Wheat is needed in the quest Cook Assistant.
As of 1.18 (The economy update), The wheat merchant has been removed. And wheat has to be harvested using a Gathering Scythe.
- Before 1.14.1 Gameplay Update, you needed to travel from Ternaves to Maltic or Ragni for the quest Stable Story. It is now a boss fight rather than a fetch quest.
- Wheat can also be obtained by killing the Scared Wheat mob in the Katoa Ranch.