by John and Sally Perkins, Salem, NH
Click on image to enlarge
Species of Diploid Clade
Rhododendron cumberlandense in center
Clockwise from top left: periclymenoides, prunifolium, prinophyllum, viscosum, flammeum
Not shown: alabamense, arborescens, canescens, eastmanii, occidentale
S and J Perkins
Species of Tetraploid Clade
Rhododendron calendulaceum in center
Clockwise from top left: colemanii, luteum, atlanticum, pink austrinum, austrinum
S and J Perkins
'Margaret Abbott' (Abbott)
Rule #3: prinophyllum (diploid) X calendulaceum (tetraploid)
S and J Perkins
Crosses onto R. calendulaceum 'Cherokee', a tetraploid
Rule #4: Rejects pollen from diploids
Rule #2: Accepts pollen from other tetraploids
S and J Perkins
Bigger Than Normal Diploid Seedpod
Rhododendron arborsecens var. rubra, a diploid is next to 'Marydel', a tetraploid
S and J Perkins
The conventional wisdom, passed along in books, articles, and word-of-mouth, states “North American native deciduous azaleas, excluding Rhododendron vaseyi and canadense, interbreed freely”; however, the rules of engagement for these azaleas are far more complex when crossing diploid and tetraploid azalea species.
The 2 Clades
The latest work by Benjamin Hall and Tom Ranney indicates that native deciduous azaleas break into 2 major clades that correspond to their ploidy levels. For those unfamiliar with the discussion of clades and ploidy we will digress and explain that when genetic material is used to try to determine how closely related species are, the term clade refer to species that have a common ancestor. The analysis done so far, strongly suggest that there are 2 different clades when examining the deciduous azaleas. These clades seem to separate according to their ploidy level. The ploidy level refers to the number of chromosomes. Most rhododendrons and azaleas have a base number of 13 chromosomes (x=13) which is the number found in pollen and unfertilized seed. Here diploid deciduous azaleas are classified as having 2x=26 chromosomes, triploid deciduous azaleas are classified as having 3x=39 chromosomes, and tetraploid deciduous azaleas are classified as having 4x=52 chromosomes. The tetraploids have twice as much genetic material as the diploids, but they do not, based on genetic studies from Hall, have simply a duplication of the genetic material alone and the tetraploids and diploids have not been freely exchanging genes with each other for a long time. Triploids often result as a result of a cross between a diploid and a tetraploid.
The tetraploid clade includes Rhododendron atlanticum, austrinum, calendulaceum, colemanii, luteum (from Europe), and possibly, a pink-flowered species similar to austrinum. The diploid clade includes Rhododendron alabamense, arborescens, canescens, cumberlandense, eastmanii, flammeum, occidentale, periclymenoides, prinophyllum, prunifolium, and viscosum.
Although each are diploids, the two native deciduous azaleas Rhododendron canadense and vaseyi, as well as the Chinese and Japanese species of Rhododendron molle, do not belong to the diploid clade as described above. Benjamin Hall, Hans Eiberg, and K. A. Kron each found Rhododendron vaseyi to be only a distant relative of the other North American deciduous azaleas. Of the three, Rhododendron molle is the only one that has been used extensively to produce commercial hybrids involving the 2 clades described above. George Fraser and Harold Pellett each successfully crossed canadense X molle.
Rules of Engagement for Crossing Species in the 2 Clades
Rule #0: The rules of engagement for deciduous azaleas within these 2 clades are highly generalized guidelines. The rules focus on the ability to successfully create seedpods. Exceptions to these rules will occur. The only way to know for certain is to do the cross and let nature takes its course.
However, such exceptions are rare enough that hand crosses violating these rules are worthy of documentation and attempts should be made to see if the same result is repeatable between the same parents and other members of the 2 clades.
Rule #1: An individual plant in either the diploid clade or the tetraploid clade does not freely “self fertilize” to develop seeds. In those rare instances were selfing does occur, the offspring are seldom viable (if they germinate they die at a young age).
Rule #2: Species within a given clade freely cross in both directions. The resulting offspring are normally viable and fertile. The offspring normally reflect characteristics that are intermediate between the 2 species involved.
Rule #3: Species in the diploid clade freely accepts pollen from species in the tetraploid clade. The resulting offspring are often viable but usually sterile (bloom but do not produce seed) triploids. The offspring often reflect more characteristics of the tetraploid pollen parent. Offspring having pink or salmon colored flowers with a yellow blotch are not unusual. Deformed anthers, multiple petals and color streaks in the corolla are also frequently seen.
