National Conference Toronto, June 2003

1
Dr Margaret Johnston
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Seed viability

• Seed viability
• A seed that is capable of germinating to give a
  normal seedling
• The degree to which the seed is alive
  (metabolically active)
• Viability is highest at the point of
  physiological maturity and then gradually declines

3
Tests for seed viability

• Germination test (under defined laboratory
  conditions)
• Tetrazolium test
• May need to disrupt seed coverings
• Imbibe seed for 24 h at ambient temperature
• 1.0 (w/v) 2,3,5-triphenyltetrazolium chloride
  (TZ) solution and held at 40C in the dark for 24
  h
• Cut seed (embryo) longitudinally
• Interpretation can be difficult

4
Tests for seed viability

• Excised embryo test (good for dormant seeds)
• Cut seed test
• X-Ray test (allows evaluation of normal morphology

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Seed dormancy

• Survival mechanism
• Genetically inherited trait
• Wild plants show more dormancy than crop plants
• A degree of dormancy is desirable as it prevents
  premature sprouting on the parent plant
• Dormancy is defined as a state in which seeds are
  prevented from germination even when
  environmental conditions are favourable

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Seed dormancy

• Several physical and physiological mechanisms of
  dormancy, including primary and secondary
  dormancy occur in seeds
• Primary dormancy
• Exogenous dormancy
• Essential germination components (water light
  temperature) are not available so the seed fails
  to germinate
• Usually related to properties of the seed
  coverings or light

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Seed dormancy

• Causes of exogenous dormancy
• Lack of water due to impermeability of seed coat
• Impermeability of the seed coverings to gases
  (oxygen)
• Mucilaginous material surrounding seed
• Phenolic compounds
• Mechanical restriction of embryo growth
• Removal of seed coverings may also remove
  inhibitors

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Methods of breaking exogenous dormancy

• In nature this type of dormancy is overcome by
• Ingestion by animals
• Microorganisms
• Fires (smoke, heat shock)
• Temperature fluctuations
• Natural soil acidity
• Freeze-thawing

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Methods of breaking exogenous dormancy

• Mechanical scarification
• Abrasion
• Brief immersion in boiling water
• Piercing seed coats
• Duration of treatment is critical
• Prolonged treatment may cause damage

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Methods of breaking exogenous dormancy

• Chemical scarification
• Sulfuric acid
• Sodium hypochlorite
• Hydrogen peroxide
• Cellulase and pectinase
• Hazardous
• Seeds must be washed and dried
• Reduction in seed germination

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Endogenous dormancy

• Due to inherent properties of the seed
• Causes
• Environmental conditions during seed maturation
  influence the duration of endogenous dormancy
• Daylength
• Moisture status
• Position on the parent plant
• Age of the mother plant
• Environmental conditions during seed development
  and maturation

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Endogenous dormancy

• Only physiological changes are able to relieve
  endogenous changes
• Rudimentary embryo dormancy
• Seeds are shed before the embryo is mature
• Continued development can take from a few days to
  several months
• After-ripening

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Endogenous dormancy

• Physiological dormancy
• Seed dormancy is believed to be regulated by a
  balance of endogenous growth inhibitors and
  promoters
• Level of the substances is controlled by certain
  environmental stimuli, These substances may be
• Cyanide
• Phenolics
• Abscisic acid (ABA)
• Promoters
• Gibberellic acid
• Cytokinins

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Endogenous dormancy

• Osmotic inhibition
• Fleshy fruit may contain such substances
• Chemical inhibitors
• Methods of breaking endogenous dormancy
• Leaching inhibitor (chemical or osmotic)

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Endogenous dormancy

• Temperature
• Seeds with a specific temperature requirement for
  germination often contain inhibitors and
  promotors
• Stratification (3 to 10 C)
• Physical and physiological changes may occur in
  imbibed fruit
• Development of the embryo
• Inhibitor-promoter balance
• Alternating temperatures
• After-ripening 1 to 2 months at 15 to 20 C

