Research page:Mycorrhiza
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From Botany for gardeners by Brian Capon, chapter 6
MYCORRHIZAE AND ROOT NODULES Mycorrhizae (Greek: mykes, “fungus”; rhiza, “root”) are certain beneficial soil fungi that form symbiotic relationships with the tender, young roots of Orange-yellow stems of the parasite dodder (Cuscuta spp.) hang like spider webs on an unwilling host. Seen in cross section under a microscope, the parasite (near right) sends haustoria into the vascular tissues of a host’s stem (far right). Adaptations to Fulfill Basic Needs 127 many species of higher plants, including corn, peas, apples, citrus, poplar, oak, rhododendron, birch, and pine and other conifers. The fungus penetrates the root tissues, surrounds the roots, and extends into greater volumes of soil than the plant’s root hairs are able. The nutrients the fungus mycelium taps, especially phosphorus and nitrogen, are used both for its own benefit and that of the host plant. In return, the higher plant supplies the fungus with photosynthesized foods, including sugars. The beautiful, bright red snowplant (Sarcodes sanguinea), a springtime inhabitant of pine forests in western North America, is an angiosperm lacking the ability to photosynthesize. It survives through the intercession of a mycorrhizal fungus that transfers food from the roots of nearby trees. The fungus also supplies itself and the other two members of the trio with soil nutrients. Another species having the same, complex symbiotic relationship is the white, waxy-looking Indian pipe (Monotropa spp.). Another symbiosis, mutually beneficial to the two participating organisms, is that of the soil bacterium Rhizobium and the young roots of many species of angiosperms, especially members of the pea family (Fabaceae, formerly Leguminosae). All living things require a constant supply of nitrogen, White mycelium of a mycorrhizal fungus surround the root of a higher plant. Unable to photosynthesize foods, Sarcodes sanguinea (snowplant) obtains nourishment from the roots of nearby trees by way of a symbiotic mycorrhizal fungus. 128 CHAPTER 6 especially for the synthesis of cellular proteins. An abundance of the element occurs in the atmosphere in the form of nitrogen gas (N2), yet few organisms can convert the gas into a form they can use. Rhizobium (and several species of blue-green algae, or cyanobacteria) are able to perform such a biochemical trick by a process called nitrogen-fixation. The bacteria invade the higher plant’s roots, causing them to enlarge in groups of warty root nodules. There the microorganisms absorb nitrogen from the soil atmosphere, fix it into valuable ammonium ions (NH4+), and pass the product to the root’s cells. The host plant’s roots supply the bacteria with carbohydrates. As a consequence of nitrogen-fixation, peas, beans, clover, soybean, and alfalfa plants, for example, are especially rich in nitrogenous substances and become a highly desirable mulch when dug into nutrient-poor soils. Clover, soybean, and alfalfa are also nutritious species for forage. From an ecological point of view, nitrogen-fixation is a crucial link in the nitrogen cycle, a worldwide process in which the nutrient is cycled and recycled between the atmosphere, oceans, soil, and living organisms. As long as the cycle continues, nitrogen will always be available for plants and animals. The same does not hold true for the world’s supplies of noncyclable nutrients, such as phosphorus, potassium, magnesium, and iron. As these elements slowly wash out of the land and into the oceans they become unrecoverable by natural means and, therefore, unavailable to terrestrial plants.
From
- "In a mycorrhizal association the fungal hyphae of an underground mycelium are in contact with plant roots, but without the fungus parasitizing the plant. In fact the association is commonly (but by no means always) mutually beneficial. Through photosynthesis a chlorophyll-containing plant makes simple carbohydrates (using carbon dioxide, water and sunlight).
