Citrus > Cold Hardy Citrus

new thoughts on breeding hardier citrus

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I've been thinking a lot about hardy citrus breeding for some years now.  But new information and experience is changing my mind about several thing the last few weeks,

Ponciris trifoliata is hardy enough for my area,  And it is the hardiest citrus relative I know of that is readily available.  It crosses easily enough with good citrus, and I've always thought breeding from such crosses was the way to go.  The ONLY way to go.  People have told me they want to select within the pure P. trifoliata, and I thought that was stupid.  There isn't enough variation to make that way successful, I thought.

But what is really the problem with P. trifoliata as a crop?  The nasty-smelling resinous flavor, of course.  But there is at least one mutant without that problem.  Ponciris+.  And P. trifoliata is mostly nucellular.  Again Ponciris+ is zygotic so that is at least less of a problem.  In addition, P. trifoliata, its F1 hybrids, and backcrosses to mandarins (mandarin x P. trifoliata) x mandarin, are too sour and lack sweetness.  Granted, I've only tasted one mandarin backcross, Clemtriclem.  It is (Clementine x P. trifoliata) x Clementine.  And Clementine is not the sweetest mandarin.  But I have not read of any improvement over Clemtriclem.  Another problem is the size of Ponciris fruit.  They are small.  But that is a minor problem compared to flavor.

But all this means, really, is that more sugar (higher brix) and less acid (higher pH) is the whole breeding problem.  Or at least most of it.  Precocity, short time from seed to bloom, would be valuable, as with a certain set of breeding stock, progress will be set by population size, ability to measure brix and acid, and the number of generations.  Keeping generation time short would be very valuable.

But to select within P. trifoliata seems a dead end.  There isn't much variation in brix or acidity to select from.  At least I haven't found information that would make me think otherwise.  So crosses with other citrus is the only way to get the needed genes.  Fortunately, such crosses have been made.  Kumin and I and others have acquired such hybrids.  US 1279, US 1281, US1282, Us 852, 5*, are useful as seed parents as well as pollen parents.  Other varieties might be used as pollen parents if desired.  Citrangequats can't be ruled out, though they wouldn't be my choice.  Bringing in Kishu Seedless as a pollen parent seems worthwhile, as it has a dominant gene for seedless, and is said to be exceptionally sweet.

New hybrids could be worthwhile.  Precocity might be brought in from Laaz's Fast Flowering Ponciris, Etrog, or Hong Kong kumquat.  None of these are of proven value, but I am checking them out.

Hybrids of various citrus with Ponciris+ could speed up getting rid of the  Ponciris flavor.  Though a generation or two of selection should get rid of it where Ponciris+ isn't available.

The amount of Ponciris in a breeding population should be determined by your climate zone.  Zone 8, maybe zone 7, 1/4 Ponciris could be enough.  This would speed up the breeding in those areas.  I plan to make such seeds available in a few years,  when my breeding stock is older.  Kumin, in zone 6, has had some success with 1/2 Ponciris in zone 6.  What he does in the future is up to him.  He has already done in one generation what I thought would take several generations, so I'm not the one to tell him what to do.

I'm also in zone 6, and will be working with 1/2 Ponciris.  But I will also make some crosses of those 1/2 Ponciris with pure Ponciris.  This will give me a big population that can be grown without protection.  This population will include seedless plants from the Kishu Seedless mandarin ancestor, fruits lacking Ponciris famous flavor, various levels of brix and acid, and fruit size.    Of course, I'll be working on the 1/2 Ponciris population, but I'll have to provide some protection to keep seedless and some other genes in the population in the early generations.

Breeding stock I now have on hand include:

Ponciris trifoliata
Ponciris+ which doesn't have the undesirable Ponciris flavor and does have zygotic seeds.
FFP, Laaz's Fast Flowering Ponciris, which doesn't pass on its fast flowering to its seedlings when crossed with other citrus..  But it may pass it on to its grandchildren.  If so, it would speed up generation time.
B and J.  Bigger and juicier P. trifoliata.  100 seeds from Kumin.
Many seeds from a wild population.  Sent by orangedays.  These trees give monozygotic seeds.  These are to broaden the genetic diversity of my breeding population.   

Kishu Seedless.  Very small, but very sweet and has a dominant gene for seedless.  It can only be used as a pollen parent.  Half of its seedlings will be seedless.
Changsha.  Good flavor, good size, more cold tolerant than most mandarins, zygotic seeds.