Rule #4: Species in the tetraploid clade normally reject pollen from species of the diploid clade. Exceptions occur but they are extremely rare.
Rule #5: Species in the diploid clade freely accepts pollen from Rhododendron molle. The resulting offspring are often viable but usually sterile diploids.
Rule #6: Triploids resulting from interaction between the diploid and tetraploid clades are more likely to accept pollen from species in the tetraploid clade than from species in the diploid clade. This rule is much more preliminary than the others above.
Rule #7: There are no known instances of Rhododendron vaseyi successfully interacting in either direction with species in either clade.
Rule of Thumb: To maximize you opportunity for producing seed where you have the option of parents in either direction, always use the deciduous azalea of the lower ploidy or same ploidy as the seed parent.
These rules of engagement support Hall's finding that the 2 major clades of North American deciduous azaleas are divided such that species grouped in a given clade are much more closely related to other species in that clade than they are to species that are more similar in appearance in the other clade. In other words, R. calendulaceum, a tetraploid, is more closely related to the other tetraploid species R. atlanticum, austrinum, colemanii, luteum, and “pink austrinum” than to the similar looking species R. cumberlandense, a diploid. The same is true for R. colemanii, a tetraploid, being closer to R. calendulaceum, atlanticum, austrinum, luteum, and pink austrinum than to the similar looking species R. alabamense, a diploid. Rhododendron atlanticum, a tetraploid, is closer to other tetraploids than the similar looking R. viscosum, a diploid. The “pink austrinum”, a tetraploid, is closer to other tetraploids than the similar looking R. canescens, a diploid.
Our Evidence
Richard Jaynes showed that selfing deciduous azaleas failed to produce seedpods in most instances and nonviable offspring resulted in those instances where seed was produced. We have failed in our attempts to self late blooming deciduous azalea species.
Many of the late blooming commercial hybrid azaleas produced by David Leach, George, Mary, and Jeff Beasley, Ed Mezitt, and Bob and Jan Carlson involve only species of the diploid clade combining R. arborescens, cumberlandense, prunifolium, and viscosum. We have found that species in the diploid clade cross in both directions.
Tom Dodd, Earl Sommerville, and Gene Aromi produced several good-doer hybrid azaleas for the south involving primarily calendulaceum and austrinum, both tetraploids. Fred Galle produced 'Choice Cream' using austrinum X atlanticum, both tetraploids. Bob and Jan Carlson produced hybrids using calendulaceum X luteum, both tetraploids. Ian Donovan produced a cross of atlanticum X calendulaceum, both tetraploids. We found we could cross in both directions R. calendulaceum 'Cherokee', 'Marydel', and 'My Mary' with Donovan's atlanticum X calendulaceum. Both ‘Marydel’ and ‘My Mary’ were shown later to be tetraploids in the lab. Prior to the ploidy testing, many hybridizers were aware that calendulaceum accepted pollen more easily from luteum, austrinum, and atlanticum than from other deciduous azalea species.
Prior to the current information on the ploidy of North American azaleas, Frank Mossman, John Thornton, Anthony Waterer Sr., and the authors all noted that certain species crossed much more easily in one direction than the other. In retrospect, these hybridizers noticed that diploids normally accept pollen from tetraploids whereas tetraploids normally reject pollen from diploids.
In 1972, Frank Mossman wrote the following concerning his hybridization with Rhododendron occidentale, a diploid:
"We have found that Rhododendron occidentale will cross with many other rhododendrons or azaleas if Rhododendron occidentale is the seed parent, but Rhododendron occidentale as a pollen parent produces few seed."
In 1974, Mossman added the following concerning his use of three diploid species as seed parents for pollen from occidentale:
"Rhododendron prunifolium, cumberlandense, or viscosum x Rhododendron occidentale will take."
Mossman stated concerning the work of Anthony Waterer Sr.:
"Anthony Waterer, Sr, of Knaphill Nursery, England, was the first known hybridizer of Rhododendron occidentale in the 1860's and reportedly had little success for almost ten years. It is probable that he had pollen only, at first, and later had flower-producing plants to use for seed parents. His effort with the Ghent Azaleas plus Rhododendron occidentale was the beginning of the Knaphill Azaleas and later the Exburys."
