looks like heat and a touch of nute burn, back off nutes and get some type of fresh air going in the temps are probably going well over 100 i use the same light and takes some good vent to get rid of the build up heat.
r u spraying them with the lights on? looks like water and hps mix burnt them. or fert spilling on the leaves during watering. but looks like def light burn.
i have used mg for years and no problems with it, looks like either temp or lack of water, mg dries out fairly quickly on hot days, try giving some more water and check in a few hours see if it perks back up.
sectioned off the top where the light sits to make it easier to keep temp, 2 80 mm fans blowing across the top takes most of the hot air from light out and added an 80mm fan to bottom pulling fresh air in and up cooling compressor runs 1 time per hour for 15 minutes and tem is staying around 79...