Editing this post to combine some other things I’ve been working on regarding the ARS-Grin database on pecan and the previously posted info about pecan dichogamy.
Alternate Bearing search for “pecan alternate bearing conner worley” for relevant articles
I’m working through the logistics of alternate bearing as it affects pecan. There are several published articles where Alternate Bearing Index (ABI) was studied over the last 100 years. Here is a short summary.
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Pecan matures from mid July to late November. There is no correlation between date of ripening and ABI.
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Pecan varies in size of crop with age. ABI does not correlate directly with age of tree.
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Pecan varies in size of crop load with some trees producing far more nuts than others in a given year for a given tree size. ABI correlates inversely with crop load, i.e. more load in one year is associated with less production the next.
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Artificially reducing the crop load by shaking water stage nuts off heavily loaded trees will reduce the ABI.
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Pistillate flower initials are formed in the year prior to nut production and generally are formed 2 to 3 weeks before the current year nuts mature.
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There are two theories why pecan alternate bears: the carbohydrate theory and the Phytohormone theory. Neither theory fully explains observations therefore neither theory in current form is valid.
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The closest correlation I could find is that something happening in the tree during formation of pistillate flower initials determines production in the following year.
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Heavy crop load is directly associated with freeze damage and winter mortality with some cultivars severely affected.
This is my theory. Pistillate flower formation appears to be directly associated with phytonutrient availability at the time the flowering buds are formed. This suggests that breeding for more leaf canopy as a ratio to current season nut production should directly affect ABI in pecan. There is also a photoassimilation index for leaves that indicates how efficient leaves are at a given point in their lifetime. Leaves start small and newly formed, develop to full mature size at which point their photoassimilation potential is maximal, then become senescent with corresponding reduction in potential until the leaf dies and falls from the tree. Photoassimilation index is highly dependent on tree nutrition, water availability, and leaf health! Breeding for longer duration of maximum photoassimilation potential will also be correlated with reduced ABI. Finally, the plant hormone element has to be accounted for. I propose that there is a feedback mechanism in pecan where forming nuts produce hormones that tell the plant to divert photosynthate into the developing nuts. The more maturing nuts on the tree, the higher the levels of hormone produced and the greater the diversion of photosynthate. Pistillate flower initials are sensitive to low photosynthate levels and do not form where absorption by nuts exceeds production potential of the leaf canopy.
tldr; To breed pecans with consistent yearly production, we have to increase leaf canopy, decrease nut load, and improve tree nutrition and health.
Dichogamy
Some pecan varieties have near 100% overlap of pollen production and pistillate flower receptivity and are capable of pollinating themselves. There is something unusual about the genetics of varieties that overlap. Let’s find out what.
Pecan dichogamy is determined by two alleles, ‘P’ = Protogynous and ‘p’ = Protandrous. Capital ‘P’ indicates that Protogynous is dominant and lower case ‘p’ indicates that Protandrous is recessive. You might attempt to do a standard Mendelian grid and expect native pecan to segregate with 1 - ‘PP’, 2 - ‘Pp’, and 1 - ‘pp’ which would be the norm with most genes. But this is not what happens with pecan. Because these alleles affect the bloom period and because they tend to force pistillate flowers to be pollinated by the opposite genotype, the result is that a Protandrous plant most of the time pollinates a Protogynous flower and vice versa. The result is that standard segregation patterns do not apply. In native stands of pecan, trees tend to segregate 50% protandrous and 50% protogynous. This means that about half of the time, native trees will have genotype ‘Pp’ and the other half ‘pp’.
But there are exceptions. One of the most notorious is Mahan which has genotype “PP”. All of Mahan’s progeny are Protogynous. What about trees that have genotype “pp”? Well, they have a quirk or two too. A plant with this genotype will always have pollen shed either prior to or concurrent with receptive female flowers. So what did I figure out that is unusual? Pecans that have near 100% overlap between pollen shed and flower receptivity are always genotype “pp”. There are no examples that I have found so far where a tree with ‘PP’ or ‘Pp’ genotype have strong overlap. Here are some examples: Barton, Cherokee, Creek, Jubilee, San Saba, and Western Schley. What makes this interesting? Most protandrous pecans do not overlap male and female flowers, therefore there must be a gene(s) that affects flowering type to cause them to overlap. I’m going to speculate that this will be a gene or biopath linked with chilling hour requirements.
Why so? A study of chilling hour requirements for pecan suggests that catkin buds have different chilling hour requirements to break dormancy than pistillate buds. I speculate that there is a gene(s) that sets both catkin and pistillate buds with the same chilling requirement so that they always break at the same time so long as chilling hour requirements are met. In other words, pollination type of pecan whether protandrous or protogynous is linked to the biopath for chilling hour requirements to break bud dormancy. The knock on effect is that there must be more than one ‘p’ allele but not necessarily more than one ‘P’ allele! Has anyone done work to determine if there are multiple alleles for dichogamy in pecan?
Database organization of ARS-Grin
The repository database has rudimentary data on each accession. Increasing value for research and breeding would suggest some of the following could be done.
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Add notes of origin, background, and genetics for each accession. This will consume several thousand hours of labor plus tie up a lot of resources doing DNA work. Consider breaking this down into smaller tasks that can each be accomplished in 1000 hours or less. Automation can do quite a bit of the work if the background data is available.
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Addition of new accessions has to be considered as part of long term database maintenance. A huge component of this should center on sampling to capture more geographic diversity. There are also quite a few grower selections that could be considered for addition to the repository. Source would be better if a geographic lat/lon reference were used instead of a general “U.S. or Mexico” label. For example, Mexico, Jalisco, 19.527193, -104.150108 would describe an accession geographic origin in Mexico Jalisco state.
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I don’t know enough about the breeding program so this is a general comment. Reshuffling the genes in existing varieties can only achieve a certain amount of improvement. At some point, additional material will be needed both from the central range of pecan and from the farthest extended boundaries of natives. There is a strong inference that other hickory species can contribute significantly to this. The most likely species to have high value are C. Cordiformis, C. Ovata, and C. Myristiciformis. Genetics for disease and pest resistance and for soil and climate conditions would be targets.
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I am not aware of a program to develop targeted breeding, but given the effort to sequence the genome, the next step is to leverage knowledge gained to make synergistic crosses. A program with clearly identified objectives should be developed. This is an area the genome team can contribute significant expertise into developing. An early objective should be to elucidate allele function so that fewer seedlings have to be grown out and evaluated. Much of the cost of a targeted breeding program can be recovered by reducing effort required in grow-outs.
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One viable objective is to extend the range where pecan is cultivated both north and south. There are several commercial orchards in zone 6a and strong potential to develop orchards in zone 5. Very few cultivars have been selected for southern climates zones 10, 11, and 12. There is potential for pecan in Puerto Rico for example.
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Genetic modification has to be considered. There is huge potential to increase value of existing cultivars for example by developing a cassette of disease resistance genes that could be inserted into the genome of a cultivar such as Schley.
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Regional adaptation is an area that could be in the database, especially for development of improved rootstocks. Salt tolerance, PH adaptation, nematode resistance, and ability to absorb nutrients efficiently from diverse soil types are examples of some types of data that would be useful.
Putting the above together, the current static database is not going to work. There is a need to develop a transaction overlay where each fixed element is an entry in the database and a transaction is used to move that element for example into a breeding pool for crosses. This is a database concept that will require some time and effort by someone who understands data organization. Just having the data is one thing. Getting use out of the data is a different animal.