In my experience the only difference I see is a slight increase in the electric bill if you leave your lights on 24/7 ;)

This following information is more about growing plants, flowering and fruiting, rather than growing seedlings. But, whoÂs to say that what affects plants when they are "full grown" doesnÂt affect them when they are baby seedlings
This info is from the "Alaska Gardening Guide, Vol. 1" by Ann Roberts: Much of this info is exact quotes from the book.
Most plants are not fussy in their requirements for a light-dark cycle; but there are some long-day plants and some short-day plants. A short-day plant is really a long-night plant.
It seems that plants carry out some very important chemical reactions at night, and it they don't have enough uninterrupted darkness to complete these, the whole life cycle may be upset. The most striking effect is the disruption of the flowering process.
Some plants that require this dark "night time" include poinsettias and chrysanthemums. Most varieties of soybeans bloom much too late to mature beans in central Alaska. Many, but not all, varieties of squash and cucumbers produce only male flowers during the early part of the summer. Only when the nights lengthen in the fall are the female flowers formed, and by then it is too late to produce mature fruit.
A few species of veggies that are short-day plants are eggplant, butternut squash, watermelon and true spinach. Eggplants in the interior, for example, will have their large lavender blossoms will fall off without fruiting if there is too much daylight. Most members of some other species such as beets and lettuce are adversely affected by long days but a few adapted varieties of these have been developed.

Without readng the whole of two "Plant Physiology" textbooks....I understand there seems to be a maximum (light reqirement in hours) to reach the maximum chemical use of carbon dioxide, sugars etc. to cause a maximum use of all chemicals in the photosynthesis process. This process will not go any further so a dark period of sorts, whether it be total or subdued (example red light) is needed to give the leaf a chance to start over it's need for uptake of light, water, minerals(soil) & oxygen.
I may have simplified this, too much a kindergarden approach to the answer!
Each plant species has it's specific need for all. Temperature & light are the most influencing factors of each plant's reproductive abilities.
Like I said I skimmed my DH's biology books. Most of the information is how light, requires chemical reactions to occurr along with the other needs. There is a 'dark factor' to plant phsiology (so says DH, the biology teacher) which I could not find im my search.
sam

Kim, (I think that's your name, if not - sorry) it's a good point that daylight affects flowering, you can see this in Christmas cacti and all sorts.

Sharont, I was finding vague terms like "maximum chemical use" and efficiency and stuff too but not really any elaboration on net effect of 24 hour daylight.

I like the terms "long day" and "short day" plants and ones that don't care. It make more sense. They don't all have the same needs. Thanks for the input all. I hope my electric bill isn't adversely affected!

Hi again northspruce,
Once again I checked our book shelves & lo & behold I found 'The Indoor Light Gardening Book'. The author George L. Elbert describes two sensational breakthroughs: The 24 Hour Day and Continuous Bloom. Back in the 70's several nurseries went 24 hour flourescent lighting and their plants prospered. Gearge J. Ball in Chicago raised seeds that way & continued to do so. An orchid nurseryman grew his orchids under 24-hour lighting with success, actually shortening the growth period to maturity! As far as "blooming itself to death" the plant does not die. What was noted though the plants took a rest from blooming as if triggered by a genetic code and still went into dormancy. He states gardeners have raised Marigolds & Petunias under 24-hour light. They bloomed just as well in the garden and plants were just as sturdy.

There are 'Miracle Blooming Plants" under 24 hour lights.
African Violets, Begonia Semperflorens, Columnea, the Goldfish Plant, Cuphea Hyssopifolia, Episcias Moss Agate and Cygnet,Euphorbia Splendens, Exacum Affine, Gesneria Cuneifolia, Lantana, Oxalis Regnelli, Dwarf Pmegranate, Sinningia Pulsilla & Hybrids & Streptocarpus 'Constant Nymph'.
So thanks for asking the question. Those gardeners in Alaska could give us insight as to the effects of almost 24-hr daylight.
At this time of year it all comes down to comparing the electric bill to last years should you decide to annually continue the 24hr-day light method, eh?

Thank you so much for your research Sharont. Now that is info we can all use. Funny you should mention African violets; since my lights have been on longer for the seedlings, my African violets near the same lights have repeated their bloom near constantly. Now I'm wondering if I should move my cacti elsewhere though? Hmmm

An interesting observation with your African violets, northspruce. I assume the light they actually receive from your seedling's lights is minimal, which might then indicate the prevention of chemical reactions needing dark, rather than actual use of the light for photosynthesis.

Regarding the original thread subject, I do know than some hosta growers will use all night light to increase the growth rate of their seedlings. And it REALLY works.

Rick

Well actually Rick they get quite a bit of light, they are almost half under the lights because the seedling tray is elevated next to them. If that makes any sense. And the whole setup is in a south window, on top of an upright freezer. So they always did get daylight anyway. That particular light is set to come on at 4:00 p.m. and end at 8:00 a.m. so at any time they get more or less indirect natural light, direct sun, or the fluorescent lamp. The rest of my seedlings are in the basement with 24 hour lights, these were just the ones that needed warmth to germinate.

That makes a lot more sense, as I never knew African violets to be day length sensitive. Mine from my Grandmother blooms constantly all year. So then, it is merely the additional light to photosynthesize with that increases bloom on yours, I would say.

Rick