I have the same exact problem. Im growing in hydroponicaly, LED, DWC Drip, Root Riot with Hydroton surrounding. Check out my pics, the post is called "Hydroponically retarded".
Didnt get too many responses but from what I did get, it appears to be some type of nute burn. Heres is what Im gonna guess based on what Ive found out. Really need more info from you here and possibly pics. Again check out my post and if your plants looks like mine then there you go. Anyway, Im thinking your problem is probably either Nute burn or heat stress. Both easy fixes. If your using nutes, stop. If you can move your lights, then move them up. I was thinking of deficiencies as well for mine (very similar issue) particularly magnesium, but its just to early for that kind of thing.