The ideal conditions for citrus when overwintering indoors are sometimes hard to provide, as they entail moderate to high humidity, bright light, and relatively cool temperatures. Raising ambient humidity can be challenging without the use of a room-type humidifier, since humidity trays/saucers tend not to boost humidity very much, though it may be better than nothing. Our Growing Dwarf Citrus page has more information. As with many other indoor plants, fluctuations in environmental conditions can cause flowers or immature fruit to abort/drop, as can plant stress.

One of our Extension colleagues who grows his own citrus in containers follows this maintenance approach:

• to manage growth since he fertilizes often during the growing season, each summer the branch tips are pruned back a little to shorten them (around early summer)
• to stimulate renewed growth overall and faster (or more prolific) flower bud set, he also root-prunes a bit each spring to stimulate fresh root growth so the root system will more easily support the demands of the foliage and fruit; this involves unpotting the plant, shaking off some of the soil so you can see the root system, and trimming away some of the root tips to shorten them (or trimming back any tangled dense clusters of roots)
• he also gives the trees a brief period of mild drought stress prior to the root pruning in order to further trigger blooming, since plant stress can play a role in the timing or abundance of flowering; this can be accomplished by letting the plants get drier for longer periods between thorough waterings

This is not to say that your tree needs this treatment just yet; his trees are much larger than yours, so that may factor-in to the particular care regime he uses. Still, as for general plant care going forward, we suggest you monitor the potting mix acidity levels (pH) and aim for a pH in the range of about 6.2 to 6.5, which is slightly acidic and will keep the lemon's roots at peak ability to absorb the nutrients they need, including iron. Iron deficiency can be common in citrus foliage if the soil acidity becomes too strong, and if that happens the pH can be raised back to the ideal range with careful applications of lime. If the pH is too high, it can be gradually lowered with certain forms of sulfur instead. Chelated iron is a good source of iron if foliage begins to yellow due to either under-fertilization or soil pH issues. Currently, some of the foliage on your tree appears to be showing mild amounts of what could be chlorosis (whether iron or another nutrient deficiency), but it's not severe. Root health plays a role in nutrient absorption, and if roots are ailing due to overwatering (or by getting too dry too often, though this is less likely since citrus have good drought tolerance) then they won't be able to absorb enough soil nutrients, no matter how plentiful in the soil.

Nutrient deficiency symptoms in foliage can be difficult to diagnose, since some can overlap and occasionally discoloration is due to pests or disease instead. For instance, early-stage spider mite damage can mimic chlorosis from insufficient micronutrients. Check both the upper and lower leaf surfaces carefully for mites; only higher populations of spider mites make enough webbing so it becomes prominent; we don't see mites in the photo, but might not be able to if they're not too numerous yet.

Make sure you're using a complete fertilizer -- that is, one that includes micronutrients. These are the nutrients beyond just the N-P-K major nutrients, and while the "micro" in their designation may be misleading (sounding like they're not very important), they are still vital to plant health (they're just needed in much smaller quantities than N, P, or K). Micronutrients include zinc, iron, manganese, and magnesium. You normally wouldn't need to fertilize much, if at all, in winter, but if there is a soil deficiency then at least one more dose might be beneficial. Abide by the interval recommended on the fertilizer's label, though, so you don't apply too often based on when your most recent dose was.

Leaf drop usually occurs as plants moved indoors adjust to their new environmental conditions, but if it continues, might be a sign of stress due to warm temperatures (especially if the closed vent is still emitting warmth, just not an actual breeze), dry air, soil moisture level being too high or low, or insufficient light levels. Check the soil around two inches deep to monitor for watering, only watering when it's becoming pretty dry to the touch at that depth. The shedding of immature fruits could be due to insufficient pollination in addition to a stress response, so maybe you can experiment with how you're moving pollen between flowers.

Miri