Just my opinion on the situation :- I can't see how light or lack of it would make leaves at the bottom of the plant wilt. If it was light deficiency the whole plant would look sick and the more recent growth is lucious. Personally I would say that you are over watering and the soil towards the bottom of the pots has become saturated preventing the roots from getting oxygen. Looking at the quality of the soil and the stages of the plants its very unlikely a nutrient deficiency. If I was you I would not water them for a few days and see if there's an improvement. Don't take my word as Gospel but I've read alot and its most likely your problem.