1. Definition of Mycorrhiza
Botanically MYCORRHIZA is the mutualistic symbiosis (non-pathogenic association) between soil-borne fungi and roots of higher terrestrial plants. The word Mycorrhiza (fungus root, from Greek: mykes (mushroom) and rhiza (root)) was coined by FRANK (1885) to describe the mutual association of two different species to form a single, morphological organ, where the host plant provides carbohydrates to the fungus, and the fungus explores the soil for nutrients and, in turn, delivers them to the plant.
Two main types of Mycorrhiza are differentiated:
ectomycorrhiza and endomycorrhiza. Ectomycorrhiza are fungies of the
botanical class Basidiomycetes and Ascomycetes. They mainly associated
with forest trees in temperate climate zones. This kind of Mycorrhiza
is very important for the establishment and vitality of forest trees.
The most important type of Endomycorrhiza is the so-called arbuscular
Mycorrhiza (AM) denominated by arbuscles formed by the AM fungi in
cortical cells of roots (Fig. 1). The AM fungi were recently organized
in the new phylum, the Glomeromycota, which now comprises approximately
180 species. All of them are obligate biotroph symbionts, hence they
can only be cultivated on living plant roots. AM is the most wide-spread
and probably the most important symbiosis in the world. AM fungies
are present in all terrestrial ecosystems and more than 60% of all
species of the plant kingdom rely to a very large part on AM to maintain
growth.
Fig. 1: Arbuscles formation of VAM fungus in cortical
cells of roots. Fungal structure appears blue after a staining process.
Photo: Dr. Sieverding
2. The biology of Vesicular-Arbuscular Mycorrhiza (VAM)
A basic requirement for the manipulation and management
of VAM is knowledge of their biology and of the development of the
infection of plant roots, and of their identification and occurrence
in the plant kingdom.
VAM infection on roots is only visible after a staining process, where
the fungal structures turn blue. Following, they can be observed and
eventually quantified with a microscope at a >50fold magnification
in transmitted light (Fig. 2). The mycelium outside the roots is responsible
for the uptake and transport of nutrients to the host plant. In contrast,
the mycelium inside the roots provides the exchange of nutrient (taken
up from the soil) from the fungus to the plant and the receipt of
photosynthetic products from the plant to the fungus. As already pronounced,
vesicles are fungal storage organs, where lipids are stored, which
are used by the fungus in times of low supply with photosynthetic
products from the plant.
Fig. 2: Fungal structures in roots turn blue after
a staining procedure. Note the root external (ext) and internal (int)
mycelium, arbuscles (arb) and vesicles (ves).
Photo: Dr. Sieverding
Spores of the fungi (Fig. 3) are formed in the root external mycelium and sometimes in roots (Fig. 4). Spores can survive in the soil for a very long time and they serve as fungal propagules. The morphological characteristics of spores are often used to taxonomically identify the respective fungal species. Infective propagules are also fungal mycelium, which also can survive in soils for some time, and infected living roots or dead root segments with fungal structures, which remain in soils after a plant has been harvested, or dies at the end of a season. The named infective propagules germinate under favourable conditions of soil humidity and temperature, and can infect a newly formed growing root. The process of germination and infection can last up to 5-10 days. Products of the BioMycTM International Corporation contain all three sources of infective propagules.
Fig. 3: Formation of spores of the AM fungus Glomus
intraradices in the root external mycelium.
Photo: Dr. Sieverding
Fig. 4: The AM fungus Glomus intraradices frequently forms spores in roots.
Photo: Dr. Fritz Oehl
3.Function in soil aggregation
Fertile soils have a high percentage
of stable aggregates (BURNS and DAVIES, 1986). VAM fungi can bind
and aggregate soil particles through the intensively growing mycelium.
SUTTON and SHOPPARD (1976) showed that Mycorrhiza plants grown in
sand dunes aggregated five times more sand at the roots than plants
of equal root biomass but without VAM association. The formation of
aggregates can be important to improve physical soil conditions.
