Thank you for the question. Growing citrus indoor in Minnesota in the winter is a challenge but it can be done.  It’s not uncommon for citrus trees to drop leaves and buds when brought indoors for the winter.  Ideally, the process is done over several weeks and involves gradually increasing the amount of shade the plant receives.  If brought in suddenly, it will drop leaves and buds alarmingly.  Once indoors, the tree needs 5-6 hours of direct sunlight minimally, and for best results, needs 8-10 hours.  Indoor humidity should be in the 40-50% range and this is very hard to achieve in our winter Minnesota houses.  Even with a humidifier running, I can’t get my rooms above 35%!  According to Mary Meyer, University of Minnesota Extension Horticulturist in the February 2014 issue of Yard and Garden News at http://blog.lib.umn.edu/efans/ygnews/2014/02/yes-you-can-grow-oranges-in-mi.html:  Citrus requires a lot of nitrogen and iron to grow well, and iron is often unavailable in high pH soils, which tends to happen over time with the alkaline water we use on indoor plants. Yellow foliage is a common sign of iron deficiency in citrus and means you need to add a fertilizer that has available or water soluble iron. Throughout the summer, I use a readily available, water soluble fertilizer, such as 20-20-20 once or twice a month. In the winter, I rarely use fertilizer. And about once a year, I use an iron supplement to keep the foliage a healthy green color. These guidelines apply to lemons as well as oranges.  Watering is also very important to winter health.  The soil should be moist but not wet.  Water when the top 2 inches of soil are dry and then water until water drains out the bottom of the pot.  Pour off this standing water so the plant doesn’t have constantly wet feet.  Also, citrus can have trouble with pests in the house over winter.  Spider mites, aphids, mealy bugs and scale can be problematic.  Inspect your tree weekly and if you see any sign of these, spray the plant with mild soapy water and repeat weekly.  Wipe the leaves with water and soft cloth, paying special attention to the undersides.  These practices should help control mild infestations. Here is a link to a non research based website that has a lot of information and I have seen it listed as a source in some university articles about growing citrus indoors:  https://www.fourwindsgrowers.com/tips-and-advice.html  Here is another article about growing Meyer lemons indoors:  http://extension.oregonstate.edu/gardening/meyer-lemons-can-be-grown-pnw-if-protected-winter