Cold-hardy citrus?

[walt]

I'm having to water every day now.  I wish I had C. glauca (desert lime) in the pedigrees here.  But that would mean another thing to select for, and then I'd need a bigger population.
I have 58 P. trifoliata now, big enough to graft onto.  They are planted 2M. X 2M.  Some of them are a meter or more tall.  All have been in the ground 2 months or more, some for 5 years.  I wonder how long until I see bloom.  3 are precocious, but those are the youngest and therefore the smallest.  Maybe next year.

[walt]

tropicalfruitforum seems to be down the past 2 days.

[walt]

I got into the tropicalfruitforum today with no problem.

[socal2warm]

Apr 27, 2018 15:49:54 GMT -5 walt said:

Triploidy is indeed usually a dead end.  

I don't believe triploidy is always a dead end, I think most of the people breeding citrus just might not really understand the science behind it.

I'm planning to try using seedlings of Oroblanco as the female parent in crosses, and from one of the research articles I've looked into I have reason to believe about half the seeds will be zygotic (assuming it was pollinated by another variety that is a regular diploid). 

That's higher than it would be for regular grapefruit, which would only be 10-30%.

Admittingly, I'm not absolutely completely sure about this, but this is what I've pieced together from the information I've looked into.


The other intriguing thing about triploidy is the chance (maybe 1 out of 25? at least in Oroblanco) chance that one of the seeds could turn out to be a tetraploid.
This results when an unreduced 3n gamete combines with a normal 1n gamete. 
Obviously if you have a tetraploid, you can cross that with a normal diploid and get a new "seedless" triploid variety (low seed count).
Spontaneous tetraploids arise from regular citrus too, through a different mechanism, but happen with much lower frequency in most varieties.
(I'm not going to go into too much further detail on this here)

Anyway, the zygotic seeds from a triploid will be almost all regular diploids, so the seedlings won't be "seedless" anymore.
The nucellar seeds will all be triploids, obviously, but those won't be of much use to breeding since nothing has changed from the fruit parent.

[socal2warm]

It was my understanding that Kishu has a low seed count (nearly seedless) for at least two or three different reasons. One of those reasons is that it is a very poor pollinator. If there is pollination from other citrus varieties around, most of the fruits will have a seed or two in them.

Kishu is one of the mandarin varieties that produces 100 percent zygotic seeds, so it's a good choice for female parent in attempted crosses. I think it would also be possible to use it as the male parent, despite it being a poor pollinator.

[socal2warm]

Jan 27, 2018 14:44:56 GMT -5 walt said:

I've not addressed the problem of seedy fruits.  Much of my breeding stock is very seedy.  But a long-term problem.

I wouldn't worry too much about seedy fruits right now if you're thinking long-term. 

Once you manage to develop of a cultivar that is both cold-hardy and edible, it can be developed into a suitable triploid in another generation or two. 
Compared to the challenge of combining cold-hardiness and edibility, that won't really be too difficult. 
The other option is to send in the cultivar and have it be irradiated and they can develop a new seedless version of it. Apparently it's not too difficult for them to do this.

[walt]

Kishu mandarin looks like a good bet for breeding, assuming it is quite pollen fertile.
Triploids need not be a dead end, but it would likely take more pollinations to get some triploids that crossing 2 diploids, my breeding population x Kishu mandarin.  Just backcross my populations x Kishu mandarin and its decendants in each generation, using my populations as seed parent.  It seems I'd have my very seedy populations and a few seedless seedlings in each generation.
I mean I'd be breeding the populations for flavor, zygotic seeds, precocity, and winter hardiness, and using the pollen from Kishu mandarin and its most recent seedlings on the side to give a similar population segregating for seedless.

