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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.
| Ectomycorrhiza on juniper
roots |
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.
 
| Left:Transverse section
of an ectomycorrhiza on roots of Pseudotsuga menziesii showing
the mantle (deep yellow). Right: Cross section of a root
cell completely surrounded by hyphae of the Hartig net. |
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.
The commercial production of
mycorrhiza innoculants is now a reality. In the UK, try Vaminoc
Bonsai Formula from Bonsai Mart. In the USA there is a product
called Transplant One-Step, from ROOTSinc 800 342-6173.
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