Their Desciption in Ulimate Wilderness makes no mention of photosynthesis and while they have the plant type the plant type only says that Plants breath and eat but do not sleep.
As the Plant Type says they must eat then they do need to eat. I would say that it would vary with GM interpretation as to what counts as eating. I had a player buy nutritious soil at the same cost as rations. She had fun going to local gardens and complaining about the “taste”.