[socal2warm]

Here is some data that might be useful for breeding:

As many of you know, many types of common citrus cultivars are considered to be polyembryonic (that produce seeds which are genetic clones of the parent). This can cause problems for breeding because most of the seeds in a polyembryonic cultivar will just be clones of the parent, rather than inheriting any traits from the parent the tree was pollinated with. However, not absolutely all of the seeds in a polyembryonic citrus variety will be clones, a few will be zygotic (the result of sexual recombination).

seed parent ... Seedlings/seed ... % nucellar
__________________________________________
Lemon: Eureka, Lisbon, etc ... 1.05-1.06 ... 32-33
Rough Lemon ... 1.24-1.96 ... 54-98
Mexican Lime ... 1.29 ... 78
Mandarin: Dancy, Kara ... 1.37-1.71 ... 100
Mandarin: Satsuma ... 1.44 ... 90
Mandarin: Kishu ... 1.00 ... 0
Mandarin: King, Ponkan ... 1.01-1.42 ... 21-98
Grapefruit: Marsh ... 1.08 ... 96
pummelo: 11 cultivars ... 1.00 ... 0
Sweet orange: 4 cultivars ... 1.09-2.00 .... 39-97
Sour orange ... 1.21 ... 85
Tangelo: Orlando, Minneola ... 1.31-1.49 ... 83-97
Trifoliate orange ... 1.03-1.26 ... 13-73
__________________________________________
data in this table comes from Frost and Soost (1968)
H.B. Frost, R.K. Soost, (1968) Seed reproduction: development of gametes and embryos.
In: W. Reuther, L.D. Bachelor, H.J. Webber, The Citrus Industry, Volume II. Division of Agricultural Sciences. University of California Berkeley, pp 292-334



Clementine and Temple Orange are two varieties that have frequently been used in breeding as the female parent because they produce all zygotic seed.


Crossing a tetraploid citrus with a regular diploid citrus will result in a triploid citrus. Triploids are generally sterile and seedless. This is a strategy that has been used in many cases to breed seedless citrus varieties (although there are other strategies).

It turns out that most regular citrus varieties—the ones that are polyembryonic producing clonally from seed—will convert to tetraploids at least once in a while. That is, if you grow 300 seeds, at least one of them will likely turn out to be tetraploid, even though it is otherwise a clone of its diploid parent. Tetraploids are often slightly bigger and slightly deeper in color than their corresponding diploid parent.

Some citrus varieties are more likely to convert to tetraploids than others. For example, out of 78 seeds grown from a Duncan grapefruit, 5 turned out to be tetraploids.
'Mapo' tangelo (7 out of 73) and 'Tardivo di Ciaculli' mandarin (2 out of 38) also had high rates of tetraploid seed. For Minneola Tangelo, 3 tetraploids were observed out of 166 seeds.
Something else notable, it appeared that plants producing fruit in colder conditions in marginal climatic areas had an increased likelihood of producing tetraploid seed.

Troyer citrange had very high rates of polyploidy, as high as 10-20 percent of the seedlings. (Carrizo was almost as high too)

_______________________________________

P. Ollitroult found half diploid and half triploid among the seeds of 'Oroblanco'.
Citrus genetics, breeding and biotechnology, Iqrar A. Khan, p205

Only diploids were found in the progeny of clementine fertilized by Oroblanco pollen, the same source says.


Triploid pollen can induce the formation of haploid embryo from the female fruit parent; the single chromosome set would always come from the female parent in such a case.

"This study reports haploid plantlet regeneration through gynogenesis in Citrus clementina Hort. ex Tan., cv. Nules, induced by in vitro pollination with pollen grains of Oroblanco, a triploid cultivar of grapefruit. It indicates that parthenogenesis induced in vitro by triploid pollen can be an alternative method to obtain haploids in monoembryonic cultivars of Citrus."
( Gynogenetic haploids of Citrus after in vitro pollination with triploid pollen grains, M.A. Germanà, B. Chiancone. Plant Cell, Tissue and Organ Culture, July 2001, Volume 66, Issue 1, pp 59–66 )

_________________________________________

One study looked at the seeds of triploid citrus (this particular cultivar having resulted from a tetraploid cross between Hamlin orange x rough lemon, which then was crossed with diploid tangerine) which was open-pollinated, and out of 25 well-developed seeds, 13 were triploid, 10 were diploid, 1 was tetraploid, and 1 was aneuploid. What was particularly interesting was that all of the triploids appeared to be nucellar.
( Ploidy variation and genetic composition of openpollinated triploid citrus progenies, Shi-Ping Zhu,Jian-Kun Song, Botanical Studies (2009) 50: 319-324 )

The findings suggest very low likelihood of triploid meiosis being able to produce a diploid gamete, and so presumably the single tetraploid seed must have resulted from an unreduced triploid (3n) gamete being sexually combined with a normal haploid (1n) gamete. The zygotic seeds of a triploid variety would be diploid because two normal haploid (1n) gametes combine together as normal.

