GeraniumsOnline   

                                               

 

 

   

   THE COLOR OF FLOWERS

    BY WAYNE L. HANDLOS

 

Calliope Dark Red        Designer Red

Who has not been impressed by the bright red color of the common, bedding geranium

(Pelargonium x hortorum)? The red geraniums continue as a favorite bedding plant in the

US and many other parts of the world. Several plant breeders have developed their own

cultivars to capture their share of the available market.

 

Plant fanciers over the decades/centuries have been intrigued by the

appearance of new and novel colors in the flowers of their favorite plants.

Early on, these changes occurred by chance and were coveted, saved and

propagated by gardeners. As the science of plant breeding developed,

these variants became the basis for hybrids and the development of additional

colors, patterns and combinations. Even Gregor Mendel, father of modern

genetics, in the 19th century, carried out classical hybridizing experiments between colored and white

flowered pea plants.

 

As our chemical expertise and sophistication have developed, the basis for color differences

has been analyzed on the chemical level. During the 20th century the fields of genetics and

cellular chemistry have come together so that the basis for color development in terms of

chemical pathways has made great strides. The steps in chemical pathways/synthesis within

cells/organisms have been found to be under genetic control, so the two fields have developed

together and helped us understand both plants and animals in great detail.

 

One has only to look at the various proprietary lines of Pelargonium to see the basic

variations in each subgroup of these popular plants. For example, Ball has its Designer

zonal line with 15 different cultivars; Oglevee (now owned by Ecke) has its

Patriot zonal line with 12 cultivars; Goldsmith (now part of Syngenta) has its

Americana zonal line with 15 different types; in Europe, Dummen has it Pacific ivy

line with 17 cultivars, PAC has its Aristo regal line with 13 cultivars and Fischer

(now part of Syngenta) has its Contessa ivy line with 13 cultivars.

 

In flowers in general, color has been found to be due to three

groups of natural pigments: flavonoids, carotenoids and betalains. While each group is

different chemically, they may produce the same color, or they may be present in combinations

to produce distinctive colors.

 

Flavonoids are the most common and most important of the floral pigments. They are

the best understood and have proven useful for the study of gene expression and gene

regulation in the cells of flowers.

                                  

                        Parade of Pigments   (See molecular structure)

Flavonoids are water soluble pigments found in the vacuoles of cells. The vacuole is

the central part of a cell, usually containing primarily water with substances dissolved in

it. More than 3,000 forms of flavonoids are known. They all have the same basic structure

of two aromatic/carbon rings and a heterocyclic ring containing oxygen. Within the

plant, these pigments provide protection against the effects of UV wavelengths of light,

provide defense against plant pathogens e.g., inhibiting the fungus Fusarium, attracting

the bacterium Rhizobium, which aids in nitrogen fixation in the root nodules of legumes,

and produce plant and flower colors (important in attracting animal pollinators). Flavonoids

are responsible for most orange, scarlet, crimson, magenta, violet, purple and

blue flower colors. This is one of the chemical groups commonly known as “antioxidants.”

 

Carotenoids, as well as the green photosynthetic pigment chlorophyll, are lipid soluble

pigments in the plastids (small structures within the plant cells). Over 400 carotenoids

are known, but only a few are common in flowers. β-carotene and violaxanthin are examples.

Carotenoids are long chains of carbon and hydrogen units joined head to tail.

There are two types known: lycopenes, which are composed solely of carbon/hydrogen

units and xanthophylls, which contain oxygen attached to the carbon/hydrogen units.

Carotenoids contribute to most of the yellow hues in flowers like roses, tulips, daffodils,

gerberas, lilies and pansies. In combination with anthocyanins (reddish flavonoids) they

produce colors like orange-red, bronze and brown.

 

Betalains are water soluble nitrogenous pigments. They are restricted to the flowering

plant families formerly referred to as the Centrospermae and now expanded and called

the Caryophyllales. The families included are the Aizoaceae, Amaranthaceae, Cactaceae,

Caryophyllaceae, Droseraceae, Nyctaginaceae, Phytolaccaceae, Plumbaginaceae,

Polygonaceae and Portulacaceae among others (see p. 4). There are two types of betalains:

beta-xanthins which are yellow and orange in color and the betacyanins which are

red and violet. They are never found in combination with anthocyanins (the flavonoid

pigments). They are produced by very different chemical pathways. Betalains are

named after the common beet (whose Latin, scientific name is Beta vulgaris). These

pigments are also considered “antioxidants” and may have some fungicidal properties..

