If not mites, I would go with high N also. The new leaves usually start off slightly pale and then gradually darken to a nice healthy green. But your smallest/newest leaves are already pretty dark out of the gate. Though, this could be from slower growth as a result of another problem... Just thinking 'out loud' I guess Also, high iron/sulfur might darken those leaves up too...
What I would do: check for mites under a 100x scope. If no mites, then I would wait for those curled leaves to grow out a bit and see if they rectify themselves. If they do, great! Just up-pot and enjoy the grow. If not... well then... I would look further into that Nitrogen possibility.
Some people also attribute curled leaves to high and/or low (depending on the person you talk to) calcium and/or magnesium. Pretty easy to fix this one. Do you use Cal/Mag when you water/feed?
Totally a side theory of mine, but you may be experiencing this as a result of high water, low light. Meaning that the leaves aren't able to aspirate as much water as they are growing in, so they sometimes curl (not to the point of edema though). This happens similarly with cannabis, but with cannabis, the leaves usually curl a little less abruptly - almost more of a bowl (or boat) shape if you can imagine that.
Well, I just laid out three or four possibilities, best of luck! Please tell us what ends up happening in a month from now!
