Benefits of Mycorrhizae

Research is continuing into the many benefits to plants of mycorrhizal associations and there are probably many yet to be discovered. However, in the light of current knowledge, benefits can be divided into six categories.

Water and Nutrient Uptake

Mycorrhiza greatly increases the roots efficiency at nutrient and water uptake largely because of the vastly increased absorptive surface area. The combined surface area of the millions of hyphae is far greater than that of non-mycorrhizal roots. Increased water uptake by mycorrhizal plants is due to physiological improvements which are made as a result of increased nutrient uptake into the plant.  The water uptake is through the roots, not the mycorrhizal fungi.  In addition, the extending hyphae are able to draw on more distant or inaccessible supplies of nutrients from than the roots can reach. (Bear in mind that the rhizosphere is always in effect nutrient deficient by virtue of the presence of the functioning root which has taken the available nutrients!)

Using radioactively-labeled nutrients, scientists have shown that ectomycorrhizae are especially clever at absorbing phosphate and potassium as well as alkali metals. VAM were shown to be efficient at absorbing phosphorus, copper, iron, zinc and calcium.  Potassium uptake by VAM is also indicated, but is governed by levels of nitrogen, potassium and calcium that are present.

Alleviation of Stress and Disease Environmental and cultural stresses influence the plants susceptibility to and ability to combat bacterial diseases and are known to actually cause some non-bacterial diseases. VAM greatly reduce the environmental stresses - nutritional (too much or too little), drought, root pathogens, soil toxicity etc - which predispose a plant to disease. The increased uptake of nutrients, particularly micro-nutrients which are "locked" to soil particles and unavailable to the roots, make the plant less susceptible to the ingress of plant pathogens, and more resistant to other environmental stresses such as cold and heat. Protection Against Root Pathogens Ectomycorrhiza, in particular, have recently been shown to resist attack by soil-borne pathogens. For example, there are several mycorrhizal fungi known to protect pines from pathogens such as phytophthora, Fusarium and Rhizoctomia. There are several mechanisms by which this occurs, many of which are thought to operate simultaneously. production of antibiotics by the fungus itself, which inhibit root pathogens the physical barrier created by the mantle of ectomycorrhizal hyphae production of chemical inhibitors by the host, induced by their reaction to invasion by the mycorrhizal fungus the establishment of populations of protective microbes in the rhizosphere. Altered Root Physiology Researchers have demonstrated that ectomycorrhizae produce growth hormones and regulators which are responsible for the altered metabolism and growth of the roots themselves. These substances enhance the ramification of root tips, the proliferation of roots, enlargement of cells, and enhanced rooting of cuttings.

Fossilised structures from the roots of early land plants thought to be vesicles of an early endomycorrhizal fungus   Jurassic Mycorrhiza Phyto-archaeologists have discovered correlations between the geologically sudden disappearance of tree species from large areas with the disappearance of evidence of associated mycorrhizae. The interesting aspect is that the mycorrhizae seems to have disappeared first, indicating the potential long-term effect of mycorrhizal deficiency.

Detoxification of Soils
This is still a very sketchy area, as research is still in the early stages. However, scientists are now investigating what appears to be the capacity of mycorrhizae to assist plants to colonize soils which would otherwise be chemically toxic to the plants.  

Maintenance of Soil Structure
Mycorrhizae accelerate the decomposition of primary minerals and secrete organic 'glue' (extra cellular polysaccharides) which bond the finer soil particles into larger, water-stable aggregates.

Significance of Mycorrhizae in bonsai culture

If your bonsai is in a pot that is large enough, with a suitable soil and an adequate regular supply of water, nutrients and micro-nutrients, it's probably in reasonable health and growing well. But that doesn't necessarily mean it's performing to its full potential. One can get so accustomed to experiencing early autumns, weak second growth flushes, mid-summer shut-down, poor leaf condition in late summer and so on, that it becomes accepted as the norm. We're familiar with the benefits to pines from mycorrhiza, but lets see how they can benefit bonsai in general by looking at the five points again.