[Examples of commonly grown plants in the plant families of the Centrospermae or Caryophyllales.

Aizoaceae—New Zealand spinach, ice plant;  Amaranthaceae—pigweed, beets, spinach, cockscomb;

Carophyllaceae—pinks, carnations, chickweed, babys breath; Nyctaginaceae—four o’clock, bougainvillea;

Phytolaccaceae—pokeweed; Plumbaginaceae— leadwort, statice, armeria;

Polygonaceae—knotweed, buckwheat, rhubarb, sorrel; Portulacaceae—purslane, spring beauty, moss rose.]

 

Studies of Pelargonium cultivars, show the presence of six different flavonoid pigments.

These are: pelargonidin, cyanidin, peonidin, delphinidin, malvidin and petunidin. Pelargonidin

is responsible for orange, salmon, pink and red colors in the flowers; cyanidin

produces magenta and crimson flowers; delphinidin results in the color purple, mauve

and blue. Through chemical changes cyanidin is converted to peonidin while delphinidin

can be converted to malvidin and petunidin.

 

On the following table, you can see that eight of the ten cultivars contain all six pigflavonoid

ments, however the proportions of each pigment varies from one cultivar to another; and

the predominant pigment varies from cultivar to cultivar. Undoubtedly, this leads to the

great variation in shades of red flowers from one cultivar to the next.

Relative percentage of different anthocyanidins in 10 cultivars of Pelargonium

Cultivar name                                        Pelargonidin     Cyanidin     Peonidin     Delphinidin     Malvidin        Petunidin

Diablo - zonal                         7 2.7       1.6         15.4         0.3         10.0             0

Tango Dark Red - zonal        70.0        5.0         16.4         1.2           6.6             0.9

Rocky Mt. Dark Red - zonal  68.2        2.4         19.6         0.7           8.6             0.5

Tango ‘09 - zonal                   77.0        1.6         18.5          0             2.9              0

Designer Dark Red - zonal   53.5        2.2         24.2         0.7        18.7              0.8

Samba - zonal                        67.2        2.6         21.7         0.7          7.3              0.5

Calliope Dark Red - inter*    13.9        20.7       56.1         3.4          4.3              1.0

Freestyle Dark Red - ivy        25.7       16.0       51.9         2.1          3.3              1.1

Americana Dark Red - zonal 57.6       10.1      19.1         1.9          9.9              1.3

Eclipse Velvet Red - inter*     18.6       10.5      50.2         6.5        12.6              1.8

(* inter=interspecific hybrid) (Table from Syngenta patent application 12/876291)

White, cream and ivory flowers result when the cells contain colorless “pigments”.

These result when the pathways to colored anthocyanin are blocked in one or several

places. Or there are yellow flavonoids called chalcones and aurones which produce a

yellow color. These pigments give yellow color to snapdragons, dahlias, pinks and

Aeschynanthus.

 

In addition, more colors can be produced by interactions between the pigments and/or

with other compounds in the cells. As shown in the above chart, the co-occurrence of

different pigments produces different shades of color. Orange may be produced by the

presence of reddish anthocyanins on a yellow flavonoid background (found in snapdragons

and dahlias) or by the presence of anthocyanins and yellow carotenoids (found in

freesia, gerbera, roses and zinnia). Co-pigmentation is the presence of different anthocyanins

with other compounds (the bluish rose contains cyanidin and gallotannin). In

some plants, metal ions form a complex with anthocyanins. In blue bachelor buttons

cyanindin is complexed with iron. In pink flowered hydrangeas (the color is in the sepals)

when delphinidin is combined with aluminum, the color changes from pink to

blue. The weedy day flower (Commelina) produces a blue pigment called commelinin

which is a complex between delphinidin, magnesium and a flavone.

 

In yet other plants, the pH (the acidity or alkalinity) of the vacuole changes the color of

the pigment. A lower pH results in reddening while a higher pH results in a bluing of

the color. The wonderful blue of ‘Heavenly Blue’ morning glories is an example of this.

In a future newsletter, I will cover the genetic bases for the determination of flower color.

 

“Flowers don’t speak in words. They use colors. So, listen and enjoy.”

                                                                                             - perhaps, George Sand