The seedlings are individuals, not exact copies. One has a root problem, the other one doesn't.

Why are they indoors?

What did you fill the pot with? -- potting mix? soil? a combination of the two? or your own recipe?

Any drain holes?

Do you water until water comes out the drain holes?

what jean said .. but also ... the insult could have been weeks ago .. and is just now showing in the leaves ... recall any event????

ken

@Jean - I'm planning on keeping them as houseplants, if possible. Depending on how large they grow, I may end up moving them outdoors next spring. They're potted in a mix of perlite, citrus soil, and potting mix, ~ 1/3 of each. Drain holes in the pot, and I water until the water comes out of them.

I took a peek at the roots. The ones high up in the pot are withered and brown, but the ones growing near the bottom are still healthy and white. I've noticed a few gnats around... wondering if they're the culprit here.

@ken - nothing besides repotting about a month ago, and I tried to avoid as much root trauma as possible.

Leaves that look sunburned could be exhibiting signs of environmental leaf scorch, even on a plant growing in the house.

http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/environmental/scorch.aspx

kimmq is kimmsr

you might want to check in in the houseplant forum.. maybe someone there has done this ...

as well as the citrus forum ...

transplanting a month ago.. could be the cause ... think in tree time ... when things happen .. rarely with trees.. do things happen fast.. so we have to track back in time ...

ken