Usually you read that Poncirus hybridizes "freely" or "readily" with Citrus. Thus it is even used in scientific experiments to count the number of zygotic seeds that a Citrus species has. They pollinate with Poncirus pollen and then count all seedlings with three merous leaves. And that number is taken as the number of zygotic seeds the cultivar in question does usually produce.

This is the one thing you read. But on the other hand I found articles of Swingle and Webber where they say that only 2% of cross-pollinated orange flowers set fruits of which only a low percentage produced hybrids. And if Poncirus is used as the mother than even less. This is confirmed by Dr. Brown of Texas who said that Poncirus as mother does not produce hybrids. Therefore Dr. Brown completely refrained from using it as mother tree.

Now, I am wondering about these two very different statements. I suspect that the success in hybridizing depends very much on the individual cultivars used. But which cultivars promise success? Are oranges exceptions and hybridizing with other species is usually easier? In "The Citrus industry", I could only find that Lemon is difficult to cross with Poncirus and Kumquat almost impossible.

Mitschurin observed that the very first flowers of seedling plants are more open to receive foreign pollen. Is that also true for Citrus?

And how does the rootstock affect the success? Does, for example, a Mandarin grafted on Poncirus produce more Poncirus hybrids than a Mandarin on its own roots? I have only red somewhere that the number of zygotic seeds is, among others factors, influenced by the choice of the root stock.

Finally, how does the kind of pollen influence the number of zygotic seeds? I found the interesting statement of Swingle (or Webber?) that Eustis Limquat produces almost 100% hybrids. - I have forgotten with what variety . - That stands in contrast to the common opinion and in fact experience that Eustis Limquat is highly nucellar.

A lot of questions! We will probably not find fast and easy answers. But I wanted to draw the attention to these matters because the contradictory observations make it clear how limited our knowledge in fact is and how much we often depend on wrong generalizations and mere hearsay. I can at least say that about myself.

I don't know. But there are surely a lot of others.

Today I have red that Rough Lemon produces over 46% hybrid seeds with Poncirus although it is 98% nucellar when self-pollinated. It is all complex...

If Poncirus hybridizes freely with Citrus, there should be numerous spontaneous hybrids coming out of Citrus collections where both are housed.
Not sure if their flowering totally overlaps everywhere?

Most members have probably grown seed from their own plants that seem obviously crossed, ie Citrus X Citrus, or Microcitrus X citrus, or recieved such seed from others.

Any such random Poncirus hybrids ?

Citranges + Citrumelos had to be bred to be obtained, maybe not strictly freely crossing.

I do not have much experience in crossing Poncirus with citruses, but even for citrus to citrus hybridizations it is not possible to generalize.

This spring I castrated 5 poncirus flowers and pollinated them with Rubino Rosso (probably AMOA8 tangor).
Only two fruits were formed, one without seeds, another with two of them. One seed was monoembryonic resulting in the seedling with typical poncirus appearance.
But the second produced two seedlings, with the smaller one having broad leaves characteristic for citranges.
So, the overall efficiency was rather limited.

The Chinese and Japanese have been using poncirus as rootstock for centuries without any records of spontaneous hybrids between pt and citrus.

A spontaneous hybrid was found in France about 1890 and is occassionally still available from European sources as x citroncirus webberi or citrange du montauban.

The Americans began trying to make hybrids around that time. Only one successful cross has ever been identified from citrus pollen on pt. The reverse cross was also totally ineffective until pollen from an out of season flower pt was put on citrus. The majority of pt-sweet orange crosses that have been made since have the same variety of sweet orange as parent.

There are several things going on here. Both poncirus and most citrus are highly nucellar. So a cross using pt as seed parent is almost impossible. Using flying dragon will result in hybrids as it is about 20percent zygotic.

As a rule where the parent is nucellar you will not get hybrids.pumelo should be easier as seed parent as zygotic.

Lemon and kumquat are different problems. Firstly you have to right variety, with lemons most are nucellar but with right parent some hybrids can be produced. There is then a genetic incompatability and seedlings either do not produce leaves or they are not properly formed. Eventually the majority of seedlings die, the few that survive are strong and vigourous.

There is a pollen incompatability with nagami kumquat and seed rarely forms. On the occassions it does the seedling are weak and inherit poor root growth from the kumquat parent. They eventually die. Miewa kumquat has disfunctional pollen and even crosses using it as pollen parent on other kumquats is ineffective.

