More info needed. How far from the lights and what kind of lights?
How often do you feed and with what?
What kind of medium are you growing in?
How often do you water? do you use a ro filter?
PH?
Looks burned, either with too much fertilizing or do to lockout. You state that its closest to the top of the plant that the problem starts, than look to the temps near the top of the plant.
Sorry I could not be of more help, pictures of the entire plant would be a help as well.