Almost all plants can get their nutrients from the leaves, including terrestrial plants. Aquatic plants will do very well in inert gravel as a substrate, if the gravel is sized so the plant can be held down and there is room for roots to grow. Even those plants commonly called "heavy root feeders" are only assumed to be that because they have big vigorous root systems. But, many aquatic plants grow roots just to anchor them in place, not to feed themselves. I have grown "heavy root feeding" sword plants that completely took over the tank in a few weeks, with only water column dosing of ferts. The ideal substrate would have lots of nutrients so the plants can obtain their nutrients from either or both the water and the substrate, but that isn't essential.