Daylength sensing by orchids - which wavelengths?

[estación seca]

As the days shorten, those of us still working frequently leave for work and arrive home during the dark. The only way to work with our orchids is with artificial lighting. This might disrupt growth and blooming cycles related to day length.

Does anybody know which wavelengths trigger day length responses in orchids? Is it possible to use some particular color of light so we can see, but the plants won't think the sun came up at 9 pm?

[gngrhill]

Good question. Hope someone chimes in.

[Ray]

Google is your friend! Red-, to far red, particularly 660 nm.

[estación seca]

I asked here because most of what I found was non-specific, and I found different plants respond to different wavelengths, though the same type of enzymes seem to be involved in all plants. However, to answer my original question, it seems light of short wavelength will not prevent orchids from flowering. I'm going to look for a blue or purple LED headlamp to wear at night around my plants during fall and winter.

Photoperiodism
Look down to "What is the receptor pigment involved in night interruption?" Note that Xanthium is maximally inhibited by orange light, and this can be reversed by later exposing them to far red light (that we can't see.) Morning glories are kept from flowering by red light, but not by far-red light. But I didn't find this kind of information for orchids.

I found lecture notes
OrchidSafari ARCHIVES* Flower Induction in Orchids
saying the light that prevents some orchids from flowering is far-red, but I didn't track down original research. I highly recommend everybody read this lecture. It mentions some of the kinds of orchids that are daylength sensitive, and some that are not.

I will simplify a few other things I read. Phalaenopsis absorb most of their CO2 at night. They use crassulacean acid metabolism to fix the CO2 at night and store it for use in photosynthesis during the day. In this they are similar to cacti and a great many succulent plants. Phals can and do also perform C3 carbon fixation and photosynthesis, which is what most plants do - opening their pores during the day, and using the CO2 which diffuses into the plant for photosynthesis concurrently.

I found one paper reporting white Phalaenopsis NOID hybrids on a strict 8-hour natural light day in a greenhouse flowered a week earlier than those exposed to natural day length variations over the summer. Those grown at a constant 28C / 82 F did not flower. Those grown at a constant 22-23C / 71-74 F did flower.
https://www.researchgate.net/publica...enopsis_Plants

Another paper had more complicated results. Everybody who grows Phals under lights should read this paper. It focused on maximal CO2 absorption and not as much on flowering.

Phalenopsis amabilis cultivar TS97 had maximum photosynthetic efficiency (as measured by carbon dioxide uptake and fixation) at about 200 micromoles per square meter per second (mm/m^2/sec) when this illumination was held constant through the 8-hour day. Higher light intensity decreased photosynthetic efficiency. Much of the light was natural daylight, but supplemental light was also used. They did not say what kind of lamps provided the extra light.

When grown under lower light intensity of 30 mm/m^2/sec, they adapted. They reached maximal CO2 absorption at 125 mm/m^2/sec.

Phals grown under only 5 mm/m^2/sec were able to use the CO2 they absorbed during a night.

However, a 12-hour day resulted in more CO2 uptake and fixation. The plants opened their pores at around 8 hours into the day and began using C3 metabolism to fix additional CO2.

The 12-hour daylength promoted earlier flowering and higher flower counts than the 8-hour daylength.

They concluded 8 hours of 200 mm/m^2/sec or 12 hours of natural daylight are adequate for P amabalis TS97. However, they note in the discussion that other Phal hybrids are slower growing and need less light than Phal amabilis.
Photosynthetic Light Requirements and Effects of Low Irradiance and Daylength on Phalaenopsis amabilis

[u bada]

interesting notes/info/links... thanks for all that...

hydroponics people have tipped me before on green light to be used for that purpose... since plants don't use much of that spectrum it wouldn't through things off with plant cycles.

[estación seca]

I bought a blue LED bulb that screws into a standard household socket. Spectrum information isn't on the label, and I haven't had time to go find it. It is certainly blue.

[burz]

I used a green light(while doing roughly 4 hours of work) and it seemed to have no effect on the photo period to plants that are deemed to be photo sensitive.
I've not had to use the green light with orchids, but for me day/night tempreture and just Temps alone have been more the deciding factor on bloom or not bloom with the orchids I grow in Windows in zone 5b (phals, brassidium, oncs, and....acacalis cyeanea).
I have no data to back, but don't feel that a normal household lamp/bulb is enough light to dramaticly effect a photo period given the light source is far enough away from the plants yet enough to manage to do what ever it is you have to do during that time. But I'm willing to say it would if when natural daylight runs out and you have a light on for most of the duration of "night" plants probably would not be at peak performance.
Now that I've hastily posted I'll read the article links shared above lol

[Tindomul]

Can't add to the conversation, just want to say thank you for the taking the time to inform us. I learned some.

[Regelian]

Very interesting topic. Yes, red is the light that triggers flowering in many plants. It is most abundant when the sun is high (Summer) and in Winter, with the sun at a low angle, the blue light dominates. Of course the sun hasn't changed, but the air filters (attenuates) the light removing the red part of the spectrum in Winter due to the longer path the light travels through the atmosphere coupled with the longer (less energetic) red wavelegths. We see something similar under water when we dive. At about 10m under the surface only blue light is available and we do see this! Shorter daylengths play a role, as well.

Essentially, plants require 380nm-500nm for their main photosynthesis, while the red spectrum between 630nm-680nm adds more energy, as well as stimulating-regulating certain biological processes. It is the strong variable during the yearly light changes. It is a fascinating study and we have found that light at 670nm combines with light at 720nm actually creates more photosynthesis than the sum of its parts! Still, the photosynthesis is minimal when compared to the blue range, which is the bread and butter of photosynthesis.

We still do not have much info on the spectrum between (500-600nm), other than certain pigment production.

To view your plants in the evening without influencing possible bloom stimuli, then avoid the red spectrum over 600nm. Violet to green to yellow orange should be fine.

All this said, I am basing it on experience and my own research in marine systems. I have not found any firm data that truly defines spectral influence in terrestrial plants. Only that oriented to greenhouse culture, which tends to be focused to fruit and flower production, often excluding long-term effects to the plant itself.