JARS v56n2 - Let's Talk Hybridizing: Hybridizing with Elepidote Polyploid Rhododendrons

Let's Talk Hybridizing: Hybridizing with Elepidote Polyploid Rhododendrons
Jim Barlup
Bellevue, Washington

Hybridizers are experimenting with polyploid breeding with the hopes of creating stronger growing plants with heavier foliage and larger, thicker flowers. Working with elepidote tetraploid rhododendrons is a challenging experience. What I have learned is limited, but perhaps it is enough to be of interest to other hybridizers following this same path.

One way of categorizing rhododendron polyploids is according to the number of chromosomes. Most rhododendron are diploids with 26 chromosomes. A tetraploid has a double set of chromosomes, or 52. An even number of chromosomes is needed for a plant to accept pollen and produce seed. A triploid has an additional half set of chromosomes totaling 39. It is assumed that you cannot cross onto a triploid because of its uneven number of chromosomes. Polyploidy is defined as any plant with more than 26 chromosomes. In the absence of scientific testing, it is difficult to categorize plants accurately. In general, our only way to detect a tetraploid is by its increased leaf, flower and plant size as compared to a normal or diploid plant. In other words, we are guessing which plants might be tetraploids. Trial and error in hybridizing efforts can, to some degree, confirm or deny these assumptions. Without scientific chromosome counts, we may make wrong assumptions.

Probable tetraploid seedlings
"Probable" tetraploid seedlings, 2½ years old: ('Phyllis Korn' x 'Trude Webster'),
('Horizon Monarch' x 'Gargantua'), ('Phyllis Korn' x 'Virgo'*).
Photo by Jim Barlup

I've heard that 'Phyllis Korn', 'Gargantua' and 'Horizon Monarch' are all triploids. If we assume that a plant is a triploid, we may erroneously avoid using it for hybridizing. I have successfully crossed onto each of these plants. Therefore, I conclude that they are not triploids. I crossed 'Phyllis Korn' and 'Horizon Monarch' with 'Point Defiance', 'Trude Webster', 'Very Berry'*, 'Virgo'* and 'Gargantua' pollen. The crosses all took and the seed germinated. Repeating the crosses gave the same results. 'Phyllis Korn' pollen worked just once on 'Whitney's Late Peach'*, which I assume to be a tetraploid. It had large, heavy leaves and 5-inch (12.5 cm) flowers. I lost the plant during a very cold winter and cannot remake the cross. Was that cross accurate? I think it was, but the only way I can prove it is to have 'Phyllis Korn' pollen work on another tetraploid. The pollen of 'Horizon Monarch' ('Nancy Evans' x 'Point Defiance') so far has proven to be sterile, as most, but not all, 'Nancy Evans' crosses have sterile pollen. So far, I have not been successful in producing seed using the pollen of 'Horizon Monarch'. However, an established plant at Meerkerk Rhododendron Gardens was observed to have many seed pods. If it self-pollinated, then the pollen is viable.

Once you identify the plants that seem to be tetraploids, you can try to make crosses between these plants. Usually the offspring will be tetraploids. If you cross a diploid with tetraploid pollen, you may get wonderful seed pods, but germination is very difficult. I have found the germination rate with 'Point Defiance' crosses to be less than 3 percent, if they germinate at all. The results may be a diploid or a tetraploid. A cross of 'Janet Blair' with 'Point Defiance' produced one plant that appears to be a diploid. A cross of 'Calsap' with 'Point Defiance' produced five plants, probably diploids. The seed from at least thirty other such crosses did not germinate. Frank Fujioka crossed Jim Elliott's 'Flirt' ('Britannia' x R. degronianum ssp. yakushimanum ) with 'Point Defiance' and germinated one seedling. It has a huge, beautiful truss and is probably a tetraploid, but the pollen appears to be sterile. Ned Brockenbrough crossed 'Nancy Evans' with 'Point Defiance' and got just a few seedlings, but among them were 'Horizon Monarch' and 'Patricia Jacobs', both beautiful tetraploids. This year I managed to cross 'Mindy's Love' ('Nancy Evans' x 'Lionel's Triumph') with 'Trude Webster' pollen and had very good germination. There are some very strong growing plants in this group of seedlings, which appears to be a mixture of diploid and tetraploid seedlings.

