Remember Film Z?
Sanji fights the dude who has some kind of literal plant abilities
Greenbull appears in a sun chair initially
Then he mentions that he hasn't bothered to eat for 3 years
I can only think about Photosynthesis.
And it actually makes a lot of sense.
Kizaru is Light, Fujitora is Gravity, and Greenbull is the Flora.
The perfect environment for life to grow.
And besides, we shouldn't expect Greenbull being a literal Bull, cuz their epiteths doesn't work like that.
But i would love seeing Greenbull with a cool Zoan, but as the best Rokushiki Master out of there, to the point he creates new techniques to teach the Marines, and doesn't bother fasting for 3 years somehow