yellowish seed leaves on 1 seedling
 

WHY??? And how to correct?

Only 1 tomato seedling (out of ten) does not have the normal green seed leaves like the others; ALL the tomato seedlings are under lights.

This is the only seedling for this particular variety (Legend) and it is a favorite of mine that I have started and grown for the past three years. Seedlings from previous years have had the normal seed leaves.

I'm hoping it's not a serious sign, that this seedling will still grow and produce normally, as it is my last seed . . . it's too late to re-order and start for this season.

If it is a favorite, I would gently turn it over, holding the soil of the others in with my fingers, and pop that one out. Then check the soil condition and roots if you can see any. If the soil is wet all the way down, I would repot in dryer soil. In my experience seedlings that size don't need fertilizer yet so I would think a dificiency is less likely. I can't think of any other cause.

It could just be genetics. Might be a mutation. The plant is growing new leaves and looks healthy, although a bit slower developing than your other varieties. . Since it's too late to replace I'd treat it as you have previous Legends and hope for the best. It might come around to what you're expecting or might give you something new and different. Either way, save seeds. :-)

Sometimes trying to correct a problem this early can do more harm than good.
I might, maybe, give it a transplant into fresh sterile soil. After a soil wash with
dilute hydrogen peroxide. Might be a funk bit of soil.

I would not try and correct with any fertilizers just yet. I'd like to see the next grow
spurt first.

I'd like to say I never have just one last seed but it has happened a couple times this
season already. And I save seeds every season. Those two are the only hybrids I
grow. Need to re-stock.

I usually assume it's a root problem when the leaves start to yellow. I overwater sometimes, causing yellowing. Or the fungus gnats, which stunt the roots, cause the plants to yellow.

I had one seedling like that and it fixed itself.
A one of a kind I would consider cloning when it gets bigger.
Some people let the suckers get big and use those for cloning material.
On a determinate type tomato you can get more mileage out of them.