Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

Legumes

I.  Introduction/General

• produced by plants in the Bean family (also called Fabaceae or Leguminosae)
• probably the second most important crop after the cereals
• every civilization with an important legume - usually combination of cereal and legume which are nutritionally complete (soybean/rice; bean/maize; barley/lentil)

II.  Characteristics of the Bean Family

A.  Leaves - compound (dissected)

B.  Fruit.
    The fruit type is called a legume.  These fruits are 1-several seeded and splits open along two sides to release the seeds.  Note that modern cultivars are selected to remain closed.

C.  Seeds.

D.  Flowers.
    The flower structure depends on the specific type.  One characteristic group of legume plants has "papillonaceous" flowers (butterfly shaped, Papilio - recall the film "Papillion"?).  These flowers have five petals - the uppermost petal is largest and called the banner (or standard), there are two side petals (wings) and two lower petals (keel petals) that are fused into a canoe-like structure (keel).  There are 10 stamens that are fused together.  The flowers are designed for bee pollination.

E.  Roots
    The roots are nodules that contain bacteria (Rhizobium) that can fix nitrogen.  

III.  Nitrogen Fixation
    Points to consider:

1. The main forms of nitrogen in the environment are:  N₂ (air is comprised of 80% nitrogen), ammonia (NH₄), nitrate (NO₃), & nitrite (NO₂).  The latter three are mostly dissolved in water.
   

2. Nitrogen is used in many molecules in plants and other organisms especially in proteins and nucleic acids (DNA)
   

3. Nitrogen fixation - converts nitrogen gas into ammonia.  This is the result of the action of:  (a) aquatic microbes like Anabaena and Nostoc, which are types of blue-green algae that are better called cyanobacteria; (b) electrical discharges ("the grass is always greener after a thunder storm"), (c) free-living soil microbes (Azotobacter); and (d) symbiotic nitrogen-fixing bacteria (Rhizobium) in nodules on the roots of legumes (called poor-persons meat, rich in protein cause they fix nitrogen) and a few other plants.
   

4. Plants absorb ammonia (conifers, grasses) or nitrate (most others).
   

5. Nitrification -  conversion of ammonia to nitrate by soil microbes (Nitrosomas converts ammonia to nitrite; Nitrobacter converts nitrite to nitrate).  Favored by warm temperature and neutral pH.
   

6. Wastes and decayed organic materials decomposed by another set of bacteria into ammonia - called ammonification.  Various microbes are responsible, favored by cool temperatures, all pH's.
   

7. Denitrification - closes the cycle.  Returns nitrogen to the atmosphere.  Conversion of nitrate, nitrite, and/or ammonia back to nitrogen gas by other microbes.
   

8. Note the heavy reliance on microbes for the function of this cycle.
   

9. Nitrogen-fixing plants can usually out compete others in nutrient poor soil
   

10. Nitrogen fixation is "expensive" -  it requires a lot of energy (high metabolic cost)
    

11. Legumes are excellent green manure because they have high levels of nitrogen.  Nitrogen in many systems is usually in short supply so legumes are valuable soil enhancers.
    

12. Fertilizers rated by the nitrogen (N) - phosphorus (P) - potassium (K) amounts or abbreviated, NPK.  For example, Osmocote, a slow release pelletized fertilizer is 14 - 14 - 14, which means it has a nitrogen content of 14%, phosphorus content of 14% and potassium content of 14%.

IV.  Uses of Legumes

1. seeds (e.g., lima beans, navy beans, kidney beans, peanuts) and/or pods (snow peas) eaten for food

2. sprouts (e.g., alfalfa or mung beans)

3. forage crop (e.g., alfalfa)

4. nitrogen fertilizer (green manure)

5. erosion control (many grow quickly and good at stabilizing soil and adding nitrogen - Crown vetch is a good example)

6. ornamental plants (for example, honey locust trees are widely planted on campus)

7. wood pulp (Leucaena trees are an important global source of wood pulp for the paper industry)

8. dyes (indigo comes from a legume plant)

9. flavoring (carob comes from the pod of a legume plant)

10. insecticide (rotenone is obtained from the roots of a legume plant)

V.  Limitations as a food source.
    Although legumes provide an excellent source of protein in the diet, there are limitations to using legumes for food.  These include:

1. Toxins - Many legumes have toxic chemicals in them.  For example, lima beans, especially wild types, have lots of cyanogenic glycosides in them.  Other legumes have trypsin inhibitors (block our protein digesting enzymes) and toxic amino acids.  Lathyrism is an irreversible paralysis caused by eating a vetch and favism is a hemolytic anemia from eating Broad beans.  This latter condition results in genetically susceptible individuals who lack the ability to make a critical enzyme.  Pythagoras may have been a victim.
2. Digestibility - some legume proteins tend to resist digestion
3. Flatulence (undigested sugars)

VI.  A Quick Bean Survey

A.  Soybean (Glycine max)

• animal feed

• rich source of commercially important oil (cooking, margarine, paint, lubricants, soaps).  The oil is expressed

• food (soy sauce, tofu, sprouts, soy milk)

• Tofu is prepared by soaking → rinse →  crush  →  heat →  a. liquid (soymilk, used as beverage or can make cheese) →  salt →  curd; or b. solid →  okara

• Soy sauce:  okara + wheat + fungi (Aspergillus oryzae) →   salt (18%)  →  age year

• annual plant

• domesticated in NE China about 7000 ybp

B.  Peanut  (Arachis hypogaea)

• native to central/south America
• took to Africa then came back with the slaves
• alfatoxins - extremely toxic (carcinogenic); chemicals produced by mold contaminants
• Virginia - brown skinned, 2-seeded
• Spanish type - red skinned, 3 or more seeded
• flower above ground, pedicel elongates and buries fruit (legume)

C.  Lentils (Lens culinaris)

• among most ancient crops
• Middle East (8-9000 ybp)
• drought resistant
• used for "dals"

D. Peas (Pisum sativum)

• domesticated about 9000 ybp
• Old World

E.  Broad Bean or Horse Bean (Vicia faba)

• Mediterranean
• domesticated about 6000 ybp
• cool crop

F.  Chick pea or Garbanzo (Cicer arietinum)

• domesticated about 7500 - 5400 ybp
• India is a large producer

G.  Phaseolus Beans (Phaseolus sp.)
    These are New World beans.  There are several common species:

• Common bean (P. vulgaris) - many varieties including Black, Navy, Shell, Pinto, Great Northern, Snap, String.  Often intercropped with corn.
• Scarlet runner (P. coccineus) - native to South  
• Tepary (P. acutifolius) - native to N. Mexico
• Lima or Butter Bean (P. lunatus) - Andeas, 5000 ypb

H.  Vigna Beans (Old World)

• Cowpeas or Black-eyed peas (V. unguiculata) - Africa
• Green gram (V. aureum) - Africa
• Black gram or Urd (V. mungo) - India
• Mung Bean (V. radiata)

Last updated:  01/07/2005 / � Copyright  by SG Saupe / URL:http://www.employees.csbsju.edu/ssaupe/index.html