I used to keep a potted fig tree on a southeastern glass loggia for a few years. I had to move it to a northwestern open balcony last spring. When I did it, I repotted it to a slightly larger pot. I use plastic pots for it and used to keep the existing soil. In the autumn before night frost, which is somewhere in October here, I moved it indoors to a cold room once it shed leaves. The temperature in the room was around 15 degrees Celsius and is still around that, except for around a month ago when it was about 20 degrees Celsius. The room wasn't shaded, so, in the last month of the winter, the fig tree started to grow new leaves, but they were unhealthily white. I suspected that it might be missing some nutrients resulting in a lack of chlorophyll and fertilized it with an off-the-shelf fertilizer for citrus houseplants for several weeks. It might be also relevant that in the room it stayed on a bench in a tight space between a calamondin tree and a bay tree. I also had to keep storage boxes with apples in the same room for months.
A month ago, I removed the soil. The roots did not grow into the new soil that I added last spring. The roots had grown over time into a tight cylinder. I cut the bottom of it and removed as much of the foul-smelling soil from the inside as it was possible. Removing it was difficult because it was thick and sticky. I identified and cut rotting roots. They were reddish inside, so I cut them until they were white on the inside. I had to cut about a third of the roots. Then, I washed the roots clean, though the sticky substance was reluctant to go. I let it dry and powdered the cuts with hand-ground charcoal. I potted it into fresh soil labelled for citrus trees.
I also cut the branches. The tree started to grow pretty quickly, but the new leaves grow white. Some leaves have green in them, but the amount of green in a leave does not seem to change over time. A few older leaves dried up. Sprinkling the leaves with a water-diluted fertilizer for a week or two had no apparent effect on the amount of green in a given leaf.
A few days ago I tried to take the plant outside during the day, but the white leaves started curling and a few got brownish burns. I took a picture of the plant the first time I took it outside. Please, see below.
I suspect that the roots could get overheated last summer on the balcony, or the plant got overwatered in the winter, or it was too cold when it started to grow, or it was overfertilized then, or that it got little light lately. It is still cloudy in here. How can I save the plant?