Soil To Power Grower

I recently transplanted two 2 month old outdoor plants (purple urkle) into my power grower. They seem to be doing fine. The new growth is nice and green. the only problem is that the lower leaves on both plants seem to be turning yellow. what could be causing that?
i keep my ph leveled out with lemon juice and aim for 5.5-6.5 at all times. im using General hydroponics flora series nutes.

One more quick question. How did going from soil to hydro work. i just tried it because i didnt want to wait on seedlings and some how they have continued to grow for about 5 days . How is this Possible?

If the tops are growing green the plants should be okay. If it continues you have a lighting problem, or possibly the nutes- but the entire plant would yellow, not old growth.