Prechat handout:

Mycorrhizae - What are they?

Mycorrhizae or more correctly, endomycorrhizae, are filamentous fungi that assist with the
germination and growth of orchid seeds and plants. The mycorrhizal fungi that assist with
germination can be very host-specific. The many fungi that inhabit the roots of mature orchids
can include some which would not be expected to assist with germination.

How do the fungi enter the seed or roots?

Orchid protocorms and orchid roots produce tiny hair-like structures called rhizoids or root
hairs. Typically, a thread or hypha of the fungus enters a root hair near its tip and moves along
the hair toward the protocorm or the root.

What function do mycorrhizal fungi serve?

Protocorms are dependent on the mycorrhizal fungus for early nutrition. Many terrestrial orchids
exist for years below ground and completely dependent upon their fungal partner. Once the fungi
enter protocorms or roots, they form coils (pelotons) in certain cells, such as the cortex cells of
the root. At first the coils are easy to distinguish microscopically but as the orchid begins to
digest the fungus, the peloton becomes an amorphous blob. The peloton absorbed by the host
orchid as food. More and more pelotons form, are absorbed, and the orchid grows. Once orchids
become photosynthetic and have sufficient reserves to support their life, the fungi may become
less important but much investigation is still needed to clarify this. There is evidence that the
mature Coral Root (Corallorhiza trifida) receives nutrients via its mycorrhizal associates. Mature
photosynthetic plants may benefit from mycorrhizae including access to mineral nutrients, water
and protection from root pathogens.

What function does the orchid serve the mycorrhizal fungus?

This is good question for which we have yet to have a good answer. Some say the orchid
parasitizes the fungus but many orchid seedlings are parasitized by the fungus before they reach
maturity. Why? The best guess is that the relationship is one of delicate balance that can easily
be disturbed.

Do artificially propagated orchid roots ever become infected with mycorrhizal fungi?

If we examine the roots of epiphytic orchids that have been growing in potting medium for
several months, we can often find fungal pelotons in the root tissue. Such evidence demonstrates
that secondary infections do occur but what benefit these relationships may have is as yet
unclear.

What are the functions of roots?

Orchid roots do more than anchor a plant to a tree or keep it snug within a pot. Roots are the
primary structures used by a plant to absorb water and mineral nutrients. In leafless orchids,
orchids serve the dual function of anchor and photosynthetic organ. Roots are used as storage
organs especially in monopodial orchids such as Phalaenopsis and Vanda, and also in deciduous
orchids such as Cypripedium. Roots store starch and water. In orchids that grow in seasonally
harsh and dry environments, seedlings may have a very short time to become established. Seeds
can germinate very quickly, seedlings of some species producing large thick storage roots first,
then leaves and stems in subsequent seasons.

What is the difference culture-wise between fine-rooted and thick-rooted orchids?

Orchids with fine roots such as many of the Oncidiums and Tolumnias require finer more porous
potting mix or are perhaps best mounted to permit the fine roots to ramble over the bark. Plants
with thick roots require special care. It may seem that a plant has more than enough roots but
root pruning may seriously set back a plant. We are not generally attempting to 'bonsai' an
orchid. Our goal is to get blooms and bloom production is dependent on an adequate nutrient
store to support the blooms.

How does one deal with orchids exhibiting seasonal root growth?

Some orchids grow year round, others seasonally. Where growth is more or less continuous as in
Phalaenopsis, re-potting can be safely done year round. With orchids whose growth is seasonal,
care must be taken to repot when roots are just starting to grow otherwise we risk damaging or
interfering with root development. Catasetum, Pleione, certain Dendrobiums and Cattleyas
produce roots once a year. These should be repotted when root initials become visible or shortly
before.

Why should we conserve roots when repotting an orchid?

Roots are important organs of absorption and absorption. Loss of some of the absorptive surface
area might critically affect the ability of the orchid to recover after potting. Loss of stored
nutrient can also be deleterious. Many orchids use the roots to store reserves. Removal of many
roots reduces the food and water store of a plant. Without this storage capacity, a plant may need
perfect growing conditions and much time to be able to recover. Some ways that we can employ
to limit root loss when re-potting is the potting-on technique where the mounted plant is attached
to the new mount and allowed to grow onto it.

 Discussion Begins:

MarilyninOttawa
I have devised this session to be a series of questions and answers. Please feel free to pose further questions but remember that I may not have all the answers!

Mycorrhizae - What are they? Mycorrhizae or more correctly, endomycorrhizae, are filamentous fungi that assist with the germination and growth of orchid seeds and plants. The mycorrhizal fungi that assist with germination can be very host-specific. The many fungi that inhabit the roots of mature orchids can include some which would not be expected to assist with germination.

