IN RECENT YEARS we've read and heard a lot about the benefits of mycorrhiza in association with pines - how that tacky-looking white fluff can actually benefit the tree. But this is only part of the story and there is a lot more to this miracle of nature.

What are Mycorrhizae?

Before we go further, lets just examine exactly what we mean by mycorrhiza.

The term mycorrhiza (plural mycorrhizae) is rather like the term "marriage". It describes an association, a relationship. If both parties to the relationship are compatible, and conditions suitable, they will both benefit. Otherwise the relationship is of no benefit to either and it ceases.

One party to this relationship is your tree, the other is a microscopic beneficial fungus. The fungus forms a sheath-like structure at the root tips through which it passes to the tree various nutrients it has gathered from the soil in exchange for food the tree has produced through photosynthesis (remember, fungi are not able to produce their own food in this way).

In fact almost all vascular plants (which excludes mosses, other fungi etc) benefit in nature from a mycorrhizal association of one kind or another. Although mycorrhizae are by no means essential to the well-being of any plant, their associations are of tremendous benefit in less than ideal circumstances. For example, a tree planted in fertile, moist yet well-drained soil with a good supply of readily available nutrients will already be growing at its maximum rate with maximum health, and so has little need of mycorrhiza. Indeed, as we will see later, mycorrhizal fungi would probably not survive for long in such conditions anyway. On the other hand, trees planted in marginal conditions would probably not survive without a mycorrhiza and it is in these conditions that mycorrhyzae will thrive.

This begs the question; "Is bonsai soil in a bonsai pot ideal or less than ideal?" The answer has to be less than ideal. Bonsai containers provide "marginal conditions" for any tree and it's only the dedication and knowledge of the grower that enables the tree to thrive. In a bonsai pot the roots are subjected to the extremes of temperature - becoming as cold as the ambient temperature in winter and as hot as a tin roof in summer. They are also subjected to daily drenching and drying during the growing period. The soil is largely inert or lifeless (Akadama, baked clay, grit, pumice) and any nutrients are rapidly leached out with daily watering. These are exactly the conditions in which mycorrhizae can be of profound benefit to plants.

Are Mycorrhizae species-specific?

Not as a rule. There are a few mycorrhizal fungi species that will only associate with one host species, but the vast majority have a broad range of potential hosts. Likewise, virtually all plants - and almost certainly all trees - are perfectly happy to form mycorrhizal associations with a number of different fungi, the eventual choice being dictated largely by which fungi are available in those particular soil conditions.

A closer look

Types of Mycorrhiza
There are two major types of mycorrhizal fungi based on the anatomy of their association with the host roots: ectomycorrhizae and endomycorrhizae.

Ectomycorrhizae typically grow in the intercellular spaces of the root cortex (outer layer or "skin") and for a thick mantle of tissue around the exterior of the root tip. Some hyphae (fine, thread-like filaments) extend out from the roots and into the surrounding soil to gather water and nutrients. The network of intercellular filaments, the Hartig net, forms the exchange sites where the host swaps carbohydrates for nutrients from the fungus. Ectomycorrhizae occur primarily on members of the Pinaceae, Betulaceae and Fagaceae families.

Endomycorrhizae grow mainly inside the cortical cells (INTRAcellular spaces These don't form any external mantle so they are impossible to detect with the naked eye, but they do also send out extensive hyphae into the surrounding soil.

Some endomycorrhizae form structures called vesicles and arbuscles within the root's cortical cells. These are known, naturally enough, as vesicular-arbuscular mycorrhizae, or VAM for short. This is the type of mycorrhiza we find on 90% of the world's higher plant groups. The arbuscles are tightly bunched hyphae which take carbohydrates from the cells, growing as they do so. Once they have completely filled the cells, they break down, releasing their nutrients to the host and the fungus proceeds to colonise another cell. As for vesicles - nobody has yet discovered their function.

There is a third group called ectendomycorrhizae which, as you might have guessed, combines some of the features of the other two groups.

How do Mycorrhizae get there?
First, the potential fungal symbiont must produce viable mycelium (see panel on Anatomy of Fungus, below) in the vicinity of the roots of the potential host. Usually this involves the germination of either spores or "resting" hyphae. This mycelium must then find its way to the roots of the host, which it does not entirely by chance. The area of soil around a plant's roots - the rhizosphere - contains millions of minute organisms (microflora) which are influenced by the presence of the roots. By detecting this, the mycelium can navigate its way to the roots remarkably efficiently. Having reached the roots, the mycelium must penetrate them.

Reproduction
VAM reproduce from chlamydospores, which are long-lived, thick-walled spores, produced by the fungus near the surface of the host root, and are able to withstand the rigors of underground life until the roots of a potential host grow close by.

Ectomycorrhizae can reproduce from spores or vegitatively from various types of clusters of hyphae or from resting hyphae.

In all cases, germination is stimulated by near proximity of roots of a potential host plant, via their effect on the microflora in the rhizosphere.

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 absorbtive surface area. The combined surface area of the millions of hyphae is far greater than that of non-mycorrhizal roots. In addition, the extending hyphae are able to draw on more distant or inaccessible supplies of water and 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-labelled 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 and calcium.

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.

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 colonise soils which would otherwise be chemically toxic to them.

Maintenance of Soil Structure
Mycorrhizae accelerate the decomposition of primary minerals and secrete organic 'glue' (extracellular 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.

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 utilising 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 fertilisers. 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 discoloured 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.

Although there is no evidence to suggest that VAM increase resistance to or decrease occurance of viral diseases, they do seem to limit the severity of attack.

Protection against root pathogens
Traditional bonsai wisdom states that if there's a problem with the tree's vigour, 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 fertilisers, 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 proliferaton, 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 colonise 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 recolonisation 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 innoculated 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.