In the 1990's, we found 'Marydel' and 'My Mary' rejected pollen from late blooming deciduous azaleas such as R. arborescens, cumberlandense, flammeum, prunifolium, and viscosum but accepted pollen from each other and R. calendulaceum.
As an exception, John Thornton has found that Rhododendron austrinum normally rejects diploid pollen but was able to successfully cross Rhododendron canescens 'Crane Creek' onto Rhododendron austrinum on one occasion. The resulting seedlings were “sickly and sterile”. R. canescens 'Crane Creek' is a lab-tested diploid.
Mike Oliver cross raised by Dick Cavender’s of Rhododendron calendulaceum ‘Colossus’ X occidentale ‘SM-30’ and Jim Skonieczny's self of Rhododendron calendulaceum 'Colossus' X occidentale ‘SM-189’ are 2 possible additional exceptions of a tetraploid accepting pollen from a diploid. Augie Kehr was successful crossing an evergreen azalea onto Rhododendron calendulaceum 'Colossus'. Dick Jaynes in his work was only able to successfully cross evergreen azaleas onto diploids and not tetraploids. Carlson crossed luteum, a tetraploid, onto R. calendulaceum 'Colossus'. Britt Smith’s crossed of R. calendulaceum 'Colossus' onto occidentale, a diploid, producing at least some fertile offspring. We have found no documentation of a member of the tetraploid clade accepting pollen from Rhododendron calendulaceum 'Colossus'. What is interesting to us is that Rhododendron calendulaceum 'Colossus', believed to be a calendulaceum by David Leach, Augie Kehr, and Clarence Towe, behaves by the evidence above as a member of the diploid clade. In other words, our rules suggest, probably falsely, that R. calendulaceum 'Colossus' is more likely a R. cumberlandense. We have never used R. calendulaceum 'Colossus' in any of our crosses. 'Pumpkin 3.1416' is a Carlson cross of R. calendulaceum 'Colossus' X luteum.
Lab testing the ploidy of 'Colossus' would be informative; however, the more interesting question is how general is 'Colossus' in violating Rule #4 above. Are the crosses mentioned above repeatable? Does 'Colossus' accept pollen from most or all occidentale. This is doubtful since Frank Mossman used 'Colossus' only as the pollen parent in his hybridization program involving occidentale. Does 'Colossus' accept pollen from other diploid species besides occidentale? Does 'Colossus' accept pollen from tetraploids?
Frank Abbott of Saxtons River, Vermont produced 'Margaret Abbott' using prinophyllum, a diploid X calendulaceum, a tetraploid. In a personal letter from Joseph Gable written in the 1940’s, Frank was reminded to always put the seed parent first in listing the cross as this convention had not always been followed in the past and led to confusion.
Ron Rabideau of Rarefind Nursery grew 2 orange flowered seedlings from seed he collected from a native Rhododendron prinophyllum on his parents' property in Ashburnham, MA. This Rhododendron prinophyllum was growing near an orange Exbury azalea, most likely Rhododendron 'Gibraltar' which is a lab-tested tetraploid. This interaction provides support for the possibility of diploid X tetraploid occurring in nature. Tom Ranney and Clarence Towe have documented natural occurring triploids. T.G.R. Eeckhaut, L.W.H. Leus, A.C. De Raedt, and E.J. Van Bockstaele ploidy testing of Ghents showed a mixture of triploids and tetraploids.
Research by Ernest Henry Wilson and Alfred Rehder indicates that 3 of the earliest (1830) English deciduous hybrids where the pollen parent is known were each diploid X tetraploid, namely R. viscosum X luteum, flammeum X luteum, and periclymenoides X calendulaceum. In this timeframe many native deciduous species were grouped under viscosum and periclymenoides.
An examination of ARS seed exchange lists from 1990 to 2008 indicates that for crosses involving an azalea from each of 2 clades, the crosses where the diploid is the seed parent far out numbers those where the seed parent is the tetraploid. Moreover, most of these interclade crosses where the tetraploid is the seed parent, R. calendulaceum is listed as the seed parent. It is interesting to point out that Rhododendron cumberlandense, a diploid, is often misidentified even by experts as R. calendulaceum, a tetraploid.
