I seriously doubt its a straight root problem, if that were the case, the entire plant would have folded not just the lower sections. For one I would feed your plants, a good healthy dose, giv efull feed of bloom with some additional veg nutes, rason for the veg nutes is to boos tup nitrogen in the plant. It is a common thing for lower leaves to die off if they are not collecting enough light, but solid nutes are needed. Wont hurt to add a little epsom salt for magnesium will be an up. If you have anything like Dynagro Protekt or any silcate nute normally a 0-0-3 or like additive, would be a good thing to toss that in the mix.
Dont over nute them but make sure its a nice healthy feed, give them 2 days of feed then 1 day water.