University of Vermont
Department of Plant and Soil Science

gmg logoSummer News ArticlelineTHE HOW AND WHY OF PLANT COLOR

Dr. Leonard Perry, Horticulture Professor
University of Vermont

While some now grow flowers for pollinators, the main reason most of us grow flowers is for their colors of blooms and sometimes leaves. What imparts color to leaves, flowers and fruits, and why these are in various colors, may seem simple on the surface but really is more complex.
 
The color that you see in flowers is actually the result of reflected light from various chemical compounds called “plant pigments.”   Before humans were interested in these for the aesthetics they impart—flower, leaf, and fruit colors—they used pigments for dyes such as indigo, and herbal medicines.  There is still some use of these for dyes in crafts and natural art, but also in dietary supplements.  Pigments more recently are being studied for their antioxidant health-promoting properties.  Some may inhibit bad cholesterol, prevent blood from clotting, and help to prevent cancer in cells.

The real role of color in plants, flowers in particular, is not for humans but for their ecological roles.  Plant colors serve to attract insects, birds, and animals for both pollination and seed dispersal. Often what they see in colors is different from what we see. Forget-me-nots (Myosotis) or other members of the Borage family, as well as some other flowers such as larkspur (Delphinium), change color between pink and blue.  This usually indicates to insects that a flower has aged and is past pollination, so move on.

There are three main groups of plant pigments.  “Anthocyanins” are a group of flavonoid chemicals (phenolic compounds) that are responsible for many colors, from orange and red to
violet and blue.  Scientists have identified over 300 different anthocyanins which occur in nature.  Colorless (to us) flavonoid pigments, and their flavonol versions, absorb ultraviolet light so they are readily seen by insects—a pollinator cue of some flowers.  

Anthocyanins are composed of anthocyanidin chemicals to which sugars are attached. These chemicals may sound familiar, as they’re named after flowers in which they’re found.  Delphinidin imparts the blue color to delphinium, as well as to violas and grapes producing Cabernet Sauvignon wine.  Malvidin imparts blue to the flowers of some primroses, is the main pigment in red wines, is found in perennial geraniums and petunias, and of course is in mallows (Malva). Pelargonidin is of course in the red annual geranium (Pelargonium), as well as in many red fruits from strawberries to raspberries and cranberries.  Purplish-red colors in peonies are from peonidin.   Dark red or purple in grapes, Saskatoon berries, Indigo rose tomato, and of course petunia is from petunidin.

It is these anthocyanidin pigments that biotechnologists are studying to change flower colors. For instance, they've taken the scarlet pelargonidin-producing gene from corn and placed it into petunias to give this flower a novel orange color. The gene for delphinidin has been placed into carnations to make some blue. Other factors within the cell, such as the acidity (pH) and even cell shape, are making the genetic production of blue mums and roses a bit more challenging.
 
Roses are red, and some potatoes blue, due to anthocyanins.  Fruits with the highest levels of anthocyanins include black currants, black raspberries, blackberries, cherry, elderberry, some red grapes, and wild lowbush blueberries. Red cabbage and eggplant also are high in anthocyanin.  Many of the new brightly-colored sweet peppers owe their colors to anthocyanins.  While plant colors often attract animals and insects, in some plants anthocyanin pigments may deter herbivores (plant-eating animals). 

A second group of plant pigments, the “carotenoids”, are terpenoid chemicals.  They are responsible for yellows, oranges in carrots (a good way to remember carotenoid), and reds in tomatoes.  The specific carotenoid in most red tomatoes is lycopene, named from the scientific species name.  Carotenoids absorb wavelengths of light that chloroplasts (those parts of plant cells where photosynthesis occurs) can’t.  They also may protect plants from damage caused by both ultraviolet and visible light—think sunscreen.  The two main groups of carotenoids are the “xanthophylls” and the “carotenes”.  The latter are what make cantaloupes and carrots orange.

The term xanthophylls comes from the Greek words for yellow (xanthos) and leaf (phyllon), and are the main yellow pigments found in leaves.  They are present during the season but masked by the green chlorophyll, except in plants that may be stressed or with yellow leaves normally.

A third group of plant pigments are the “betalains”, composed of those that appear reddish to violet, and those that appear yellow to orange. This group is much less commonly found in plants than the first two groups of pigments, primarily found in the group of plants--  Caryophyllales-- containing dianthus, cacti, and beets (hence the pigment name) where it replaces anthocyanin.

Many flowers may not change color on an individual plant but may change color, even if slightly, among locations or various conditions. Temperature affects color, hence, there are often more vivid colors in cool northern gardens than hot summer ones. Plant stress, such as from drought, insect attack, or plant nutrition (too much or little) also can cause different levels of pigments in flowers, and. as a result, different colors. If a plant doesn’t appear to have the color leaves or flowers that it should, consider these factors.


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