The Tropical Fruit Forum

Excellent idea! I think backcrossing has great potential to combine all the desired traits in a single plant, even more than the F1 itself—especially for cold hardiness, which is what I’m most interested in.

This description of Lubera has simply been copied from elsewhere. Lubera does not grow anything itself, but only sells goods from other nurseries, mainly Tintori, but recently also varieties of Eisenhut (although it is unclear where they get these plants from).
In any case, the name Ichangponsuma is a creation by Lubera....

I x P x S can be found at
Eisenhut
https://www.eisenhut.ch/en/product/18773
Tintori:
https://www.oscartintori.it/en/prodotto/c-ichangensis-x-poncirus-x-satzuma/
Lari
https://www.floricolturalari.com/prodotto/ichangensis-x-poncirus-x-satsuma-citrus-ichangensis-x-poncirus-trifoliata-x-citrus-unshiu/?v=5f02f0889301

There are some promising reports about IxPxS, so I think you simply have to give it a try.

In 2015, I received fruit from Karel. He didn't tell me where it came from, but it seemed to be from his own tree.

Are the fruits ripe now?

Just to be clear this is the legal situation:

(everyone take his decisions and maybe you have just luck)
 

At the EU border, checks are compulsory.
Any plants coming from outside the EU must enter via an EU Border Control Post (BCP) and get document, identity, and physical checks.

Who is the importer?
The person the parcel is addressed to is treated as the “operator responsible for the consignment.” A seller or “middleman” in the EU doesn’t change that if the box ships to you directly from a non-EU country.

Pre-notify in TRACES (CHED-PP).
Before arrival, the operator responsible must file an online Common Health Entry Document for Plants & Plant Products (CHED-PP) in the EU’s TRACES system and present the parcel at a BCP.

If something’s wrong, you pay.
If the consignment fails the checks (wrong papers, pests, wrong routing, etc.), authorities can order destruction or special treatment and bill the operator responsible for the costs.

Phytosanitary certificate is mandatory.
Plants for planting must be accompanied by a valid phytosanitary certificate; only a few narrowly defined items are exempt.

Some plants are classed “high-risk.”
The EU keeps a High-Risk Plants list that are temporarily banned from import until a risk assessment is done. That is the case with Citrus.

Bottom line:
Even with an EU “middleman,” if the parcel ships directly from China to you, you are the importer. Without BCP entry, CHED-PP, and a valid phytosanitary certificate, the shipment will be stopped and usually destroyed at your expense, and you can face national penalties on top.

e.g. Germany: fines up to €50,000.
If you break plant-health rules, German law allows administrative fines up to €50k (on top of losing the goods).

Hypothetically speaking, a phytosanitary certificate is mandatory, even within Europe. A phytosanitary certificate is not an optional service; it is a prerequisite for selling plants.
But in this case it is theory, because it is generally strictly prohibited to import citrus fruits into the EU in any form other than as fruit; even pollen is officially not allowed. There are also no exemptions. The same applies to the USA.
Even citrus institutes have virtually no way of exchanging material with each other. (I speak from personal experience.)

This does not mean that it cannot be sent from China, as customs does not check everything. But it is not legal.

And honestly, the risk of introducing citrus greening to Europe is not worth it.

Hi Luis, of course—grafted trees fruit earlier because they’re already mature; seedlings take much longer.

It's very interesting that this hybrid combination flowers after only 5 years, as Maxima tends to be very slow to flower for the first time.
Have you made similar observations with other Genova hybrids?

If this mutation proves to be stable, it would really be great. Normally, most breeders aim to move away from trifoliate traits toward monofoliate ones, but to me a polyfoliate leaf also has its own appeal.
Perhaps it could be useful to preserve the polyfoliate sections by grafting them as separate plants? Just as a hypothesis: if the polyfoliation were limited to a single bud, further cultivation of that specific bud might allow the trait to be reinforced. The mother plant, on the other hand, might eventually produce normal buds again and lose the characteristic.
But as I said, this is just a hypothesis without knowing the deeper background.

How do you protect the plants during winter?

the specific answer is written here, one only need to read it

Terol, J., Naranjo, M. A., Ollitrault, P., & Talon, M. (2010). The aconitate hydratase family from Citrus. BMC Plant Biology, 10, 222. https://doi.org/10.1186/1471-2229-10-222

For a well-balanced flavor, a certain amount of acidity is essential in fruit—perhaps slightly less than in oranges—but the complete absence of acidity makes them taste rather bland; the comparison to sugar water is quite accurate. It’s easy to test whether you like it.

Mandarins have a more pronounced sweetness and less noticeable acidity in the fruit.  Maybe this is a better choice?

The nursery Eisenhut also has Dolce Lemon, it is also named in the Citrus book by B.Voss
https://www.eisenhut.ch/de/shop/citrus-limon-dolce-z15_z15

That's exactly what I had in mind. These monogenic models are useful as a conceptual guide, but in reality, the process is unpredictable. In the end, it's a numbers game — and a matter of time.