With rose seedlings, I use 24/7 lighting.

I remember this topic coming up last spring and I believe I was the one to ask it--I was told then that seedlings and plants need a bit of 'rest' time where they are not converting the light to food. I have my lights on about 16 hours over my seedlings, but I keep them on 24 hours over my germination mat (for the seeds that need light to germinate) and when I am trying to grow cuttings. I also keep my fans on 24 hours. Hope this helps.

I have done it both ways and have not noticed a difference. Theresa.

no way

plants produce food when it is dark

They'll die 24/7 light

See:

http://www.actahort.org/books/56/56_9.htm

Here is a link that might be useful: link for above

See:

http://www.usu.edu/cpl/research_dwarf_pea4.htm

Here is a link that might be useful: link for above

have also had mine on 24/7.......it wont hurt them

Quote
no way
plants produce food when it is dark

They'll die 24/7 light
Unqoute

I dont think the above quote is accurate.

Plants produce food by photosynthesis, that means that the sun's rays (or light energy from suitable artifical lightsource) are harvested by cholophlast, which is the photosynthetic cells in the leaves and which by the way is also responsible for the green color in leaves. The light (not darkness) is harvested, and together with fertilizer, water, and carbondioxide, these elements are turned into carbohydrate which is energy.

Carbohydrate is then used to facilitate growth, for storage, transported to roots, etc etc.

Light is the primary energy source which facilitate the conversion of ingredients into plant food (plant energy).

During the dark period, plants metabolize, e.g. convert stored "food" reserves (aka carbohydrates.. produced during the light phase) into plant energy.

Sometimes, the plants are less stressed during the dark phase, and therefore you often find growth is faster at night. This is especially true for plants which are grown under very warm conditions.

However, the plant energy (foodstuffs, aka carbohydrates) is produced during the lighted period. Not during the darkperiod.

I usually put my lights on a timer, but I have also grown plants under full 24/7 light with no problems.

Kenneth, most succulents and cacti, and several other plant species, do only produce "food" when it is dark. They have adapted their metabolism to turn light energy into intermediate products during the day, and then metabolise these to carbohydrates at night. The main reason in succulents is to avoid carbon dioxide exchange during the day which would cause water loss. The majority of plants metablise all the way to carbohydrates during photosynthesis.

Most plants have a metabolism at night, called respiration, that is different from during the day. They actually consume carbohydrates and emit carbon dioxide. Thus they can continue to grow even when there is no light.

Research has shown that some seedlings benefit from 24/7 light, some don't, and most mature plants do not perform better under constant light. Anecdotally, most succulents grow significantly faster with a period of darkness each day.

One of the problems of discussing whether a seedling/plant will benefit from 24/7 lighting is what type of light are we talking about? Weak 24/7 indoor light intensity such as produced by fluorescent lighting may have a different effect than the light intensity produced by outdoor sunlight or the light intensity produced by indoor high pressue type lights (what I have called in previous posts as World War II government surplus searchlight setups) .

Also temperature can be a factor. I came across one article that stated that tomato plants have a problem with 24/7 lighting when kept at constant temperatue but did not have a problem when there was a normal change in temperature from what would of been day to what would of been night (even though the lights were kept on 24 hours per day.

I appreciate all of the different opinions that I see posted in this thread. Since the opinions vary greatly we have to make some sense out of them in order for the information to be truly useful. I would personally like to see statements qualified by the conditions under which they are observed, such as:
1. What kind of lights were used? How many? What wattage?
2. What plants were grown?
3. How old were they (were they grown to be planted out, or were they grown to maturity under the light)?
4. What happened when the lights were on 24/7? Better growth than with shorter periods of light, or about the same? Was a control group used with normal on/off cycles for comparison?
5. What is the temperature at soil level with the lights on and with them off (at night)?

I can see an advantage to continuous lighting if plants grow much faster and are healthier; this could happen if the room is very cold and the lights produce needed heat, even if the additional light didn't help the plants. But if there is no improvement in growth then we are just wasting electricity and shortening our bulb life.

Thank you for your input.
Bob

Title: Treatments of fluctuating temperature under continuous light enabled the production of quality transplants of tomato, eggplant and sweet pepper

Authors: Omura, Y.; Oshima, Y.; Kubota, C.; Kozai, T.

Authors affiliation: Chiba Univ., Matsudo, Chiba, 271-8510, Japan.

Published in: Hortscience, volumn 36, pages 586-587, (2001).

Abstract: "In transplant production under artificial lighting conditions, continuous lighting is desired to achieve high production efficiency while potentially reducing costs. However, such physiological disorders as chlorosis and necrosis on leaves are often observed under continuous light. Tomato (cv. Momotaro), eggplant (cv. Senryonigo), and sweet pepper (cv. Kyomidori) seedlings were grown for 14 days either under a 12-h photoperiod with a photosynthetic photon flux (PPF) of 300 mumolcntdotm-2cntdots-1 at 24/24 degreeC (temperature in photo-/dark period), or under a 24-h photoperiod (continuous light) with a PPF of 150 mumolcntdotm-2cntdots-1 under one of three temperature regimes: 24/24 degreeC, 28/20 degreeC and 32/16 degreeC (temperature in the former (12 h)/latter half (12 h) of each day). The average temperature was 24 degreeC and daily integrated PPF was 13.0 molcntdotm-2 for all treatments. The dry masses of tomato, eggplant, and sweet pepper seedlings under a 24-h photoperiod were 1.1, 1.5, and 1.6 times, respectively, greater than that under a 12-h photoperiod at 24/24 degreeC. Chlorosis was observed on leaves under a 24-h photoperiod for tomato at 24/24 degreeC and eggplant at 24/24 degreeC, and 28/20 degreeC; but, it was not observed for tomato at 28/20 degreeC and 32/16 degreeC, and eggplant at 32/16 degreeC. For all three species, the stems of the seedlings were shorter on day 14 and the seedlings set the first flower at lower nodal positions when grown under a greater diurnal range of air temperature in the 24-h photoperiod. Fluctuating the air temperature under continuous lighting enables the production of tomato, eggplant and sweet pepper transplants of quality (i.e., having accelerated growth and early flowering) without causing undesirable chlorosis on leaves."
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Henry,
Thank you very much for locating the article about tomato, pepper, and eggplant seedlings. Your previous post regarding picea left me less than excited, but this one on tomatoes had information I could 'sink my teeth into!'
This really explains why there was such disagreement among contributors to this discussion; they were probably all correct, depending on the circumstances.
Bob

My understanding has always been that the real magic of growth takes place during the dark period. IMO, dark time is required to process all the goodness the plant has recieved during the light period. Personally, I do an 18/6 cycle w/ great results...

JW