Nelson Bays Mycorrhizas www.mycorrhizas.co.nz

MYCORRHIZAL SYMBIOSIS: "myco" means "fungus", & "rrhiza" means "root", i.e "mycorrhizal symbiosis" literally means "fungus-root partnerships"

 

The following sub-topics of mycorrhizal symbioses are detailed & brief analysie of 2-species / population interactions, examining the effects or outcomes of plant-plant, plant-fungal, & plant-fungal-plant relationships & environmental impacts on the growth & survival of 2 species & or populations.

 

Mutually beneficial plant-fungal interactions (+/+): "mutualisms" or "proto-cooperation" ?

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Symbiotic or 'host' plants with actively photosynthetic shoots & or leaves provide roots & their mycorrhizal fungal partners with Carbon, i.e sugars / energy. In exchange mycorrhizal fungi provide their plant 'hosts' with mineral nutrients and water.

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In the scientific literature describing mycorrhizal symbiosis, it is relatively common for researchers & educators to refer to mycorrhizas (aka "mycorrhizae") as "mutualisms".  Is this regular use of the term 'mutualisms' to describe mycorrhizal benefits an example of subjectivity of science writers? ... or an ethusiastic passion for all things symbiotic? ... or else an attempt to redress a common opinion amoung growers that all or most fungi are crop diseases, pests, parasites or decay organisms, instead of being a symptom of healthy soils & plant roots?

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In contrast to the energy needs of most obligately symbiotic  mycorrhizal fungi, most plants can use photosynthesis to produce their own energy needs.  With very few exceptions, most plants are able to survive & reproduce in soils or 'soil-less' growing media that contain adequate plant available nutrients. Population ecologist the Late Eugene Odum used the term "prot-cooperation" to define non-obligate mutually beneficial inter-species relationships. 

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Commensal (0/+) plant to plant interactions via a shared  mycorrhizal fungal partner:

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The image above visually illustrates that different aged Pinus sylvestrus seedlings can share a common underground network of the same species of ecto-mycorrhizal fungi. Read (1997) The laboratory use of glass-sided chambers as used to provide the above image, are sometimes also fitted with an ultra-fine nylon mesh to exclude roots but allow smaller diameter fungal mycelia to connect adjacent soil compartments & plants.

 

Radioactive chemical isotope tracer based evidence can been used to corroborate visual based root chamber observations in the laboratory. However it is more difficult but not impossible to conduct similar field studies to show that underground networks of mycorrhizal soil mycelia can connect different plant species compatible with the same species of mycorrhizal fungiplant & enable plants to be connected to one another by a common mycelium inter-plant transfer. Studies of Carbon, Nitrogen,& Phosphorus inter-plant transfer via interconnecting mycelia has been measured frequently in laboratory experiments, but it is not as easy to prove whether such transfer is bidirectional, whether there is a net gain by one plant over its fungal connected partner, or whether trans-fungal transfer affects plant performance in the fielld.

 

Using laboratory-based isotope tracer studies it has been shown that the magnitude of one-way transfer can be influenced by shading of 'receiver' plants, fertilization of 'donor' plants with phosphorus, or use of nitrogen-fixing donor plants and non-nitrogen-fixing receiver plants, thus indicating that movement may be governed by source–sink relationships.

 

Parasitic (+/-) fungal/plant interactions? ; & or Competitive (-/-) resource type fungal/plant interactions?

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During winter in temperate climates deciduous plants lose their leaves & thus are unable use photosynthesis to make sugars to share with mycorrhizal fungal partners. The reduced availability of winter season sunlight causes simillar changes in the available energy budgets & reduced photosynthesis ability of evergreen plants & their root-fungal mycorrhizal partners. Under these light-limiting seasonal circumstances, it is possible to record evidence of reduced growth in plants that have mycorrhizal fungal partners, & likewise increased plant growth in the same plant species grown under identical environmental conditions, but without mycorrhizal fungal partners. 

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Amensal (0/-) or (-/0) fungal/plant interactions  ( & or absense of fungal/plant symbiotic inter​actions )

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Mycorrhizal fungal partners that do not have a capacity to decompose non-living organic matter are obligately dependant on living 'host plant' partner(s) for photosynthetically derived energy (carbohydrates).

