I remember seeing in the Igros Moshe a while back which quotes the Saba Kadisha's opinion that one can technically use a lemon instead of an esrog. It was a side idea in the Igros Moshe. Does anyone know where the Saba Kadisha's opinion is stated and his reasons?
In Orach Chaim 4 siman 123, Rav Moshe stronglly asserts that a lemon is not kosher for an esrog and says kol shekein a canary palm is not good for lulav.
Later in the tshuva he quotes the Saba Kadisha in his tshuvos chelek 3 siman 12 who seems to lean to possibly allowing the lemon to be used as an esrog, but points out that even he would not allow the canary palm, the focus of his tshuva there.