I agree, it's from the heat most likely. I know in my garden, the greens always get better tasting after a frost in the fall. I won't grow greens in the summer. Even Swiss Chard which is pretty tolerant to heat just fades so badly with out daily high temperatures it always looks wilted.
Yeah, to grow kale, chard, lettuce, beets and other cool weather crops in summer, I grow them under the shade of my tangerine tree. They still need a lot of water so they don't wilt. If the temperature hits 90 degrees, then even watering briefly at midday is not always enough to save them.
