BSCI 124 Lecture Notes

Undergraduate Program in Plant Biology, University of Maryland


I. Importance

  1. Global importance:
    1. Grasses are the ecologically dominant plants in the temperate zone areas with rainfall between 10 and 30 inches/year.
    2. Grasses are found in almost all land areas, plus shallow aquatic habitats. Grasses equal 25% of the world's vegetation.
  2. Human importance:
    1. Grasses include the grains, the single most important food group; major supplier of calories for human nutrition, also for nutrition of animals for human food.
    2. Human agriculture produces roughly 600 million tons/yr each of the three most widely-used grains: wheat, rice, maize (=corn), equals half of all calories in human nutrition.
    3. Advantage of grain as food: Dry seeds can be stored for years; are a concentrated food source.
    4. Grasses are used extensively as fodder for livestock.

II. Characteristics of grasses

  1. Systematics: Grasses belong to family Poaceae (alternative name Graminae), has 857 genera, 10,500-11,000 species according to Wielgorskaya (1995). See the GRIN Database produced by USDA for details. Grasses are monocots (Magnoliophyta, Liliopsida).
  2. Vegetative features.
    1. Leaves are linear, strap-shaped, have parallel veins
    2. Stems may vary:
      1. Upright stem is a culm, supports the leaves and inflorescence; usually surrounded by leaf sheaths.
      2. Tillers = secondary stems growing from the base of the main stem; very important for grain production.
      3. Horizontal stems may spread the grass: thin green stolons or runners at the soil surface, or thicker underground rhizomes. New shoots can grow from nodes on stolons or rhizomes.
    3. Root system is fibrous, many small roots growing from the crown; the tap root dies in the first few days after germination of the seed.
  3. Inflorescences and flowers
    1. Inflorescence = structure containing all the flowers; very different in grasses. Agriculturally, the mature inflorescence with seeds is referred to as a seed head.
    2. Flowers may be perfect (= bisexual), as in wheat or rice, or imperfect (= unisexual), as in maize.
    3. Flowers are adapted to wind pollination: small, inconspicuous, many. The anthers of the three stamens are large (compared to the whole flower), produce much pollen. The two stigmas are large and feathery, easily catch pollen.
  4. The caryopsis or grain:
    1. The fruit is a caryopsis or grain, has one seed which makes up most of the fruit; only several thin layers of fruit wall remain.
    2. The grain normally dries until it is only 5-15% water; most plant tissue is 75-90% water. The dry grain can easily withstand dry conditions over a hot desert summer, or cold winter.
    3. The grain has several parts [see the illustration] that have specific biological functions:
      1. Endosperm is the bulk of the grain. It is composed of large cells, mostly inactive, filled with starch grains. This is the part that supplies sugar to the germinating seed for energy and for synthesis of all cell components.
      2. The embryo is the young plant of the new generation, the product of the developing zygote. This is commercially known as the germ, and is rich in proteins, vitamins, and oil.
      3. The bran consists of the outer layers of old fruit wall, seed coat, and aleurone layer. Most layers are fibrous and contain little of nutritious value. However, the aleurone layer is rich in proteins and vitamins; this layer produces and secretes the enzymes that digest the stored starch and other stored foods of the endosperm.
    4. Milling of grain separates the parts. In whole-grain products, only the chaff has been removed; the product is brown from the presence of the fibrous bran. Whole-grain products are more nutritious mostly because of the presence of the germ or embryo. Advantage of white flour or rice: less insect attack, less rancidity (oxidation of oils) in storage.

III. Wheat: Triticum spp.: most widely cultivated grain; top producers US, Ukraine, China. Adapted to cool dry climate, 12-36 in. rain per year.