How do the fungi enter the seed or roots? Orchid protocorms and orchid roots produce tiny hair-like structures called rhizoids or root hairs. Typically, a thread or hypha of the fungus enters a root hair near its tip and moves along the hair toward the protocorm or the root.

All orchid roots produce root hairs. These are more easily seen with terrestrial orchids and on the roots of seedlings in flask. As roots mature, and especially with epiphytes, the root hairs are confined to the part of the root next to the substrate and often only in the younger parts of the root. Here we see root hairs of the terrestrial orchid, Epipactis helleborine. If you look closely, you can just make out a fungal hypha within one of the hairs.

Here we see the fungal thread within a root hair of terrestrial wetland species, Pogonia ophioglossoides. The fungus may be Epulorhiza sp.

The roots of this orchid ramify throughout the substrate composed of sedge and other herb roots, mosses and decaying vegetation. Although we cannot see the root hairs with the naked eye, close examination would reveal an extensive network of fungal links between the orchid and the substrate.

What function do mycorrhizal fungi serve? Protocorms are dependent on the mycorrhizal fungus for early nutrition. Many terrestrial orchids exist for years below ground and completely dependent upon their fungal partner. Once the fungi enter protocorms or roots, they form coils (pelotons) in certain cells, such as the cortex cells of the root.

At first the coils are easy to distinguish microscopically but as the orchid begins to digest the fungus, the peloton becomes an amorphous blob. The peloton absorbed by the host orchid as food. More and more pelotons form, are absorbed, and the orchid grows.

Here we can see some caramel brown areas within a section of orchid root. These are pelotons in the process of digestion. If roots are normally cream to white, we can readily identify areas of root with pelotons as they will appear light brown. picture

Once orchids become photosynthetic and have sufficient reserves to support their life, the fungi may become less important but much investigation is still needed to clarify this.

We know that orchids grow in some harsh situations. On the Pacific slopes of Mexico, there are some particularly harsh environments where orchids grow epiphytically without rain for up to six months of the year. Of course, there are fogs which provide moisture but it is dry.

N_Calif_Kathy
Pelotons are only in terrestrial orchids once they've grown past the protocorm stage? Terrestrials like those of the orchis subgroup? Or also ones like Goodyeara? Or paphs?

MarilyninOttawa
Good question, Kathy. All orchid roots infected with mycorrhizal fungi develop pelotons. This is true for all orchids including epiphytes and terrestrials. It is also true for all orchid protocorms germinating under natural conditions.

N_Calif_Kathy
Is this why some people say that orchids are saprophytic? Because they digest pelotons?

MarilyninOttawa
There are several terms used to describe the orchid-fungus relationship. Saprophyte is generally used to describe plants dependent upon fungi for their continued existence. Parasites would be orchids, even green plants, which exploited another organism for food. I believe that most orchids are mutualists. There is benefit for both partners although the pendulum may swing different directions during the relationship.

In flask, seeds of one such orchid, Encyclia kienastii (Domingoa kienastii), germinate within 24 hours and begin a rapid development of a large storage root seen in the picture. While we do not know for certain what happens in nature, it is quite possible that seedlings germinating in the wild take rapid advantage of moisture, growing quickly to secure a niche in a bark crevice. If what we see in flask is any reflection of nature, seedlings grow six months then rest six months. It is only a year after germination that a small shoot begins to elongate and leaves form. picture

Do non-green orchids have fungal partners? There is evidence that the mature Coral Root (Corallorhiza trifida) receives nutrients via its mycorrhizal associates. Mature photosynthetic plants may benefit from mycorrhizae including access to mineral nutrients, water and protection from root pathogens.

An interesting investigation into the algae, fungi and bacteria associated with the roots of epiphytes has been reported by Tsavkelova et al (2003). They found that when the aerial roots of Acampe papillosa, Dendrobium moschatum, Phalaenopsis amabilis and Dendrobium phalaenopsis were maintained under conditions of 90% relative humidity at 25-28C in bright light, the aerial roots became covered in a dark green mass of micro-organisms up to several millimeters thick. Some of the organisms are known to fix nitrogen which is especially interesting.

N_Calif_Kathy
People who grow their orchids in clear pots note that the roots will turn green. Most folks agree that algae grow on roots in these pots and it is not detrimental. However Roy Tokunaga thinks that the roots may become photosynthetic. What do you think? Algal overgrowth? Or the root responding to the presence of light and becoming photosynthetic?

