Your plant can only withstand a certain amount of heat and light. After a certain point, your plant will start exhibiting signs of stress on the leaves near the sources of light and/or heat. Your leaves will get yellow or brown brown spotting and may appear generally burnt in places. Find a way to lower the temperature and/or increase the circulation in the grow room or grow area if heat is the problem. If your plants are just getting too much light, try removing some of the lights or moving your lights further away from the tops of the plants. It's best to try to keep things at a comfortable room temperature at all times for optimal growth.