Thanks for your question.

With respect to the lemon tree in the pictures you sent, I have the following impressions:

1). There is widespread chlorosis in the leaves as evidenced by yellowing between the leaf veins. Although you did not mention it, if the variety of your lemon tree is either Meyer or Lisbon they have the highest light demand of all citrus trees. Even under your grow light, I suspect your lemon tree now being inside is getting only 10–20% of the summer light it had outside. This significant reduction in light can lead to leaf chlorosis, leaf mottling, and leaf drop. Often these symptoms occur after two to six weeks of being moved indoors. Since the stems of the lemon tree appear green and healthy, I would expect that the tree has not been significantly damaged.

2). With respect to your lime tree, it is far less susceptible to light reduction than is your lemon tree. This is the primary reason for the different appearances between the two trees.

3). Your lemon tree probably experienced stress in being brought inside. This transition would have affected the entire plant. Roots might have experienced a slowdown in growth. For sure, photosynthesis would have been reduced. Sometimes this transition will stimulate growth in the abscission region of the leaf. This is the area where the stem of the leaf attaches to the branch. Growth in this layer leads to leaf drop. As is true for light requirements, lime plants are far more resilient when moved inside than are lemon trees

4). Finally, your house is probably typical of most Minnesota homes during the winter months. The indoor humidity is significantly lower than outside humidity. This reduced humidity can promote leaf drop in lemon trees. Lime plants have a greater tolerance for dry conditions.

Moving forward, I suggest the following:

1). Increase the light Intensity around the lemon tree. Be sure that your grow light is between 6 to 10 inches above the top of the tree. Exposing your lemon tree to a second grow light would be an excellent thing to do. Use a full-spectrum LED grow light with a wattage of at least 150. Have these lights on for about 14 hours/day.

2)l. The humidity around the lemon tree should be at least 40%. Avoid placing your lemon tree near any heating vent or radiator. Ideally if you have a portable room humidifier, have this as close as possible to the lemon tree.

3). Do only very light fertilizing during the winter months. Whatever fertilizer you might use, apply it at only ½ the amount as specified on the product label.

See the following for information on these and related points:

https://ask.extension.org/kb/faq.php?id=426121 - :~:text=Expert Response,to dry out between waterings.

https://forums.botanicalgarden.ubc.ca/threads/leaf-drop-on-indoor-meyer-lemon-tree.94953/

https://extension.umn.edu/houseplants/growing-citrus-indoors - :~:text=Citrus plants grow best indoors with 65° days, dropping,before bringing them back indoors.

Good luck. Be patient. If you follow these suggestions, I believe that you will see significant improvements in your lemon tree within six weeks. The yellow and mottled leaves will probably not improve so they can just be removed at this time. New leaves should grow to replace them.

Thanks for using our forum. Please get back to us with any further, related questions.

Glad to be of assistance.  As long as the stems remain a vibrant green, the plant is alive and kicking.  If you have a chance, send us a progress response around the middle of February.  Many thanks.