Maybe the lights are too bright. Lower leaves are greener because they're more shaded + further away from the light. Just a guess.

Thanks for posting! I’m curious as well: My lemons are not quite as light as yours, but close. My Valencia and key limes are both dark green. All three types are on the same fertilizing schedule and receive the same light.

Do your fertilize? When i started my grapefruit the leaves started to pale and the members here advised me to fertilize, that was the problem!! Also, mine was not getting enough full sun, within weeks she improved

I’ve heard some say lemons require more Nitrogen than other citrus (don’t quote me on that :)).

it does look like nitrogen deficiency.

And did you know that the fruit of citrus seedlings won't be identical to those of the parent tree? Maybe similar, maybe better, maybe worse.

If you are providing that much light that is what happens when you don't fertilize and use a low pH solution...Looks like the other plants are lacking nutrients and rich color too.

How often should I be fertilizing? I'm going by what's on the box, and it doesn't seem to be hurting my others near as much. I don't want to overdo it and burn them up either. Not worried about the tree not being true to the parent seed... I grow these in my classroom just for the fun of it. Never intend to get fruit off of any of them anyway. We have 20 species growing there right now of various things (papaya, lemons, couple different kinds of mango, three different kinds of grapefruit, dragonfruit, etc). Usually end up keeping one for myself and just giving the rest away at the end of the year.

What a cool thing to do in class! Papaya trees especially are so cute. Watch out for dragonfruit; mine just ripped the cover of my greenhouse. Nasty thorns, too.

Best advice on fertilizing is "Weakly, weekly". So if the box says to fertilize witn one cup once a month, use 1/4 cup once a week. If you are already doing that, increase the amount of fertilizer by 1/4 and see if that helps.

have you ever grown dragonfruit before? I'm curious as to how small they can be to successfully root from cuttings... I'm growing them in clumps of 5 or 6, but some of the individual plants are about 10" long now. They all have that ridiculously thin little stem at soil level, so I'm thinking if I can root a cutting it'll have a stronger root presence. It seems like I might need to wait awhile longer, so in the meantime I just put a little extra dirt around the base of the plants in the hopes they'll root. They're putting all kinds of aerial roots up top anyway.