Q.Lemon tree only has a few leaves.
I would like to know what I can do with my lemon tree which grows in full sun in a pot. It
has very few leaves, please tell me if it is lacking something.
Many Thanks
A good rich potting soil, adequate water and fertilization are the first considerations.
If the plant has been in the existing pot more than a year or two, consider re-potting into a larger pot with good potting soil like this:
https://www.amazon.co.uk/COMPO-SANA®-Citrus-Potting-Soil/dp/B001B1NVHG/ref=sr_1_fkmr0_1?ie=UTF8&qid=1538486571&sr=8-1-fkmr0&keywords=down+to+earth+potting+soil
If it's a new planting this year or last, and you think the soil is as good as you can get, then the next issue is water. Too little or too much can be a problem.
Consider using a soil moisture meter to take the guesswork out of when to water. Don't under water or over water, excess water can cause root rot and the signs you describe. Too little water can cause drought stress, loss of leaves and branch die-back.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
Fertilize at least spring and fall with a slow release natural fertilizer like this:
https://www.amazon.co.uk/Down-Earth-Organic-Fertilizer-25-Pound/dp/B00KO4KTRG