US 852  85% zygotic seeds
US 1279  More than 95% zygotic seeds
US 1281  More than 95% zygotic seeds
US 1282  More than 95% zygotic seeds
Bishop Citandarin  Seedling of US 852.  An improvement in flavor.

010 Citrange  Survived 3 winters in Pennsylvania.  Zone 6  Fruit quality unknown.  % zygotic unknown.
058 Citrange  Survived 3 winters in Pennsylvania.  Zone 6  Fruit quality unknown.  % zygotic unknown.
067 Citrange  Survived 3 winters in Pennsylvania.  Zone 6   Fruit quality unknown. % zygotic unknown.
These 3 are seedlings from the same tree, C-35.  Mother tree was 15% zygotic.  These were all grown and selected by Kumin.  He has reported his work elsewhere on this forum.  Very impressive work.

Kumin has been reporting on his citrus breeding.  Of about 3,000 zygotic F2 seedlings from C-35 citrange, he had 12 survive in good health for the past 3 winters in Pennsyvania, zone 6.  Others survived with serious damage.  But lets look closer at those that survived in good health. 

12 out of 3,000 is 1 in 250.  The odds of getting a given plant is the odds of a suitable pollen x the odds of getting a suitable egg cell.  We don't know the pollen parent of these seeds, but it seems likely they had been self pollinated.  I'll assume they were.  So 16 x 16 = 256.  256 is close enough to 250 for my use just now.

So C-35 citrange gametes have a 1 in 16 chance of giving hardy seedlings.  I expect plain P. trifoliata gives 100% potentially hardy gametes.  So if C-35 had been pollinated by P. trifoliata, 1 in 16 seedlings would hardy in Kumin's area.  I am also in zone 6, but sudden temperature changes and lack of snow cover here might mean less survival here.  But this is still good enough for what I'm saying.

So I'm thinking about crossing citandarins back to P. trifoliata and to mandarins.  I want to get the seedless gene from Kishu Seedless into my breeding.  There are a few ways to do this. 

One is to pollinate a citandarin with Kishu pollen.  But such seedlings couldn't be grown outside in my area, nor Kumin's, with any chance of survival.

Another way is to pollinate a zygotic P. trifoliata with Kishu pollen.  Then use Pollen from those seedlings in further crosses with other citandarins, citranges, etc.  This is a good way. but I don't know how soon this all could happen.

A third way would be to make the cross (citandarin x P. trifoliata) x (citandarin x Kishu).  The resulting seedlings would still be 50% mandarin and 50% P. trifolitata.  The advantage of this method would be that someone in zone 7 or 8 might be willing to grow out the citandarin x Kishu seedlings in hopes of getting hardier mandarin-like fruit.  It could happen.  How much improvement would be found remains to be seen.  This would save me limited greenhouse space.  And I could grow out the citandarin x P. trifoliata seedlings outside.  Again, this would save me greenhouse space.  And perhaps 1 out of 16 would survive.

Back to Kumin's citrange F2 plants that have done well through 3 winters. With such strict selection, I'm sure that they will produce more than 1 in 16 useful gametes.  Using their pollen on citadarins is a logical plan.   No one knows what fraction of such seedlings will survive, but it should be much better than 1 in 250.

Thanks for your roadmap and report, Walt. Just a few comments on the citrandarins and selections 010, 058, and 067. This Winter will allow me to compare 1279, 1281, and 1282 in regards to cold resistance. Initial appearances are that 1281 and 1279 may be more cold resistant than 1282. 1281 shows the greatest Poncirus influence on the foliage.

Segentrange 010 is more cold hardy than 058 and 067. 010 shows more Poncirus influence, 058 and 067 are the most Citrus-like of all the trial survivors.


--- Quote from: Walt on November 04, 2021, 05:43:39 PM ---The advantage of this method would be that someone in zone 7 or 8 might be willing to grow out the citandarin x Kishu seedlings in hopes of getting hardier mandarin-like fruit.  It could happen. 

--- End quote ---

Oooh! Oooh! Me! Pick me!

I'm currently trialling pretty much every variety of citrus that's reputed to be cold-hardy. Anyone needs a clay-soil, zone 7(b) test out here in north alabama across a few separate microclimates I prepare, just let me know :D

Thanks for the offer.  It will be at least a year, maybe more.  W


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