Plant Secondary Metabolites • Plants make a variety of less widely distributed compounds such as morphine, caffeine, nicotine, menthol, and rubber. These compounds are the products of secondary metabolism, which is the metabolism of chemicals that occurs irregularly or rarely among plants, and that have no known general metabolic role in plants. • Secondary metabolites or secondary compounds are compounds that are not required for normal growth and development, and are not made through metabolic pathways common to all plants. • Most plants have not been examined for secondary compounds and new compounds are discovered almost daily.
Plant Secondary Metabolites • Secondary compounds are grouped into classes based on similar structures, biosynthetic pathways, or the kinds of plants that make them. The largest such classes are the alkaloids, terpenoids, and phenolics. • Secondary compounds often occur in combination with one or more sugars. These combination molecules are known as glycosides. Usually the sugar is a glucose, galactose or rhamnose. But some plants have unique sugars. Apiose sugar is unique to parsley and its close relatives.
Functions of Secondary Compounds • The most common roles for secondary compounds in plants are ecological roles that govern interactions between plants and other organisms. • Many secondary compounds are brightly colored pigments like anthocyanin that color flowers red and blue. These attract pollinators and fruit and seed dispersers. • Nicotine and other toxic compounds may protect the plant from herbivores and microbes. • Other secondary compounds like rubber and tetrahydrocannabinil (THC) from cannabis plants have no known function in plants.
Alkaloids • Alkaloids generally include alkaline substances that have nitrogen as part of a ring structure. More than 6500 alkaloids are known and are the largest class of secondary compounds. They are very common in certain plant families, especially: • Fabaceae – peas and beans • Asteraceae - sunflowers • Papaveraceae - poppies • Solanaceae – nightshade, tomato • Apocynaceae - dogbanes • Asclepiadaceae - milkweeds • Rutaceae - citrus
Terpenoids • Terpenoids are dimers and polymers of 5 carbon precursors called isoprene units (C5 H8). • Terpenoids often evaporate from plants and contribute to the haze we see on hot sunny days. They are expensive to make; they often take 2% of the carbon fixed in photosynthesis; carbon that could otherwise be used for sugars.
Phenolics • Compounds that contain a fully unsaturated six carbon ring linked to an oxygen are called phenolics. • Salicylic acid (basic part of aspirin) is a simple phenol. • Myristicin is a more complex phenol that provides the flavor of nutmeg. • Flavonoids are complex phenolics. They are often sold in health food stores as supplements to vitamin C. The most commonly available flavonoid is rutin from buckwheat. • Anthocyanins are a type of flavonoid that give flowers red and blue pigments.
More Phenolics • Some phenolics form polymers. • Tannins are astringent to the taste. They give dryness (astringency) to dry wines. They can also be used to tan leather. They often give water a tea-colored look. Tannins are common in pines and oaks. • Lignin is a major structural component of wood. The exact structure of lignin is complex and not known.
Minor Secondary Metabolites • Mustard oil glycosides are nitrogen-sulfur containing compounds that occur in cabbage, broccoli, horseradish, watercress and other members of the mustard family (Brassicaceae). They give the group its characteristic taste and odor. • Cyanogenic glycosides occur in several families of plants, but are especially common in roses (Rosaceae) and peas (Fabaceae). They are sugar containing compounds that release cyanide gas when hydrolyzed. • Cardiac glycosides effect vertebrate heart rate. Especially common in milkweeds Asclepiadaceae. • The parsley/carrot family Apiaceae is noted for having aromatic and poisonous 17 carbon polyacetylenes, though a few species have alkaloids like Coniium.
Cardiac Glycosides Common Milkweed Purple Foxglove
Apiaceae - Polyacetylenes Water Hemlock
Ethnobotany Old and New
What is Ethnobotany? • Ethnobotany is the study of plants used by primitive and aboriginal people. – John W. Harshberger 1895
What is Ethnobotany? • A better definition is: Ethnobotany is the study of the interactions of plants and people, including the influence of plants on human culture. Oaxaca, Mexico
Assyrian Bas-relief Of gods Pollinating Date-palms
Aristotle 384-322 BCE
Theophrastus 370-285 BCE
Cover of Gerard’s Herbal – 1597