As Petunia is a far relative of the tomato, why not looking for a recipe for tomato?

Thanks S. Wenkart for your reply. That makes sense, following the same ideia maybe a recipe for potato could also works.

To start the initial culture from nodal explants you should have healthy young plants, bette before flowering or at the beginning. Sterilize it with 70% etanol 30',15% sodium hypochlorite 8', sterile distilled water 3 times 5 '. If the fungi still remain, merthiolate is needed. Leaves, internodes, flower stalks can regenerate on MS medium with 1 BA, 0,1 NAA. Then transfer to large culture vessels with good gas exchange. Then you can cut the soots to nodes with 1 bud, and they are rooted spontaneously on plant growth regulator free MS.