Toning the lower abs can be done by doing many abdominal exercises. However, there are ways to target the various parts of the muscle.
This is easily done and is similar to targeting certain regions of the back of the thigh, which you can isolate and target to become more firm and tight.
The whole area works, you just put more attention on the region you would like to work.
In this video is a great exercise to firm and tighten the lower abs and to assist in getting a more firm and toned stomach.
There is a huge added bonus in this exercise when it is done correctly.