We have several citrus trees, and one of the orange trees always has drooping leaves, or curled up leaves. We have watered it and it will spring back, but next day same thing. It had a small root ball when we planted it. Have tried to replant, but nothing seems to work. No leaf drop.
Curling Leaves on Citrus Tree
Certified GKH Gardening Expert
You may not be watering it enough. You need to make sure that the water gets all the way through the soil. If the soil is very dry, it may actually start to repel water. So enough water for another plant would not be enough to rehydrate this one. Give the soil a good, deep soaking at least once a week (in addition to regular watering) and that should insure that it gets enough water.
I had the same problem with a myers lemon. You might want to go after the roots by using super thrive and high phosphorus. Sounds like you have alot more above than the roots can keep up with.Bare in mind that citrus require alot of water in summer.I would work on roots and about 3inches of mulch I hope this helps. fellow gardener