Roses display a vast spectrum of colors, ranging from pure white to deep red. This palette is the result of centuries of selective breeding and hybridization. However, rose color is fundamentally determined by a limited set of natural chemical compounds and the plant’s genetics, which govern the production of these hues.
The Pigments That Create Natural Hues
The natural coloration of a rose petal is controlled by two main classes of pigments: anthocyanins and carotenoids. Anthocyanins are water-soluble flavonoid compounds stored in the cell vacuole, responsible for red, pink, and purple shades. The intensity of these colors depends directly on the concentration of anthocyanins.
The specific shade is fine-tuned by the type of anthocyanin present; for example, pelargonidin creates orange-red tones, while cyanidin produces true reds and pinks. The acidity (pH level) inside the petal cells also influences the final color, causing the same compound to appear red in an acidic environment or shift toward violet in an alkaline one.
Carotenoids are fat-soluble compounds stored in chromoplasts, responsible for yellow and orange hues. Yellow petals accumulate carotenoid compounds such as zeaxanthin and \(beta\)-carotene. Orange roses result from the co-accumulation of both red-producing anthocyanins and yellow-producing carotenoids in the petal cells.
The Genetic Barrier to True Blue
Despite the diversity in rose color, a true, naturally occurring blue rose remains an impossibility due to a specific genetic limitation. Roses lack the gene necessary to produce the enzyme flavonoid 3′,5′-hydroxylase (F3’5’H). This enzyme is required to synthesize delphinidin, the anthocyanin that creates true blue pigment.
Since the blue pathway is blocked, “purple” or “mauve” colored roses are not chemically blue. These purplish shades are merely a darker, more violet expression of the red-pigment pathway, resulting from high concentrations of cyanidin-based anthocyanins in a less acidic cellular environment. The so-called “black” roses are cultivars bred to maximize the concentration of red anthocyanins, resulting in a very deep maroon or velvet-red color.
Achieving Colors Beyond Nature
To achieve colors the rose genome cannot naturally produce, two distinct methods of human intervention are used. The simplest method is artificial dyeing, responsible for the bright blue, rainbow, and other novelty roses seen commercially. This technique involves placing the cut stem of a white or light-colored rose into water mixed with a vivid dye. As the flower draws water, the dye is carried into the petals, staining them.
For the permanent creation of novel colors, genetic engineering has successfully overcome the natural color barrier. This process involves introducing the missing F3’5’H gene, often taken from other flowers like pansies, into the rose’s genome. This modification allows the rose to synthesize the blue pigment delphinidin, resulting in the commercialized “Suntory Blue Rose Applause,” which exhibits a bluish-lavender hue.