This was sent to me in a PM and while I could have answered it there, other members would not benefit from my answers nor would any other members be able to add their input. I have transfered the message here so all can learn from and add to it.
Andi

Hi, I just joined the forum as a very new "guardian" of a container Meyer lemon tree. have been reading your responses to other users problems and I am wondering if you could help me. I dont want to give up on the small tree. but the leaves continue to yellow and fall off. I bought it about 3 weeks ago; transplanted it into a slightly larger pot; am careful about the watering; used citrus fertilizer with magnesium once; and it is on my terrace which gets direct sun from dawn until 12:30PM; then it gets indirect sun. Is it in shock from the transplant? does it need much more direct suntime? Thanks for your time. Peggy

Hi Peggy and welcome to the forum.

Your new tree is suffering from transplant shock and this is to be expected. The sunlight it gets should be sufficient to allow it to grow but I would move it into a shadier area until it has a chance to recover from the transplant. After a couple weeks you can start bringing back into direct sun a little at a time. Maybe start with an hour the first day, 3 hours the 3rd day and so on until its in direct sun for the full time until 12:30

I would check the soil to make sure its not staying too wet and leave it alone for a couple more weeks. If everything is ok you should see new leaf buds swelling along the branches. IF it continues to drop leaves we can investigate further reasons.

The only thing you did that I would not have done was give it fertilizer. Plants are usually fertilized by the grower just before shipping to help them survive. These plants would not need to be fertilized for about 3 months depending on where you live. Is the fert you used a water soulible or slow release type or is it one of those spike things?

You didn't say how big the tree was and whether it was fruiting/blooming.