I continue to use pollen from what appears to be tetraploids (such as 'Trude Webster' and 'Point Defiance') on low growing hardy plants such as 'Ingrid Mehlquist', 'Fantastica' and Rhododendron degronianum ssp. yakushimanum . Yes, I do get seed pods from some of these crosses, but so far no seed has germinated. One of my goals is to work with a dwarf tetraploid. So far all of the possible tetraploids that we have to work with are large growers. We only need one breakthrough with a dwarf, semi-dwarf or low grower to start a whole new generation of plants. Don Wallace crossed 'Orange Marmalade' with 'Point Defiance' and has what appears to be a low growing tetraploid plant. Unfortunately, the pollen appears to be sterile. I consider his cross a major accomplishment. A cross of R. proteoides with 'Point Defiance' or 'Trude Webster' pollen would be of great value to hybridizers if it turned out to be a dwarf tetraploid. One must have visions for the future.

Briggs Nursery worked with colchicine treated plants in an attempt to obtain tetraploid forms of known plants. At this point, two elepidote plants have been tested and have obtained that status: 'The Hon. Jean Marie de Montague', now known as 'Briggs Red Star', and 'Nova Zembla', now known in its tetraploid form as 'Supernova'. 'Supernova' has accepted pollen from 'Gentle Giant', 'Trude Webster' and 'Gargantua', and I currently have seedlings growing from those crosses. The pollen of 'Supernova' has not worked so far on other tetraploids, but it is too early to know if it is sterile or viable. I have not worked with 'Briggs Red Star', but I have a plant budded for hybridizing in 2002. New excitement!

There are many mysteries in this field of endeavor. 'Gargantua' (a registered name) is a seedling of Rhododendron decorum ssp. diaprepes and has been tested and supposedly thought to be a triploid yet its pollen is the most powerful and fertile of this whole group. Clint Smith and Loyd and Edna Newcomb have made successful crosses using 'Gargantua' pollen. This year 'Gargantua' set seed with pollen from four different tetraploids. The seed has germinated and the seedlings are growing well. There were no takes using diploid pollen. I am not a scientist. I do not know the answers to this intriguing puzzle. I am thankful that plants cannot read so they don't know what they are supposed to do. All I know are the results. Harold Greer has crossed plants that appear to be diploids with 'Trude Webster' as the seed parent. ('Trude Webster' x 'The Hon. Jean Marie de Montague') produced 'Very Berry'*, and ('Trude Webster' x 'Lydia') produced 'Grand Slam'*. I consider both of these hybrids to be tetraploids. I tried fourteen different diploid pollen crosses on 'Trude Webster' this past year. No seed set. I'll try again next year. However, crosses with 'Gargantua' and 'Point Defiance' set large seed pods. I also got smaller pods from 'Summer Peach' ('Whitney's Late Peach'* x 'Phyllis Korn'). Does this mean that my 'Phyllis Korn' cross has polyploid capabilities? Will the seed germinate?

From my limited observation, it appears that 'Trude Webster' pollen applied to dwarf diploids would be the most logical way to obtain a dwarf or semi-dwarf tetraploid plant. The pollen takes are considerably more than with 'Point Defiance' and germination is easier to obtain. In December 2001, I planted eight crosses using 'Trude Webster' pollen on probable diploids. It will be interesting to see what germinates and whether they appear to be diploid or tetraploid. It is usually obvious as polyploid seedlings have triple the growth rate. I will continue to use 'Point Defiance' pollen on diploids even when I know that germination is close to impossible. It is a challenge.

