Tuesday, March 25, 2008

The Fungus Among Us

(Here's a portion of the chapter called "The Fungus Among Us" on mycorrhizal association from my most recent book Roots Demystified, Change Your Gardening Habits to Help Roots Thrive.)

According to experts on the subject, the intriguing life-forms known as fungi comprise about 52% to 55% of a forest’s biomass. Since a whopping 80% of all green plants depend on at least one fungal relationship in order to survive, it’s appropriate here to discuss the magical relationship of beneficial fungi with the roots of trees as well as with a range of annual, herbaceous, and woody plants. This relationship is known as mycorrhiza, or fungus root, from the Greek: mykes [mushroom] and rhiza [root]. The plural is mycorrhizae.

Fungi, in general, form masses of tiny filaments known as mycelia, which frequently interact with plant roots. There are two major kinds of mycorrhizae: ectomycorrhiza and endomycorrhiza. In ectomycorrhiza (usually abbreviated as “EM”), these filaments remain outside of the plant, living on the cells of the root hairs. With endomycorrhiza, these filaments actually live between and inside of the cells of the feeding roots. There are numerous kinds of tongue-twisting endomycorrhizae: arbutoid, monotropoid, ericoid, orchidioid, and vesicular-arbuscular or arbuscular mycorrhiza, all of which interact with plants in different ways. The most important and widely distributed type of mycorrhiza is the vesicular-arbuscular mycorrhiza or arbuscular mycorrhiza (abbreviated as “VAM” or “AM”.) These are the new terms for endomycorrhiza. (Some plants utilize neither EM nor VAM; examples of plants with no mycorrhizal association include all the species of brassicas—cabbage, broccoli, cauliflower, Brussels sprouts, etc.)

All mycorrhizal associations are beneficial and are characterized by the movement of plant-produced carbon to the fungi and fungal-acquired nutrients to the plant. Rather than a parasitic relationship, it is a mutualism in which both life-forms benefit. In general, most plants are dependent upon this union, as it is estimated that about 80% of all plant species in the world are mycorrhizal symbionts.

In general, mycorrhizal plants are well-fitted to endure environmental stress. Nutrient-poor or moisture-deficient soils show improved capacity for supporting plant growth and reproduction when mycorrhizal fungi are present. As if to return the favor, the plant allows the mycorrhizal fungi to extract sugars, starches, proteins, and lipids from its lateral roots. (We’ll go into more detail a few paragraphs down.) Mycorrhizal fungi may also improve water absorption, increase drought resistance, and exude substances that reduce infections caused by some soil pathogens.

Phosphorus is the most common nutrient transferred via VAM association into the root system of a plant, especially if it is growing in soil that is low in this essential nutrient. All trees need phosphorus but are not always able to absorb soluble phosphorus efficiently; they are thus dependent upon the mycorrhizal relationship. The mycorrhizal fungi produce phosphatase enzymes that breakdown phosphorus compounds. (The absorption of micronutrients such as zinc and copper is also improved by mycorrhizal association.) The extensive mass of a fungal mycelium produces a huge surface area that allows the fungi to “mine” a much greater amount of soil and duff than the root hairs of the tree are capable of exploiting on their own.

As an example, the mycorrhizae can increase the absorbing surface area of pine seedlings by 80%. Some horticulturists maintain that the absorption surface area of a tree can be increased by 700-1000% by mycorrhizal fungi. Fortunately, there are plenty of these helpful organisms to go around. One scientist in Europe documented 101 species of mycorrhiza fungi associated with a single tree species—Norway spruce (Picea abies)—and 117 mycorrhizal species associated with Scotch Pine (Pinus sylvestris).

See the photo of Redwood seedlings with (right) and without (left) mycorrhizae. Photo credit: Mike Amaranthus, USDA

The B&W illustration is from Roots Demystified. It shows to two types of mycorrhizae-EM (on the right) and the VAM (on the left).

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