I've been gone quite a while. I just wrote a letter to Mr. Hong in Wichita, KS, which is a summery of where my citrus work has gotten to. I decided to post a copy here.
Jun 2 at 3:55 PM
Dear Mr. Hong,.
I doubt you would remember me, except that you likely don't meet many citrus breeders in Kansas. As you were so generous giving me so many poncirus fruit for seeds, I thought I should let you know if there has been any progress in breeding hardy citrus for Kansas.
When I started, I thought I'd have to make all the crosses except for a few F1 plants bred about a century ago. But once shared my plans on a citrus forum online, help came in from all directions. And I was suprized that no one made fun of my plans. Most of the online help was helpful advice, but some was breeding stock.
First, I must tell you why the attempt to breed hardy citrus a century ago failed. Many citrus, and poncirus, make seedlings that are clones of the seed parent. The pollen parent has no contribution at all. These are called nucellar seedlings. They are clones of the mother. They are useless for breeding. But the pollen of these citrus varieties can usually be used if a variety can be found that makes normal hybrid seedlings. These are called zygotic seedlings, and they are true hybrids and are useful for breeding.
Many citrus varieties have a few per cent of zygotic seedlings. This wasn't known a century ago, when hardy citrus breeding was given up as impossible. But knowing which varieties have higher percent zygotic seedlings tells us how many extra seedlings you have to grow to get a given number of hybrids to get a given number of hybrids to select from.
So a man in North Carolina sold me some fruit of US 852. It is tangerine x poncirus. It has 60% to 70% zygotic seeds. I now have about 50 seedlings from it. About 30 to 35 of the seedlings should be zygotic.. It is hardy in zone 8, and most years it can take a zone 7 winter, but not reliably enough to grow there except in protected spots, The fruit the seeds came in were very sour. Extremely sour. But I mixed the juice with water and sugar and it was good. Not great, but OK. Much better than I expected.
Someone else sent me fruit of Taitri. Taitri is Taiwan lemon x trifoliate orange. Trifoliate orange is of course poncirus. Taiwan lemon is not a lemon. DNA test shows it has no lemon ancestors. It is a sour orange, but it is used as a lemon in Taiwan, because it is much hardier than a lemon. True lemons are very sensitive to cold. Taitri is hardy in zone 8, less so in zone 7, much like US 852. No one knows the percent of zygotic seedlings Taitri has, but i see segregation for leaf shape, which tells me many are zygotic. I have about 50 of these seedling, too. The Taitri fruit had green flesh like limes, There was none of the unpleasant poncirus flavor, except it was very sour. I mixed it with sugar and water and it was OK.
I also received fruits of (Clem x tri) x Clem. That is the name it goes by. It is (Clementine x trifoliate orange) x Clementine. So it is 3/4 tangerine, 1/4 poncirus. I found the fruit to be more sour than I would want, but it could be eaten as is. It could be used in cooking or as a small part of a salad. It is hardy in zone 8. It has little or none of poncirus bitterness. I have about 10 seedlings, all zygotic.
Other helps I have received over the internet include:
-Precocious Poncirus trifoliata that blooms when only one year old, when grown in the south with a full year growing season. This will speed up breeding, but the trait is recessive. It won't show up until the second generation when crossed with good citrus,
-Flying Dragon Poncirus trifoliata. It is a dwarfing root stock for many citrus. But it also has 50% zygotic seedlings. That is very valuable for breeding. Most P. trifoliata are 0% zygotic,
-A mutant P. trifoliata called poncirus+, which lacks the bitter flavor of other P. trifoliata. It is also 100% zygotic! The lack of bitter flavor is also recessive. While Pocirus+ is in Europe, seeds were sent to someone in the states, and that person is sending me a tree grafted with all 7 seedlings from those seeds. -Seedless Kishu mandarin. Its seedless gene is dominant, so when its pollen is used on other citrus, half of the hybrid seedlings will be seedless. It is also an excellent flavored mandarin, though the fruits are size of a golf ball.
Other people working on more winter-hardy citrus that I know from the internet include:
-Ilya, in France. He discovered that the leaves of seedlings from poncirus hybrids can be tasted, those that will have the bitterness of poncirus will have bitter leaves. Thus seedlings can be sorted and useless ones can be discarded. Ilya has been breeding and selecting for many years and has a good collection. Included are some 3/4 poncirus, 1/4 grapefruit, which are only a year old. He used poncirus+ so they should not taste bitter.
-Kumin, in Pennsylvania, who planted 20,000 seeds from a orange x poncirus hybrid last year. This hybrid has 15% zygotic seedlings. So he had 3,000 second generation hybrids and 17,000 clones of the seed parent. The clones all died during last winter. But of the 3,000 zygotic seedlings, 12 healthy plants survived the winter! That is in zone 6! I never thought any of them would survive. I admire this work. -Mikkel, in Germany, is working on hardy citrus.
-SoCal2warm is working toward excellent flavored citrus for the state of Washington. His resulting seedlings selections can be crossed with my hardier but not so good flavored selections.
I'm sure there are others, but I am on an English speaking forum, so others working on hardy citrus might not use it. I am aware of a French forum, and mention is often made of Eastern European, Chinese, and Japanese breeding work.
So progress is being made much faster than I expected. And it is great fun. Thank you again for seeds,
I have learned most of what I know about citrus there. That is true even though I've read books on citrus, read scientific journal papers on citrus research, and asked and received answers from professional citrus scientists. In it is a link to an older citrus forum which is no longer active but can still be read. Many in the new forum were in the old one.