Q.lemon trees
leaves are turning yellow…
what’s the cause?
(trees are indoors…
The most likely causes of the yellow leaves are low light levels, overwatering, or both. If the tree has been brought indoors recently, it is normal for it to lose some leaves as it adjusts to the lower indoor light levels. It will also need less water when it is indoors.
If that's not the issue, here are some other possibilities to check:
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-leaf-drop.htm
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm