Yellowing on top leaves

No curling, no brown spots, just yellow that starts from the stem of the leaf and working its way out. Plant looks nice, green and juicy. Does this indicate that the light is too close? Or, does it mean that I need to add some more plant food? Or sulfur deficiency? I am growing in a aeroponic system.

The problem actually looks a lot like this, which is labeled as a sulphur deficiency by this pic. But maybe one of the other indicators that I mentioned also.

http://www.growweedeasy.com/sites/growweedeasy.com/files/sulphur-deficiency2.jpg

This is not a pic of my plant, but the outside of the leaves are green and the middle is yellowing out.

This is actually more like it, but its only leaves on top. The leaves not getting as much light are not doing this, and smaller plants that I have in this same veg, are also not yellowing. Which makes me think it's the light. Now, it has been getting cooler here at night. This pic here indicated cooler temps causing this. It never gets below 69-70 in the room.

They look healthy. I would bump up the nitrogen

Yeah they are healthy as fuck. This is my first go with synthetics.