Lights

The light may not be reaching all the way down to the leaves at the bottom of the plant. You could get a couple CFL's and some desk lamps put shining up at the plant from the ground, or from the side.

If the yellowing starts climbing up your plant you might have nutrient deficiencies, like if the teeth around the edge start turning yellow first followed by the rest of the leaf it may be a nitrogen problem. But it sounds like my main suspect in this case would be the light.