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National Conference Toronto, June 2003

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Viability is highest at the point of physiological maturity and then gradually declines ... Tests for seed viability. Germination test (under defined laboratory ... – PowerPoint PPT presentation

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Title: National Conference Toronto, June 2003


1
Dr Margaret Johnston
2
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

5
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

6
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

7
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

8
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

9
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

10
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

11
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

12
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

13
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

14
Endogenous dormancy
  • Osmotic inhibition
  • Fleshy fruit may contain such substances
  • Chemical inhibitors
  • Methods of breaking endogenous dormancy
  • Leaching inhibitor (chemical or osmotic)

15
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

16
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

17
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

18
Germination factors
  • Centre for Native Floriculture
  • The University of Queensland Gatton

19
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

20
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)

21
Chemical promotion of seed germination
  • Ethylene
  • Stimulates germination of some species
  • Regulates auxin levels in dormant seeds
  • Peanuts and sunflowers

22
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

24
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

25
Chemical promotion of seed germination
  • Thiourea
  • Like KNO3 thiourea promotes germination of many
    species
  • Perhaps replacing temperature and light
    requirements

26
Chemical promotion of seed germination
  • Other chemicals
  • Plants can produce substance that promote or
    inhibit germination
  • Scopotelin (phenolic)

27
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.

28
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

29
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

30
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

31
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
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