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Endogenous dormancy

• Light
• Dormancy is broken by exposure to red light (670
  nm)
• Continuous light may inhibit germination of some
  species
• Circadian rhythms
• Time measuring capacity
• Appears to influence the pattern of seed
  germination
• Interaction of primary dormancy mechanisms
• Seed may show a hard seed coat and physiological
  dormancy
• Embryo excision can be used to overcome dormancy

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Secondary dormancy

• Non-dormant seed encounter conditions that cause
  them to become dormant
• Spring wheat and winter barley
• Exposure of dry barley seed to temperatures
  between 50 to 90 C
• Seven days storage of winter barley at high
  moisture contents at 20 C
• One day storage of spring wheat at air-tight
  container at 50 C
• Placement of seed under water and in darkness for
  1-3 days at 20 C
• Secondary dormancy is temperature, light or
  darkness imposed

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Germination factors

• Centre for Native Floriculture
• The University of Queensland Gatton

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Chemical promotion of seed germination

• Gibberellins usually GA3
• Promotes germination in many species
• Can substitute for light and temperature
• Important role in the regulation of seed dormancy
• 50 to 500 mg/L

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Chemical promotion of seed germination

• Cytokinins
• Kinetin
• Known to break primary dormancy in some seeds but
  appear to be more effective at overcoming
  secondary dormancy
• Exact role is unclear
• Overcomes chilling requirement of sugar maple,
  Proteaceae (Leucodendron and Protea)
• Overcomes light requirement (Rumex, lettuce,
  celery)

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Chemical promotion of seed germination

• Ethylene
• Stimulates germination of some species
• Regulates auxin levels in dormant seeds
• Peanuts and sunflowers

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Chemical promotion of seed germination

• Hydrogen peroxide
• Stimulates germination of several species
• Conifers, legumes, tomatoes and barley
• Respiration stimulant accelerates breakdown of
  food reserves
• Disinfectant

23
Chemical promotion of seed germination

• Auxins
• Auxins and other plant growth regulators are
  universal components of plants and common
  constituents of seeds
• IAA reported to increase lettuce seed germination
• Effect is temperature dependant
• High concentrations inhibit while low
  concentrations promote or a ineffective
• May interact with light in influencing germination

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Chemical promotion of seed germination

• Potassium nitrate (KNO3)
• Widely used to promote seed germination
• Concentrations used 0.1 to 0.2
• Most seeds that are sensitive to KNO3 are also
  sensitive to light
• May interact with temperature and light
• May act cooperative with plant growth regulators
  (GA3 and kinetin)
• There are reports that KNO3 can inhibit
  germination
• KNO3 may influence the respiratory system, it may
  stimulate O2 uptake or serve as a co-factor to
  phytochrome

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Chemical promotion of seed germination

• Thiourea
• Like KNO3 thiourea promotes germination of many
  species
• Perhaps replacing temperature and light
  requirements

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Chemical promotion of seed germination

• Other chemicals
• Plants can produce substance that promote or
  inhibit germination
• Scopotelin (phenolic)

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Other factors

• These factors may affect germination
• Osmotic pressure (high osmotic pressure make
  imbibition more difficult and usually retards
  germination)
• Halophytes germinate better in saline
  environments
• pH
• Germination of most species occurs readily
  between pH 4 to 7.6.

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Other factors

• Presoaking
• Can speed germination
• Presoaking at 20 C protects seeds from chilling
  injury during subsequent germination at lower
  temperatures
• Prolonged soaking can be damaging.
• Need to avoid low O2 levels
• Osmoconditioning

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Other factors

• Frost and cold nights prior to seed harvest
• May injure seed
• Depend on
• Temperature
• Exposure
• Moisture content of seed
• Physiological maturity of seed
• Husk protection
• Variety

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Other factors

• Radiation
• Exposure to gamma radiation above 10 Krad may
  retard seed germination
• Effects more pronounced at high temperatures and
  high seed moisture content

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Other factors

• Mechanical damage
• Harvesting, processing and handling
• Susceptibility to mechanical damage increases as
  moisture content decreases
• Varieties can vary in their susceptibility to
  mechanical damage