I heard it theorized that the potion Dumbledore drank at the end of HBP may have been made of nightshade, judging by its effects.
Wikipedia says about nightshade:
Deadly nightshade (Atropa belladonna), also known as belladonna or dwale, is a well-known perennial herbaceous plant, with leaves and berries that are highly toxic and hallucinogenic. It is in the nightshade family (Solanaceae), which it shares with potatoes, tomatoes, eggplants, jimsonweed, tobacco, and chili peppers. In addition, Solanum nigrum is also called Deadly nightshade.
"Bella donna" is Italian for beautiful woman. I really don't see that applying to Petunia, as she is most definitely not described as beautiful. Maybe it's hinting at the two sides of her, though. So far we've seen the harsh, bitter side. Now maybe we may see the softer, caring side.