Jukka Kallijarvi wrote the following in an email conversation comparing Azaleas to rose hybrids:
"Rules #3 and #4 are, in fact, a rule of thumb in rose hybridization. Pollen from tetraploids works on diploids, but not vice versa. Also, tetraploid roses are generally much easier to hybridize than diploids."
In 2008 we performed 18 tests using Rhododendron calendulaceum 'Cherokee', a tetraploid, as a seed parent. In each instance, pollen from the 5 different tetraploids produced seedpods. In each instance, pollen from the 13 different diploids failed to produce seedpods. In the same year, pollen from 12 of these diploids produced seedpods when applied to other diploids. In 2009 we placed pollen from R. colemanii and “pink austrinum”, both tetraploids, onto several tetraploids producing seedpods in each instance. In total over the years, we have done 50 crosses placing diploid pollen onto tetraploids and all have failed to produce seedpods.
In 2009, we performed 42 tests placing tetraploid pollen on fertile diploids. Forty of these crosses produced seedpods. Our experience indicates that, for the same diploid seed parent, pollen from a tetraploid produces larger seedpods than pollen from a diploid. In some cases, much larger seedpods result. Hans Eiberg states that the size of a seedpod is determined by the number of seeds and the amount of DNA in each seed so for a fixed number of seeds the seedpod would increase in size depending on whether the resulting seed was diploid, triploid, or tetraploid.
In 2009 we placed R. molle pollen on 8 diploids producing seedpods in each instance. Frank Abbott produced 'Jane Abbott' using R. prinophyllum X 'Miss Louisa Hunnewell' where 'Miss Louisa Hunnewell' is a cross between the Japanese and Chinese forms of molle. Ed Mezitt and Harold Pellett produced hybrids using R. prinophyllum X molle hybrids. Felix and Dijkhuis produced hybrids using R. viscosum X molle.
There are no documented interspecies crosses involving Rhododendron vaseyi. All of our attempts at crossing other species onto Rhododendron vaseyi have failed.
Caution about Historical Documentation
Tetraploid X diploid crosses of deciduous azaleas are mentioned in the literature. The reader when reviewing such crosses has to be mindful of three things:
1. Many deciduous azaleas documented as “natural hybrids” of tetraploid X diploid species are selections found in the wild. The parentage is based on the conjecture of knowing both the physical characteristics and the distribution of the species in the immediate area. Which species is the seed parent is unknown.
2. In the past, especially in the 19th century and early 20th century, the order of the parentage was alphabetical. The use of seed parent first and pollen parent second is now the accepted way of listing parentage of a cross.
3. Our native deciduous azaleas are often misidentified even by experts.
Labels at arboretums and display gardens can be wrong by benign neglect, malicious or helpful label switching, and incorrect identification by the initial source. In fact, an examination of Galle's wonderful tome, Azaleas shows that once natural hybrids are excluded and one discounts 'Galle's Choice', documented as R. calendulaceum X alabamense as very likely R. calendulaceum X colemanii, there is not a single occurrence of a documented cross of a tetraploid deciduous azalea species as the seed parent with the diploid native deciduous azalea species as the pollen parent.
Worthy of mention is that no matter how careful one performs the multiple steps involved in producing a named hybrid, including accurate identification of parents, proper hybridization techniques, proper seed handling, and proper labeling of seed, seedlings, and transplants, one unintentional mistake may result in an inaccurate documentation of the parentage. We believe that reproducibility, using multiple crosses on the same parent and the same pollen across many different parents, and the distribution of seeds to the seed exchanges will address some of these issues.
Home Tests for Ploidy
Prepare for hybridization the unopened flower buds from known tetraploid and known diploid deciduous azaleas by removing the corolla and immature stamens, then wait one to two days allowing their styles to straighten and stigmas to become receptive. For clarification the use of the terms pollen and seed fertile are based on prior experience. “Pollen fertile” refers to a deciduous azalea’s pollen that has been used previously to successfully produce seed from a hand crosses. “Seed fertile” refers to a deciduous azalea that has produced seed from either hand crosses or known to set open pollinated seed freely.
A deciduous azalea that accepts pollen from both diploids and tetraploids and, also, produces pollen that takes on both diploids and tetraploids is indeed rare.
Speculation is a fertile triploid would act similar to a tetraploid.
To increase confidence, perform both of these tests for multiple parents of known ploidy. For conclusive knowledge send the azalea to a lab for testing.
