I have noticed in the past that many rhododendron enthusiasts have heard the words “polyploid”, “triploid” and “tetraploid” but are not really sure what the words mean and how the plants with this dscription came about. So here is an unscientific description that I hope will help you understand the words.
Rhododendrons have 26 chromosomes. These are found in the nucleus of every cell in the plant. Housed on these chromosomes are the genes. These genes control everything about the plant: flower color, size and shape, leaf shape and when they are to be produced, etc., everything about the plant. The plant cells are microscopic and the chromosomes are even smaller and the genes are even smaller.
The plant grows by cell division. When it is time to grow, the chromosomes line up and an exact copy of each chromosome is made. This new set pulls away from the old ones and a cell wall forms between the two sets. The new set of chromosomes coalesces into a new nucleus and presto a new cell is formed with exactly the same chromosomes as the original had.
There is one exception to this process. When the plant wants to make a gamet cell a different process takes place. (A gamet cell is a reproductive cell, a pollen grain or an ovule in the ovary of the plant.) Only half the chromosomes are placed in the gamet cell. So the rhododendron with 26 chromosomes places only 13 chromosomes in gamet cells. THIS IS CALLED A REDUCED GAMET. That number “13” is an important number in botany and is indicated by the letter “N”. So a reduced rhododendron gamet has 1N chromosomes. When a reduced gamet with 1N chromosomes fuses with another reduced gamet with 1N chromosomes the resulting seed will have 2N chromosomes, one from each parent and the plant that grows from that seed will be called a diploid. Virtually all the rhododendrons we grow are diploid plants.
Now every once in while when the plant is making a gamet cell it makes a mistake and instead of putting 13 chromosomes in the gamet it puts a complete set of 26 chromosomes in the gamet. So this gamet will have 2N chromosomes and IS CALLED AN UNREDUCED GAMET. If this unreduced gamet fuses with a normall reduced gamet, the resulting seed will have 3N chromosomes (2N + 1N) and the plant that grows from the seed will be called a “triploid”.
In the unbelievablely rare occation. if thnunreduced gamet fuses with another unreduced gamet the seed will have 4N (2N + 2N) and the plant that grows from that seed will be called a “tetraploid”.
Now every once in a while when the plant is making a gamet cell it makes a mistake and instead of putting 13 chromosomes in the gamet it puts a complete set of 26 chromosomes in the gamet. So this gamet will have 2N chromosomes and IS CALLED AN UNREDUCED GAMET. If this unreduced gamet fuses with a normal reduced gamet, the resulting seed will have 3N chromosomes (2N + 1N) and the plant that grows from the seed will be called a “triploid”.
In the unbelievably rare occasion if an unreduced gamet from on plant fuses with another unreduced gamet the seed will have 4N (2N + 2N) and the plant that grows from that seed will be called a “tetraploid”. Tetraploids and triploids are called “polyploids”.
These polyploids have some interesting ornamental characteristics. The flowers and truss are larger than you would expect with thicker corollas and greater saturation of color. The leaves are thicker than normal and the proportion of length to width seems to change; the polyploid’s leaves are slightly wider in proportion to the length. The stems are thicker, sometimes noticeably so. The plant is usually more robust. Of course the real impotant characteristic of these plants is the flower and truss.
The history of these plants is very interesting and goes way back to the early 19th century for it was then that an English hybridizer made a very logical cross. He crossed the hardy East Coast species, rh maximum onto the brand new, just bloomed for the first time Indian species rh arboreum. Arboreum had several features: it was very tender, it could be only grown outdoors in extreme southern England, it grew into an enormous size (thus its name meaning “tree like”) and it had RED FLOWERS. When it bloomed, the English had never seen anything like it, imagine, a shrub with red flowers. There were no wild flowers in Europe with red flowers as bees cannot see red. So the hybridizer did the obvious thing, he crossed the super hardy light pink to white maximum onto the tender red arboreum. One of the plants from that cross was named ‘Broutinii’. It was and is a triploid. Of course at the time no one knew anything about polyloids, they just knew it was superior.
A few years later another hybridizer made a similar obvious cross, he crossed the other hardy East Coast species the mauve colored rh catawbiense onto another brand new Indian species, rh Griffithianum with its enormous white flowers and also being very tender. From that group of seedlings he named a plant ‘George Hardy’. It was and is a diploid. It took 70 years before someone was able to cross ‘Broutinii’ onto ‘George Hardy’ and got a plant that was the darling of the roaring twenties, ‘Pink Pearl’, the plant that brought rhododendrons to the attention of causual gardeners in England. ‘Pink Pearl’ was and is a triploid.
We have to back track now the the middle 19th century when the ‘George Hardy’ cross was reversed: rh Griffithianum was crossed onto rh catawbiense and rh ‘Cynthis’ was named from that cross. ‘Cynthia’ was and is a triploid. It was in the early 20th century that ‘Cynthia’ was crossed with ‘Pink Pearl’ and resulted in ‘Countess of Derby’ a TETRAPLOID probably the first tetraploid in cultivated rhododendrons. Harold Greer selfed ‘Countess of Derby’ and grew a plant from that cross that he named ‘Trudy Webster’ a tetraploid. He then cxrossed ‘Trudy Webster ‘ onto ‘Jean Marie de Montegue’ and got ‘Very Berry’.
If a rhododendron has more that the normal number of chromosoms it is called a polyploid. Most polyploids have either 1 1/2 times the number, called a triploid, or twice the number, called a tetraploid.
John and Sally Perkins have taken an enormous interest in rhododendron polyploids and have had many cultivars analyzed to determine whether they are polyploids. Here is a partial list of some of the rhododendron polyploids:
Anita Gehnrich Triploid
Anna Rose Whitney Triploid
Antoon Von Welie Tretrploid
Beauty of Littleworth Triploid
Cotton Candy Triploid
Countess of Derby Tetraploid
Doreen Gale Tetraploid
El Camino Triploid
Gentle Giant Tetraploid
Gomer Waterer Triploid
Gorgeous George Tetraploid
Grace Seabrook Triploid
Lem’s Monarch (AKA Pink Wallaper) Tetraploid
Lucky Strike Triploid
Mariness Koster Tetraploid
Phyllis Korn Triploid
Pink Pearl Triploid
Platinum Pearl Triploid
Point Defiance Tetraploid
Phyllis Korn Triploid
Trudy Webster Tetraploid
Now you might wonder why polyploids are important for hybridizers. All you have to do is look at the list above and you will see some very important hybrid rhododendrons. Most of them are West Coast plants, too tender for most East Coast gardens. Many of them grow in my garden, but that is not saying much, as I have a very “tender” climate. So the goal going forward is to see if polyploids can be developed that are hardy in cold climates. There is one particular hybrid rhododendron that has a standard number of chromosomes, called a diploid, and has shown in the past to produce polyploid hybrids. It is ‘Jean Marie de Montegue’ which I shorten to ‘Jean Marie’. If you look at my list of selected seedlings, you will see that I have used it frequently in the past and gotten, I think, some polylpoids in its offspring.