  1. Origin and evolution: Wild wheats in Near East. Original diploid wheat = einkorn (Triticum monococcum), hybridized with goat grass (Aegilops sp.) to make emmer (T. turgidum), with doubled chromosomes (tetraploid). Durum wheats descended from emmer. Emmer hybridized with another goat grass to produce hexaploid bread wheat (T. aestivum).
  2. Modern wheats and their uses: Six classes of wheat
    1. Durum or spaghetti wheats, grown in North Dakota, Canada, southern Europe, India. Used for pasta, noodles.
    2. Bread wheats: high gluten content causes the wet dough to be elastic and sticky. It will trap bubbles of gas. A leavening agent (baking soda or yeast + sugar) is added to provide CO2 bubbles, which are trapped, and the dough rises. Bread wheat flour is the only flour that can make a dough that rises; the dough of flat breads made with other grains does not rise. Bread wheats are grown in United States, Russia, Ukraine, China, Australia, Canada.
    3. Wheat growing regions in the United States; US production third, globally, behind China and India, and ahead of Russia, France and Canada; see the statistics!
  3. Wheat is a good source of carbohydrate and fiber and a variety of flours

IV. Maize or corn: Zea mays. Maize is preferred name, because corn refers to other grains in Europe.

  1. Types of modern corn and their uses
    1. Popcorn, oldest. Has hard seed coat, high-moisture cells internally. Heating produces steam, explodes, turns endosperm inside out. Widely used by Indians because it didn't have to be ground.
    2. Flour corn, soft endosperm, easy to grind, but attacked by insects.
    3. Dent corn, has soft starch in center, hard on outside; soft starch shrinks more on drying, leaves dent in kernel. All modern field corns are dent corns. Major uses are for animal feed, corn meal, cornstarch, high fructose corn syrup, ethanol.
    4. Sweet corn, endosperm contains sugar instead of starch, harvested and eaten immature.
  2. Hybrid corn: Seed from cross between two inbred (self-pollinated) lines, each with specific characteristics, but not very productive. Hybrid has hybrid vigor, more productive. Seed must be purchased every year; because seed from the hybrid is F2, segregates for many characters. This type of corn is widely planted and used throughout the world
  3. The uses of corn are many and varied. Processing of corn is complex because of its multiple uses.
  4. Kernel colors: pigments may be present in the pericarp (fruit wall), aleurone layer, or endosperm
  5. Origin and evolution: Central Mexico. Selected from, and evolved from, wild grass teosinte. In native language, teosinte means mother of corn. Widely planted by numerous groups of Native Americans [REQUIRED READING]. Recent report in Science (28 Jun 1996) is that "change from inedible to edible could come from mutations in just one small stretch of teosinte DNA" supporting the notion that "large evolutionary leaps" can results from "minor genetic changes".
    1. Characteristics of both:
      1. tassel (inflorescence of male flowers) at end of stem.
      2. ear (inflorescence of female flowers) on lateral branch. Silk protruding from ear is mass of individual stigmas, each attached to an ovule that will become a seed.
    2. Characteristics of teosinte:
      1. ear has 6-10 triangular seeds, each surrounded by hard case called cupule.
      2. Several small ears, several tassels at ends of main and side branches.
      3. Spike shatters easily, scatters seed.
    3. Characteristics of maize:
      1. tassel on main stem only.
      2. one to three ears at ends of short lateral branches.
      3. many seeds on ear, in 14 or 16 rows in modern corn; cupule reduced to red glumes in modern corn, stays on cob.
      4. Nonshattering seed; no dispersal mechanism, modern corn can't survive in wild.

V. Rice: Oryza sativa.

  1. Importance: Rice feeds more people than any other grain; especially Asia. Rice is the only grain grown entirely for human food, not animal feed.
  2. Origin and evolution; see also this view and this: Rice began as a marsh grass in southeast Asia; evidence of cultivation 7000 years ago. Majority of cultivated rices are paddy rice, transplanted to flooded fields; upland rice grown on dry land.
  3. Vegetative and cultural characteristics
    1. Extensive air chambers in stem and roots, allows aeration of submerged parts.
    2. Water fern (Azolla spp.) grows in paddies with rice. Fern has pockets in leaves, filled with cyanobacterium Anabaena which fixes nitrogen from the air, makes it available to fern. When fern dies and decays, nitrogen is available to rice; natural fertilizer.
    3. Grain cultivation, harvesting and milling
      1. Paddies are drained for harvesting, either by hand or combine.
      2. Brown rice is dehulled whole grain, has more vitamins and oil than polished white rice. Diet based on polished rice may result in beriberi, = thiamine deficiency.
      3. Due to hybridization and natural selection, there are now several kinds of weedy rice which are a threat to cultivate rice
  4. Extensively cultivated and globally consumed, including the United States. Made into sake (rice beer) or awamori (distilled liquor)

