Off the top it looks like heat stress.....but you stated only one plant has the issue (not the one closest to the light is it?). I'd check your ph runoff and see if it is in the proper range for soil. Also could be a bit of overnute......hard to say.....so check everything.

Also not sure about your water source ph......if you are watering with anything outside the range of 6.2-6.8 I'd certainly stop doing it and make sure you water within those ranges.