BSCI 124 Lecture Notes

Undergraduate Program in Plant Biology, University of Maryland


  1. Legume family (Fabaceae or alternatively Leguminosae). Legume is synonymous with pod, the fruit; the seeds are inside the pod or legume. Examples: peas, beans, soybeans, clover, alfalfa, peanuts.

    A. Importance of legumes

    1. Major plant sources of protein, oil.
    2. Major nitrogen fixers (with symbiotic bacteria in roots), adds N from air to plants.
      The Nitrogen Cycle:
      1. Nitrogen fixation- conversion of atmospheric nitrogen (N2) to ammonia (NH3), which reacts with water to form ammonium (NH4+) . N2 has strong bonds (N=N)
        1. Performed biologically by nitrogen-fixing bacteria and cyanobacteria.  Nitrogen-fixing bacteria may by free-living in the soil, or may live symbiotically with plants.  Legumes are the only important agricultural families that have synbiotic N-fixing bacteria.
        2. Some nitrogen is also fixed during thunder storms, volcanic explosions, etc
        3. Industrial (fertilizers) - takes a lot of heat and pressure
      2. Nitrification- conversion of ammonium to nitrate (NO3-)
        1. Performed by several species of nitrifying bacteria that live in the soil: NH4+ --> NO3- (nitrate)
      3. Assimilation- the intake of either ammonium or nitrate by plants and its conversion to protein and other N-containing compounds
        1. When animals eat plants and convert plant proteins to animal proteins, they are also assimilating
      4. Ammonification- the conversion of organic nitrogen (in animal wastes and in dead organic matter) to ammonium (NH4+)
        1. Performed by ammonifying bacteria in the soil
      5. Denitrification- the conversion of nitrate to nitrogen gas
        1. Performed by denitrifying bacteria in the soil: NO3- --> N2

    B. Vegetative characteristics: Legume plants are mostly herbs, usually compound leaves, mostly annuals but some perennials.

    C. Flower, fruit, and seed characteristics:

    1. Flowers are irregular, bilaterally symmetrical, usually 10 stamens, one pistil of two carpels. Pollination is by bees; stamens may be cocked during growth, triggered by bee, hits bee with pollen.
    2. Fruit is a pod or legume, a long fruit with two rows of seeds, easily splits on two seams, two rows of seeds; example, peas in a pod.
    3. Seeds are large, have two large cotyledons (seed leaves) that make up most of the seed, filled with stored food. No endosperm at maturity.

    D. Nitrogen fixation was discovered in legumes, due to symbiotic association of Rhizobium species of bacteria that inhabit nodules on roots. Bacteria take N2 from air, make it into ammonia for their use, also secreted to plant; plant supplies carbohydrate to bacteria for energy and synthesis of cellular material. Today, strains of Rhizobium are sold to enhance agricultural productivity and is the subject of considerable research. In fact, some cultivars have been developed with the bacterium as a requirement.

    E. Examples of food legumes:

    1. Beans (Phaseolus vulgare): green bean, plus many other genera and species. Common beans are native to Mexico and the Andes.
      1. Rich in protein, also some carbohydrates
      2. Embarrassing problem: Intestinal gas, caused by bacterial breakdown of indigestible (by humans) complex carbohydrates in colon. Alleviate three ways: long cooking time, treatment of cooked beans with enzymes ( "Beano" is commercial trade name), plant breeding to eliminate complex carbohydrates. Beans are not the only food that causes flatulence.
      3. Examples of Phaseolus vulgare are red kidney bean: black bean; kidney bean, there is considerable production in the United States; pinto beans; navy beans, and green beans
      4. Example of Phaseolus lunatus, the lima bean; some production
      5. Fava bean or broad bean (Vicia faba): popular in Mediterranean. Some people have genetic defect in an enzyme, suffer from favism = hemolytic anemia, when they eat fava beans.
      6. Many different kinds of beans eaten as cooked seeds; or immature pods also eaten as green beans , since 1700s.
        1. For example the mung bean (Phaseolus aureus) which is grown for bean sprouts, and
        2. adanka bean or adzuki bean, which is cultivated in Asia
        3. field bean and
        4. black-eye pea
      7. Beans are widely cultivated in the United States with harvesting depending upon the bean.

