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  #1  
Unread 09-01-2007, 12:07 AM
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Default Orchids and temperature

Why do orchids need a difference between day and night temperatures? I am sure there is science behind this requirement, please share.

Dave
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  #2  
Unread 09-01-2007, 12:28 AM
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In their natural habitats, most orchids have adapted over time to natural variations between day and night temperatures. In tropical regions, temps are more or less constant throughout the year seasonally, but they may drop significantly from day to night. When you bring an orchid into your home, which has evolved to survive in such conditions, you can't expect it to perform at its optimum if conditions are any different.

Typically changes in temperature will trigger an orchid to produce flowers. This is another result of evolution - temperature changes let the plant know what time of the year it is, and generally an orchid will flower at a time of year in which a particular insect species is active, so the flowers can be pollinated. By replicating such conditions in the home or whatever growing area you might have, we are simply fooling the plant into thinking that it is the right time of the year to flower. Some orchids are more responsive to temp changes than others; species from high elevations need a pronounced cooling period in fall and winter, while warmer growers from lower elevations need only a little day/night variation to flower.

To give you an example, most slipper orchids are pollinated by various bee species, which are active generally in the spring and summer. To get a slipper orchid to flower, usually you will lower temperatures, especially at night, in the fall and winter. Then in the spring, temperatures rise again, (just like in spring in nature) and the plant flowers. Paph callosum, a warm grower, will fllower with only a little day/night variation, while Paph micranthum needs a cool rest in the fall to flower. Even more extreme, a Cyp will not even survive unless it gets a 2-4 month cold dormant period.

I hope this answers your question.
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Last edited by slipperfreak; 09-01-2007 at 12:38 AM..
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  #3  
Unread 09-02-2007, 05:09 AM
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Default A Simple Explanation

Imagine an orchid growing high up a tree. It can get extremely dry up there, when the sun is shining and a good breeze blowing. So to protect itself from drying out, it closes its pores. But in so doing, it now has a problem with the intake of carbon dioxide (CO2) it needs for photosynthesis.

Over the years, many orchids have evolved to deal with photosynthesis in two stages. It absorbs light during the day with its pores closed, then, in the cool of the night when the air is relatively moist, it opens its pores to take in the CO2 it needs to finish off the second stage of the cycle. So, for these orchids, the difference in day/night temperature serves as the trigger to flip between the two stages of metabolism.

It is important to note that not all orchids exhibit this type of metabolism - thin leafed orchids and terrestrials don't.

Weng

Cultural note: Those orchids that need a good day/night temperature difference have their pores closed during the day, and are blissfully unaware of the loving care you take in maintaining humidity, misting, or any folia feeding that you do

Last edited by weng; 09-02-2007 at 05:18 AM..
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  #4  
Unread 09-02-2007, 10:21 AM
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The orchids that close their stomatas during the day follow a crassulean acid metabolism (CAM). It is their way of conserving water, however if humidity is high and the plants are watered correctly, the orchids that normally close their stomatas during the day will keep them open. So this may only be one factor for the neccesity of dropping the temperature. The main reason for the drop in night temperature is for the storage of carbohydrates. The plants produce carbohydrates during the day, and at night they spend all of it if temperatures are diurnal temperature differences are small. However when night temperatures are much cooler than day temperatures, the plant stores the carbohydrates instead of spending it. And these carbohydrates are neccesary to produce flowers. I hope this helps.

Jeremy
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  #5  
Unread 09-02-2007, 11:49 AM
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Cockburn et al. (1979) showed that stomata of CAM plants respond to the internal concentration CO2 in the leaf. Daytime closure is due to internal CO2 release from breakdown of malate prior to photosynthetic fixation. Opening commences as the CO2 level falls late in the day.

Clearly water stress will cause the stomata to close, but does the absence of water stress actually cause the stomata to remain open?

Can you point us to a scientific paper?

Weng
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  #6  
Unread 09-02-2007, 12:09 PM
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Good replies, thank you. I second the request for scientific papers.

Dave
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  #7  
Unread 09-02-2007, 12:11 PM
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I would like to point out that if you grow almost any orchid at constant room temperature, it will survive but will not flower, nor will it appear particularly healthy. All of the information posted here makes sense, though.
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Last edited by slipperfreak; 09-02-2007 at 12:14 PM..
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  #8  
Unread 09-02-2007, 03:22 PM
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Here is a passage from my Advanced Placement Biology book word for word, I hope this may assist everyone to understand CAM.
"A second photosynthetic adaptation to arid conditions has evolved in succulent (water-storing) plants (including ice plants), many cacti, pineapples, and representatives of several other plant families. These plants open their stomata during the night and close them during the day, just the reverse how other plants behave. Closing stomata during the day helps desert plants conserve water, but it also prevents CO2 from entering the leaves. During the night, when their stomatas are open, these plants take up CO2 and incorporate it into a variety of organic acids. This mode of carbon fixation is called crassulacean acid metabolism, or CAM, after the plant family Crassulaceae, the succulents in which the process was first discovered. The mesophyll cells of CAM plants store the organic acids they make during the night in their vacuoles until morning, when the stomata close. During the day, when light reactions can supply ATP and NADPH for the calvin cycle, CO2 is released from the organic acids made the night before to become incorporated into sugar in the chloroplast."
Source: Campbell, Neil A. and Reece, Jane B. Biology sixth edition. San Francisco, CA: Benjamin Cummings, 2002. 6:192-93.
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  #9  
Unread 09-04-2007, 08:18 PM
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Thank you for this info, and it makes much sense. So now I am wondering if it is the light or temp change that triggers the switch in the plant. I assume it is light, because temp isn’t playing a role (at least in my understanding). That is unless the chemicals (enzymes?) involved in the conversions only operate at certain temperatures.

Are there any books out there that explain the biology of orchids? What books are out there that explain biology of plants in general. I need to know more.

Dave
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  #10  
Unread 09-04-2007, 08:35 PM
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Well actually, I think that the most important is temperature since it cause the plant to store carbs. But light can also play a factor since some orchids like unifoliate cattleyas are sensitive to decreasing daylength as they flower when the nights are getting longer. I really dont know about enzymes or hormones that change orchids although there are some hormones that are discussed in general biology books which affect plants. But I also do wish there was a biology book specific to orchids only.
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