Apparently it has actual luminescent cells which don't need any kind of special treatment or food in order to glow. The company which created the Firefly Petunia has been experimenting with bioluminescence since the 80s.
Here's the blurb:
The firefly petunia glows brightly and doesnāt need special food thanks to a group of genes from the bioluminescent mushroom Neonothopanus nambi. The fungus feeds its light-emitting reaction with the molecule caffeic acid, which terrestrial plants also happen to make. By inserting the mushroom genes into the petunia, researchers made it possible for the plant to produce enzymes that can convert caffeic acid into the light-emitting molecule luciferin and then recycle it back into caffeic acid ā enabling sustained bioluminescence.
