rootbound motherplants?

How do I go about preventing my mother plants from becoming rootbound if I want to keep them for an extended period of time? Theyre already in 5 gallon buckets, but I think theyre still becoming rootbound. How do I keep the size of the plants, and more importantly, the roots in check when trying to keep them for an extended period? Heres what they look like that leads me to believe theyre becoming rootbound again, because they were like this before I transplanted them the last time..

I don't think it's a rootbound problem, I have seen some pretty large plants grown in quart size pot's. Maybe you could provide more pic's and the experienced growers on this site may have some input.

This is my tallest, oldest plant, theyre all in the same containers, and this is the only one that has this problem. THanks in advance for any advice.

Is it possible that she is shedding lower leaves due to being shaded out?