Yuri, Mage of Life and Hope

Green mana doesn't empty from your mana pool as steps and phases end.
At the beginning of each of your upkeeps, you gain 1 for each white mana in your mana pool.
{1}: Add {g/w} to your mana pool. ({g/w} provides both {g} and {w} mana, but provides only one mana at a time.)