This is my problem with Orthodoxy....Jewish or otherwise. People take things literally and then, though the use of analogy and arguement, move into new and previously unspecified rules, prescriptions and regulations that were never actually specified by the original scripture (the alleged word of God).
In Leviticus, it states words to the following effect (depending on which translation you want to quote):
23-40------ And ye shall take you on the first day the fruit of goodly trees, branches of palm-trees, and boughs of thick trees, and willows of the brook, and ye shall rejoice before the LORD your God seven days
Nowhere does the fruit used in the ceremony specify that it must be a citrus fruit called Etrog....never mind whether it is from a grafted tree or not. So...way back when, wherever this custom began, someone used a lemon cv. etrog and that became a tradition passed orally from generation to generation. But if you accept the old testament verse from Leviticus as the complete and infallible word of God, either directly or through the inspiration to whatever man first put the verse into writing......there is nothing that really prevents you from using any fruit from any goodly tree. This whole business of worrying about whether the tree is grafted or not is taking the issue of compliance with the law to a ridiculous extreme. This mostly occurred from centuries of rabbis sitting around analyzing, hypothesizing and specifying how one could make themselves more holy by sticking the rigors of the law to the nth degree.....and beyond. It really does get to the absurd. But, if this following of tradition as it has been handed down is comforting to some....then by all means, go for it. For me, it just makes no sense.
Harry