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Sphagnum vs fir bark

This is a discussion on Sphagnum vs fir bark within the General Orchid Culture forums, part of the Orchid Culture category; You're right that the general structure does not change, but the cells themselves DO change ...

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  1. #11
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    You're right that the general structure does not change, but the cells themselves DO change in size, cell wall permeability, and the like.

    If that did not happen, I cannot see how a single type of plant could thrive under wildly-different conditions, which is the case we see routinely - cattleyas grown in coarse bark and in S/H, for example - two, entirely different sets of conditions.

    Many years ago (25-30, maybe?), a grad student at some university did a study, and clearly showed differences between phalaenopsis "submerged" and aerial roots, and those grown in water culture. The work was initiated by the now-closed Venger's Orchids in Colorado Springs (they originated water culture, by the way), and I'll be damned if I can recall more details, but it really helped me understand what was going on with transitions to S/H culture from others.

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    It's still based on the availability of water, not the medium. The same amount of water and periods of drying is required by an orchid root no matter what media it is grown in. If grown in bark it needs to be watered often and will rely more on the humidity level. If grown in S/H it doesn't need to be watered at all, just keep the reservoir full.

    Velamen cells rapidly increase in size when exposed to water. I suppose without searching for supporting evidence for it that the cell walls of passage cells in the exodermis could alter their water absorbing capability, but this sort of thing can happen in the existing roots making for fine adjustments in the availability of water. I doubt given the slow growth of roots that something is telling the orchid to grow a different kind of root because it is now in a different medium.

    If you transfer an orchid into S/H does it languish until new roots are grown, not root tips, but whole new roots from the base of the plant?

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    Quote Originally Posted by sciencegal View Post
    It's still based on the availability of water, not the medium. The same amount of water and periods of drying is required by an orchid root no matter what media it is grown in. If grown in bark it needs to be watered often and will rely more on the humidity level. If grown in S/H it doesn't need to be watered at all, just keep the reservoir full.
    The difference might very well be the relative availability of water, but that can change with the medium, can't it?

    In regard to that second sentence: if it's true that "the same amount of water and periods of drying is required by an orchid root no matter what media it is grown in", then that plant in bark - if it does well when allowed to dry out between waterings - cannot possibly do well in S/H, which is wet 100% of the time, and that has been proven false time and again.

    My contention is that those roots that did well in the wet/dry cycle of bark, will not do well in the 100% wet environment of the S/H pot, so the plant will need to grow roots that are optimized for that environment.

    Velamen cells rapidly increase in size when exposed to water. I suppose without searching for supporting evidence for it that the cell walls of passage cells in the exodermis could alter their water absorbing capability, but this sort of thing can happen in the existing roots making for fine adjustments in the availability of water. I doubt given the slow growth of roots that something is telling the orchid to grow a different kind of root because it is now in a different medium.
    Again, maybe it's the available water content that is triggering the changes.

    If you transfer an orchid into S/H does it languish until new roots are grown, not root tips, but whole new roots from the base of the plant?
    That's going to depend on the degree of difference between the "old" conditions, and the "new" ones. Take a plant that has been growing in constantly moist sphagnum, and move it to S/H and it's a "walk in the park", with the existing roots. Take a plant that has been in coarse bark that dries rapidly, and move it, and it's roots will fail in fairly short order. Repot it just as new roots are emerging, and those new roots will thrive.

    Another example of the cellular adaptation: When we move plants to S/H culture, it is important to make sure the root depth in the pot is above the reservoir. if you plant them too deep, so that they are submerged, they will die and rot. However, just about every plant in S/H will eventually grow roots down into the reservoir, where they stay constantly submerged, and because they have grown there - optimized for being submerged - they are fine.

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    This link from the Orchids of Asia book has some interesting information on orchid roots and how they can change.

    According to the author, the root tip, which is not covered with velamen, is the only part that conducts water to the plant [I would wonder about that being true]. The velamen in the rest of the root supplies nutrients to the root tip by leaching [downward toward the tip rather than up toward the plant]. This would explain why the root tips you see submerged in the water can survive. They are being nourished by the roots that have velamin while the tips are supplying water to the plant. I'm going to guess that the roots you see in the water have no velamen layer. Since it is the velamen that absorbs nutrients the root tips in the water could not get nutrients and I'm guessing necessary gasses, unless part of the root were above the water. The roots in the water are not a different type of root they are only missing the outer layer of cells. The velamen grows behind the root tip. If the root is completely submerged the velamen may not grow.

    Additionally, he says that any root can change into a support or feeding root. If a root touches a surface it will grow root hairs and the velamen will be thin or absent so water and nutrients can be absorbed directly. In monopodial orchids some roots can go into the ground and form naked feeding roots that do not have the water absorbing velamen around them. However, they are always associated with a fungi to break down food. Maybe the presence of the required fungi triggers this change. He says that he grew a Phal in a hydroponic system and it had one huge feeding root that was supporting the entire plant.

    So, this kind of says that the existent roots can respond to the environment and lose (or never develop) its outer water absorbing layer. I would guess that once the root loses its velamen can't grow it back but it would be interesting to know what would happen if you take the orchid which has developed submerged roots, put it in dry medium and see if the roots survive or develop velamen.

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    I don't have first-hand knowledge of the "water absorption only at the green tip" assertion, but like you, I have to challenge that, as I cannot tell you how many plants I've seen from dry environments that don't have green root tips, yet seem to do OK when the roots are wetted.

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    When I looked into Ray's idea, there aren't any direct scientific evidence, but what Ray has been seeing is possible. At least the acclimation of root anatomy to different root environment is likely. I'm not completely sure about the irreversibility. The tissues which have been fully developed are unlikely to change as Ray says, but the new tissues created from the root acclimated in a different environment is likely to change.

    This is not the response to water, but here is an example of how the root structure of Laelia changes due to urea concentration:
    Changes in anatomy and chlorophyll synthesis in orchids propagated in vitro in the presence of urea

    The claim of the book Karin linked is somewhat bogus. Or it is probably not clearly explaining the function of epiphytic roots. With this regard, Zotz and Winkler's 2013 paper is interesting. Since I can't link it, here is the citation:

    Gerhard Zotz & Uwe Winkler, 2013. Aerial roots of epiphytic orchids: the velamen radicum and its role in water and nutrient uptake. Oecologia 171: 733-741.

    Plants can't uptake mineral nutrients if the concentration becomes too high. So in epiphytic situation, the water dries up quick (and the concentration becomes too high) in the root tip, so the root tips may not be able to absorb nutrients. Velamen (the dead tissue) can maintain the appropriate concentration. Maybe this is what they mean? The nutrients uptaken by any part of roots are surely redistributed to the other parts of the plant (including the root tips, which need more nutrients than the older part of the root, and the shoot system).

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    Quote Originally Posted by naokit View Post

    Plants can't uptake mineral nutrients if the concentration becomes too high. So in epiphytic situation, the water dries up quick (and the concentration becomes too high) in the root tip, so the root tips may not be able to absorb nutrients. Velamen (the dead tissue) can maintain the appropriate concentration. Maybe this is what they mean? The nutrients uptaken by any part of roots are surely redistributed to the other parts of the plant (including the root tips, which need more nutrients than the older part of the root, and the shoot system).
    I'm thinking that is what they mean. Since the growing root tip isn't covered with velamen it is nourished by the nearby velamen.

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