Citrus > Cold Hardy Citrus

Long term cold hardy citrus breeding project

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I've started a citrus breeding program about 3 years ago.  Goal is hardy edible citrus for zone 6.

The plan is to have 9 breeding populations, based mostly on P. trifoliata, oranges, and mandarins.  The populations are pop. 1, pure P. trifolata;  pop 2, 7/8 P. trifoliata 1/8 Citrus species, most orange and mandarin;  pop 3, 2/8 , 6/8 Citrus;  pop 4, 3/8 Citrus, etc, up to pop 9, 8/8 Citrus.
Pop 1 is to be only monoebryonic P. trifoliata x precocious P. trifoliata.  If the F1 isn't monoembryonic, then, it could take a couple more generations to give a monoembruonic, precocious P. trifoliata.  This will be useful in later breeding.
Pop9, at the other extreme, will be to produce good-tasting oranges or mandarins with as much hardiness as possible without poluting it with P. trifoliata.  Others are working on this, so I won't be working on it.
Pop 5 is where my energy has been so far.  Half P. trifoliata, half citrus.  I have about 25 Sanford open-pollinated seedlings, and 2 US 852 seedlings.  I have been promiced some open-pollinated C. ichangensis x P. trifoliata seeds. I have some other citranges which I'll be using for pollen, so my gene base will have some diversity. I am working mostly on this population at first because until these bloom, I can't do selection or crossing to make other populations.  Plans are to select only for precocity, monoembryony, cold hardiness, and flavor.
Flavor selection will only be against P. trifoliata  flavor.  It will still be too sour for most people, but until the P. trifoliata flavor is dealt with, lack of sweetness is a minor problem.
Selection for cold tolerance will be done by keeping cuttings in a freezer for a week and see which survive.  Each population will have its own temperature. which will change slightly in each generation.  I'm doing experiments now to see what temperatures I'll be starting with.
Precocity will be easy to detect, I think, as well as monoembryony.  I have read that one (or more) citrus variety makes monebryonic seeeds via appomixis.  I'll have to watch  out for that by checking that seedlings aren't too uniform.
When population 5 blooms, crossing it with populations 1 and 9 will give populations 3 and 7.
If any of population 3 can survive here in zone 6, then populations 1 and 2 will not be needed.  But I'm not counting on that.
If any of population 7 has good fruit, population 8 and 9 won't be needed.  But I'm not counting on that either.  I have been encouraged by  reading of Dr. Brown's work with cold hardy citrus.  Some of his would be like population 7.  But I haven't tasted any of his citrus, so I don't know how accurate the reports are.
Population 7 would be the place for Troyer citrangequat, which some people enjoy.  I read that its seedlings don't do well, and I am not planning to use it.  Comments about this are welcome.
Crossing populations 7 and 9 give pop. 8.  Some of these should be edible, if not really good.  I'm prepared to select for flavor for a several generations.  And selection for cold tolerance for several generations might give measurably more cold tolerance.
Crossing populations 7 and 9 will give pop 8.  I expect some to taste quite good even in the first generation.  Perhaps I'm too optimistic.  We'll see.  Likewise we'll see if 1/8 P. trifoliata will give any increased cold tolerance.
Crossing populations 5 and 7 will give population 6, 3/8 P. trifoliata and 5/8 citrus.  Selections will be as for pop. 5.
Crossing populations 3 and 5 will give population 4.    Selection will be as for pops 5 and 6.  Selections is most effective generally when genes are around 50% frequency.  So pops 4, 5, and 6 will give greatest improvement.  Pops 7, 8, and 9 will give good fruit sooner though and somewhat increase the range of good citrus.
Crossing pops 1 and 3 will give pop 2.  If pop 3 gives any winter-hardy seedlings, pop 2 will never exist.  And pop1 would be dropped.
I plan to grow out at least 200 plants per population per generation.  More would be better, of course, but space is limited.
Populations will be dropped, or better still, turned over to other people when they have served my purpose, or better still, as someone else sees their potential and wants to take them on.
Monoembryony should quit being an issue in 3 or 4 generations.
Precocity is very important and will always be selected for, but there will be a limit.
Flavor and winter-hardiness will need work longer than I expect to live, and I expect to live a long time.  That is why I'm hoping others with the means will take an interest in this.

I know that cold tolerance alone doesn't give winter hardiness.  A winter-hardy plant must become cold tolerant before the first hard freeze of winter, and maintain cold tolerance in spite of mid-winter warm spells.  But the lower number populations should include winter survivers because they are mostly P. trifoliata, which survives well here.

From what I know precocious flowering gene in poncirus is recessive , you need two copies of it to express this feature.
There exist "edible" poncirus clones like Swamp Lemon and Poncirus+ that could be  better starting points at  your pop1 level to produce further F1 populations.
Another comment- since your breeding goal is hardiness for z6, you necessary should select for deciduous trait, even in your pop 5 population crossings.   

Towards the goal of starting with more desirable poncirus, I can offer my flying dragon variety which originated as a seedling. It is very resilient and has a sour but not bitter, slightly floral taste. It is small and seedy though. I'm am actually beginning my own effort at breeding with it in which I hope to stick mainly to poncirus but maintain this seedlings flavor but get to a larger less seedy fruit.
If you are interested in cuttings from my variety in spring let me know.
Have you looked at the earliest fruiting mandarins for crossing efforts. This could be important for controlling for dormancy.

Thanks for posting this somewhat detailed plan, I hope you keep us updated as it  progresses.    My one suggestion is that you actively work to find someone in your region to collaborate with.   All too often these sorts of long term individual projects get side track by unexpected life events of the originator. 

I'm also breeding cold-hardy citrus, but I'm focusing more on things that could survive in zone 8.

I believe zone 8 is pushing it, but still within the reasonable range of what something good tasting could grow in.

As for cold hardiness testing, my plan is just to breed everything to two generations and then, out of all the offspring, see which ones can survive. That's the fastest way to accelerate things. I'm also planning to propagate a clone of each sample before it undergoes testing for cold hardiness, that way if it survives the cutting will have been already growing the entire time, so there won't be any Winter setbacks in the breeding process. The drawback to this approach is having to grow more cultivars without knowing whether they're going to be cold-hardy.

If I can make a suggestion for you, zone 6 is going to be really difficult. You might try to make a backcross that contains more than 50% poncirus in its ancestry.
Something like this:
 ([citrumelo x mandarin] x poncirus) x (citrange x poncirus)


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