R. 'Legend'
'Legend' ('Point Defiance' x 'Lem's Monarch').
Photo by Jim Barlup

Tetraploid plants to consider for hybridizing would be 'Horizon Monarch', 'Virgo'*, 'Very Berry'*, 'Point Defiance', 'Lem's Monarch' or any plant from the Walloper Group. 'Gentle Giant', 'Reverend Paul', 'Canadian Beauty' and her siblings seem likely possibilities. My own plant of 'Legend' ('Point Defiance' x 'Lem's Monarch') has accepted tetraploid pollen and set seed. 'Point Defiance' is not an easy plant to get pollen from but it appears in some trusses. I'm sure other hybridizers could list even more possible tetraploids. If you find a plant you think is a tetraploid, try 'Point Defiance' or 'Trude Webster' pollen on it. It just may work. If it fails try again another year. A cold wet spring can certainly affect a plant's ability to set seed. I continue to test questionable pollen and questionable plants for three or four years to determine if either is viable or sterile.

'Taurus' remains a mystery to most of us. I have concluded it is not a triploid, just a very difficult and temperamental tetraploid or diploid, but which is it? Over the past few years I have made many crosses onto 'Taurus', both with diploid and tetraploid pollen. Only the tetraploid pollen showed faint signs of working. If what I planted was indeed seed, it never germinated. Unfortunately, my 'Taurus' plants have always been in pots and are not well established. I believe an older, well established plant would give more positive results. This year I got tiny seed pods, all from tetraploid pollen. I have four seedlings with 'Trude Webster', two with 'Very Berry'* and three with 'Gargantua'. Thank you 'Taurus' for such generosity! These results have all the characteristics of tetraploid pollen taking on a diploid plant. The seedlings are small, possibly suggesting they will all be diploids. It is too early to tell. This year Merle Sanders sent me an open pollinated seed pod he found on his huge, well established 'Taurus'. Will it germinate and what will it produce? Merle has many tetraploid-type rhododendrons in his garden. It would appear that a bee can do better than I.

Last year I obtained pollen from a colchicine treated 'Besse Howells'. Much to my surprise, it took on 'Point Defiance' and on 'Horizon Monarch'. This had to be tetraploid pollen to take on probable tetraploid plants. Another breakthrough for hardiness! Hopefully they will be tetraploid plants. I have many seedlings growing from the 'Point Defiance' cross but only a few from the 'Horizon Monarch' cross. This is normal. 'Horizon Monarch' is a difficult plant to work with and seed germination is minimal even when crossed with another tetraploid.

If you choose to work with these giants, do not be concerned about their taking forever to bloom. They vary considerably in bloom time. My hybrid 'Legend' ('Point Defiance' x 'Lem's Monarch') bloomed in three and a half years. A sibling took ten and a half years to bloom. It is still a reluctant bloomer, whereas 'Legend' has set multiple buds from a rooted cutting in its second year. My cross of ('Phyllis Korn' x 'Trude Webster') produced beautiful well branched plants. All seem to be tetraploids. One of this group set a bud in two and a half years. I can hardly do that with a diploid cross.

By this time you have obviously concluded that I am calling certain plants tetraploid with no scientific evidence to justify such claims. Research on the chromosome count of these plants, especially 'Taurus', would be of great value. I find hybridizing with elepidote tetraploids to be a very exciting field with many new challenges and potentially outstanding results. Until we have more scientific data on which plants are truly polyploid, the best we hybridizers can do is to share what we observe from our own experimentation.

* Name is unregistered.

Jim Barlup, a member of the Cascade Chapter, authored the Tips for Beginners series on hybridizing in the following issues of the Journal: Vol. 50, No. 2; Vol. 50, No. 3; Vol. 50, No. 4; Vol. 51, No. 1; Vol. 51, No. 2; Vol. 51, No. 3; Vol. 51, No. 4.