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  • 4 Post By PaphMadMan
  • 4 Post By Halloamey

Do orchids photosynthesize from all parts of the leaf?

This is a discussion on Do orchids photosynthesize from all parts of the leaf? within the General Orchid Culture forums, part of the Orchid Culture category; The reason I ask is, many of my orchids (one Phal . in particular) leaves ...

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  1. #1
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    sand_tiger86 is offline Senior Member
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    Default Do orchids photosynthesize from all parts of the leaf?

    The reason I ask is, many of my orchids (one Phal. in particular) leaves twist and grow every which direction, as orchids tend to do. So, the result is many leaves backs facing the window, rather than the front. But does it matter? Does the entire leaf photosynthesize? I feel like I remember reading that only the front of the leaf is useful in sunslight.

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    I'm not sure about the bottom of the leaf, but in addition to the top of the leaf, roots in clear containers also with perform photosynthesis. That is why most commercial growers put their phals in clear plastic pots.

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    PaphMadMan is offline Senior Member
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    Any green tissue can photosynthesize, though the capacity and efficiency varies. A leaf is going to do a fairly good job of it no matter what the orientation relative to the light, though not necessarily as good as it could be if it maximized the exposure of the top surface.

    Most plants, even orchids, will orient new leaves to best catch the available light as they grow. If it doesn't it could mean that there is enough light that the plant doesn't need to maximize it. Or if you move or turn your plants often as they grow you could be causing the twisting as the leaf tries to turn towards the light that keeps changing direction. This might be especially noticeable in a Phal since they have large leaves that grow over a long period of time. Changing the direction of the light will cause twisting of Phal spikes too and for me that is enough reason to avoid turning them, and it usually keeps the leaves all oriented together too.

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    Orchid leaves can photosynthesize from all parts of the leaf but there is considerable difference between the photosynthesis activities that go on on the adaxial (upper leaf surface) and the abaxial surface (lower leaf surface) of Orchid leaves. Orchids carry out a specialized highly efficient photosynthesis called Crassulacean Acid Metabolism (CAM). In other plants called C3 plants the leaves capture the light and use its energy to absorb CO2 and make sugar from it, the advantage is that it is an instantaneous and simple process, the disadvantage is that the plants have to keep their stomata (small pores for allowing exchange of gases) open during day time in full Sun when temperatures are high, this causes rapid loss of water in the form of water vapor due to transpiration. As orchids do not have constant access to water (except for a few species) they have adapted and separated the two processes. During the day time they keep their stomata closed utilize the solar energy and store it in the form of NADPH+ (consider it like plant battery) . Later at night when the Sun is down and the temperatures cooler, they open their stomatas and then use the stored energy to fix and absorb CO2. So to efficently separate the two reactions the leaves are divided, the upper surface has very few stomatas and it functions to absorb the light during the day and store the enery, whereas the lower surface has all the stomata and functions to store the CO2.

    So basically the upper surface does the light absorption, so it should face the light, if that is not the case, there is something wrong with the light source, either the orientation or the spectrum. The lower side of many orchid leaves are red in color due to anthocyanins (red coloured pigments) this is to absorb the the high energy blue light radiation that is reflected back from the surfaces on which orchids are growing.

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