Many edible mushrooms are found only in association with the roots of certain trees. In these intimate symbiotic relationships the fungus benefits the growth of the tree by helping with the uptake of essential elements, and the plant provides the fungus with carbohydrates and a place to live. These are the mycorrhizal mushrooms, and among them are some of the most expensive foods in the world.
To understand the high value placed on edible mycorrhizal mushrooms and why attempts to cultivate them have so often proved fruitless, it is useful to understand mycorrhizas and how they function. Many people are surprised to learn that most higher plants, from ferns to flowering plants, do not technically have roots but instead possess mycorrhizas (literally "fungus roots"), the composite structures formed when a mycorrhizal fungus infects the roots of a higher plant (Smith and Read 1997, Mycorrhiza Information Exchange 2002). This relationship generally benefits both the plant and the fungus and is therefore an example of symbiosis. Plants in a few families—the bras-si cas (Brassicaceae) and nettles (Urticaceae), for example—do not form any type of mycorrhiza, but collectively these amount to only 10-15 percent of all species. Mycorrhizal relationships have existed for a very long time; in fact, mycorrhizas have been reported in 460-million-year-old Ordovician fossils (Redecker et al. 2000, Sanders 2000). Over this long span of time, mycorrhizal fungi have become very specialized so that most cannot survive unless they are in contact with their host plant. Many plants have become equally dependent on mycorrhizal fungi and without them become stunted and yellow— often due to a lack of phosphorus. A lack of these fungi is one reason why some plants, raised in a sterile, soilless potting mix and purchased from a garden center, die when transplanted in the garden (Hall 1988).
The most common mycorrhiza is the vesicular-arbuscular mycor-rhiza (VAM), which is formed by almost all agricultural and horticultural plants. In this type, most of the fungus is found inside the root, where the fungus produces small storage organs called vesicles (literally "small bladders") and arbuscules (literally "little trees"), which are
well adapted to the transfer of nutrients between the host plant and the fungus. These fungi do not produce mushroom fruiting bodies.
Another mycorrhiza is the type known as an ectomycorrhiza. This is the type of particular interest, because some of the fungi that form ectomycorrhizas produce mushrooms, some of which are edible. An ectomycorrhizal fungus forms a layer of fungal material over the surface of small roots, much like the fingers of a glove. This layer is called the mantle. Short fungal threads, called hyphae, extend from the inner surface of the mantle and grow between the outer layers of root cells, producing a three-dimensional structure called the Hartig net. You can get some idea of the structure of the Hartig net if you imagine that it looks like the mortar between the bricks (equivalent to the outer cells of the root) in a brick chimney. From the outer surface of the mantle, hyphae extend out into the soil. There they absorb nutrients such as phosphorus and nitrogen and pass them back down the hyphae to the mantle, eventually reaching the plant via the Hartig net. The fungus obtains carbohydrates from the plant in return, but the loss is more than offset by the increased plant growth made possible by improved access to nutrients.
Host plants for ectomycorrhizal fungi include birches (Betulaceae), oaks and beeches (Fagaceae), eucalypts (Myrtaceae), pines and spruces (Pinaceae), poplars and willows (Salicaceae), rock roses (Cis-taceae), and dipterocarps (Dipterocarpaceae) (Becker 1983, Smith and Read 1997). Edible ectomycorrhizal mushrooms are never found associated with, for example, horse chestnuts (Hippocastanaceae), maples (Aceraceae), and apples, apricots, or peaches (Rosaceae), since all of these form vesicular-arbuscular mycorrhizas. Because of the importance of ectomycorrhizal trees in most of the world's forests, it is unusual to find areas where there are no ectomycorrhizal fungi. But there are a few exceptions. For example, all but a handful of New Zealand's native plants form vesicular-arbuscular mycor-rhizas. Consequently, ectomycorrhizal fungi, which are dependent on their host plants for survival, are generally restricted to those areas where their southern beech (Nothofagus) and tea tree (Lep-tospermum and Kunzea) host plants are found (Johnson 1977). Moreover, forests in many areas of the tropics are predominantly composed of vesicular-arbuscular mycorrhizal trees.
Because of the importance of mycorrhizas and the fungi that produce them, more than ten thousand specialist scientific publications deal with the subject. Some of these provide good summaries of the salient points. The Mycorrhiza Information Exchange Web page is a good place to start, while Mycorrhizal Symbiosis (Smith and Read 1997) is an excellent book for those wanting a more in-depth treatment.
Mycorrhizal mushrooms are renowned for the flavors they impart to meals, but also for the high prices some of them command—up to $30,000 per kilogram. Compared with their saprobic cousins, the market in dollar terms is disproportionately higher than the relatively small quantities that are harvested. Since only a handful have ever been cultivated commercially, the chef and gourmet have to rely on professional or amateur collectors harvesting the mushrooms from the wild during the few months that they fruit.
Just what quantities are in fact harvested is unknown. Estimates that have been made are very approximate, with official statistics probably containing some gross errors. One reason for this is that many mycorrhizal mushrooms are sold under the counter so that the picker and purchaser can avoid paying taxes. Many more are taken home after a day's collecting without ever passing through a market.
Portia Barnes, one of a small number of competent wild mushroom collectors in Victoria, Australia, with a basket of grade-one Lactarius deliciosus (saffron milk cap). EDIBLE. (Hall)
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