Rhododendron Flower Color: Genetic/Cultural Interaction
R. J. Griesbach, Ph.D.
USDA, Florists & Nursery Crops Laboratory
Reprinted from Rhododendron Society of Canada Bulletin (Vol. 12:2)
Chlorophyll, Flavonoids and Carotenoids
Flower color is due to three different pigments - chlorophyll, flavonoids, and carotenoids. Chlorophyll is located in small "packets" called chloroplasts found throughout the petal and sepal cells. This pigment is responsible for green color and is fat or lipid soluble. The carotenoids are also found within small "packets" in the cells. The "packets" containing carotenoids are called chromoplasts. These pigments are responsible for yellow and orange colors and are also lipid soluble. The flavonoids, unlike the other two pigments, are located within the cellular vacuole which occupies most of the cell volume. Anthocyanins are responsible for red and blue color and are water soluble.
Each pigment is the result of a different sequence or pathway of biochemical reactions. The production of each pigment is independent of the other two. Thus, a block in the flavonoid pathway has no effect on the carotenoid and chlorophyll sequences. For example, in white flowered forms of many of the red spotted rhododendron, the flavonoids which are usually present in the spots are lacking due to an absence of a critical enzyme in the flavonoid biosynthesis pathway. The carotenoids, however, are unaffected. Therefore, the spots are yellow on a white background.
Flower color is the result of mixing the three pigments (flavonoids, chlorophyll and carotenoids) in different proportions. For example, a flower of 'Vulcan' appears red because of the presence of red flavonoids and the absence of both chlorophyll and carotenoids. On the other hand, the flowers of R. japonicum appear orange because of the presence of red flavonoids combined with orange carotenoids. Similarly, R. sanguineum flowers appear brown because of the presence of red flavonoids combined with green chloroplasts. By mixing and matching the three pigments, an endless array of different colors can be created.
Very little is known about the biochemistry of carotenoids and chlorophyll as related to flower color. However, a lot of information is known about flavonoid biochemistry and flower color. The flavonoids can be subdivided into several groups - anthocyanins, flavonols, aurones, chalkones and gossypetins.
The remaining part of this paper will discuss the anthocyanins. There are six major anthocyanins - pelargonidin, cyanidin, delphinidin, malvidin, petunidin and peonidin. There are several factors which influence anthocyanin coloration. These factors can be subdivided into two types, those with a genetic basis and those with an environmental basis. Light intensity, temperature and even soil pH can affect flower color.
pH Changes and Color
In general, the cells of blue flowers are more alkaline than red ones. However, in hydrangeas a soil pH of 6.0 will produce pink flowers while a pH of 5.5 will produce blue ones. At acidic pHs aluminum becomes more available and is found at a higher concentration in the sepals than at more alkaline pHs. The availability of aluminum overrides the effect of pH. Aluminum, when it complexes with anthocyanins can change the color of the anthocyanin from pink to blue. The type of fertilizer can also affect the color of hydrangeas. A 25-5-30 formulation will lead to blue flowers while a 25-20-20 formulation will lead to pink ones.
Color changes associated with flower aging are also controlled by pH. In morning glory, the fresh flowers are pink with a petal pH around 6.5. As the flowers age the pH increases to about 7.5 and the flowers appear bluer. When the flowers are ready to close the pH decreases to about 6.0 and the color changes to pink.
In most rhododendrons the flowers are buffered. This means that soil pH has no effect on flower color. In addition, aging does not change the color of flowers (aging may change the intensity of coloration). In general, the pH of rhododendron flowers is predominantly under genetic control with very little environmental interaction. This fact is very important in breeding and judging, for it tells us that the type of potting medium will not effect flower color. In addition, in order to create redder or bluer flowers one can breed for pH. The pH of petals appears to be controlled by a small number of genes. By crossing flowers which are reddish in color with the flowers which are acidic in pH, one can produce a redder flower.
Light and Temperature
Light and temperature can also dramatically affect flower color. High light intensity during flower development can also lead to more vibrant coloration. At high light intensity, photosynthesis is occurring at a very rapid rate which leads to the production of increased amounts of sugar. At cool temperatures the plant's growth is slowed down, limiting the amount of sugar needed for respiration. Cool temperatures and high light intensity thus allow the plant to accumulate a reserve of sugar. Sugar molecules are bound to anthocyanin molecules and have the effect of stabilizing color. In addition, at high light intensities, increased anthocyanin production occurs. Anthocyanins help protect the cell from harmful effect of increased irradiation. All these factors coupled together lead to an increase in anthocyanin under cool temperatures and high light intensity. High light intensity and high temperatures can cause the anthocyanins to break down and lead to fading. In order to retain the vibrant color, the flowers, after opening, could be placed in a low light intensity, cool environment to prevent fading.
Besides environmentally induced fading or intensity differences, there are genes which control the amount of anthocyanins produced. These genes can either increase the amount of pigment per cell or increase the number of cells producing pigment. When comparing plants for differences in color intensity, one must be careful to separate differences due to genetics from differences due to culture or environment. To make matters even more difficult, there is a genetic component to environmental-induced fading.
The co-occurrence of anthocyanins and other flavonoid pigments can lead to a blueing of flower color. This effect is called copigmentation. At normal cell pH (between pH 3 and 5) pure anthocyanins are not as strongly colored as at acidic pH (pH 2 or less). The addition of flavonols at physiological pH causes an increase in the stability and intensity of anthocyanins. A sport of 'Red Wing' Azalea with orange rather than red petals was discovered at Beltsville, MD. This change in color was the result of a reduction of the concentration of copigments.
With any given anthocyanin it is possible to obtain all colors between red and blue by ranging varying either the pH, the concentration of that anthocyanin or the ratio of anthocyanin to flavonol. A good example of this is seen in the blue cornflower where the anthocyanin is cyanidin, which is red in vitro. As now should be quite evident, the color of a pure anthocyanin in vitro has little relationship to its color in vivo. By breeding for such traits like increased or decreased flavonols or pH instead of breeding for anthocyanin, it is possible to create an almost endless range of different flower colors. One should also realize that there are many environmental factors which will effect flower color. A thorough knowledge of both parentage and cultural conditions are necessary to adequately breed or judge flower color.
Dr. Griesbach presented this paper as part of a panel discussion on "Breeding Rhododendrons and Azaleas for Yellow and Blue Colors" at the Breeder's Roundtable, 1986 ARS National Convention, Cleveland, Ohio. Dr. Griesbach is a research geneticist with the USDA, Beltsville, Maryland.