Rose Bloom Color and Chemistry

by Carolyn Elgar, OCRS Master Rosarian

March 2018

The color of a rose bloom is a defining feature of the variety. The softness of a white or cream colored flower, the vibrance of a red one, and the drama of a bicolored or striped bloom increases every rose lover's heartbeat. But sometimes your neighbor's Mr. Lincoln seems to be a different red from the Mr. Lincoln in your garden. Or the blooms on one Double Delight are much whiter than on those on a different bush. What affects the way the color develops in the bloom? As usual in plant science, there's chemistry involved.

The color of a rose comes from the pigments in its petals. Primarily, these pigments are anthocyanins and carotenoids. Carotenoids produce the bright yellow, orange, and red pigments such as you would find in lemons, oranges, and tomatoes. They are enclosed in compartments called plastids that are part of the plant cells' cytoplasm. As a result, they are stable and little affected by environmental factors or plant health. Anthocyanins create the deep red, magenta, purple, and blue colors. They are water soluble and carried by the plant's sap. Because they are in fluid surroundings, they are much less stable and subject to environmental factors.

Research has shown that a bloom's level of anthocyanins changes, along with sap pH, as the flower transitions from bud to bloom. As the flower ages, the sap pH becomes more alkaline and the amount of anthocyanins decreases. As those who grow hydrangeas know, high alkaline levels result in blue colorations whereas more acidic conditions favor the pink/red side of the color scale. Thus the red of your rose bloom will tend to the darker, bluer side of the spectrum as it ages and its pH increases. Anthocyanins are at their highest levels when the flower is in bud, continue to be high as it blooms, and decrease quickly as the flower fades. 

Other, more variable factors can impact anthocyanin levels. Levels increase and color intensifies in sunlight, but heat also degrades anthocyanins and colors fade. The deep reds and purples of spring become less vibrant as temperatures increase during the summer. Roses in low light are not as intense in color. In contrast, the oranges and yellows of carotenoids brighten with sunlight and heat. In cooler temperatures flowers with these pigments will soften in color.

The amounts of anthocyanins and how they chemically combine with other pigments is genetically determined. As a result, some rose varieties are more subject to anthocyanin flucuation than others. Their rose blooms can seem completely different, depending on the amount of sun and heat they get. These roses are sometimes referred to as phototropic roses. They include Double Delight, Paradise,

and Color Magic. Some other varieties have blooms that change colors as they age, depending on heat and light. Distant Drums is an example of this.

(The 'Double Delight' on the top has a much larger cream center and less red than the one on the bottom. Sunlight has raised the level of anthocyanins and intensified the red color.)

Interestingly, anthocyanins interact with metals; they act as natural chelating agents by binding with the metal molucles, reducing their positive electrically charged ions to a neutral state that is more easily absorbed by the plant. Accordingly, it could be assumed that an increase in metal ions in the plant sap will affect anthocyanin levels. How metals, such as zinc or iron, affect anthocyanin levels has been researched not only for reasons of color stabilization but because they are known to be strong antioxidants that help boost the imune

system and promote health when they are in foods such as grapes and berries. Studies have shown that foliar sprays of zinc increase anthocyanins in grapes and lemon balm; one experiment's results determined that combining zinc with the anthocyanins in a grape beverage increased the stability of the pigment content. This explains why rose exhibiters recommend applying zinc to roses to intensify bloom color, especially before a rose show. 

So the instability of athnocyanins is what causes the variation of color in roses of the same variety. Environmental factors play a big factor in the levels of this pigment. As stated in the article, The Chemical Pigments of Plants, Journal of Chemical Education, v.59, no.3 - "The anthocyanin colors can be modified by a number of factors including: the presence of small quantities of other pigments, copigmentation with other flavonoids, metal chelation, and the pH of cell fluid."

('Distant Drums' blooms can be dominated by a warm apricot color, TOP, when it is hot and sunny and the level of cartonoids increases. Spring and cooler temperatures allow the more delicate pink shades to emerge, BOTTOM, as the anthocyanins respond to sunlight.)

Despite the romantic allure of a rose bloom, its color is largely determined by chemistry and science. Pigments like anthocyanins and carotenoids co-mingle, while environmental factors like heat, light, and fertilizers affect the amounts of these color-producing molecules, especially anthocyanins. All of this results in paler or more intensely colored blooms in your

garden and in the vase.

(The pinker Color Magic on the top is from the same bush as the one on the bottom. The paler bloom occurred during in early May, while the pinker one is a bloom from late June when the sunny days are longer).