Breeding For A New Type Of Blue Rhododendron
J. Pat Halligan
This article first appeared in the Seattle Chapter Newsletter, "Seattle Rhododendronland", May, 1986. It was written as the result of a discussion between Marge and Bob Badger and Pat Halligan regarding new color possibilities for elepidote rhododendrons.
The breeding of blue elepidote rhododendrons has followed a rather predictable course over the years. The purple species, R. ponticum, has been bred with an assortment of other rhododendrons (mostly unknown) to produce plants with purplish blue flowers, such as 'Purple Splendour', 'Blue Ensign', and 'Blue Peter'. Occasionally R. campanulatum, another bluish species, has been used as a source of blue or purple for hybrids such as 'Susan'. More recently these primary hybrids have been interbred, producing second generation hybrids such as 'Blue Boy' and 'Blue Pacific'. Of course, these have all been "Yaked" at least once or twice.
In breeding for blue or bluish colors using R. ponticum and R. campanulatum people have selected for both type and quantity of flavonoid pigments. There was also selection for flavonoid co-pigments which are invisible in themselves but influence the absorption spectra of the visible flavonoid pigments (Asen, et al, 1972). All of these flavonoids are chemically related and compete with one another for substrate. Thus we are limited in the type and amount of bluish pigments available through the traditional species used in breeding for blue or purple.
Another pigment is available which is totally unrelated to the purple and blue flavonoids. It resides in a species which does not have blue flowers at all. First, let's speculate what would happen if you added green pigment to a blue flower. Such a combination might result in a turquoise, aquamarine, or jade type of color. The question is how do you add green to a blue flower?
One species of elepidote, R. sanguineum, in its black crimson and brown forms provides the key. Like many other rhododendrons, it has an intense concentration of flavonoid pigmentation in its epidermis. But this species is unique in that it also has a great number of green chloroplasts in its sub-epidermal layers. The red of the flavonoids combined with the green of the chlorophyll causes the flower to become almost black in color (Spethmann, 1980). Since the flavonoids and chlorophyll are unrelated chemically and found in different layers of the corolla, they do not compete for substrate and can both be found in full concentration in the same flower.
So how can the green chlorophyll of R. sanguineum be integrated with the blues and purples of the standard hybrids? First, the obvious crosses must be made both ways. Why both ways? Because the chlorophyll is produced by plastids, and plastids are partially independent of the nucleus. They carry their own genes, which are passed down from generation to generation to the cytoplasm. Only the female parent contributes significant amounts of cytoplasm and the included plastids to their progeny. However, plastids are also controlled to some degree by the chromosomes that are contributed by both parents. Until you've tried the crosses, you can't say what the effects of nuclear and plastid inheritance will be on the distribution and color of the plastids. Thus you must breed in two separate lines, one using R. sanguineum as the ultimate female parent, and the other using a traditional blue as the ultimate female parent. You cannot mix the two lines without one of the female parents being lost in the shuffle. The results from one line may be quite different from the results from the other line. Only experience will tell if there are any differences between the two lines and which line is better.
The job of obtaining blue or bluish flavonoid pigmentation from the combination of the blue of the traditional parent and the red of R. sanguineum will take at least two generations. Hybridization of blues with reds tends to result in loud offensive colors that clash with everything, or muddy wine tones. Be intrepid, and carry on to further generations. The colors will sort out. It's just a matter of generations and numbers of seedlings raised.
Green is quite another matter. Selecting for green is quite outside the ordinary scope of rhododendron hybridizing. The green coloration is masked by the overlying red, purple or blue coloration. When masked by red, it causes the flower to appear almost black. When masked by reddish purple, it would cause the flower color to become muddy and quite dark. As progress is made toward blue, the color would hopefully become less muddy and actually enhance the blue coloration, making the flower a turquoise or similar color. In short, instead of selecting for bright clear colors, you have to select for dark muddy tones while at the same time selecting for blueness...not an easy task. The results are not predictable, and they could be just awful...or great.
Asen, S., R.N. Stewart and K.H. Norris (1972) Co-pigmentation of anthocyanins in plant tissues and its effect on color. Phytochemistry 11: 1139-1144.
Spethmann, W. (1980) lnfragenerische Gliederung der Cattung Rhododendron unter Berucksichtigung der flavonoiden und carotinoiden Bíuteninhaltstoffe und der Entstehung der Blutenfarben. Dissertation. University of Hamburg. Pg 78.
Spethmann, W. (1980) Flavonoids and carotenoids of rhododendron flowers and their significance for the classification of the genus Rhododendron. Contributions Toward a Classification of Rhododendron. New York Botanical Garden. Bronx, N.Y. Pp 247-275.
Pat Halligan is a member of the Seattle Chapter and the Northwest Hybridizers Group. Read more about Dr. Halligan and his hybridizing goals in Smithsonian, Vol. 17:2, May 1986.