A portion of endomycorrhizal mycelium bearing some of the very large spores

Water and nutrient uptake
Newly repotted or collected trees don't have access to the entire growing medium simply because their roots don't fill the container. Mycorrhizal hyphae will extend from the existing roots throughout the container in a fraction of the time it would take non-mycorrhizal roots, thus utilizing all available moisture and nutrients. They also regulate the rate of nutrient uptake, thereby reducing the danger of 'root burn'. Later in the season, when the tree's water demand is higher, mycorrhiza can still help, even though the pot appears to be full of roots. Many soil ingredients such as the harder grade of Akadama, calcined (baked) clay, pumice and even some bark chips, are impenetrable to roots. The hyphae, however, are able to penetrate the micro-pores in these particles and and retrieve nutrients and micronutrients stored therein. In addition, they supply these to the tree in a form which the tree can use immediately.

Alleviation of stress
Bonsai, by definition, are always under some form of stress (albeit controlled, hopefully) and this is made all the more significant with the increased usage of non-organic, inert growing media and synthetic fertilizers. Therefore, bonsai are more susceptible to serious damage by disease and stress-related disorders than field-grown trees. Symptoms such as mid-summer shut-down and early autumn, or discolored and tired foliage are all indications of stress or stress-related disease. In fact, if was only one category of plant crying out for the additional protection offered by mycorrhizae, it would be bonsai.

There is evidence to suggest that VAM increase resistance to some viral diseases, but no evidence they can decrease occurrences of viral diseases.  In certain cases they can limit the severity of attack.

Protection against root pathogens
Traditional bonsai wisdom states that if there's a problem with the tree's vigor, the cause is in the roots. Not all root pathogens are fatal - but more become fatal in a bonsai container than in the field, simply because of the slow rate of root growth and absence of the roots of other plants. Good tool, pot, soil and water hygiene, plus the choice of reputable organic fertilizers, should prevent most soil pathogens from entering the container. However, some are air-borne and can arrive at any time, but many of these are unlikely to become a danger in a good bonsai soil. That still leaves the the few that could become a danger. The added protection afforded by mycorrhiza could give the bonsai grower the confidence to say that if there's a problem with the tree, it's probably NOT caused by the roots.

Altered root physiology
Increased ramification, increased root proliferation, enlargement of cells (greater efficiency) and enhanced rooting of cuttings. Need more be said?

Detoxification of soils
Once again, good soil and water hygiene should eliminate the possibility of accidental toxicity of bonsai soil. But there is some concern that calcined clays and other mineral soil ingredients can accumulate a toxic level of salts which could eventually harm the plant. If mycorrhizae can assist here, and it's not yet certain that they can, then better with than without!

Maintenance of soil structure
As bonsai soil's organic matter - as well as it's akadama, loam or clay content - naturally breaks down into fine particles, they are re-bonded by the mycorrhiza's polysaccharide secretion, thus maintaining an open, free-draining and well aerated soil. You may have noticed how the soil in the pot of a pine with mycorrhiza is more 'friable' and granular than that in the pot of a non-mycorrhizal pine.

Mycorrhiza inoculation

We all save some old mycorrhiza from our pines, and re-introduce it into the new soil when we repot. Does this work? Well, yes it does.

In fact there could well be enough spores, chlamydospores, sclerotia, rhizomorphs and resting hyphae left on the remaining roots to colonize the pot ten times over. But because you have pruned away the root tips, where the mycorrhiza forms, and your loose, granular soil has left you with an almost bare-rooted tree, you can never be sure, so re-introducing it is a very good idea.

The same goes for other species with endomycorrhiza, which you can't see. Re-introducing chopped-up pieces of the pruned-away root tips will help to ensure re-colonization of the pot.

However, there is one other important point. Remember we discovered that when the fragments or spores germinate, they are stimulated to do so by the microbial changes in the rhizosphere - which you don't have in your new soil and clean roots. The roots that the inoculated mycorrhiza is adhered to are now dead. One answer is to make sure that, when you introduce the chopped-up mycorrhizal roots, they are in good close contact with living FEEDER roots. Another is to include a proportion of the previous soil in your new mix. Since the entire pot was probably completely filled with roots, practically all the soil would qualify as rhizosphere.