MarilyninOttawa
There are the green algae and the blue green algae, the latter group being the ones most often considered to fix nitrogen. I expect that both kinds could exist in a transparent pot. Epiphytic orchid roots are likely photosynthetic although to what degree their photosynthetic production aids the plant is unclear. In leafless orchids, the benefit is clear but in larger leafy plants, the leaves may be the more important producer.

Greg_in_MI
We try to preserve the Mycorrhizae that occurs in the pots of pine raised as bonsai. They seem to do much better. In fact some add commercial Mycorrhizae solutions to try to build the colony.

John_in_Arcadia_CA
Yet there are some that are dangerous to trees. We lost our big camphor tree to oak root fungus!

MarilyninOttawa
Pines would have ectomycorrhizal associates. These fungi live on the outside of the roots and yes, they are important to the well being of the tree. There are products available now that can be used to infect tree and other plant roots with such fungi. There is not a product available for orchids, however. John, there are many different fungi and some are pathogens like the Phytophthora fungus you are referring to. This is the same genus of fungus (not the same species) that causes Black Rot of Cattleya, and the Potato Blight.

John_in_Arcadia_CA
It also has killed about 5 of our 30 year old camillias (I think that was the cause anyway!)

Greg_in_MI
Has anyone produced treatments that are useful to orchids?

John_in_Arcadia_CA
I think that would be an interesting experiment.

pecteilis_in_KY
I remember reading a paper, German, about growing orchid seed in an open flat of Oat meal? innoculateds with orchid fungi.

MarilyninOttawa
The Oats Medium, an agar-based, sterilized medium, has been used together with the appropriate mycorrhizal isolate, to support the symbiotic germination of orchid seeds.

MarilyninOttawa
What function does the orchid serve the mycorrhizal fungus? This is good question for which we have yet to have a good answer. Some say the orchid parasitizes the fungus but many orchid seedlings are parasitized by the fungus before they reach maturity. Why? The best guess is that the relationship is one of delicate balance that can easily be disturbed.

N_Calif_Kathy
As an example of evolution you'd have to wonder which came first? The orchids capturing the fungus for its purposes? Or the orchid adapting to the presence of fungus in the environment?

MarilyninOttawa
The jury is still out on that one although one must expect that orchid seeds have lost capabilities over time and become more dependent on a fungal partner for germination and early growth.

MarilyninOttawa
Do artificially propagated orchid roots ever become infected with mycorrhizal fungi? If we examine the roots of epiphytic orchids that have been growing in potting medium for several months, we can often find fungal pelotons in the root tissue. Such evidence demonstrates that secondary infections do occur but what benefit these relationships may have is as yet unclear.

pecteilis_in_KY
Marilyn the set back that most orchids experience if drenched with an anti fungal product seems to suggest that the fungus is beneficial, even in mature plants.

N_Calif_Kathy
Good point

MarilyninOttawa
There have been some studies where the effect of fungicide was tested. I do not have those at hand but suggest that only particular fungicides could be damaging. The mycorrhizal fungi are basidiomycetes although later infestations could be with ascomycetes. Fungicides disrupt the lives of particular fungi. Some products such as those for potato scab will affect basidiomycetes. Most fungicides will affect ascomycetes but again there is some specificity. Generally speaking, you could expect an effect.

pecteilis_in_KY
Marilyn, one of your pics showed a hypha in the root hair of a mature plant? If it has not function, what is it doing there? The orchid has defenses against MOST fungi.

MarilyninOttawa
A good question but we really do not yet know the answer. My assumption is that there are mutual benefits to those fungi which invade orchid roots. In some orchids, this must happen every year since the roots are annual. Such is the case with Catasetum, with Platanthera, and Ophrys.

MarilyninOttawa
What are the functions of roots? Roots have a central conductive core where water, minerals and nutrients can flow. This core is surrounded by a thick layer of cortex cells. This is where pelotons form and are digested. On the outer surface, there is an epidermis or covering layer of cells. As the aerial roots of epiphytic orchids develop, an external layer of specialized dead cells forms. This is the velamen, a usually silvery reflective coating seen on many orchid roots.

The velamen protects the internal cells from dessication and reflects light which possibly keeps roots cooler. Gases can move through the velamen. In large thick aerial roots, we can usually see pores in the form of elliptical slits. It is sometimes easier to see them if we first wet the root surface. These pores mechanically shrink and expand as the roots are alternately wetted and dried.

Orchid roots do more than anchor a plant to a tree or keep it snug within a pot.

