Don't let the soil get completely dry; citrus like moist root zones, but not flooded root zones. Give it some food, a good balanced fertilizer in diluted dosage and often.
Put a little vinegar in the water with the fertilizer... maybe a teaspoon.

So basically all this plant needs is a fertilizer and it should be fine? Do I need to buy a fertilizer for citrus trees or would any fertilizer get the job done? (this is probably stupid question, but can I use orchid fertilizer?)

Also...I found 3 different fertilizers at home..is any of those suitable?
First one is NPK 10-6-8 ratio
Second is 6-8-8
3rd one is 8-8-8 ratio.

I guess you mean 1 teaspoon of vinegar in a gallon of water?

Looks like the symptoms of oedema to me, brought about primarily by over-watering, exacerbated by any one or a combination of low light, soil compaction, cool temperatures, lack of air movement ..... Take a close look at your soil and change it if it's warranted - use a lighter hand on the watering can.

Al

@AI

Thank you for your answer. For my plant I used well drained potting soil. There are even weep holes in pot so there never stays any excess water. When it comes to light, my plant gets at least 8 hours of direct sunlight so I'm not sure how lack of light could cause the problem.

The pot looks a bit small, but I can't see the entire plant.

The potting mix looks too heavy, too much peat.

Indoor or outdoor sunlight?

10-6-8 fertilizer would be the best of those that you have available.

Josh

FWIW - how well a soil drains has little to do with the fact that water flows through the soil and out the drain holes. The key is how much water the soil retains, and in particular, how tall the layer of soggy soil at the bottom of the container is. It's not uncommon for commercially prepared soils based on peat or other fine composted products to support 6" or more of perched water. This means that after saturating the soil, up to 6" of it could be completely saturated, and would remain that way until it evaporates or it's used by the plant. Unfortunately, a lot of water in the soil inhibits water uptake AND root function, often causing symptoms exactly like those in the picture you posted.

I'll leave you a link that emphasizes the importance of a soil's structure and it's ability to hold enough air to keep the root system happy to your ability to consistently bring along healthy plant material. A healthy plant isn't possible unless you can keep the root system happy.

Al

Here is a link that might be useful: More about soils if you click me .....

Thank you all for your answers. AI, I'll listen to you and read an article you posted.

Btw, can you recommend me what kind of soil should I use, so I repot my lemon tree?

This might sound vague, but what you use as a soil doesn't matter, as long as you can water correctly and the soil holds a reasonable amount of water. You'll understand what I mean perfectly if you work toward an understanding of the info at the link I left. You'll see by other's posts to the thread (almost 3,000 now) that soil choice makes a dramatic difference in how much opportunity you're able to offer your plants to realize their potential.

Al

edema is caused by factors other than light; it is most commonly a result of soil that is too cool relative to the air temperature. Citrus roots do not grow well in cool soil; and the existing roots tend to absorb more water than the plant, which is not growing, can transpire. The simplest solution is to put some sort of heating pad to warm the root zone.

"[Looks like] oedema to me, brought about primarily by over-watering, exacerbated by any one or a combination of low light, soil compaction, cool temperatures, lack of air movement ....." When I wrote this, I had the uneasy feeling I was forgetting one of the important influences ...... and I was. That is, high humidity levels.

Cool soil temperatures slow water absorption, so would decrease turgidity and the likelihood of oedema. High soil temperatures increase the probability of oedema becoming a problem. Cool AIR temperatures slow respiration and cause higher turgidity and thus increase the potential for manifestation of oedema. A reduction in the amount of leaf surface a plant has (defoliation) can also cause symptoms of oedema.

Al