Photosynthetic roots.

[bil]

OK, let's get another argument/discussion going.

I keep hearing people say that the green tips of epiphytic orchids need to go in clear pots so that they can photosynthesise.

I just don't buy it.. It's true that the tips DO photosynthesis, but I don't think that is the reason for that. I think that the photosynthesis is there to give the roots negative photo taxis, and drive them into the medium or against the branch.

Normally the growing tip of a shoot displays positive phototaxis. On the side of the shoot towards the sun, auxin production is suppressed, and on the shady side it is encouraged, so that the shade side grows more, bending the stem towards the light.

In orchid roots, I believe that the reverse is usually the case, and growth is encouraged on the bright side, so that the root curves downward and dives into the medium. This is hard to explain if the photosynthesis is significant for the plant's well being, but once you have seen the roots on a mount rising out of the medium to immediately dive back into it, it no longer appears to be anything but a guidance system.

I'm attaching a pic of one mount where the orchid was hung near to the support beams of the greenhouse that create a quite shaded area there. The orchid has reacted to this by throwing roots out as tho they were magnetised and attracted to the steel.

[Roberta]

I don't know that the green tips do a significant amount of photosynthesis, but the green that appears when the root is wet (rending the velamin translucent) indicates that the full length of the roots can perform photosynthesis. In fact, in the large number of leafless (or nearly leafless) orchids, that's where all the photosynthesis happens (Microcoelia for sure, also some higher-elevation Phals are leafless or nearly so for part of the year and totally depend on the chlorophyll in the roots for their energy)

[orchidsarefun]

Hmmm.....people use clear pots so that they can see the condition of the roots as well as being an indicator of whether a watering is required. I've never heard anyone say it's an aid for photosynthesis.
This phal - just watered - is a good example of green roots. I wait until they dry out ( change colour ) before watering again.

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It also has plenty of exterior roots.

[Roberta]

Originally Posted by orchidsarefun

Hmmm.....people use clear pots so that they can see the condition of the roots as well as being an indicator of whether a watering is required. I've never heard anyone say it's an aid for photosynthesis.
This phal - just watered - is a good example of green roots. I wait until they dry out ( change colour ) before watering again.

Take a look at this Microcoelia stoltzii Not a leaf in sight (it doesn't have any). Those roots green up when they're wet so that one can see thorough the velamin, but they're photosynthesizing all the the time (in the light of course) This species (and others of the genus) won't survive in pots. This one is just splayed over the top of a wood basket. This year is looking like it will be even better than last, so it's getting what it wants.

[rbarata]

I keep hearing people say that the green tips of epiphytic orchids need to go in clear pots so that they can photosynthesise.

I use clear pots with phals to know when to water. Nothing more.

Photosynthesis of root tips is contradicted by most of my orchids: the roots always grow from the light and not towards it.

[orchidsarefun]

Originally Posted by Roberta

Take a look at this Microcoelia stoltzii Not a leaf in sight (it doesn't have any). Those roots green up when they're wet so that one can see thorough the velamin, but they're photosynthesizing all the the time (in the light of course) This species (and others of the genus) won't survive in pots. This one is just splayed over the top of a wood basket. This year is looking like it will be even better than last, so it's getting what it wants.

I agree. Leafless means they are adapted to photosynthesise via their roots. Having a leaf means that organ (?) is the primary photosynthetic apparatus. (?).

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Lovely plant btw.

[Roberta]

Originally Posted by orchidsarefun

I agree. Leafless means they are adapted to photosynthesise via their roots. Having a leaf means that organ (?) is the primary photosynthetic apparatus. (?).

Lovely plant btw.

Thanks. As far as the source of photosynthesis, I don't think it has to be "either-or" , more like "both-and". Leaves certainly have more photosynthetic surface available. But in nature, these epiphytic orchids grow on trees, and may also have significant root area. Of course the primary job of the roots is to capture water, but they're clearly dual-purpose. That's also a clue to their need for a wet-dry cycle. How good is the drainage on the side of a tree? How much air do they get on the side of a tree? Of course, it also rains frequently and humidity is high... we have to compromise on the air and drainage part to provide enough moisture in our homes and yards. But if that natural environment is kept in mind, you find the "sweet spot" in that compromise.

[Ray]

I don't think chlorophyll plays the slightest role in phototropism, and that the direction of root growth is controlled by it either; roots are looking for the moisture, and that's going to be away from the brighter light....

[bil]

Originally Posted by Ray

I don't think chlorophyll plays the slightest role in phototropism, and that the direction of root growth is controlled by it either; roots are looking for the moisture, and that's going to be away from the brighter light....

Not the chlorophyl. I think that the photosynthesis is simply the way of the plant 'seeing' the direction and if it is that way then it is interesting that the mechanism is the opposite was round.

On this pic, they are reaching for the shade. There's no moisture there.
If that top root was headed for moisture, it would be going downwards to the moss.

[katrina]

I would say there is no One-Size-Fits-All in this. The roots absorb nutrients and water and, to likely various levels, perform photosynthesis but they also function to anchor the orchid/plant. In the case of epiphytic orchids...they would seek out the tree (or whatever) and that is usually going to be away from the light.

Like bil, I have some where the roots grow away from the light and not down into the pot. It's like they are reaching out to grab onto something. Plenty of roots in the pots...but still those crazy ones reaching out toward something they can't "see" and they all seem to be going in one direction...away from the light source.

I also have some that grow roots in all directions. Some mounted orchids where the roots attach to the mount as well as growing all over the place. Some mounted orchids that attach but just as many roots growing straight down. And then I have those orchids that are orderly and obedient and all the roots grow where I prefer them to grow - either down into the pot and/or anchoring onto the mount.

Really, I see just about every scenario in my collection so I don't know that there any single answer to the question/situation.

EDIT - some of my Hoya do the same growing away from the light. The vines w/roots along the stem will grow out in one direction...away from the light. As if reaching out to grab onto something. It's most noticeable when they are outside in the summer, under a giant tree...they will grow toward the fence as if they are trying to attach.