Extreme Heat Tolerant Plants?

I have a certain spot in my backyard that takes EXTREME sun and heat. I have had a pumpkin and a sunflower wilt over, lose all crispness of the leaves, and die. First of all, I want to know why that crispness of the leaves goes away if it isn't from extreme heat, and I also want to know what kind of plants that are nice could take that kind of heat. I am a zone 8 area. Thank you so much.