In Jainism there isn’t a canonical rule that specifies “one leaf per day.” There are several fasting and dietary practices, but they’re not described as a universal mandate of eating exactly one leaf daily.

What you might be thinking of is a form of fasting or a simple daily intake:

• Ekaahaar (one meal a day) is practiced by some Jains during vrata or austerities. In this practice, the person eats once, usually once before noon, and abstains the rest of the day.
• There are various vrata (vows) with differing strictness in Digambar and Shwetambar traditions, including partial fasts, selective fasting, or eating only certain foods on specific days.

General dietary principles in Jainism emphasize non-violence and minimizing harm to living beings. Many Jains also avoid root vegetables (like onions and garlic) and follow region- or tradition-specific rules about what may be eaten on fast days.

If you meant something else (for example, a particular rule from Digambar or Shwetambar texts, or a specific practice observed by a community), tell me exactly what you have in mind and I’ll explain it in detail according to the tradition.