I wasn't sure if I was supposed to post this in the "Rootstock varieties" section, but I'll ask here. If indeed I was supposed to post there then the moderators can move it.
I read something on a Russian discussion about citruses that made me think, and I've said somewhere that I don't trust any flower care information, especially citrus, from the Russians, but this sounded interesting and I wonder if it's true.
This was coming from a person that said a tree in question was Dutch, but there was a card that obviously said that it's a product of Portugal.
They said that most ornamental citrus nurseries graft most of the dwarf citrus varietes (like Calamondin) on a trifoliate orange rootstock. And because of that the reasoning behind keeping citruses at cooler temps during winters is that because the trifoliate orange not unlike a birch tree is not an evergreen plant like all citruses and becomes dormant during that time (if we had a seedling of a trifoliate orange apparently it would drop all of its leaves for winter). So according to the person the rootstock being dormant moves very little juice to the grafted canopy and at cool temps it's enough, but if kept at higher temps that would be a big problem and we'd be risking to lose the tree.

Is this all true?

I would think that if temperatures are high enough to harm the canopy (due to the dormacy of the trifoliate orange rootstock), then those same temperatures would be warm enough to bring the trifoliate orange rootstock out of dormacy and the problem would be a non-issue.

The reason that citrus are kept cool in the winter is so they will flower in the spring.

In most woody plants (and so I presume it's true with citrus/Poncirus as well, but have no direct proof there), it is the scion that controls growth vs. dormancy, not the rootstock. So with a citrus scion on Poncirus roots, those roots should "think" it's summer, even in winter. Certainly, in Florida, which gets rather warm weather for much of the winter, if you put lemon or lime on Poncirus, it will continue to grow quite happily throughout the winter, without a pause.

On the other hand, oranges or grapefruit on Poncirus or some citrumelos do tend to go into a deeper dormancy in winter than they would on some other rootstocks, so there is some sort of rootstock effect happening there.

But in any case, no, growing a poncirus-rooted tree warm in winter definitely will not kill, nor even harm, it.

Malcolm