Leaf rolling problem

A few of my clones and moms have some leaf curling. It seems to be on the upper leaves and new growth the most. I don't see much of it on the flowering plants. Here is a picture. I've read how it could be a larger number of things, most not very serious. Do you know what it might be?

That could be heat stress. What is the temp in the growing area?

70-78 degrees F

So........I'm going to give them straight water for a day or two, then come in with nutes at 50% and see if it improves. I don't think it's a light problem.