Form 6 Biology – GROWTH AND DEVELOPMENT

SEED DORMANCY

Refers to a condition where seed will not germinate despite the presence of those environmental conditions for germination.

Causes of seed dormancy

1. Immaturity of the embryo  

 Newly harvested seed need some period of time for the embryo to become mature .The seed undergo some internal transformation before it can be able to germinate .This period where the seed undergo internal changes for maturation is called the after ripening. To terminate this type of seed dormancy allow the seeds to have enough period of time before they can be sown again .

1. Hardness of the testa

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           Hard seed coat or testa makes it impermeable to water and oxygen or being physically strong enough to prevent embryo growing.

How to break this type of dormancy.

• physical damage or scarification to the seed coat
  
• By action of micro organism such bacteria.
  

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-soaking for a long period of time and   by chemical   action in the soil.

BREAKING OF SEED DORMANCY

The mechanism of breaking seed dormancy depends on the type of dormancy under consideration.

                 For primary Dormancy.

1. The seed need to be stored for long periods until the embryo matures
   
2. Wetting of seeds with appropriate solvents such as water and gibberellins so as to remove growth inhibitors.
   
3. Removal of the testa by mechanical secretion or by weakening chemicals such as enzymes so as to enable the emergence of the radical and plumule.
   
4. Using organic solvents to dissolve the waxy coating over the testa rendering it permeable to gases and water.
   
5. Soaking the seeds in water for sometime soften the testa and therefore makes it permeable to water and oxygen
   

                  For Secondary dormancy.
   

6. The seeds have to be supplied with appropriate temperature. This may be by chilling in deep freezers / refrigerators or by temperature shocking in which the seed are either boiled for a while or washed in hot water.
   
7. The seeds have to be provided with specific light intensity so as to rise the levels of gibberellins to work (activated) 
   
8. They have to be provided with enough moisture and oxygen.
   

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                                       SEED VIABILITY AND GERMINATION

a).  SEED VIABILITY

1. Viability is the capacity of the seed to remain capable of germination when conditions are not favorable.

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• A viable seed is that which is capable of germinating when all causes of dormancy are broken.
  
• Seed viability ensures that the seed will germinate once supplied with all conditions necessary for germination and all causes of seed dormancy are broken.
  

  Factors governing seed viability:-
  

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1. Seed maturity
   

   Immature seeds die, thus when sown they never germinate since they are inviable due to the fact that their embryos are not completely formed after seed formation, seeds need time to completely form their embryo so as to be able to carry active growth.
   

2. Activity of enzymes
   

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• Since germination is an enzyme controlled process, depends on enzymes for its take off.
  
• In the seeds where enzymes are inactive, they cannot be activated, germination is impossible and the seeds are therefore inviable
  

  3.Storage condition of
  

  4.State of health of the seed
  

  • Temperature: Optimum temperature is acquired for the seed in the store for them to remain viable. Extremely high temperature denatures the enzymes in the seeds. This results into failure of seeds to germinate
    
  • Moisture: high moisture content of storage site can cause seed to hence lose their viability. Dry atmosphere is the preferred one.
    
  • Aeration: Enough supply of oxygen is required to cater for the minimum metabolism in seeds. Lack of aeration will hinder metabolic reactions in the seed and seeds may lose their viability
    

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• This diseased seed may lose its viability as its embryo may be infected by fungi or bacteria.
  

  5.Time of storage of seeds.

  This varies from seed to seed and from species. Most of the annual plant seeds lose their viability in a period of one year.
  

  However there are other seeds e.g. those of cassia bicaspsularis and cassia maltijuge retain their viability for about 115 and 158 years respectively.
  

• Also long storage may result into seeds being destroyed by insects.
  

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