I had the same problem with my first crop, I learned however that the problem is likely caused by two factors. First, the lamps might be too close to the plant and might cause the leaves to burn, try raising the lamps a bit, even just an inch or two. Second, the problem might be caused by some sort of nutrient deficiency, when the young leaves start to yellow or wilt that's likely to be a nitrogen deficeincy problem, try adding something called soda of nitrate, something you could pick up at any gardening store. soda of nitrate is simply sodium nitrate which helps stimulate growth and basically when there isn't enough of it the younger leaves yellow, wilt, and die.