Hence, the aggregation of soil particles by VAM fungi is a potential
instrument to control and minimize erosion of soils. The function
of VAM fungi for soil aggregation has often been underestimated. Today
it is known that VAM mycelium not only loosely aggregates soil particles,
but also that the hyphae are bound to them through amorphous polysaccharides
(BURNS and DAVIES, 1986).
4. Function in climatic stress situations
Common climatic stress situations
in the tropics and subtropics are high temperatures combined with
high evapotransipiration rates, which often results in droughts.
Soil temperatures of 25°C to 30°C and soil water content of 40-80%
of the maximum water holding capacity were found to be the optimum
for VAM development and effectiveness (SIEVERDING, 1980). These ranges
are within those of the physiological optima for the growth of most
tropical plants.
However, in some tropical regions the diurnal changes in temperature
can be dramatic and differences of 40 K - 45 K can be observed during
one day. When soil temperature rises to this high level, VAM development
and plant productions are markedly reduced. On the other hand, these
high temperatures will not usually be found in soil zones deeper than
5 cm, due to the thermal damping capacity of the soil, and hence may
not seriously affect the Mycorrhiza there. Temperatures of less than
17°C-18°C, common in tropical highlands, may be more problematic,
because the VAM fungal effectiveness is then reduced.
Water stress is often considered to be a more severe problem in the
tropics than high temperatures. Several beneficial functions of VAM
on water relation of plants have been reported (see NELSON, 1987)
such as: decreased resistance to hydraulic conductivity, positive
effects on phytohormones and stomata regulation, and a finer more
branched root system. The effects of VAM fungi on transpiration rates
reported in literature are contradictory, sometimes increasing and
sometimes decreasing in comparison with non-Mycorrhiza plants.
The results of most investigations clearly suggest that the improved water relation of VAM plants is an indirect effect via improved plant nutrition, especially phosphorus (P) nutrition. Potassium (K) uptake can be enhanced too (SIEVERDING and TORO, 1988), and it is well known that this element plays a fundamental role in water regulation of plants (MENGEL and KIRKBY, 1982). There is no evidence that water is transported by VAM fungal hyphae: hence, it is unlikely that VAM fungi can make water directly available to the plant.
On the other hand, at maximum transpiration the root diameter shrinks and the water film around the roots is ruptured (BERNSTEIN et. al., 1959). It is suggested that, in this case, VAM fungal hyphae may function as physical bridges and that they may maintain the contact between the root and the soil water: in this way the preservation of a water film (and flow to the root) and the transpiration may be maintained (SIEVERDING, 1980).
The improved plant nutrition via VAM can have considerable influence on drought resistance of tropical crops. Plants grow faster, and through an in tenser, deeper root system water can be extracted more efficiently. Also, the water use efficiency, i.e. dry matter production per unit available water (either by rainfall or irrigation) is significantly increased. There is also evidence that VAM plants recuperate faster after short period of water stress than plants without Mycorrhiza.
In many regions of the tropics it
is of great importance that plants are able to survive during shorts
periods of water stress (10 days without rainfall can seriously damage
beans, for example). The rapidly decreasing water content in the uppermost
(0-10 cm) soil layer during short periods of drought naturally affects
the growth of feeder roots in this chemically more fertile soil horizon.
There fertilizers are mainly located. However, the availability and
diffusion of elemental nutrients decrease in the case of drought.
It appears that under such conditions VAM fungal hyphae are more resistant
to drought than the nutrient-absorbing feeder roots of plants (AHMADSAD,
1985). After new rainfall, nutrients are once again present in the
soil solution and these can immediately be taken up by the still functioning
fungal mycelium.