VI. Other grains

  1. Barley: Hordeum vulgare, brewing, animal feed. Fourth most important cultivated grass.
  2. Rye: Secale cereale, important in eastern Europe.
  3. Oats: Avena sativa, mostly for horse feed.
  4. Sorghum is the fifth most important cultivated grass. Grain sorghum. Sweet sorgham or sorgo, and millets: several genera and species, origin Africa and India, used for human and animal food.
  5. Sugar cane: Saccharum officinarum. A major tropical crop and principal source of sugar
  6. Forage grasses: numerous genera and species. Leaves digestible only by ruminant animals. Examples include Kentucky bluegrass (Poa pratensis), foxtail millet (Setaria italica) which is also found in birdseed, various of the wild-ryes such as Elymus glaucus, and crested wheatgrass (Agropyrum desertorum), now widely planted in the West
  7. Wild Rice (Zizania aquatica), a native American grain of increasing importance, not closely related to plain rice (Oryza sativa).

VI.  Alcoholic beverages from grains

      A.  Beer and ale:  beverages made by fermentation of sugars from malted barley.  Beers and ales contain 0% to 12% (commonly 5%) ethanol or grain alcohol, plus aromatic flavor compounds from grain and hops, the rest is water.  Beers and ales are made in 4 stages:  Tour the Tyne Brewery site for a more complete explanation, photos, and diagrams.

            1.   Malting = germinationof barley, until just the new root has emerged from the seed, about 4 days.  Malt = germinated barley.  Malted barley is then dried and ground.  Malting produces amylases, enzymes that digest starch to sugar.

            2.  Mashing = incubation of malt in warm water for 2-6 hours, during which time the starch in the grain is digested to glucose.  Additional grains (adjuncts), such as rice or corn, may be added during brewing.  At the end of the incubation, the brew is filtered; this liquid is wort.  Wort is immediatley boiled with hops (female flowers of hop plant, Humulus lupulus) for flavor, and sent to fermenting tanks.

            3.  Fermentation = growth of yeast on the wort in the absence of air.  The yeast may be a top-fermenting yeast (Saccharomyces cerevisiae), which floats, and produces ale; or a bottom-fermenting yeast (Saccharomyces uvarum), which sinks to the bottom, and produces beer. Fermentation produces ethanol and carbon dioxiide.  Fermentation takes 7-10 days. Product is green beer.

            4.  Ales are aged for  2-3 weeks, lagers (which include most beers) for 2-3 months, during which time flavor develops, and dead yeast and undigested proteins settle out.  Beers and ales may then be pasteurized or filtered for sterility and increased shelf life; so-called "real ales" and beers are unsterilized, have superior flavor but short life. Beers and ales are then bottle, canned, or kegged.

     B.   Whiskey = an alcoholic beverage made by distilling a grain beer.   Scotch and Irish whiskies are made from barley beers, not the same as commercial beers.  Bourbon, or corn whiskey, is distilled from fermented corn mash (corn plus water), invented in colonial North America.  By distilling, the beer is boiled to a vapor, and the vapors that boil off at the lowest temperatures are condensed; this concentrates the alcohol and flavor and aroma compounds, leaves much of the water to boil at higher temperature.


Review these notes for section of grasses
A reminder about scientific names
Maize Genetic Database: Technical
All about rice
Rice Genetics Project

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Last revised: Aug 1998 - Barnett