Looks like a magnesium deficiency. The symptoms of a magnesium deficiency first shows up on the older leaves. The lower center of the leaf blade remains green, while leaf tips and sides turn yellow. Many times the green portion is in a delta shape. Dissolve one tablespoon of Epsom salts in a gallon of HOT water, and pour the cooled solution over the soil. Repeat weekly until symptoms disappear. - Millet (1,086-)

Since I will fertilize tomorrow can I mix it in withe the fertz

Maybe Mg, but it does occur on some younger leaves. I'm more inclined to think biuret toxicity from impure urea in the fertilizer. Is that a possibility?

I don't think this is a nutritional problem. It's winter, and the seedlings appear to be housed, therefore they are in a delayed dormancy. Some of my FD's look like that also. The leaves are yellowing up before dropping. My FD's did this also and they appear perfectly healthy. I don't think you have a problem at all. The trees appear to be trying to enter dormancy but the warmer environment is delaying the process.

Has it been a long time since you have applied any magnesium? Dr. Manners thinks it might be a magnesium deficiency, but is more inclined toward a biuret toxicity. Is the nitrogen source in your fertilizer mainly from Urea? Citrusgalore could certainly also be correct. Anyway, what ever, you can certainly apply Epsom salts with your fertilizer in the morning, especially if you have not applied any magnesium in a while. - Millet (1.086-)

These were in containers in a greenhouse at our local citrus center millet. So , the background on the fertilizer I don't have. We did have that cold weather pass thru and now it's in the 80's.
I don't know if that helps any.

I'm sharing view of citrusgalore on it, seedlings looks dormant to me.
Leave them alone, don't force anything.

Hey, I've got the same problem with my lemon seedling. It started when I forgot to preserve the plant from sunlight (summer) when I went on holidays, so after a week it was completely dry and burnt. It experienced a shock, but stayed alive. Unfortunately, older leaves started to turn yellow just like in David's photos and they're still doing it. Turning yellow on the borders, then whole leaf turns yellow, almost white and then it falls off. New leaves are growing, older falling down. 6 months passed and nothing changed.

So, David, maybe your seedlings experienced a shock like heat, frost or lack of water? Just a thought, sorry - I don't know how to cure it.

I have several FD outdoors inground unprotected--they have all dropped their leaves, but they all turned yellow before droppping.