Pollen Parent Test
One can test the likely ploidy of a “pollen fertile” deciduous azalea as follows:
Place the pollen of the deciduous azalea of unknown ploidy onto known tetraploids and known diploids stigmas from the 2 clades mentioned above.
If the known tetraploid(s) X unknown ploidy produces seedpods, then the unknown is very likely a tetraploid.
If the known tetraploid(s) X unknown ploidy fails to produce seedpods but the known diploid(s) X unknown ploidy produces seedpods, then the unknown is very likely a diploid.
If no seedpods are produced for either known set of seed parents, then no conclusion is reached.
Seed Parent Test
One can test the likely ploidy of a “seed fertile” deciduous azalea as follows:
Place the pollen of known tetraploids and known diploids from the 2 clades mentioned above onto the deciduous azalea of unknown ploidy.
If the unknown ploidy X known diploid(s) produces seedpods, then the unknown is very likely a diploid.
If the unknown ploidy X known diploid(s) fails to produce seedpods but the unknown X tetraploid(s) produces seedpods, then the unknown is very likely a tetraploid.
If no seedpods are produced for either known set of pollen parents, then no conclusion is reached.
Conclusion
Hall's clade work and Ranney's ploidy work divides the deciduous azaleas into a 6 species tetraploid clade and an 11 species diploid clade where Rhododendron canadense, molle, and vaseyi are excluded. Their work caused a paradigm shift in how we think about our deciduous azaleas and how we approach doing hand crosses involving these azaleas. The rules of engagement address this shift in our thinking and have dramatically increased our ability to predict the possibility of producing seedpods.
Our evidence suggests that the ability to produce seedpods is clade dependent but is not species dependent with respect to the 2 clades. In other words, in general, species within a clade behave similarly with respect to accepting pollen from within the clade and between the 2 clades. All species within a given clade accept pollen from one another. All species in the tetraploid clade reject pollen from all the species in the diploid clade. All species in the diploid clade accept pollen from all the species in the tetraploid clade.
It is highly likely that individual plants exist, such as calendulaceum 'Colossus' and canescens 'Crane Creek', that fall outside these rules of engagement especially Rule #4 above. The questions are as follows:
Are the exceptions repeatable? Were viable offspring produced? Were fertile offspring produced?
Are there more such plants? How do such plants get identified and documented?
Are such plants associated with certain species?
What is the actual lab-tested ploidy of these plants? Are most such plants diploids, tetraploids, or possibly triploids?
Do such plants fall outside Rule #4 when crossed with only a few plants in the other clade, or for an entire species in the other clade, or for several or all the species in the other clade?
Notes and References
Rhododendron colemanii by Ben Hall, Tom Ranney, and Ron Miller
Ploidy Levels and Relative Genome Sizes of Diverse Species, Hybrids, and Cultivars of Rhododendron by Jones, Ranney, Lynch, and Krebs
Clade Ploidy of Deciduous Azaleas by John and Sally Perkins
Ploidy Research of Rhododendron canadense by John and Sally Perkins
Status of Ploidy Research for Rhododendron canadense by John and Sally Perkins
Dr. Tom Ranney is now testing the ploidy of living samples of Rhododendron canadense.
F1 Crosses of Evergreen and Deciduous Azaleas and Other Wide Crosses of Rhododendron by Richard A. Jaynes
With Camera, White Umbrella, and Tin Pants In Rhododendron occidentale Heartland by Frank Mossman
The Western Azalea, Rhododendron occidentale by Frank Mossman
ARS Seed Exchange Lists: 1990 to 2008
A Monograph of Azaleas: Rhododendron subgenus Anthodendron by Ernest Henry Wilson and Alfred Rehder
Rhododendron periclymenoides X calendulaceum
Lindley (in Bot. Reg. XVI. t. 1366 [1830]) states under A. calendulacea sub-
cuprea, quoting from Mr. Gowen's letter, that this form was raised at Highclere
between A. nudiflora rubescens impregnated with pollen of A. calendulacea
triumphans.
Rhododendron flammeum X luteum
In the Botanical Register Mr. Gowensays: "This Azalea was raised at Highclere
in the same year with those already figured in previous parts of this work and is
a seedling from Azalea coccinea major impregnated by the pollen of Azalea pontica.
The flowers are bright pink with the narrow lobes of R. nudiflorum and with the
upper lobe of a pale buff color or nearly whitish."
