We have a 30yo lemon tree in a large container. It spends winters inside and summers outdoors.
We moved it outside a few weeks ago, when day temperatures were between 10 and 20 C and night temperatures at 5C and above, and it seemed initially quite happy.
It was lush and dark green (best winter ever, so far), but the top leaves may have gotten less light than the main crown while indoors. It had none of the huge etiolated leaves that indicate lack of light, though. Temperatures indoors were around 20C, because we inhabit the room.
It was fertilized regularly, following the instructions on the bottle.
After a very short time outside, leaves started to drop and roll a bit lengthwise. We suspected lack of water (although we had thought the soil to be humid enough) so we watered very thoroughly, letting the excess run off. Leaf shape went back to normal.
Since a few days we see a massive leaf drop (ca. 100 leaves in total) and discoloration: Leaves take on a yellowish, almost coppery tinge overall, in some the veins are lighter, and some are spotting grayish circles on the top that can be rubbed off (but need vigorous rubbing). The leaves are more leathery than usual and the scent is missing.
What is especially strange is that it seems as if the tree has difficulties drawing the water from the soil - after a week of dry and sunny weather, the soil is still more humid than we would expect it to be based on our experience.
We did have a very light frost, but that’s not the first time the tree was exposed to it and it never caused this kind of symptoms.