[walt]

I have my first finger lime seedling.  Finger lime seeds are much smaller than citrus in general, so the seedling is a tiny little thing.  It will be a window sill plant this winter.  And it will be even less cold tolerant than most citrus.  But it is supposed to be precocious.  And it is supposed to be good eating or drinking.  And it makes zygotic seeds.

[walt]

I posted today on tropicalfruitforum.com/index.php?topic=26278.msg337328#msg337328  
That post will tell my progress so far.  Basicly, it says I've aquired my breeding stock and can get going.

[walt]

I've been posting about my citrus breeding on tropicalfruitforum.com , but today I'll get folks here up to date. 
I bought 14 fruit of US 852 from Stan Mckennzie, a grower and seller of more cold hardy citrus.  US 852 is  mandarin x trifoliate orange.  It was bred as a root stock for citrus, but has not been used as a root stock because it has 60% to 70% zygotic seedlings.  Citrus growers want high percentage nucellar seedlings for uniform root stocks.  But as a breeder, I want lots of zygotic seeds.  So I have about 50 seedlings from US 852, about 30 of them will be of use in my breeding.
Someone who goes by eyeker sold me a box of taitri.  Taitri is Taiwan lemon x trifoliate orange.  Taiwan lemon is a sour orange that is used as a lemon.  Most of the seeds were monoembryonic, which usually means zygotic.  So I have about or more 50 F2 sour orange x trifoliate orange.
And I got a box of (Clem x tri) x Clem,  That is (Clementine x trifoliate orange) x Cementine.  These should be all zygotic.  Again over 50 seedlings.

And someone who goes by Kumin planted 20,000 seeds of an F1 citrus x trifoliate orange.  The variety he used has about 15% zygotic seeds,  so they should be 17,000 clones of the seed parent, and 3,000 zygotic seedlings.  They seedlings were left outside over the winter in Pensylvania this last winter.  10 seedlings seem to have survived.

Ilya in France has been breeding for cold hardy citrus for some time.  He now has some 3/4 trifoliate orange, 1/4 grapefruit.  Given Kumin's success, I think Ilya might have a winter hardy tree if it were in my location.

[Srdjan Gavrilovic]

Over last few years I planted 3-4 kg of different F2 seeds (seeds extracted from fruits on F1 plants from regular citrus x trifoliate crosses). Only 2 plants are still alive.
Out of ~0.5 kg of trifoliate seeds I planted ~ 100 plants surviving multiple winter in ground in northern Estonia. I would call it a success
The idea is that first generation cross cant survive here thus only zygotic seedlings have small (very small) chance to survive.

Still looking to source any F2 seed I can.

[walt]

Kumin ordered seeds that were being to nurseries or farmers for use as root stocks.  They were sold by the liter, or maybe quart, since it was in the USA.  He bought a variety that has 15$ zygotic seeds.  You can read details on tropicalfruitforum.com .  Kumin will also answer questions if you ask.  Also Mikkel in Germany and Ilva in France report there experiences and answer questions there.

[walt]

Ypu may be able to get US 852.  Mikkel in Germany has it.  It is a mandarin x P. trifoliata.  It is terribly sour, but the juice mixed with water and sugar was ok to drink.  More important, Its seedlings are 60% to 70% zygotic, so it is worth breeding with.  I have several F2 seedlings growing and I hope for improvement.

[Srdjan Gavrilovic]

Ilya and Mikkal are known to me. We've been exchanging materials.
I have few F1 hybrids myself growing in pots. They are simple not able to produce enough seeds. I keep getting/buying different amounts from time to time.
Myself, I'm from south of Europe and I'm in process of setting a collection of F1 hybrids on my father land. Hope is to have them growing and sending seeds to harder climate to select only zygotic seedlings with improved cold hardiness. First goal is hardiness (even as decorative plants), taste is unfortunately just a dream atm but I keep striving towards that goal as well.