Docstoc

seed-dormancy

Document Sample
seed-dormancy Powered By Docstoc
					                              Seed Viability and
                                  Dormancy

                                 Dr Margaret Johnston

                              Centre for Native Floriculture
                           The University of Queensland Gatton


1   Dr Margaret Johnston
                   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



 2
           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 40°C in the dark for 24 h
     – Cut seed (embryo) longitudinally
     – Interpretation can be difficult

 3
       Tests for seed viability

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




 4
              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


    5
                 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

 6
                 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
 7
     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


 8
     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




 9
      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

 10
           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

  11
        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




 12
       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


13
       Endogenous dormancy

• Osmotic inhibition
  – Fleshy fruit may contain such substances
• Chemical inhibitors

Methods of breaking endogenous dormancy
• Leaching inhibitor (chemical or osmotic)


 14
        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

 15
          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

 16
            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

  17
     Germination factors




        Centre for Native Floriculture
     The University of Queensland Gatton

18
       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




 19
      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)
 20
      Chemical promotion of seed
            germination
• Ethylene
  – Stimulates germination of some species
  – Regulates auxin levels in dormant seeds
  – Peanuts and sunflowers




 21
      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




 22
      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
 23
      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

 24
      Chemical promotion of seed
            germination
• Thiourea
  – Like KNO3 thiourea promotes germination of
    many species
  – Perhaps replacing temperature and light
    requirements




 25
      Chemical promotion of seed
            germination
• Other chemicals
  – Plants can produce substance that promote or
    inhibit germination
  – Scopotelin (phenolic)




 26
                  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.



 27
               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

 28
                    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

 29
               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




 30
               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




 31

				
DOCUMENT INFO