The lemon tree is a fruit-bearing tree belonging to the citron family that can grow in both temperate and tropical climates. Its delicate yellow leaves are wide and somewhat heart-shaped, they are often used as a food coloring.
The leaves of the lemon tree turn yellow when they get exposed to direct sunlight or if it has been too wet for a long time. This is a natural process and the leaves will eventually drop from the tree, this is called "dieback".
The leaves could be also turning yellow because of the following reasons:
The tree has been pruned too much.
There might be a lack of direct sunlight on the tree, which is why it is losing its chlorophyll.
The tree may have been exposed to too much water, or there has been too much rain.