Rose Science Gary A. Ritchie Master Rosarian

Transcription

1 Rose Science Gary A. Ritchie Master Rosarian Flowers: Part 4 Color Plants produce color through an astounding array of biochemical processes that animals do not possess. Animals have evolved a complex of what are known as primary metabolic pathways that lead to normal metabolic reactions such as respiration. Plants, in contrast, have gone way beyond this, having developed innumerable secondary metabolic pathways that lead to a dazzling assortment of chemical compounds. Many of these, such as the alkaloids, are highly toxic and protect plants from herbivorous insects and animals. While others include the various pigments that give flowers their color. So first, let s mention a few things about pigments. There are essentially two types of pigments. The first type is made up of countless tiny, solid colored particles. This is what you would find if you were to examine a commercial paint microscopically. In the second type of pigment the molecules, rather than being solid, are dissolved in solution. We call these dyes these are what impart to plants and flowers their colossal pallet of colors. Dyes produce color using what is known as a subtractive process. This means that they reflect a portion of the light that falls upon them while absorbing or transmitting the rest. What we see is the light that is reflected. All pigments absorb color based on the nature of the chemical bonds that hold their atoms together. Each of these different types of bonds absorbs light in very specific wave lengths 1/. For example, all pigments contain carbon atoms. Many of these are connected by what are known as double carbon bonds, diagrammed as C=C. This bond (represented by =) absorbs light with a wave length of exactly 180 nm (nanometers), which is ultraviolet light. Other bonds absorb light of other wavelengths, the net effect being that every pigment and every variation on every pigment absorbs slightly differently, thus imparting to flowers their near infinite pallet of colors. 1

2 Plant pigments are grouped into four main categories depending on the biosynthetic pathway (a series of biochemical reactions) that leads to their formation. I don t intent to try to describe these pathways here; each one would require several pages. But I will list the main pigment types and talk a little bit about each one. First are the porphyrins. While not very important in flower color, porphyrins are critical in many other ways. For example, chlorophyll and hemoglobin are porphyrins. As we have learned, chlorophyll is an essential light harvesting pigment in photosynthesis. It is the most abundant and one of the oldest pigments on earth, having evolved over 3 billion years ago. It absorbs largely in the red and blue regions of the spectrum, hence it reflects green to give leaves their green color. Second are the isoprenes. This large group of pigments imparts the yellow-orange colors to plants. It absorbs greens and blues and transmits colors of longer wavelengths ( nm). In flower petals, these pigments are often located in specialized structures called chromoplasts. The pigment lycopene, a type of isoprene, gives tomatoes their redorange color and derives its name from Lycopersicon, the botanical name for tomato. Isoprenes also assist in light capture in photosynthesis and help protect plants against photodamage (injury from excessively high light intensity). Next are the flavonoids. They comprise a bewildering array of chemicals among which are some of the most important and abundant pigments contributing to flower color. They are all built upon a molecule called phenol, which has six carbon atoms arranged in a simple ring structure (Figure 1). Most of the reds pinks and purples we see in our roses are the result of various flavonoids. A large group of flavonoids called anthocyanins are the most important color-producing pigments in plants. There are at least thirty different types of anthocyanin pigments. They are responsible for many of the various colors seen in flowers and senescing fruits along with the beautiful autumn colors we enjoy. Two groups of flavonoids called chalcones and aurones produce yellow and orange. Another group of flavonoids are the quinones, which have an important role in photosynthesis, but contribute little if anything to flower color. 2

3 The betalains are a group of pigments that create color in Bougainvilleas, Portulaca and certain cacti. They are apparently not involved in rose flower colors, however. Finally, I am often asked whether we will ever see a blue rose. The rose family doesn t contain any blue pigments, but breeders have tried for years to produce a rose that looks blue. So we have Wild Blue Yonder, Blueberry Hill, Blue Moon and other roses that look sort of blue - but are actually mauve. I heard an interesting story many years ago about a genetic engineering firm in Australia that had successfully introduced the gene that codes for delphinidin into rose plants. Delphinidin is an anthocyanin that imparts the true blue color to delphiniums and violets. The problem was that it caused the entire plant to turn blue not just the blossoms. Next challenge figure out how to express the gene in flowers only! Science is fun / In an earlier Clippings article (September, 2009) we noted that colors are defined by their wave length in nanometers (a nanometer is a billionth of a meter). The color blue has a wave length of roughly nm, green is nm, yellow, orange and red are from nm. The human eye can perceive wavelengths from about 400 to 700 nm. 3

4 Figure 1. All flavonoids are built upon a phenol molecule shown here. Phenol has a simple 6-carbon ring structure with a hydroxyl group (-OH) bonded to one of the carbons. 4

5 Figure 2. The reds and pinks in this hybrid tea rose Maggie Barry are produced by flavonoid pigments called anthocyanins. 5

6 Figure 3. This wonderful floribunda, Blueberry Hill, may look blue but is actually mauve. The Rosaceae family is not capable of producing true blue pigments. 6