So to summarise using nucellar as parent is a waste of time. Even when successful when using a zygotic seed bearet may has genetic incompatabilities.

Hello yuzuquat,

thank you for your many examples of what functions and what not. Still, I hesitate to totally agree to you. I have given two examples where a highly nucellar variety does produce of lot of hybrids. And I have a friend who liked to use Poncirus as a mother plant. That Poncirus is nucllar is probably a generalizion that is perhaps only true as a rough rule because nurseries dislike zygotic plants. So they preselect what we get. But have found a lot of Poncius varieties on a scientific Japanese side that are highly zygotic. And most seeds of a Poncirus from Mainz botanical garden were monoembryonic. So it seems to me that it is all about the right variety and the right combination. There may be important exceptions to the rule you have found. It seems, however, that Poncirus and sweet orange do not fit very well. and I have also red what you write about Poncirus and Lemon, Poncirus and Kumuat and Meiwa Kumquat in general.
I hope that I will have own data this year. Despite of my many crosses last year I had much bad luck with the wheater and because of beginner mistakes like storing pollen in a warm room.

Till

General rule on storing pollen is cold and dry. Fridge with desicant is fine, between 6 and 12month depending how good your storage is.

I feel you must be right about poncirus
There is an immediate genetic advantage to an individual plant to be nucellar, which is then exploited for rootstock varieties. Being nucellar is disadvangeous for the species as awhole as it limits genetic variability and thus disease resistance.

So we end up back at your other question on low seeded poncirus
To really test the hybridisability of Poncirus we need to go back to the wild gene pool. The plants that are establishing in the wild in America seem to be gentically varisble viz-a-vis fruit size, juice and seed content. The wild stock from korea may be even more variable.

Cultivars in US may be variable in number of zygotic seedling they produce, flying dragonw and rubidoux being the cases in point. If they are in bloom at same time a flying dragom pollenated with sweet ormay be expected to produce one seedling in five as hybrid, rubidoux one in a hundred or less.

The problem for you is obtaining a named cultivar of poncirus that produces a significant number of hybrids as seed parent or growing lots of wild seed to fruiting size to find the one that does.

It the same problem that faces all of us wanting to do scientific hybridisation.

Poncirus polyandra is zygotic from what I read in literature. Chances of getting it out of china are impossible at present. Watch my messages for updates, have contact with someone who collected in Yunnan/Burma border region with Kumming Botanic Institute.

Yes, the key is to screen wild plants of Poncirus. I personally do not search for named cultivars. I just check the few trees in botanical gardens. And I think I have already found a tree with mainly zygotic seeds. From what I have red nucellar embryony is incompletely dominantly inherited. That means a zygotic plant crossed with a nucellar one gives offsprung that is mainly nucellar but uses to have some zygotic seeds. So two mainly nucellar trees crossed with each other may result in a mainly zygotic tree if the allel for zygotic seeds was present in both parents.
Another point is that Swingle and Webber wrote that the difficulty of pollinating Poncirus with Orange was that Poncirus did not set fruits with Orange pollen. That is a problem not related to polyembryony. It means that Poncirus usually does not accept Orange pollen at all. (Setting fruit means to accept pollen no matter what later happens to zygotic embryos. Only exception: varieties that do not need pollination and bear seedless fruits.) This at least was their observation with the Poncirus they had. But that it is in principle possible to cross both species is also evident from the many citranges that were finally produced.

Poncirus polyandra stands in the botanical Garden of Darmstadt (central Germany). It is a neighbour town of my hometown. But I have this information only from a plant forum, not from the site of the garden itself. I will check that soon.

In primula there are multiple sections.

One of the differences between sections is the form of the pollen. The stigma accepts only pollen from its own section and only then can proceed to the ovules and effect pollination.

Not sure what the barrier is in citrus.

I had a short conservation with the botanical Garden in Darmstadt.
They do have Poncirus polyandra inground with winter protection. They also wrote that it struggles with it`s situation. Don`t know what that means, some has to check this...
There is an other plant in a private collection in Germany, I wrote him and wait for an answer... this one isn`t inground.

Their are various methods used to overcome pollen incompatability, used by plant breeders who hybridise species, such as,
Use a mix of same + foreign pollen, the same allows reception for both and chance of X-pollination.
Use a mix of live + killed pollen, as above, killed is less recognised as foreign.
Pollinate either very early flowers or very late flowers, incompatability can vary at times.
Cut off the stigma and pollinate directly into the style or ovum, to bypass receptor.