The reason I am growing from seed is I am wanting to develop some new hybrids, ones with fairly good fruit quality but also with a fair degree of cold hardiness.

In Olympia, WA, so pretty far North. The summers have plenty of heat here (during the daytime), although the summers are short, and the temperatures don't really begin to become consistently warm until very late in May.

The seedlings started inside a grow chamber to hold in the humidity and keep the temperature warmer. They generally grow very well inside the grow chamber, altgough at some point they begin to develop root rot issues (yellowing leaves) from the excessive constant moisture on their roots, and spider mites were also an issue, so periodically they all had to be treated with a special spray. Now that June has come, they grow just fine outside (though have to be kept consistently watered because of the intense daytime sun and heat).

For hybridizing purposes, usually you want to select a female parent that produces zygotic seeds, because if it's a nucellar variety all (or the vast majority) of the seeds will tend to be genetic clones of the fruit parent. (zygotic varieties include pomelo, citron, most mandarin varieties, C. ichanghensis, and kumquat, but not Orange or Calamondin)

Very interesting! Thanks for sharing your progress with us! I've been growing some seedlings as well but for the purpose of wanting nucellar seedlings of varieties I eat often. I know they may take years to fruit but I would still enjoy having a beautiful citrus plant. The seedlings varieties I have now are cocktail grapefruit/Mandelo, minneola tangelo, Moro, Rio red grapefruit, navel orange, and a mandarin unknown. The only 100% confirmed zygotes so far are the navel and mandarin.

whats your way of determining zygotes from nucellar seedlings? People here suggest growth rate and weakness. However I've looked online for different studies and I have read research papers that found that growth rate can be high for either zygote or nucellar. And some polyembryonic seeds that contained twins/triplets were all zygotic contrary to our assumption that polyembryonic seeds always contain nucellar seedlings. After reading that I'm having a hard time deciding when to cull mine and how I'll go about it when the time comes

There's really no sure way to differentiate nucellar seedlings from zygotic ones. The general rule is that multiple seedling sprouts coming from one seed indicate that they are nucellar. (But in rare cases, sometimes one of these seedlings can be zygotic, often one of the weaker ones, and single sprout seedlings can also be nucellar)

In some cases they'll hybridize trifoliate leaf citrus with regular leaf varieties to be able to easily identify seedlings that are nucellar, if they have different leaves from the parents.

The other option is to wait many years until the seedling begins producing fruit, to see if the fruits are substantially different from the fruit parent.

The specific citrus variety you're growing from will also give you a good idea what the chances are the seedling is zygotic or nucellar. Lemons and grapefruits often tend to have about 70-90% nucellar seed. Hybrids are much more likely to be nucellar than the original species.

Nucellar = genetic clone of the fruit parent

Also keep in mind that citrus which is not grafted onto a grafting variety of rootstock can become much larger trees, and takes many more years until it begins fruiting. (stunting the growth a little bit redirects the tree's energy toward fruit production earlier in its lifespan). So if you grow a fruit tree from seed, without using a different rootstock variety on which to eventually graft it, expect different behavior from the plant.

Where did you get the keraji seed?

You might make a drive to South Carolina. There are several nurseries in that region that have it.

growing small citrus seedlings

It's covered by a transparent plastic sheet enclosure to hold in the humidity, which is absolutely necessary if grown inside (at least in the climate here).