Citrus trees maybe dying after front?

I live in southern Utah, zone 7 or 8.
Last spring I bought four dwarf citrus trees (Valencia orange, bearrs lime, eureka lemon, ruby red grapefruit) and kept them in their pots. They got hit by some frost outside a couple of times, and their leaves got brown and started to fall off. I brought the four trees inside.
The orange tree recently got leaves again. The other three seem to be greenish on the root stock, but I can't see green under the bark on the upper part. The branches are brittle, but there seems to be a little softness under some of them.
Little sprouts started coming out on the root stock...I figured those were suckers, and I removed them. Was that the right thing to do?
If a tree grew well the previous year, then does a healthy root stock mean a healthy tree, or is it possible for the upper part to die while the bottom stays fresh?
Thanks for any info, advice, etc. you can give. --S.
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    Yes
    Yes. It is quite common. Common practice is to graft desired varieties to more cold-hardy rootstock.
--
Derald

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