Hitzestress ?

I don't think this is heat stress, I think they are getting to much water by the looks of the medium and do not, I repeat DO NOT, spray them when the lights are on. Learn to feed via the roots. spraying when lights are on causes the water droplets to turn into mini magnifying glasses and burn the leaves. Only time I spray plants is when their is a pest present. and I don't see any present on the plant, Sure some gnats but kill em all off with BTi or nematodes via the roots. yellow sticky traps are for monitoring only they do not solve the problem. You also need to chill on the defoliating and HST on the plant. you are in week 10 and your plants are smaller then others in week 3 or 4. Could be lack of nutrition in the soil as well but I doubt it as I seen mild tip burn from overfeeding. I also think your lights to close/intense their is little to no spacing between the nodes. This will be keeping the plants structure compact. Good Luck!