The tree was left outside and the leaves had begun turning yellow due to cooling night temperatures, so it was finally brought in.
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Check the root system fot hte one outside. The one inside I think it has N deficiency coupled with poor light conditions. I have kaffir lime who stay outside at 5C and they don't turn pale or yellow. Of course these are from seed acclimatized over 4 years. The ones grafted with scion from Thailand, don't stand a chance outside.
To make a plant hardier, I believe one must stress it. Cold, dry, little light then recover with warm, moist soil then again stress it. The plant will learn to adapt to survive. Plants don't run like animals, they don't hide, they adapt. Growth retardants are also good.