Some plants need particular photoperiods to trigger blooms so with plants like tomatoes many times seedlings and young plants are grown under light 24/7 until they are a few months old and then people normally switch to a more normal photoperiod, but there are some that grow them under 24/7 as well.
The dark reactions are called that just because between the light and dark reactions only 1 can take place at night (the dark, lol) but they can take place in light as well, they do not require darkness.
This is from another forum, garden web, about growing 24/7, it's anecdotal and does not prove it's better, but it sure didn't kill his plants.
"If you're just using 24/7 light to start plants to go into the garden, then it works great. I did an experiment last year. Two sets of plants..one 16 hours of light, one 24 hours. The 24 hour plants were much bigger, produced faster, and produced much more over the season. I leave my lights on 24/7 for everything now. There is a thread out there that I documented my experiment in from last year. I used the peppers as examples but tomatoes were very similar.
Good luck"