    2. Peas (Pisum sativum): garden pea; plus other genera and species. Garden pea is native to Near East.

    1. Rich in protein, carbohydrate.
    2. Immature pod also eaten, as snow peas or sugar snap peas.
    3. Peas were subject of Mendel's investigations of genetics.
    4. Peas grown in the United States as a field pea used for livestock, a garden pea, and as an edible-podded pea.

    3. Peanut (Arachis hypogea): Native of South America; introduced to Europe, from there to Africa, from Africa to U.S. with slave trade.

    1. Rich in oil and protein
    2. Unusual growth characteristic: After fertilization, the flower stalk dips and grows into the ground, where the pod matures. (Peanuts are called ground nuts in many parts of the world.)
    3. Uses: Half of U.S. crop for peanut butter; rest for snack food, candy, peanut oil. Oilcake remaining after pressing oil is rich in protein, used for animal feed.

    4. Soybeans (Glycine max): Native of China; introduced into Georgia in 1765. Now the most valuable crop in the United States, grown in Midwest and South.

    1. Oil used for cooking oil, salad dressing, margarine, shortening, mayonnaise.
    2. Oil cake is rich in protein, used for animal feed, also to make textured vegetable protein (TVP), used as meat substitute for humans, can be spun or shaped in many ways, flavored to taste like any meat.
    3. Traditional uses in the Orient:
      1. Soy sauce = fermented soybeans and grain in brine
      2. Tofu = soy milk curds
      3. Miso = fermented soybean and rice paste in Japan
      4. Tempeh = fermented soy cake in Indonesia.
      5. Soybeans also eaten as sprouts.
      6. Widely used as a health food
    4. Industrial uses: Oil can be used as diesel fuel, or made into plastic, paint, ink, soap.
    5. A major crop in the United States with production increasing rapidly, with greater yields.

    5. Forage legumes: several can be dried and made into hay, usually with grass for nutritional balance. Examples include:

    1. Medicago sativa, alfalfa Some 20 million acres of alfalfa are planted currently in the United States.
    2. Trifolium spp, true clovers; many species. Red clover and white clover are found widely introduced, and in many instances naturalized, in pastures throughout the United States.
    3. Vicia spp. Vetches are now planted for hay and erosion control, but often are weedy.

    6. Kudzu (Pueraria lobata), an important albeit minor economic plant (as a starchy root) in China, is now a major weed in the southeastern United States

  2. Starchy staples
    1. Starch: most widespread storage form of energy in the plant kingdom
      1. Starch is a linear polymer (chain) of glucose molecules.
      2. Most common starchy crops have a tropical origin.
      3. Many plants have specialized organs for storing starch.
    2. Modified storage stems [Consult this site for critical information]:
      1. Rhizome = thickened underground horizontal stem, may spread the plant, new shoots grow from nodes. Examples: ginger, iris.
      2. Tuber = thickened and rounded organ on a rhizome. Example: potato.
      3. Bulb = short vertical stem with thick leaves. Example: onion, tulip, daffodil
      4. Corm = short vertical thick stem, surrounded by thin leaflike scales. Example: gladiolus, banana,
    3. Modified storage roots: definitions
      1. Storage tap root: thickened main root. Examples: carrot, turnip.
      2. Tuberous root: thickened portion of a lateral root. Example: sweet potato, cassava.