Rhododendron viscosum X luteum
Sweet states that he received the specimen figured the end of May, 1830, and that "the seeds were raised by Lord Carnarvon's gardener from Azalea viscosa var. rubescens, fertilized by A. pontica under Mr. Gowen's own inspection."
Rhododendron flammeum x molle.
The plant is stated to be "a hybrid produced from A. coccinea fertilized with the pollen of A. sinensis. The size of the flowers and the foliage with the entire habit of the plant is quite like that of the A. sinensis with the exception of the color which differs only in the light orange-red margin of the corolla." The hybrid was raised by Mr. Cunningham of Comely Bank near Edinburgh, in whose collection it flowered February, 1835. The parentage given is probably correct as far as can be judged from the colored plate.
Rhododendron arborescens x calendulaceum = R. Anneliesae (Rehder)
This hybrid originated accidentally at the Arnold Arboretum and was raised probably in 1896 from seed of R. calendulaceum or R. arborescens collected in the Arboretum. It is exactly intermediate between R. calendulaceum and R. arborescens and I have little doubt that it is a hybrid between these two species. The first flowers of R. arborescens are usually just beginning to open about the middle of June when late blooming forms of R. calendulaceum bear the last flowers; and with dichogamous plants this is just a favorable condition for cross-fertilization. From R. calendulaceum it differs chiefly in the glaucous and glabrous under side of the leaves, only the midrib being furnished with strigose hairs and slightly pubescent toward the base, in the very sparingly hairy branchlets and in the longer corolla-tube of the pinkish white fragrant flowers marked with a large yellow blotch and in the style puberulous only near the base. From R. arborescens it differs in the slightly pilose and slightly puberulous branchlets, glabrescent toward the base, in the pubescent and strigose midrib of the under side of theleaves, in the large yellow blotch on the upper lobe of the pinkish white flowers, in the shorter ovate calyx-lobes and in the style being puberulous near the base.
It is a shrub of vigorous habit, very handsome in flower with its large fragrant, pinkish white flowers marked with a conspicuous yellow blotch; the pinkish corolla-tube is rather densely furnished with short glandular hairs and the style is purple toward the apex. The leaves are elliptic or broadly elliptic and somewhat bluish green above.
R. Anneliesae is named for my wife.
Species in Our Mist by American Rhododendron Society Massachusetts Chapter Species Study Group
"A Revision of Rhododendron Section Pentanthera", Edinburgh Journal of Botany, Volume 50, No. 3 (1993) by Kron, Kathleen A.
KONSTRUKTION AF RHODODENDRONFAMILIENS STAMTRÆ by Hans Eiberg
Phylogenetic tree published by Hans Eiberg
Eiberg, Hans (1997) Rhododendronfamiliens stamtræ 2. Rhodo-Nyt 1: 14-18
Eiberg, Hans (1999) Rhododendronslægtens stamtræ. in Rhododendron i Danmark i 25 år page 186-199. Copenhagen.
"Occurrence of polyploidy in Rhododendron luteum Sweet, Hardy Ghent, and Rustica hybrids", Eeckhaut, T.G.R., L.W.H. Leus, A.C. De Raedt, and E.J. Van Bockstaele. 2004. The Azalean 26:32-37.
Sakai, K., Y. Ozaki, K. Ureshino, I. Miyajima, A. Wakana and H. Okubo.,"Interploid crossing overcomes plastome-genome incompatbility in intersubgeneric hybridization between evergreen and deciduous azaleas.",Scientia Hortic.,115: 268-274.,2008.02.
molle X 4x deciduous azaleas produce seed capsules whereas 4x deciduous azaleas X molle do not
We found in 2009 that the 4x 'Marydel' rejected molle pollen but we did no other such 4x X molle crosses. We found in 2010 that luteum rejects molle pollen.
George Woodward has found that tetraploid lepidotes reject pollen from diploid lepidotes but diploid lepidotes do accept pollen from tetraploid lepidotes.
Mike Creel wrote the following:
"I well-pollinated two different Mountain Flame Azaleas - 'Gamecock' and 'Walhalla Gold' - with fresh pollen from a nearby eastmanii this May, and NONE of the flowers formed a seed pod, acting like they had never been pollinated."
"Can anyone try to explain this apparent incompatibility between calendulaceum and eastmanii?"
"A friend of mine in Oregon has had several successes with crossing occidentale and eastmanii, so those species appear compatible."
