Phytochrome and daylength.
 

The plant pigment phytochrome exists in 2 forms called Red (Pr) and Far-Red (Pfr). When Pr absorbs light red light of around 670 nm it is converted into Pfr. During the night Pfr slowly changes back into Pr. It has been shown that the photoperiodism of some plants is determined (at least in part) by how low the Pfr concentration drops during the night. In the winter the long nights allow more time for Pfr to convert back to Pr and some plants that require long nights to initiate flowering use Pfr concentrations to determine when floweing begins.

Another way to convert Pfr back to Pr is to expose it to light at 730 nm which quickly converts Pfr to Pr.

Phytochrome

What I would like to see are some experiments with long night flowering orchids where 730 nm far red lght is used to artificially create long nights.

Since my orchid solarium connects to my living room via large glass doors and I also like to work out there late at night I am worried that some of my orchids that require long nights to flower may not flower. I am considering adding some 740 nm LED lights to use during winter nights to simulate long nights.

Another interesting topic, though I think most of the orchids we grow aren't terribly sensitive to daylength since many are from tropical and equatorial regions where days don't vary as much over the course of a year. This might be more of an issue with temperate terrestrials or species further outside the tropics, but in general blooming in orchids seems more commonly temperature or growth cycle mediated. Not that I can cite anything to back me up on this... :roll:

--Nat

I believe Euanthe sanderiana is a short day bloomer and Ascocentrum curvifolium is a long day bloomer. I have heard that there are some Catt species that are short day bloomers that will refuse to bloom if there is a nearby street light shedding light on it during the night.

I was reading Withner's book on Cattleyas and he mentioned one that was daylength sensitive, I think it may have been labiata. I believe that commercial Phalaenopsis growers have also used daylength to control flowering.

Edit:
I just did a goole search on "cattleya daylength" and got a bunch of pertinent hits.
Google

Interesting. I have always heard that commercial Phal growers move plants into a cool house for several weeks to induce spike. Seems like day length would be difficult to manipulate in a greenhouse?

Yes, seems that that was a false memory on my part. This AOS article says that Phalaenopsis are _not_ affected by daylength but suggests that a number of Catts are.
http://www.aos.org/Default.aspx?id=415
Since I am going to be growing a number of Cattleya species daylength will be a concern for me.

Edit (again):
This looks like pertinent article, titled "The Flowering of Orchids, A Reality Check"
http://sharepoint.agriculture.purdue...%20orchids.pdf

Interesting article! Here's a link to another one by Dr. Runkle discussing photoperiod, among other aspects of lighting: http://www.hrt.msu.edu/faculty/Runkl...s_of_Light.pdf.

I find it very curious that plants from tropical regions (where daylength doesn't drop below 11 hours per day) respond to much shorter days by blooming. I guess it's because there's a physiological threshold beyond which a plant will bloom, regardless of whether that was reached due to many slightly longer nights or a few much longer ones?

Yes, I agree, very curious. At the lines of 23 degrees N and S latitude that demarcate the Tropic of Cancer and the Tropic of Capricorn, hours of daylight at winter versus summer solstice varies by about 2 hours and 50 minutes (10:33 hours to 13:43 hours) . Even if we go to 30 degrees N (Northern Mexico) or 30 degrees S (Southern Brazil) the hours of daylight varies by only about 4 hours. It is hard to see that this would be significant enough for plants to synchronize their flowering.

Perhaps in the tropical orchids that require nights of some minimum length to flower, high Pfr concentrations from days significantly longer than 13 hours out of 24 can still inhibit flowering even though this is not what the plants normally use to determine their annual cycles.

I am just taking shots in the dark here, as you say; very curious.

or that it is other factors than Phytochrome levels that messed up flowering behavior?

And can it be simple as the plants has grown under the "lighter" part of the year and are ready to flower when they are fully grown, in the "darker" part of the year?

I have no idea, just through out suggestions. and as I have no indication that my orchid acctually have any problems on this issue I will not invest my moniey on far red night light.
David, have you poblem with nonflowering plants that you fullfill every other aspect of growth or is it just preventing problem that is non excisting?

What is your target problem plants?

Cattleya species, I just started establishing my collection about a year and one-half ago so many are not flowering size yet. By the end of this year I expect to have plants of most every Cattleya species. It seems a well established fact that a number of Cattleya are influenced by daylength, there are a number of references. Here is from an AOS "Orchids" magazine article (Lopez and Runkle, Orchids, March 2004, page 200):
http://sharepoint.agriculture.purdue...%20orchids.pdf
Cattleya is a genus composed of 60 species native to tropical regions of Central and South America. This epiphytic plant is generally found growing on trees of moist and wet forests from sea level to 4,900 feet (1,500 m) in elevation. Several published scientific studies indicate that flowering of Cattleya species and hybrids is promoted by exposure to short daylengths and cool temperatures. For example, in Cattleya warscewiczii, Cattleya gaskelliana and Cattleya mossiae, flower induction occurred only when plants were placed under photoperiods of nine hours (nine hours of light per day) at 55 F (13 C), while flowering was inhibited under 16hours of light (per day) at 55 F (Rotor, 1952, 1959). These represent but a small sample from this genus, which also contains long-day species, as well as both short-day spring- and autumn flowering types.

Another article by Lopez and Runkle:
https://sharepoint.agriculture.purdu...0-%20color.pdf

Ohhh.
I am impressed that you will have accuired most of the Cattleya´s within 2 years! I would like to grow more Cattleya´s but have not the space :-(

And I agree that they can be dependent on the length of the day. It will be interesting to follow your ecperiments, especially if you have the ability to compare the far red light treatment you plan with normal darkness.