How much do we know about graft chimeras and hardy citrus?
Graft chimeras are a sort of hybrid, but not a genetic one, between two different species that resulted through grafting, typically from growth offshoot coming out of a graft union area and then separately propagated. The graft chimera is comprised of a mixture of cells between the two citrus types.
There are different types of graft chimeras. The most homogeneous ones, and the ones of most interest are periclinal chimeras, which typically involve a single layer of cells distributed throughout the growth of the plant.
I'm also experimenting with joining together different seedlings together at the earliest stage of their development, so that the seedling sprout consists of a mix of cells from two sources. (This takes some very fine precision and a good eye)
How much hardiness can a cold hardy variety confer to another normal citrus variety when they are part of a chimera together?
Could this be a viable strategy for developing new cold hardy citrus?
From what I've seen, many obscure citrus varieties that are believed to have originated as a graft chimera have not actually been confirmed as being so, so it's not truly known with certainty.
The only way to be sure is if there's obvious phenotypical differences in different parts of the tree, or on different parts of the fruit, but in that case its not a very homogenous chimera, and not a periclinal type of one, which would be expected to give the best hardiness because the cells are more evenly distributed throughout the plant.
Say for instance we had a Satsuma graft chimera together with a Satsuma-trifoliate (citrandarin) hybrid.
The Satsuma-trifoliate hybrid within the chimera system could be a triploid with only one of its three sets of chromosomes coming from trifoliate.*
That could potentially make the resulting chimera nearly indistinguishable from normal Satsuma.
* (This could come about through hybridizing a tetraploid Satsuma with a normal diploid trifoliate, or the pollen may have been unreduced coming from the Satsuma, or the female parent being used could have been a "seedless" triploid, and so any rare seeds that did manage to form would be much more likely to have originated from an unreduced female gamete, since triploid cells that undergo meiosis have a fairly high chance of turning out aneuploid and won't develop. Also, you have to have a non-nucellar citrus variety for the triploid to turn out seedless, otherwise the seeds are still going to form from nucellar tissue even though the zygote failed to develop.)
Prague Citsuma is believed to be a graft hybrid, but it has not yet been positively confirmed with certainty. (A few basic tests were done but were inconclusive)