Mycorrhizae: Root — fungus partnership
If you know anything about mycorrhizal relationships, then you probably are making sure you have them happening in your garden. If you don’t know anything about them, you should. They are in and amongst the roots of up to 95 percent of all land plants and doing great things for them all. Let me tell you about one of the most amazing wonders of nature.
Mycorrhizal relationships are between a root and a fungus and together these form a symbiotic relationship. This is a relationship that is beneficial to both parties. Since the fungus lives in the in the ground, in the dark, it cannot photosynthesize, so it has to get its food from other sources. (Fungi, by the way, are not of the plant kingdom, they’re in their own kingdom.)
So the mycorrhizal fungi go looking for a partner, or thousands of partners. They go up to a root tip and give it a chemical signal to let it know it is there and available, and “would you like my help?” The root then may respond, with a chemical signal, “yes, I would.” And it will soften its tissue and allow the fungal hyphae in amongst its root cells. The hyphae are like long, very thin filaments. After taking up residence in the plant root the filaments grow outward in search of the minerals and water the plant needs. In exchange, the symbiosis part, the plant gives the fungus a variety of sugars to build its body structure.
Remember, the plant is taking in carbon dioxide through its leaves, plus water, and using sunlight energy to build sugars, i.e. photosynthesis. Well it turns out that trees may be giving up 20 to 80 percent of their carbon to fungi. Then these crafty fungi go mining, hunting, and drilling for water for the plants.
Those hyphae are pretty much just tubes. And once they’re all juiced up on sugars, they can grow a long way, much farther than the plant’s roots can. They can meet up with a rock, secrete acid to dissolve the rock, and tunnel their way into the rock, and all the while send back to the host plant the minerals they have released from the rock.
The fungi can reach out and do a much more efficient and thorough job of extracting water from the soil, helping to increase drought tolerance in plants. And they can hunt. They can invade the bodies of soil dwelling insects, suck them dry and send all the nutrients back to the host plant in forms that are usable to the plant.
The fungal hyphae seem to be paving the way towards better soil texture, as well as feeding their host plant. They exude a compound called gomalin, which is a sticky substance that surrounds soil particles which improves soil texture and aeration.
In a pinch of soil there can be several miles of fungal hyphae. In the forest there is so much connectivity between mycorrhizal fungi and their trees that it may well be that they are all connected and there seems to be good evidence that there is a level of communication between trees and across species. There is so much more to this. Take a listen to a podcast from Radiolab called “From Tree to Shining Tree” at http://www.radiolab.org.
There are two categories of mycorrhizal fungi. The ectomycorrhyzae and the endomycorrhizae, Ectos and endos for short. The endos are frequently called Arbusclar Mycorrhizae because of the structure of the hyphae in the roots. But I will stick with endos for simplicity.
The ectomycorrhyzae enter only the outer layers of the root cells and form a sheath around the root ends. These can be visible to the naked eye and may appear as a whitish growth. They are mostly associated with forest trees and are usually specific to a tree species. These fungi have above ground fruiting bodies, the bearers of spores for reproduction, called mushrooms. You may have eaten some, such as Chantarelles, Morels, Boletes, Truffles, and Puffballs.
The endomycorrhizae enter deeper into the root cell layers. These are microscopic. They form root associations with over 80 percent of all green plants and are the most widespread. They bear their spores underground. The Endos are the ones to nurture in your garden soil for they will help your plants, except, I’m sorry to say, for two plant families: the Chenopods (beets and chard) and the Brassicas (kales, broccoli, and mustards).
Mycorrhizal fungi are all around in the soil. The only time you may need to add any is if soil has been eroded, fallow, graded, or in some way disturbed. Mixes can be purchased.
To nurture the endomycorrhizal relationships in your garden there are several things to do. The Endos need to be happy with host roots and they don’t live on their own for long. So it’s important to always keep plants growing in the soil. Plant a cover crop between seasons, don’t allow the soil to go fallow. Tillage is an Endo killer. It breaks up the hyphae and exposes them to the light and drying. So ditch the rototiller, minimize the digging, and try a no-till approach to gardening. Soil compaction damages them, too. Stay on your paths and use mulch. Throughout the landscape add woody mulch, but not up against any trunks. The mulch keeps the moisture in and acts as food for those hyphae. I like woodchips the best.
Remember how the fungal filament offered services to the root? Well, the mycorrhizal fungi are so good at extracting phosphorous from rocks that if there’s plenty of it in the form of fertilizer in the soil, the root will not feel the need for the help, and will turn the filament down! Soft rock phosphate, which is largely insoluble, is the best form of phosphate to add to your soil, and will keep the fungi busy.
It’s a fascinating world amongst the roots.
Doreen Fogle is a Nevada City-based landscape design consultant with Delightful Gardens. She can be reached at firstname.lastname@example.org.
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