The citron is the tree's rootstock. All citrus trees are grafted to a hardy, and usually inedible, rootstock. You can see the union of the graft, usually a foot or less from the base of the tree. Any shoots or branches that come from the root stock should be removed. They take energy from the good part of the tree and will eventually take over.
Trees that have several types of desirable citrus on one tree (such as lemon + orange + grapefruit) have multiple varieties grafted to a single rootstock.
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