die unless their roots encounter an appropriate fungus. Most orchids, on the other hand, cannot germinate (in nature) without the fungus, whereas the health of the mature plant seems less dependent on this relationship.

There are several types of mycorrhizae. In the ectomycorrhizal relationship, the fungus is in contact with the root cells but does not penetrate the root, forming rather a sheath of fungal mycelia around the out- side of the root. Such mycorrhizae are thought to aid the host plant in the absorption of mineral nutrients from the soil but contribute little or no organic matter to the host. In the case of endomycorrhizae, the fungus does penetrate the host root. The orchid mycorrhizae are of this type, in that the host plant actually digests parts of the fungal body, or else digests cell contents expelled from the fungus. Typically, the outer cells of the orchid root form a "fungal host cell layer," which is penetrated by the mycelium of the fungus, but the mycelium in this layer is not digested (Burgeff, 1959). Within that layer one finds the "digestion cell layer," whose cells have been characterized as "phagocytes" by some authors. The mycelium penetrates this layer as well, and then forms dense coils or clumps in which food material (proteins, glycogen, and fat) is stored. These clumps are then digested by the orchid cell and the foods are used by the orchid—a process that may be repeated many times by the same cell. Beneath the digestion cell layer starch is stored in another layer of cells that is not penetrated by the fungus. In some cases, especially among the tribe Gastrodieae, such clumps are not formed, but the fungus expels part of its cytoplasm (cell contents) into the orchid cells, where it is digested. Clearly, the orchid plant maintains a rather effective control of the fungus and its growth, presumably by means of hormones and other chemical agents. Some authors have gone so far as to characterize the orchids as parasites on fungi, but we do not really know that it is a totally one-sided relation- ship. The fungi may be receiving vitamins or other substances from the orchid plants.

Because orchid seeds are usually tiny, they cannot contain a large supply of stored foods. When a seed reaches conditions favorable for germination, the first stages of growth may occur in the absence of any fungus, but most orchid seedlings are unable to continue growth without "infection." Some few orchids, especially members of the tribe Arethuseae, are capable of continued growth in the light without mycorrhizae, but their seedlings grow much better with mycorrhizae than without. Typically, fungal hyphae enter the seed through the suspensor end and penetrate the germinating embryo. In some cases the fungus kills the orchid seedling, and in other cases the orchid kills the fungus (and then the orchid dies, unless reinfected). lf the proper balance is established, however, the seedling is able to obtain additional food from the fungus and continue growth. For many terrestrial orchids and some epiphytes, the early development takes place within the substrate, and the seedling is completely dependent on the fungus for food until an aerial shoot or leaves are formed and photosynthesis is possible. The seeds of many of these orchids will not commence germination in the light. This is clearly an adaptive feature, for such a tiny seedling would usually dry up and die if it were exposed on the surface of the soil. For many epiphytes and terrestrials of wetter habitats, of course, the surface may be the most favorable place for germination. The relationship between the orchid seedling and the fungus is not a mycorrhiza in the strictest sense, in that the fungus penetrates not a root but the protocorm. But the relationship is homologous with that in the roots, and it would be silly to invent a new term for it. After roots develop, in most cases they form mycorrhizae with fungi that they encounter in the substrate; the fungus that penetrated the protocorm does not grow out with new roots, though it may be a part of the same mycelium that penetrates the roots. In the tribe Orchideae (and presumably also in the Diseae and Diurideae) the mycorrhiza never occurs in the root-stem tuberoid, and the plant must establish the mycorrhiza anew each growing season when new roots are formed. The tuberoids produce an effective fungistatic agent, especially after other parts of the plant have been "infected" by a fungus (Arditti, 1966a). Similarly, epiphytes have little or no mycorrhiza during the resting season; in Cattleya the mycorrhiza is reestablished each growing season (Breddy and Black, 1954). In many cases the mature orchid can grow quite well without any mycorrhiza, at least under favorable conditions of light and nutrient supply.

There has been great controversy over the specificity of orchid-fungus relationships. Some authors, especially Knudson (1922) and Curtis (1939), have maintained that there is no specificity, that the mycorrhiza may be formed with many different fungi, and that the fungus is a parasite and not necessary for the growth of the orchid. It is true that orchid seed sown on agar with adequate mineral nutrients and sugar under aseptic conditions grow quite well. However, such conditions are never found in nature. As to specificity, the situation is not clear-cut. It appears that only a few fungi are compatible with any one orchid species, and related orchid species are likely to be compatible with the same fungi. However, it is very difficult to identify most mycorrhizal fungi. Orchid mycorrhizal fungi have been classified as Rhizoctonia species, but Rhizoctonia is a "form genus/' and when fertile (sexual, spore- bearing) material is known, the "Rhizoctonias" are assigned to other genera and families (Warcup, 1975). Warcup (p. 100) sums up the whole problem of specificity rather well: "Both orchids and fungi differ markedly in the range of partners with which they form effective symbioses

Fungus (Tulasnellales) species knwn to form mycorrhizae with Orchids. These are the species of which sexual stages have been found. All others are classified as Rhizoctona species. (After Warcup, 1975.)

See also Orchids from seeds

I also recomend the website of Dr Heinrich Beyrle www.myorchids.de

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