A few weeks ago I needed some potting soil. When I got the bag of Pro Mix home, I noticed that it said it contained mycorrhizae. I knew that mycorrhizae had something to do with fungus and roots, but that was about it. So I did some research online and it’s led me on a journey of discovery about fungus, roots, water absorption and the latest hype targeted at gardeners.
Let me start off by saying that the label on the bag of potting mix is a little misleading. The word mycorrhizae (pronouned mīkəˈrīzə) comes from two Greek words – mykós meaning fungus and riza meaning root. Mycorrhizae is the symbiotic relationship between certain kinds of fungi and the roots of some plants. Since the term described the relationship between fungi and roots, it’s impossible for a bag of soil to contain mycorrhizae; what it contains are spores of the various fungi that, in the presence of roots, can form mycorrhizae.
OK, enough of my word usage obsession! What I’ve learned is that often in nature, the mycelium of some fungi (the vegetative parts of a fungi that’s made up of thread-like hyphae) can become interconnected with the roots of plants in order to help the plant absorb water and nutrients. This association is what’s called a mycorrhiza. Just as root hairs increase the surface area of the root, when the roots of plant become connected with the mycelium of fungi, suddenly the entire mycelium of the fungus becomes surface area for the plant to absorb water. While the fungi of mycorrhizae aid the plant, the plant provides the fungi with carbohydrates that the fungi can’t produce because they’re not photosynthetic. This is why mycorrhizae are symbiotic – both parties are benefiting from their association.
There are two major types of mycorrhizae found in nature: edomycorrhizae and ectomycorrhizae. Ectomycorrhizae are less common in nature but are associated with members of the beech, willow and pine family. Many of these mycorrhizae fungi are in the class basidiomycetes, the fungi that we associate with toadstools and mushrooms. In ectomycorrhizae the fungi forms a sheath around the fine roots of the plant and the mycelium extends out into the soil to aid in the transfer of nutrients and water to the plant. Many of these plants have evolved along with the fungi and don’t produce any root hairs – they don’t need them since the mycorrhizae substitutes for the root hairs.
While ectomycorrhizae may not be a huge concern for the home gardener, they’re very important to the gourmet cook. Many of the mushrooms that are prized by chefs are the fruiting bodies of the fungi of ectomycorrhizae. Truffles, morels, chanterelles and others are all edible mycorrhizal fungi. Because of the complex relationship between tree roots and fungi which produce these mushrooms, cultivating truffles or morels is difficult if not impossible. These mushrooms grow in the wild and their prices reflect this fact.
Endomycorrhizae are far more common and occur in about 80% of all plants. Instead of forming a sheath around the root, in endomycorrhizae, the hyphae of the fungi penetrate between the cell walls and also penetrate the cells of the root, forming coils and branched structures. There are only about 30 kinds of fungi that form endomycorrhizae and all of them are from the class zygomycete, the fungi class that causes black bread mold. In endomycorrhizae, the fungi along with root hairs aid in transferring water and nutrients from the soil to the plant.
In the natural world, the fungi that can create mycorrhizae are ubiquitous and most plants growing in a natural environment have mycorrhizae. While you can see the sheath of ectomycorrhizae, the hyphae of endomycorrhizae are too small to be seen with the naked eye.
It’s clear that mycorrhizae are very important in the life of plants; in fact, it appears that this has always been the case. Paleobotanists have shown that endomycorrhizae are present in the fossils of early vascular plants. This symbiotic relationship between green plants and fungi appears to be vital to plants and their growth.
But does potting mix for house plants need to have the spores of zygomycetes mixed into it? Should gardeners rush out and buy some of the “mycorrhizae” inoculants that are on the market? Are mycorrhizae something that the home gardeners needs to think about?
Those questions are for the next post!