[OGD] orchids of Alaska / mycorrhizal relationship

"Southeast Alaska has several species of orchid...

the white bog orchid, whose tall inflorescences send out... aroma.
This species is probably pollinated by moths that come to collect nectar.

The calypso or fairy slipper orchid draws in bumblebee pollinators by 
looking lovely and smelling sweet, as if it offers nectar, but it has none.
Visiting bees learn quickly that these flowers offer no food reward, so 
successful pollination depends on a supply of inexperienced bees...

Calypsos, and perhaps other orchids, should not be picked, because that 
tweaks the delicate root system and is likely to kill the plant...
...
*******************
In addition to a wide variety of relationships with pollinators, orchids... 
have an intimate relationship with fungi that connect with their roots.

This mycorrhizal (fungus-root) relationship is classically thought to be 
mutualistic: both partners get something from it.

The fungus gets carbohydrates from the plant, which typically has 
chlorophyll (green pigment) and synthesizes sugars that the fungus cannot 
make for itself.

The flowering plant gets nutrients that the fungus gleans from the soil or 
decaying organic material.
...
All orchids produce minute, dust-like seeds.
The seeds are so tiny that they contain almost no stored carbohydrates or 
other nutrients that are needed for germination and growth.

They rely on mycorrhizal associations to provide the nutrition needed for 
germination and initial growth.
Thus, all orchids begin their lives as parasites, not mutualists, of fungi 
(the fungus gets nothing from the seed).
...
Some orchids have no green pigment, so they canâ??t photosynthesize 
carbohydrates to give to the fungus.
These species remain parasitic on their fungi throughout their lives.
The fungus may extract nutrients from the soil and decaying vegetation.
However, in many cases, the root-associated fungus acts as a conduit for 
carbohydrates and other nutrients from a tree (which does have green 
pigment and can synthesize carbohydrates).
So the orchid then is also indirectly parasitic on the tree to which it is 
connected.

For example, in the yellow coralroot orchid (Corallorhiza trifida)... the 
fungal associate connects the roots of several species of tree to the 
orchid, and the orchid thus pirates nutrients from the trees.

Even orchids with green pigment and photosynthetic ability may extract 
carbohydrates from the associated fungus (and a connected tree) without 
giving anything back. So they too, are at least semi-parasitic, in many 
circumstances.

In a further evolutionary complexity, many orchids â??eatâ?? their fungal 
associates, digesting the ends of fungal filaments that connect to the orchid.
If the orchid does no photosynthesis, it thus seems to be destroying at 
least part of its essential source of nutrition.

Even if the orchid can photosynthesize carbohydrates, digestion of 
filaments would interrupt the derivation of materials by the orchid from 
the fungus or a connected tree, at least partially.

The digestion of fungal filaments opens up many questions, to which... no 
concrete answers in the literature (although this fact has been known for 
over a hundred years):

Why would the orchid destroy a major source of nutrients?
Is it not needed any more?
Or are only certain filaments eaten?
What is the contribution of the digested filament itself to orchid 
nutrition, compared to what the filament delivers from a tree?
What is the effect of filament digestion on the fungal organism?
Does destruction of the orchid-connected filament tips affect the growth 
and reproduction of the fungus, as well as limiting its expansion in the 
orchid roots?

Whether parasite or mutualist, some orchids keep their mycorrhizal 
associations all their lives,
some change their fungal associates as they grow, and some apparently 
become independent of fungi as they mature (especially if they grow in rich 
soil with good sunlight).
...
Within some orchid species, genetically different individuals have their 
own, specific mycorrhizal associates.
...
different genetic types of the spotted coralroot (Corallorhiza maculata)...
are reported to have different mycorrhizal associates, accompanied by 
subtle differences in floral shape.
A given population of this coralroot orchid may contain several genetic 
types (or races) of the orchid, each with its own floral features and 
fungal associate, potentially deriving nutrients from a variety of trees.
...
the orchid family encompasses many thousands of species, hugely diverse in 
floral structure, as well as habitat, leaf shape, life history, and so on. T
he traditional explanation for the great diversity is adaptation to an 
equivalent diversity of pollinators.

For instance, both of ... [Alaska] species of coralroot orchids are 
pollinated by small insects such as dance flies, in contrast to the 
bee-pollinated calypsos and the moth-pollinated bog orchids.
However, it has recently been suggested that some of the great 
diversification of orchids may be related to adaptations to different 
fungal partners.

The variety of floral design and fungal association within the single 
species of spotted coralroot suggests that this may be a step toward the 
origin of several new species."

URL : http://juneauempire.com/outdoors/2011-07-07/orchid-variations

photos :

1) caption : ["The yellow coralroot cannot synthesize carbohydrates and is 
totally dependent on its fungal associate."]
http://juneauempire.com/sites/default/files/imagecache/superphoto/222876.7166455.jpg


2) caption : ["The fairy slipper... has green leaves, but may still use its 
mycorrhizal to obtain nutrients."]
http://juneauempire.com/sites/default/files/imagecache/superphoto/222877.7166461.jpg
 


********************
Regards,

VB

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