Plant Pigment
Plant Pigment
A plant pigment is any type of colored substance produced by a plant. In general, any chemical compound which absorbs visible radiation between about 380 nm (violet) and 760 nm (ruby-red) is considered a pigment. There are many different plant pigments, and they are found in different classes of organic compounds. Plant pigments give color to leaves, flowers, and fruits and are also important in controlling photosynthesis, growth, and development.
Absorption of radiation
An absorption spectrum is a measure of the wavelengths of radiation that a pigment absorbs. The selective absorption of different wavelengths determines the color of a pigment. For example, the chlorophylls of higher plants absorb red and blue wavelengths, but not green wavelengths, and this gives leaves their characteristic green color.
The molecular structure of a pigment determines its absorption spectrum. When a pigment absorbs radiation, it is excited to a higher energy state. A pigment molecule absorbs some wavelengths and not others simply because its molecular structure restricts the energy states which it can enter.
Once a pigment has absorbed radiation and is excited to a higher energy state, the energy in the pigment has three possible fates: (a) it can be emitted as heat, (b) it can be emitted as radiation of lower energy (longer wavelength), or (c) it can engage in photochemical work, i.e., produce chemical changes. Flavonoids, carotenoids, and betalains are plant pigments which typically emit most of their absorbed light energy as heat. In contrast, chlorophyll, phytochrome, rhodopsin, and phycobilin are plant pigments which use much of their absorbed light energy to produce chemical changes within the plant.
Chlorophylls
The chlorophylls are used to drive photosynthesis and are the most important plant pigments. Chlorophylls occur in plants, algae, and photosynthetic bacteria. In plants and algae, they are located in the inner membranes of chloroplasts, organelles (membrane enclosed structures) within plant cells which perform photosynthesis. Photosynthesis uses the light energy absorbed by chlorophylls to synthesize carbohydrates. All organisms on earth depend upon photosynthesis for food, either directly or indirectly.
Chemists have identified more than 1,000 different, naturally occurring chlorophylls. All chlorophylls are classified as metallo-tetrapyrroles. A pyrrole is a molecule with four carbonatoms and one nitrogen atom arranged in a ring; a tetrapyrrole is simply four pyrroles joined together. In all chlorophylls, the four pyrrole rings are themselves joined into a ring. Thus, the chlorophyll molecule can be considered as a “ring of four pyrrole rings.” A metal ion, such as magnesium, is in the center of the tetrapyrrole ring and a long hydrocarbon chain, termed a phytol tail, is attached to one of the pyrroles. The phytol tail anchors the chlorophyll molecule to an inner membrane within the chloroplast.
The different types of chlorophylls absorb different wavelengths of light. Most plants use several photosynthetic pigments with different absorption spectra, allowing use of a greater portion of the solar spectrum for photosynthesis. Chlorophyll-a is present in higher plants, algae, cyanobacteria, and chloroxybacteria.
Higher plants and some groups of algae also have chlorophyll-b. Other algae have chlorophyll-c or chlorophyll-d. There are also numerous types of bacteriochlorophylls found in the photosynthetic bacteria.
Carotenoids
Carotenoids are yellow, orange, or red pigments synthesized by many plants, fungi, and bacteria. In plants, carotenoids can occur in roots, stems, leaves, flowers, and fruits. Within a plant cell, carotenoids are found in the membranes of plastids, organelles surrounded by characteristic double membranes. Chloroplasts are the most important type of plastid and they synthesize and store carotenoids as well as perform photosynthesis. Two of the best known carotenoids are Beta-carotene and lycopene. Beta-carotene gives carrots, sweet potatoes, and other vegetables their orange color. Lycopene gives tomatoes their red color. When a human eats carrots or other foods containing carotenoids, the liver splits the carotenoid molecule in half to create two molecules of vitamin-A, an essential micronutrient.
Chemists have identified about 500 different, naturally occurring carotenoids. Each consists of a long hydrocarbon chain with a 6-carbon ionone ring at each end. All carotenoids consist of 40 carbon atoms and are synthesized from eight 5-carbon isoprene subunits connected head-to-tail. There are two general classes of carotenoids: carotenes and xanthophylls. Carotenes consist only of carbon and hydrogen atoms; beta-carotene is the most common carotene. Xanthophylls have one or more oxygen atoms; lutein is one of the most common xanthophylls.
Carotenoids have two important functions in plants. First, they can contribute to photosynthesis. They do this by transferring some of the light energy they absorb to chlorophylls, which then use this energy to drive photosynthesis. Second, they can protect plants which are over-exposed to sunlight. They do this by harmlessly dissipating excess light energy which they absorb as heat. In the absence of carotenoids, this excess light energy could destroy proteins, membranes, and other molecules. Some plant physiologists believe that carotenoids may have an additional function as regulators of certain developmental responses in plants.
Flavonoids
Flavonoids are widely distributed plant pigments. They are water soluble and commonly occur in vacuoles, membrane-enclosed structures within cells which also store water and nutrients.