Roots are the primary structures used by a plant to absorb water and mineral nutrients. In leafless orchids, roots serve the dual function of anchor and photosynthetic organ. Roots are used as storage organs especially in monopodial orchids such as Phalaenopsis and Vanda, and also in deciduous orchids such as Cypripedium. Roots store starch and water.

In terrestrial orchids such as Cypripedium, roots are the primary storage organ beside the rhizome. Often, the balance of nutrient store to photosynthetic capacity is tenuous especially in shaded situations. If some roots are lost during wild collection, a plant may not have sufficient reserves to survive transplanting. Furthermore, it may lose its vital mycorrhizal network since the new environment may no longer be suitable for fungal life outside the plant. We can only speculate that this is case but certainly, wild collected plants are less likely to survive than artificially propagated, seed-raised stock. picture

When orchids are raised in flask, they are provided all the nutrients necessary to grow. In Cypripedium, seedlings properly raised have strong, starch-packed roots. picture It is little wonder that strong seedlings make the transition to the ex-flask existence. picture As I have mentioned many times before, plants with sturdy root systems are to be preferred over those with poor roots. The seedlings you see have been removed from replate flask, washed and are being conditioned before being planted into pots or beds of growth medium. They are no longer sterile.

What is the difference culture-wise between fine-rooted and thick-rooted orchids? Orchids with fine roots such as many of the Oncidiums and Tolumnias require finer more porous potting mix or are perhaps best mounted to permit the fine roots to ramble over the bark. picture

Plants with thick roots require special care. Here we see a Phalaenopsis with roots at different stages including a shriveled dead root (which can be removed) and a new growing root with a healthy green growing point. picture

Sometimes it may seem that a plant has more than enough roots but root pruning may seriously set back a plant. We are not generally attempting to 'bonsai' an orchid. Our goal is to get blooms and bloom production is dependent on an adequate nutrient store to support the blooms. picture

How does one deal with orchids exhibiting seasonal root growth? Some orchids grow year round, others seasonally. Where growth is more or less continuous as in Phalaenopsis, re-potting can be safely done year round. With orchids whose growth is seasonal, care must be taken to repot when roots are just starting to grow otherwise we risk damaging or interfering with root development. Catasetum, Pleione, certain Dendrobiums and Cattleyas produce roots once a year. These should be repotted when root initials become visible or shortly before.

Why should we conserve roots when repotting an orchid? Roots are important organs of absorption and absorption. Loss of some of the absorptive surface area might critically affect the ability of the orchid to recover after potting. Loss of stored nutrient can also be deleterious. Many orchids use the roots to store reserves. Removal of many roots reduces the food and water store of a plant. Without this storage capacity, a plant may need perfect growing conditions and much time to be able to recover.

Consider how you will deal with orchid roots when the time comes for repotting. Roots that have ramified through potting medium such as tree fern or have become firmly attached to a pot may be lost. picture

N_Calif_Kathy
Marilyn, what about the opposite. Many people wonder about roots in the wild collected plant and it survival in 'captivity'. What about the opposite? Taking a flasked plant and reintroducing it to the wild? Since these are already developed in captivity does one still have to worry about the proper mycorhizea in the wild?

MarilyninOttawa
Very good question, Kathy. A flasked plant might persist in the wild but then it might not. We really do not have enough evidence on re-introductions to make a sweeping statement. Apart from the hazards of re-collection (it happens) or loss to snails, etc (this also happens a lot!), I expect that orchid roots will become colonized with free-roaming fungi. Some species may be very dependent upon having the correct associate for life and others may not. We just do not know.

MarilyninOttawa
Some ways that we can employ to limit root loss when re-potting is the potting-on technique where the mounted plant is attached to the new mount and allowed to grow onto it. picture
This was my last slide so now we can have more questions. Ask away!

art,simpsonville_sc
Do we try to cram as many roots into pot or do we put plants in huge pots to guarantee that all the roots fit?

jeanne23_from_oz
Our 'experts" always maintain that the plant should be repotted in a pot that will last only 2 years because the roots have to reach the sides of the pot to stabilize themselves before real growth commences as the movement of the roots in the pot damages them and they go backwards

MarilyninOttawa
I feel that a lot depends on the orchid, its growth pattern, the chosen medium, etc. Some of mine must be repotted annually. Growth is that quick. You are quite right about roots moving (wobbling) in a pot. Make certain that repotted orchids are well anchored.

A lot depends on what kind of orchid. With a Phal, some roots can remain outside the pot. With a Cattleya, you may have to select a broader pot or basket. I usually choose a basket when dealing with a particularly rambling sort.

art,simpsonville_sc
So you're saying in the case of catts I need to try to select a pot large enough to cram all the roots back in instead of pruning them back and going into a smaller pot

pecteilis_in_KY
If you use the permanent mediums, lava rock, etc. You don't have to remove it, just the back part, so you don't lose all those roots.

