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Please post picture of the whole plant and close up of stems/branch with these leaves.

First off, I would never keep the roots of my trees inside that warm unless you can give provide all day full sun. I mean at least 12 hours. I like to let the roots of my trees stay at room temps if I can't and by nature cool off at night and warm by day.

Those leaves are either burning due to salt deposits, too much heat, or poor soil.

Here are a few more photos. The brown leaves are the minority. Most either have fallen off completely green or with yellow veins.

Before trying to warm the roots, I was still experiencing significant WLD with soil temp of 68F. This is getting probably 3 hours natural daylight and supplemented with another 8hrs or so of a full spectrum grow light.

Shouldn’t be soil quality. I use a relatively coarse blend of bark with minimal amount of peat. Drains well and doesn’t seem overly soggy or anything.

Losing old dying diseased and damaged leaves is normal. Are you losing perfectly fine looking leaves? What are you catching in the yellow traps? Fungus gnats?

How are you measuring soil temperature?

Had some fungus gnats outside in summer but none right now. Had mostly using it to help monitor for them or any other pest.

But yes the majority of leaves I’ve lost look either damaged and or like they have nutrient deficiency. Only a couple of green leaves have fallen.

Soil temp I measure with a soil temperature probe from a heating mat thermostat.

Your trees are growing at a window, correct? Indoors? Does your ambient temperature dip below 72? Or is 72 what your thermostat is set at. I guess in other words, do you have a way of determining air temp at your window?

Do you fertilize? Bonsai is asking because you may have a big differential between roots and leaves.

Yeah indoor air temp is about 71-72 by thermostat setting. But I know that room tends to sometimes be even warmer (exact temp admittedly unknown) but my estimate is about 2-4 degrees warmer. Which is why I thought warming the roots to 74-75 would put both roots and air in the same ballpark temp. And I get that if that’s the case, you would need a lot of light so they get some natural light and then I supplement with grow light.

As for fertilizer, yes I do. A variety of types and methods (soil and foliar application) including fish fertilizer, seaweed/kelp, and granular types of organic fertilizer which are balanced for citrus. And I recognize the controversy about organic methods in container gardening but my pots are full of soil microbes and even earthworms so I doubt it’s a nutrient availability issue.

How can you be sure your pots are full of dependable microbes in doors ?
Not for one sec do I believe they work and it’s seems to not be working for your trees.
I’ve been there and done that over the several years learning a hard lesson by killing my trees in the past. I don’t believe those huge organic companies for inside containers unless they are left outside in contact with the ground 24/7

I get what you’re saying and I can’t say that I’ve never questioned it myself. However, this isn’t year 1 or even year 2 with this tree. For this tree specificall, this is probably year 4. And I have multiple other citrus trees in containers that I’ve had for even longer (7 years or so). My practice with fertilizer has never really changed over the years in terms of using only organic fertilizer exclusively. I would think given the high nutrient demands of citrus that if these weren’t working that I would have lost more of my trees over the years.

Also, there are a number of organic fertilizers that are water soluble and as such should be available for uptake pretty rapidly. Fish fertilizer is an example of one that has water soluble nutrients (primarily nitrogen). Blood meal is another one. Even Miracle Gro sells an organic liquid soluble fertilizer that is more complete in terms of including both P and K. I’m not saying that the non organic methods aren’t likely far easier and perhaps even more effective. But I dont think a trees struggles when going indoors are related to fertilizing practices. My trees thrive outdoors. It’s just a matter of trying to develop the best strategy for overwintering them indoors which has been a challenge.

I ask about how you are determining temperature because a sgnificant difference between root temp and leaf temp can contribute to leaf drop and make it worse. Especially betwen night and day. For the most part, unless you have recently watered, your roots and leaves should maintain equilibrium with the room unless you have a crawl space below the room that is much colder and holding your floor at a lower temp. Even if the room your trees are in is warmer than the rest of the house, the temperature by the window will necessarily be cooler by a few degrees. This is because the R value of the window is much, much smaller than that of the wall and so you'll have more heat loss through your window. Even if your window were a high quality argon filled, you'll still see the temperature drop by the window relative to the temperature outside. (The colder it is, the more dramatic this drop will be). It's something I have to keep in mind for my own trees which are exclusively indoor-grown.

In addition if you're using a heating mat, but your pots are large enough, you'll have a temperature differential between the base of the pot and the surface. So if you're measuring the temp at the surface it could be much warmer lower down.

Another thing that is good to consider is that trees like a temperature drop at night. As much as 10 degrees won't hurt the trees, but can be beneficial. Think of temps outdoors. At night they'll always drop. I don't know if that's a contributing factor, but it could be.

Another thing I might look at is your water. If your trees are outside during the summer I'm assuming you're allowing the rain to do the job of watering when it can? You may have an alkalinity problem with the water indoors. I'd pour some vinegar in a glass of your tap water and see if it fizzes. If it does, you might consider correcting it.

Just a few thoughts.