 

Many seedling plants & mature plants are capable of being  grown in soils or in soil-less growing media if provided with required plant nurtrients, but in the absense of a mycorrhizal fungi. Such non-symbiotic (axenic) plant growing conditions may have negligible or neutral (0) effects on plant growth, health, or survival; whilst the absense of a plant host is detrimental (-) to the extent of being prohibitive of mycorrhizal fungal populations. Thus, as defined by population ecologists the Late Eugene Odum, & Arthur Boughey, the absense of plant/arbuscular mycorrhizal fungal interactions is a (0/-) amensarelationship.  

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Similarly, there is an increased use of anti-fungal or mycorrhizal inhibitive, i.e. (0/-) amensal horticultural practices, including soil fumigation, fungicides, intensive soil tillage, & mono-culture rotations of 'non-host' crops & green manures, & 'non-host' weed infestations (e.g. brassica, beet & amaranth families, lupins & buckwheat) in arable crops, horticultural orchards, & market garden crops.

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Allelopathic (+/-), &  or Mutually Inhibitive, or Directly Competitive (-/-) plant/plant & or fungal/plant symbiotic interactions

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Some plant species produce chemicals (poisons) in their leaves & or roots that when emitted into adjacent soils inhibit the germination of plant seeds, or else act as 'natural herbicides' to adjacently growing plants of the same or different plant species, this phenomenon is called "allelopathy".  

 

In the dry soils of the 'McKenzie Coutry' inland district in the South Island of New Zealand, a common invasive exotic weed Hieracium pilosella aka 'mouse-ear hawkweed' is an 'auto-allelopathic' (i.e. self-inhibiting) problem plant species which displaces most plants in close proximity.  "Hawkweed" is a perenial weed belonging to the Ateraceae family with a short growth habit that is, creeping, rosette-based, mat-forming, often in dense colonies that exclude all other vegetation. Hawkweed (Hieracium pilosella) & similarly Black Walnut (Juglans nigra) are common examples of alellopathic (+/-) relationships, i.e. iether benefiting (+) itself whilst inhibing other plants (-); or else 'auto-allelopathic'& mutually inhibitive plant/plant direct competition (-/-) within it's own species.

 

Although ectomycorrhizal dominant forest trees are rare in New Zealand, & thereare very few native deciduous trees, native ecto-mycorrhizal beech forests (Nothofagus genus) can often form relatively monociultural plant communities, i.e low species diversity forest floors. However, compared to Northern Hemisphere deciduous & coniferous forests, it is relatively more rare in New Zealand to encounter fungal/plant alleopathic (+/-)) interactions. Fungal/plant allelopathy is more in European countries with a heritage of naturally occuring ecto-mycorrhizal fungi that produce very valuable underground truffles (aka 'endogonaceous') mycorrhizal fungal fruiting bodies. 

 

The terms 'brûlé' and 'burnt' are used to describe vegetation-devoid areas of the ground around a range of woody plants interacting with certain truffle fungal species. There is compelling evidence that ecto-mycorrhizal mycelia, & fruiting bodies of brûlé-forming truffles have evolved diffusible metabolites for their survival, typically characterized as having harmful effects on weeds, impairing seed germination, altering root morphogenesis and plant hormonal balance, or inhibiting the native rhizospheric microflora regularly associated with the brûlé. These effects can be widely interpreted as (+/-) allelopathic fungal/plant phenomena.

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Thus, 'mycorrhizal symbiosis' is not always a mutually beneficial (+/+) interaction, but instead is more accurately descibed as a continuum of 2-species interactions (+/+); (+/0); (0/0); (-/0); (-/-), (+/-) & (-/+) or  relationships; & is more often being depicteded as such by a growing number of mycorrhiza scientists ("mycorrhizologists") including:

1. Prof. Nancy Collins Johnson, Northern Arizona University, Flagstaff, U.S.A.  http://nau.edu/cefns/natsci/seses/faculty/johnson/

2. Prof Andrew Smith, University of Adelaide, Australia  http://www.adelaide.edu.au/directory/andrew.smith

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For further analyses of population ecology issues adressed above, see information & images in other sections this website including:

• SYMBIOSIS 

• INOCCULA

• GROUND-COVERS 

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