Turnips are rarely found along the ground of Underground Jungles. They are very hard to find, as they are quite small and don't spawn in every jungle (hint: They have brown leaves, whereas the grass is bright green. If that's not enough of a hint, Turnips have pink bases-look for grass with pink bits).
Currently, Turnips only have two uses; One of which is as a low-to-mid tier food, restoring 4 hunger points per Turnip (2 hunger icons). The other-and arguably better-use for them is to breed Beetles for farming.