Interestingly, light absorption by other photoreceptive plant pigments, such as phytochrome and flavins, induces synthesis of flavonoids in many species. Anthocyanins are the most common class of flavonoids and they are commonly orange, red, or blue in color. Anthocyanins are present in flowers, fruits, and vegetables. Roses, wine, apples, and cherries owe their red color to anthocyanins. In the autumn, the leaves of many temperate zone trees, such as red maple (Acer rubrum), change color due to synthesis of anthocyanins and destruction of chlorophylls.
Chemists have identified more than 3,000 naturally occurring flavonoids. Flavonoids are placed into 12 different classes, the best known of which are the anthocyanins, flavonols, and flavones. All flavonoids have 15 carbon atoms and consist of two 6-carbon rings connected to one another by a carbon ring which contains an oxygen atom. Most naturally occurring flavonoids are bound to one or more sugar molecules. Small changes in a flavonoid’s structure can cause large changes in its color.
Flavonoids often occur in fruits, where they attract animals which eat the fruits and disperse the seeds. They also occur in flowers, where they attract insect pollinators. Many flavones and flavonols absorb radiation most strongly in the ultraviolet (UV) region and form special UV patterns on flowers which are visible to bees but not humans. Bees use these patterns, called nectar guides, to find the flower’s nectar which they consume in recompense for pollinating the flower. UV-absorbing flavones and flavonols are also present in the leaves of many species, where they protect plants by screening out harmful ultraviolet radiation from the sun.
Phytochrome
Phytochrome is a blue-green plant pigment which regulates plant development, including seed germination, stem growth, leaf expansion, pigment synthesis, and flowering. Phytochrome has been found in most of the organs of seed plants and free-sporing plants. It has also been found in green algae. Although phytochrome is an important plant pigment, it occurs in very low concentrations and is not visible unless chemically purified. In this respect, it is different from chlorophylls, carotenoids, and flavonoids.
Phytochrome is a protein attached to an open chain tetrapyrrole (four pyrrole rings). The phyto-chrome gene has been cloned and sequenced and many plants appear to have five or more different phytochrome genes. The phytochrome tetrapyrrole absorbs the visible radiation and gives phytochrome its characteristic blue-green color. Phytochrome exists in two inter-convertible forms. The red absorbing form (Pr) absorbs most strongly at about 665 nm and is blue in color. The far-red absorbing form (Pfr) absorbs most strongly at about 730 nm and is green in color. When Prabsorbs red light, the structure of the tetrapyrrole changes and Pfr is formed; when Pfr absorbs far-red light, the structure of the tetrapyrrole changes and Pr is formed. Natural sunlight is a mixture of many different wavelengths of light, so plants in nature typically have a mixture of Pr and Pfr within their cells which is constantly being converted back and forth.
There are three types of phytochrome reactions which control plant growth and development. The “very low fluence responses” require very little light, about one second of sunlight; the “low fluence responses” require an intermediate amount of light, about one sound of sunlight; and the “high irradiance responses” require prolonged irradiation, many minutes to many hours of sunlight.
The low fluence responses exhibit red/far-red reversibility and are the best characterized type of response. For example, in the seeds of many species, a brief flash of red light (which forms Pfr) promotes germination and a subsequent flash of far-red light (which forms Pr) inhibits germination. When seeds are given a series of red and far-red light flashes, the color of the final flash determines the response. If it is red, they germinate; if it is far-red, they remain dormant.
KEY TERMS
Chloroplast— Green organelle in higher plants and algae in which photosynthesis occurs.
Isoprene— Five-carbon molecule with the chemical formula CH2C(CH3)CHCH2.
Organelle— Membrane-enclosed structure within a cell which has specific functions.
Photosynthesis— Biological conversion of light energy into chemical energy.
Plastid— Organelle surrounded by a double membrane which may be specialized for photosynthesis (chloroplast), storage of pigments (chromoplast) or other functions.
Vacuole— Membrane-enclosed structure within cells which store pigments, water, nutrients, and wastes.
Additional Plant Pigments
Phycobilins are water soluble photosynthetic pigments. They are not present in higher plants, but do occur in red algae and the cyanobacteria, a group of photosynthetic bacteria.
Betalains are red or yellow pigments which are synthesized by plants in 10 different families. Interestingly, none of the species which have betalains also produce anthocyanins, even though these two pigments are unrelated.
Flavins are orange-yellow pigments often associated with proteins. Some flavins are specialized for control of phototropism and other developmental responses of plants. Like phytochrome, flavins occur in low concentrations and cannot be seen unless purified.
Rhodopsin is a pigment which controls light-regulated movements, such as phototaxis and photokinesis, in many species of algae. Interestingly, humans and many other animals also use rhodopsin for vision.
Resources
BOOKS
Corner, E J. The Life of Plants. Chicago: University of Chicago Press, 1981.
Heldt, H.W.Plant Biochemistry, 3rd ed. Burlington, MA: Academic Press, 2004. Plant Pigments And Their Manipulation, edited by Kevin M. Davies. Oxford, U.K.: Blackwell Publishing, 2005.
Peter A. Ensminger