MarilyninOttawa
If you must cram all the roots into a pot then I feel that the chosen pot is too small. There should be room for some medium also. Why not divide the plant? This way you will be able to have smaller pots.

N_Calif_Kathy
I've been wondering what it would take to re-introduce Cyp californicum to the regional parks around here. (other than a permit)

jim4eq
I know that in Florida, efforts to re-introduce have been unsuccessful. One group here has the Smithsonian helping look for the proper symbionts.

N_Calif_Kathy
Interesting! So orchids still need symbionts even past the seedling stage...

jim4eq
yeah, and is often a different myco at adulthood compared to seedling symbiont

MarilyninOttawa
Before I forget: Kathy, you should identify a legitimate source of artificially propagated C. californicum. Be certain that these seedlings can be imported into your state. Seedlings with the heaviest root systems are the best choice. You would probably want to 'garden' seedlings for a year or two before reintroduction. Seedlings will have to be caged for best results. Otherwise they become some rabbit's supper. Barring those steps, it is worth a go. Someone may already be trying this however and you will have to inquire locally to be certain that seedlings being re-introduced come from the same population. Since I do not work with this species, I am somewhat limited in terms of a more detailed response.

Gaile
I have also been told that -at least with cattleya- cutting roots back is ok, because they will branch at the cut and actually provide more root surface. This assumes, of course, that the repotting is being done at the proper time. True?

N_Calif_Kathy
I'll be interested to see what Marilyn says. I've been told that too, however in my hands the cut roots just die, and the plant has to make more. I think the cuts are just sites for infection. I could be wrong. That's why I wonder what Marilyn will say.

al_@_michigan
I was wondering about the relationship of root growth to the leaf size and growth patterns. over the growing season, can root growth be approximated by leave growth or does one precede the other.

pecteilis_in_KY
No; each species is different. Some catts make roots THEN break a bud. Others mature a growth before they root it.

MarilyninOttawa
Ok, there was a question about root pruning and something about leaf growth. Some, indeed many orchid roots will branch after being cut or broken. Some orchids such as C. dowiana will produce roots once a year. When you prune, you remove root and its starch store. The plant must produce new root and re-fill that tissue with starch. It is analagous to removing honey-filled comb from a hive. You can do it but if the bees are to survive winter, they must produce more comb and more honey as reserve.

Jade
Sounds like it would be best to just pot up in that case.

jeanne23_from_oz
Why would you prune good roots off ?

Gaile
Actually, the comment was made to me by someone who was showing me how to divide and pot a cattleya (I am very new). I think the information was given as much to reassure me I wouldn't kill the darn thing by handling it a bit less delicately.
So while the roots will indeed branch, the bad news is it could delay leaf growth or blooms until the plant has restocked the larder?

MarilyninOttawa
A lot depends on the plant, the time of year, growing conditions, and the quantity of root removed. A few centimeters of root from a huge Vanda is not as consequential as half the roots of a slower growing Phal.

Randy,_MI
Marilyn, what advice could you give about moving wild orchids from a `construction zone' to a safe area? We have a permit, and are going in relatively blind and dumb. We have set a criteria of trying to move them to an area where that species already grows--any thoughts?

MarilyninOttawa
Which species?

Randy,_MI
Will probably be mostly Cyp's, although it depends on which species is found. Our plan was to try to move anything found.

MarilyninOttawa
Generally speaking, you would want to select you transplantation site first then prepare the space for the transplant. Try to remove large areas of ground around the plants. Wait until after heavy rain if you can. Orchid roots are usually in the upper layers. Try to match the site habitat-wise including slope, drainage, companion plants. As I indicated before, dig the strips where you intend to transplant and dig a similar strip from where you intend to rescue. Same depth. This will be especially important for the Pink Lady's-slippers since their roots spread quite a lot. Finally, follow the transplant site for at least two years. Mark the plants discretely or map them so that you can monitor their progress. Your observations will assist others attempting to do the same. Keep notes and communicate your results. Good luck.

Greg_in_MI
I wonder about leafless orchids (like the FL ghost) is that totally a Mycorrhizal relationship? They certainly do get green...

jim4eq
no, the myco is in there and helps, but photosynthesis is done by the roots. Nice big fat green healthy roots, LOL

MarilyninOttawa
Leafless orchids are not without chloroplasts. They can and do photosynthesize. They likely have mycorrhizal associates also.

-the end