The word "mycorrhiza" is built from classical Greek roots. Myco
means "fungus" and rhiza means "root," so
the word mycorrhiza literally means "fungus-root." When the hyphae of certain fungi form specialized sheaths around the roots
of certain plants, that fungal root-coating is the mycorrhiza.
Having mycorrhiza on its roots improves a plant's ability to deal with
droughts, to acquire mineral nutrients, to store carbohydrates, and more. These benefits
are further discussed below.
In the image at the right, the thicker pine-tree root is covered with
mycorrhiza while the slender root at the right is not. In the drawing below you see a cross section of a root with mycorrhiza. The
fungal sheath, which constitutes the main part of mycorrhiza, is a mass
of hyphae encasing the root. Notice that some hyphae penetrate between the root's
outer cells, but they do not invade the cells themselves. Also note the hypha growing away
from the root, thus giving the mycorrhiza more surface area for absorbing water and
Mycorrhiza forming an exterior sheath on roots are called ectomycorrhizae.
Some trees with ectomycorrhizae are species of pine, oak, beech, spruce, maple, juniper,
willow and elm. Mycorrhiza with cells not forming a sheath but rather living deep
inside the root are endomycorrhizae. Plants with endomycorrhizae
include the legumes, grasses, tomatoes, apples, strawberries and peaches.
OTHER AMAZING MYCORRHIZA FACTS
- Nearly all established tree roots have mycorrhiza, and pines can't
live without them
- Tree seedlings planted with mycorrhiza on their roots are more
resistant to the poisoning effects of heavy metals in the soil
- If global warming gets bad, mycorrhiza may help us by locking huge
quantities of carbon dioxide into the soil
- But, air pollution can weaken or destroy mycorrhiza
Though some plants cannot survive without mycorrhiza -- certain
orchid species, for instance -- most plants can get along without it. Mycorrhiza simply
improves their ability to survive. The hyphae of many kinds of fungus, including some of
our most common mushroom producers, form mycorrhiza. Some fungus species form mycorrhiza
on the roots of a broad range of plant species, while others "infect" only a
few. Similarly, some plants can have any of several fungus species forming the mycorrhiza
on their roots. Douglas Firs form mutually helpful (symbiotic) relationships with around
2000 fungal species!
Here are some specific ways in which mycorrhizae help plants:
- Improved Storage of Carbohydrates: In
the cross section drawing above, the fungal sheath acts as a storage site for
carbohydrates. Carbohydrates produced by the plant seep into the fungal sheath, which acts
as a "sink" the plant can draw on when its own carbohydrate supply runs low.
- Improved Mineral Nutrition: In
the cross section drawing above, the hypha growing away from the root is very much like a
root hair in that it increases the mycorrhiza's surface area, and thus increases the
plant's access to nutrients. Also, fungal hyphae can migrate through the soil absorbing
nutrients with only about 1/100th of the energy cost of plants sending their roots there.
- Improved Water Transport: Mycorrhiza
increases a root's storage capacity for water, holding water rather like a sponge. When
the plant needs water, it can take it from its mycorrhiza.
Ectomycorrhizae can interconnect different
host plants: When mycorrhizal hyphae from different host plants -- even
from different host-plant species -- meet as they travel through the soil, often
they fuse, so that a flow of carbohydrates and other nutrients can move from one
mycorrhizal system into another, and even from one host plant to another. A dominant tree,
then, by way of the network of fused mycorrhizal hyphae below it, may provide nutrients to
tree seedlings, shrubs and herbs in its shade, thus diversifying and enriching the local
In the early 1990s mycologist Suzanne Simard and
her team at Oregon State University discovered that cobwebby networks of mycorrhiza could
connect not only many trees of the same species but also trees of different species. They
encountered birch connected to fir trees by up to ten different species of fungi.
Moreover, birch trees growing in bright sunlight seemed to be subsidizing fir trees in the
shade by sharing sugars via their mycorrhiza network.