Throw away the plant light, they are complete garbage as you can now see. It's not because it's rootbound, but there could be other issued we don't know about.
Even if there are other issues, the plant light has to go. Do not even use it after you get other lights. (HPS, Metal Halide, CFL's or even standard 4' shoplight flourescents. Any of those are worlds better than what you have.
Take a look at this pic, this is a 2 month old plant that's been raised under two 250w Metal Halide lights that used to be in a school gymnasium and which I got for free off craigslist: Notice it's in a 16oz cup... Rootbound or not plants still grow.