Definition : Deterioration of seed includes all the progressive detrimental  changes that occur in seeds as they die.

- Deterioration involves some degree of impairment in that unit:

   a. Does not differ substantially from degeneration or senescence

   b. Encompasses all of the detrimental changes that occur as seeds die

   c. Seed deterioration and vigor are inversely related

Deterioration can be characterized as follows:

 1. Is inevitable

 2. Is irreversible

 3. Is minimal @ physiological maturity (PM)

 4. The rate varies by species, variety and seed lot

Life span of seeds

Classifications:

a. Macrobiotic  (long lived) seeds

• Life span ranges from 15 to more than 100 years
  • Examples:
    1. Lupine seeds buried in peat bog for 10,000 years (Canada)
    2. Indian lotus seeds over 1,000 years old (Manchuria)
    3. Trifolium
    4. Seeds germinated that had been collected 100 years previously (Nat'l Museum of Paris)

b. Mesobiotic

• Life span varies from 3 to 15 years
  • Examples:
    1. Barley
    2. Flax
    3. Tall fescue
    4. Carrot

c. Microbiotic (short lived) seeds

• Life span does not exceed 3 years
  • Examples:
    1. Soybean
    2. Onion
    3. River Maple (may be only days)
    4. Wild rice

Ecology and morphology

a. Orthodox

• Includes most cereal and legume crops

b. Recalcitrant

• Typically have fleshy or juicy layers and impermeable testa
• Higher moisture content (50 - 70%)  than orthodox at PM
• Seed are larger than orthodox types, but embryos are smaller (15%)
• Are difficult  to store
• Do not experience dormancy

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Factors influencing the viability of seeds

 1. Internal factors

 - Damage

 - Development

 2. Relative humidity and temperature rh_temp.gif

 3. Gaseous composition of the environment

  - O₂ (20%)

  - CO₂ (0.03%)

  - N₂ gas (80%)

Seed Moisture Relationships

   a. Types of Seed-Water relationships: bound.gif

    - Bound water

    - Absorbed water

    - Free water

  b. Moisture Equilibrium

• Equivalent moisture level  in seed for mold growth = 70% RH @ 25^oC
• Above varies with chemical composition of the seed

  c. Factors affecting moisture equilibrium

• RH major factor
• Temperature has only slight influence

  d. Harrington's Rule of Thumb

1. Storage life doubles for each 1% decrease in MC
2. Storage life doubles for each 5⁰C increase in temperature
3. The sum of RH (%) and temperature (⁰F) should not exceed 100
   • Temperature should not exceed 50⁰F

Possible causes of seed deterioration

  1. Depleted food reserves

• Unfavorable storage conditions, fungi & other microbes

   2. Starved meristematic cells

• Aged barley = 50-70% lower amylase activity

   3. Toxic compounds

   4. Triggering mechanisms

   5. Inability of ribosomes to disassociate

• ATP content of aged soybean = 35% of control

   6. Enzyme degradation

• Literature shows decline in respiratory enzyme activity (oxidase, dehydrogenase, peroxidase, etc.)

   7. Lipid autooxidation

• Unsaturated fatty acids tend to form free radicals

   8. Hydrolytic enzymes

   9. Fungal invasion

• Degrade starch and lipids

  10. Membrane degradation

• Swollen and disrupted mitrochondria
• Become disorganized 2-3 days after imbibition  

11. Genetic degradation

• Chromosomal breakdown
• Impaired DNA transcription and translation processes

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Modern theories on deterioration

    - Accumulation of deleterious products

   - Wear and tear

   - Mutational changes 

Aging of Seeds

• Characteristics:
  1. Change is seed color - seed coat and/or embryo
  2. Delay ed radicle emergence and seedling growth
  3. Decreasd total germination
  4. Decline in tolerance for suboptimal germination conditions
  5. Increased abnormal seedlings
  6. Lower tolerance for adverse storage conditions
  7. Greater sensitivity to radiation treatment
  8. Mustiness
  9. Increased heat production during storage

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References:

Principles of Seed Science and Technology. 3rd Ed. L.O. Copeland and M.B. McDonald. Chapman and Hill, New York, 1995

Seeds Handbook, B.B. Desai, P.M. Kotecha and D.K. Salunkhe. Marcel Dekker, Inc., New York, 1997.