It could be heat stress, but We'd need to see a picture to know the real reason. In a sense if it is heat stress it is lack of water. Because the plant is losing moisture faster than it can uptake it through the roots. to fix it, spray some ph'd water on your plants at night, they will absorb it and it should keep your plant from being damaged by the heat. this is not a one time cure though. you'll need to do this once every few days, or every day if it's severe enough.