Do you suggest we prune this seed-grown, indoor lemon tree? It's about three years old, never been outside (we are on the 12th floor of a downtown office building). Not sure why the leaves are getting yellow/rust colored on a few lower branches. Any advice is appreciated.
The yellowing leaves are likely a watering issue; too much or too little. It's important to evenly water a Citrus. Soil should be moist, never dry and never soggy.
Your Lemon Tree is very tall, and you need to prune the tree for the health of the tree or it simply will become too tall.
Lemon Tree need full sun location.
Pruning can take place in early spring, so now would be a good time.
I've listed several links to help you.