It's cool that you have taken up the dsicussion again! And thank you for the interesting material.
Why I want to cross hybrids back to Poncirus has the following main reasons:
- I live in central Europe where we have regulary -15°C (or less) in the winter for more than a week. And I have not the space to overwinter plants in my house. So when I want to breed new hybrids it is important that my plants are quite cold hardy right from the beginning. So my rule is: cold hardiness first.
- Then I have not the space to grow hundreds of hybrids. And the space I don't have I have to compensate with time, i. e. more generations.
- I suppose that the inheritance of cold hardiness is so complex that the probability of realy hardy hybrids is small when you cross Poncirus hybrids with each other. But it is much greater in back crosses because then you always have one full set of hardiness genes.
- The phaenotype of hybrids gives you no clear idea of the genotype because the genetic material is too complex. Back crossing with Poncirus reduces the complexity of the genetic material and allows you a better guess what the genes of a F3 hybrid are like. Upon selfing in the next generation you reduce the complexity still further and can concentrate on single aspects. Such F4 plants may not be what you have searched for. But you can at least - so is my hope - make a guess what the genes are like.

But I agree that my way may cost too much time and that the probability of good and hardy hybrids is quite small in the generation of the first back crosses.

I try to enhance the probability of better F3 and F4 hybrids in that I use existing F2 hybrids with relatively good taste and good hardiness. I hope that the link between bitter taste and hardiness genes has already been broken in these hybrids so that the probability of cold hardy and non-bitter hybrids is greater in the following generations. I also suppose that a relatively tasty and hardy hybrid like the Sanford Curafora (F2) (hardy about -12°C) contains most genes of Poncirus hardiness but combined already with good taste. Time will show if that is true. And time will also show whether the late maturity of that hybrid (or others) is only on one cromosome set or on two. With other words: whether it can be easily out bred or not.

But I will also cross hybrids with each other. And I am very exited to what results you will come with your plan!
Aside from Poncirus hybrids we should also experiment with other less hardy but good tasting plants. For example: Eremocitrus X Shangsha, Eremocitrus X Keraji, Keraji X Blood orange, Keraji X Shangsha, Faustrimedin X Shangsha. That would give us some robust plants with good taste and very early maturity. And early maturity with good taste is something that we also need in breeding the hardy citrus.

I agree Ilya. My hope is only on the second generation of my crosses and they will need time. I am, however, optimistic that I get more space over the years.
Thank you for your remark on Sanford Curafora and Thomasville. Its somewhat frustrating what you write but good to know. My Curafora did well the last winter at about -8°C. So it remains an interesting plant for my project even though I was too optimistic due to some peoples books.

My greatest problem is at the moment that it is so extremly difficult to get good plants. I ordered some hybrids last summer, had to wait a year until I got them and what I got had such an iron and magnesia deficiency that they have not yet recovered from that. Patience is needed wherever you turn... Frustrating. But I keep the goal in sight.

What you write about my suggestions of Keraji X Changsha and so forth is certainly true. These are things I do not want to do myself. Only suggestions for those who life in a milder climate and manage to harvest fruits in four year after sowing.

I hope I can soon do something with Thomasville. But my plant is so tiny!