  3. Important starch crops (besides grains)
    1. Potato (Solanum tuberosum) in the potato family (Solanaceae), a tuber:
      1. Origin: Andean highlands; domesticated 8000 years ago but consumed by humans some 13,000 mya. Staple crop of the Incas. Grown at high elevations, can be stored dry for months. Chuno = freeze-dried potato mush; potatoes trampled to squeeze out water, frozen outdoors; stores dry for years.
      2. Spread to Europe by Spanish conquerors of South America in about 1570; taken to England from Virginia by Thomas Hariot via Sir Walter Raleigh and given to Queen Elizabeth in 1590; Hariot and Raleigh were also involved in the introduction of tobacco. Thought to be poisonous because other European plants of same family are (nightshade, mandrake, henbane). First used for livestock feed. Spread from Spain to England and Ireland and American colonies.
      3. Potato in Ireland: Population grew to 8 million in 19th century, supported by potato production. Late blight destroyed crop in 1845-49, causing famine. 1 million died, 1.5 million emigrated; population never recovered.
      4. Top producers now Russia, China, Poland; note the extensive parts of the world where potato is now cultivated.
      5. Production and uses in the United States: Produced in cool climate states (Idaho, Washington, Maine). 1/3 used fresh, 1/2 processed into french fries, potato chips, instant mashed potatoes, potato starch.
      6. Propagation: vegetatively by seed potatoes, either small potatoes (Europe) or pieces of large potatoes, each piece has an eye (= bud).
      7. Nutrition: 25% carbohydrate, 2.5% protein. Although protein content is low, the amino acids in it are well balanced for human nutrition. Irish laborers are said to have eaten no meat, 10-15 lbs potatoes per day as the only source of carbohydrate and protein.
      8. A popular food today in numerous forms

      B. Sweet potato (Ipomea batatas) in the morning-glory family (Convolvulaceae): a tuberous root of a vine.

      1. Origin in Caribbean; introduced to Europe by Columbus.
      2. Rich in starch; orange cultivars also rich in Vitamin A and beta-carotene. Requires more tropical climate that white potato. China produces half of world crop; about 700,000 tons produced annually in the United States. Mistakenly called yams in U.S., especially if canned.

      C. Cassava (Manihot esculenta) in the spurge family (Euphorbiaceae) :a tuberous root of herbaceous plant.

      1. Origin Brazil and/or Mexico. Also called manioc, yuca (misspelled yucca at Giant Food).
      2. Greatest production tropical Africa, next Asia, next South America (see recent production figures). Roots can be harvested piecemeal or all at once; keeps in the ground for long periods. Variously treated by different peoples. On a global basis, cassava is regarded as the one plant between living and starvation; as a result, its annual production is closely monitored
      3. Processing to remove hydrogen cyanide (HCN) released when the root is ground: grinding or grating the roots, squeezing the mush in flexible baskets to remove juice, or simply prolonged boiling or drying of strips. Cassava is cooked, eaten as porridge, baked as flat bread, or processed into starch that can be stored dry (especially in Brazil), used for animal feed, source of industrial starch. Bitter and sweet forms depending on the amount of cyanide and various acids. Eaten in U.S. as tapioca pudding.
      4. Nutrition: Cassava is 30% starch, very little else nutritionally; poorest of starchy crops in protein, vitamins. International efforts are underway to produce improved cultivars
      5. Used for a variety of purposes as a gluey starch. There is an active research program to develop low hydrocyanic acid, high yield cultivars capable of surviving in the tropics.

      D. Other starchy crops, all tropical:

      1. Yam (Dioscorea spp.) in the yam family (Dioscoreaceae): a tuber, throughout tropics. Besides food, used as source of steroids used to make contraceptive pill and cortisone and other medicinal properties.
      2. Taro (Colocasia esculenta) in the arum family (Araceae): a corm. Origin southeast Asia. Used and grown in Hawaii to make poi.
      3. Plantain (Musa x paradisiaca, a hybrid cultivar) in the banana family (Musaceae) and other species: Origin in SE Asia, originally starchy; sweet banana selected from it. Most cultivars are sterile triploids (hybrids of diploid and tetraploid), must be propagated vegetatively. Plant is herbaceous, "stem" consists mostly of leaf sheaths.

Other Sites of Interest:
quick review: Good to do now and then
Common and scientific names of crop plants
Review of edible plant parts
The population timebomb: Note the role of plants
Beans: An epicurean review
Soy*Stats: A reference guide to soybean
Interpreting the Irish famine, 1846-1850
Learn more about the Inca people and culture

Dr. James A. Duke, formerly with USDA, but now associated with the Department of Plant Biology at the University of Maryland, has prepared a number of detailed treatments for economic plants (including folk medicine, ecology, cultivation, yield data, etc.). Among those considered here are:
Black-eye pea
Broad bean
Sweet potato

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Last revised: Aug 1998 - Barnett