The thing to remember about radiation, though, is that it's not gross - it's net. In order to have net positive radiation absorbence, the thing that's doing the absorbing has to be cooler than the thing that's doing the radiating. If the broadleaf can find some way to keep its surface cooler than the surrounding gasses, then running off of planetary radiation migh work. Otherwise, it doesn't.
EDIT: I did not know what I was talking about. Thanks be to wikipedia.
EDIT: I did not know what I was talking about. Thanks be to wikipedia.