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Question: How can a rose be so many different colors besides red such as pink and yellow?
Many of us grow or purchase flowers because they are colorful and cheery. But why do roses, and other flowers, have color? There are several things involved: the chemical pigments in the flower and how we perceive the reflected light from the flowers.
The main groups of plant pigments are called carotenoids and flavonoids. Carotenoids are responsible for the yellow through orange colors. Flavonoids are responsible for the red through blue colors. Two groups of compounds are included in the flavonoids: anthocyanidins and co-pigments.
We are all familiar with anthocyanidins. When these compounds are combined with sugars, they produce the common pigments that create our colorful fall leaf display. Other pigments in leaves also include the flavonols (yellow) and chlorophyll (green).
Flower color is the result of a combination of two pigments in varying proportions, where different proportions of the combination result in different colors. For example, the red color in many flowers is the result of a combination of carotenoids and flavonoids. The red of one variety of rose will differ from that of another red rose due to the proportions of these pigments. At certain levels of acidity within the plant cells, anthocyanins are not very stable and therefore nearly colorless. The above-mentioned co-pigments act to stabilize and intensify the anthocyanin color - known as co-pigmentation. This acidity of the plant cells depends on their genes and is not normally something subject to outside sources, such as soil. In different varieties of roses, there may be the same anthocyanin, yet the differing acidity/or alkalinity (pH) levels in the cells may cause one variety to be bluer or redder than the other.
But wait, we have all seen flowers on the same plant that seem to have a different shade of color! Flowers do change in perceived color as they age, or according to the temperature zone where they grow. Stresses, for example from drought, can cause differences in the color you see. But these are a change in our perception of the color of the flower; the actual components of the pigment do not change. There are differences among the human population in eye anatomy that does create differences in perception of flower color. The color we see when we look at a flower is actually the light reflected off the surface; anything that changes this reflected light (low light levels, sunny days) will also change the color we see.
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