The ocean: It's full of salt. No saltwater-intolerant plants allowed here!
And by the way, since photosynthesis is only about ~1% efficient (compared to standard solar power being 20-40% efficient) and any fuel taken from plants is removed from our food supply (except second-generation biofuels, but there isn't much plant matter per plant that can be converted into that, currently) than the only main reason we should use it is because it can be used in petrol-burning cars.