...but not sure I'd throw in the towel on it just yet.

Try flushing with Ph 6.0 water for a wee bit, and back the light off (or the plant) until it's more than 36-48" from the little darlin'... or put it back under something more gentle for a while, like a nice 200w equiv CFL...

If you're running a 1000w MH or HPS, you shouldn't let the thing get closer than 24" to a mature plant...

But, hey, it's a weed, and they can be quite resilient...