"Photoperiodic"- Please enlighten me.

[dcarch]

This winter, I am trying to grow a few varieties of tomatoes indoors. I have been having problems of leaves turning brown, and I posted some pictures in the “Common Garden Diseases and Pests” Forum “Lime Brown Salad!” trying to find out what’s going on.

http://www.tomatoville.com/viewtopic.php?t=3401

I am shifting the topic in this forum because in trying to identify the causes I have come across a terminology “Photoperiodic” which I don’t think has been discussed much before and I think possibly may have some relevance to my situation.

I have been supplementing the windowsill sunlight with metal halide light (250w). The combination of sunlight and MH light comes to about 16 to 18 hours of light each day.

Anyone can give me some input on:

1. How many hours of illumination to give before you do damage to tomato plants?
2. Is there any data on how many hours of darkness to encourage flowering and fruiting?

Thanks

dcarch