In Beadle and Tatum's study of Neurospora, they were able to identify three classes of mutants that needed arginine added to minimal media in order to grow. The production of arginine includes the following steps:

precurs→orornithine→citrulline→arginine

What nutrient(s) could be supplied to allow for growth of the mutants with a defective enzyme for the ornithine → citrulline step?