NELSON (1987) rightly said that the improved plant growth and the
increased transpiration due to an effective VAM association can be
detrimental to plant production when water supply is limited. Bigger
plants consume available water in a shorter period of time, and thus
will suffer more severely from stress. In this case VAM reduces resistance
to drought stress. On the other hand, one important function of VAM
fungi may be that, due to the faster and increased growth, plants
can make better use of short periods (2-3 months) of optimum climatic
conditions. Plants thereby elude adverse following climatic stress
5. Function of VAM for nutrient uptake plants
The nutrient uptake of a plant is
mainly determined by the elemental absorption capacity of the root
and by nutrient diffusion and subsequent delivery of elements in or
to the soil solution. The absorption rates of ions with high mobility,
such as NO3-, from the soil solution are plant-species-and
cultivar-specific (BARLEY, 1970). The capacity for the uptake of ions
with low velocity of diffusion, i.e. for P, Zn, and Mo and to a lesser
extent for K, S, and NH4+, depends on the root density
per volume of soil. In the latter case the root morphology and the
external mycelium of VAM fungi determine the elemental uptake rate
for a plant. Several investigations (see COOPER, 1884) have shown
that the VAM fungal hyphae are not able to extract other elemental
nutrients from the soil solution than those which can also be taken
up by the non-Mycorrhiza root (Fig. 5). Hence, the principal function
of Mycorrhiza is o increase the soil volume explored for nutrient
uptake and to enhance the efficiency of nutrient absorption from the
soil solution.
In older literature it was very often stated that the host plants
benefit from VAM by incorporation of fungal metabolites in the process
of digestion of degeneration VAM fungal structures in the root cells.
In fact, only 1% (or even less) of the total benefit of VAM goes to
plant nutrition (COX and TINKER, 1976).
Fig. 5: Hypothetical increase of soil volume explored
by VAM fungi (assuming that VAM fungi grow radially around roots)
6. Increase of the rhizosphere by VAM Mycorrhiza
At the same time that VAM is formed
in the roots, VAM fungi develop a mycelium around the roots. Internal
and external fungal hyphae are in contact with up to 10 entry points/
cm root (OCAMPO et. al., 1980). Connecting points can be much less
numerous under natural conditions; our own experience showed that
it is often difficult to find them in the root system. The external
mycelium can grow to a considerable width in the soil (a distance
of 8 cm from the root has been proven but longer extensions are also
expected).
There is still no information available on the density of the external
mycelium with increasing distance from the root; indirect methods
of measurement suggest that the mycelium density is highest 0-2 cm
from the root. It is likely that fungal species (ABBOTT and ROBSON,
1985) and affected by plant and soil factors (KOUGH, 1985). There
is little information on hyphal density in natural soils. OROZZO et.
al. (1986) found 5-39 m VAM fungal hyphae ml-1 in an organic
tropical forest soil and SYLVIA (1986) reported an average of 12m
hyphae g-1 soil in a subtropical dune ecosystem. OROZZO
et. al. (1986) calculated 0,03-0,98g fungal biomass dry matter in
tropical forest ecosystem whereas SYLVIA (1986) estimated 200-1000m
of VAM hyphae cm-1 VAM-infected root length of sea oats.
Through the external mycelium, contact of the root with the medium
in which it grows is considerably increased (Fig. 6). When calculating
that 1 cm root without Mycorrhiza can explore about 1-2cm³ soil volume
with the aid of root hairs, this volume is potentially increased 5-200
times by the root-external mycelium, assuming radial growth of VAM-hyphae
around the root. Increased rhizospheric soil volume of about 200 cm³
cm-1 infected root may be an exception, but 12-15 cm³ cm-1 infected root is commonly observed.
Furthermore, VAM fungal mycelium appears to be more resistant than
the root itself to a biotic stresses such as drought, toxicity of
elements, and soil acidity (AHMADSAD, 1985). A plant with Mycorrhiza
remains in close contact with the soil for a longer period of time
than a plant without it. The life span of external mycelium is not
known, but the percentage of living external mycelium appears to decline
rapidly 3-4 weeks after the first infection of the plant by the fungus
(SCHUBERT et. al. 1987).
Further Scientific Material:
http://mycorrhiza.ag.utk.edu/
http://www.invam.caf.wvu.edu
http://www.ffp.csiro.au/research/mycorrhiza/vam.html