Jim Barlup stated the following Barlup Lecture:
"This years 'Gargantua' seed was fertilized with four different tetraploid. They germinated and seedlings currently growing well. There were no results using the pollen of diploid. I am not a scientist and I do not know the answers to this curious puzzle. I am grateful that the plants do not know not read, so they know what they are supposed to do. All I know is the results."
We found using pollen testing in 2010 that our seedling of the Oliver / Cavender of Rhododendron calendulaceum ‘Colossus’ X occidentale ‘SM-30' is both seed and pollen fertile. Diploid pollen is accepted by this seedling indicating strongly but unconclusively that this seedling is a diploid.
We found using pollen testing in 2010 that our calendulaceum seedling from 'Engine Gap' behaves as a tetraploid.
We found in 2009 that canadense accepts pollen from both luteum and atlanticum.
Two reports of vaseyi crosses occurred in 2010.
Werner Brack crossed albrechtii X vaseyi creating a single seedpod about half the size of a typical pod of self pollination on albrechtii.
Tadeusz Dauksza crossed vaseyi X 'Western Lights' to produce seedpods. 'Western Lights' is a tetraploid produced by Briggs from 'Orchid Lights'.
To date with the exception of vaseyi which does not easily cross with other species the ploidy of every deciduous azalea species in the 2 ploidy based clades plus canadense and molle could have been accurately predicted using the home pollen test for ploidy.
Research showing kuisianum X molle produces triploid due to an unreduced gamete in the kiusianum seed parent
The research shows that a lab tested diploid kuisianum crossed by a lab tested
diploid molle produced a single triploid offspring. That diploid X diploid can
result in a triploid in one generation is a lab documented fact.
Seed Size
In both monocots and dicots, when the relative dosage of maternal and paternal genomes is perturbed, the endosperm and seed size are affected (13, 14). Arabidopsis, a diploid plant pollinated with tetraploid pollen, produces large seeds. This cross generates tetraploid endosperm (2 ♀:2 ♂) with paternal genome excess rather than the normal triploid endosperm produced by diploid parents. A different endosperm and seed result is generated when a tetraploid plant is pollinated with diploid pollen generating maternal genomic excess (4 ♀:1 ♂), and the pentaploid endosperm results in smaller seeds than normal. Apart from the ploidy level and parental genome representation, endosperm development is also subject to differential expression of many genes that depends on their parent of origin.
Endosperm Degeneration
The germination of pollen grains on the stigma and the growth of pollen tubes in the style and entry of pollen tubes into the micropyles in 4X×2X are in a fairly normal manner when compared with controls 2X×2X. The percentages of pollen tubes entering ovules of the crosses 4X×2X and the control 2X×2X had no significant difference. Fertilization was normal in cross pollinated ovules. The ovules of 2X×2X,4X×4X,4X×2X were studied to determine the sequence of events leading to embryo and endosperm deterioration in 4X×2X ovules. On the 3rd day after pollination, the 4X×2X showed no signs of abnormality, Endosperm deterioration was the first indication of anatomical abnormality. The endosperm was disintegrated totally by the 7th day after pollination. Deterioration of both the 4X×2X embryo and the cells of proliferated endothelium was complete by 23 days. Therefore it might be concluded that the sterility of crosses between diploid and tetraploid was primarily attributed to the abortion of hybrid endosperm.
Frank Mossman's Occidentale Slideshow
This slideshow includes images of calendulaceum 'Colossus', occidentale SM-30, occidentale SM-189, and occidentale X calendulaceum 'Colossus'
Rhododendron 'Moorgold' from seed of 'Daviesii' a lab tested triploid.
Henry Skinner x 'Daviesii'
We have succesfully crossed tetraploids onto viscosephalum which is a similar cross to 'Daviesii'.
'Canobie Sunset' X luteum set full seedpods for all pistils. 'Canobie Sunset' is an evergreen azalea.
Pollination by Marc Colombel
In Search of Native Azaleas: Henry Skinner's 1951 Southern Collecting Trip
American Azaleas by Clarence Towe
Azaleas by Fred Galle
Frank Abbott's Village of Azaleas by John and Sally Perkins
Azalea Society of America
cumberlandense X luteum
Burce Clyburn reports the the image titled luteum X cumberlandense on the Azalea Society site is from seed Bruce received as bakeri X luteum.
The American Rhododendron Society
Rules of Engagement
Have Gun - Will Travel
