זוהר שמות — דף קכא ע”א

Parashat Mishpatim — folio 121a compares Israel to a tree and to grain. Just as grain becomes like ripened fruit after harvest, so Israel, after being gathered from exile, are called "holy to the Lord, the firstfruits of His produce." Israel is called a great and mighty tree that nourishes all — within it is Torah, nourishment from above, and prayer, nourishment from below. Even angels have no sustenance except through Israel. The etrog symbolizes the heart attached both above and below — the heart sees through the eyes which are the commandments, and the heart knows through Daat which is the tree.