SEEDS AND THEIR GERMINATION

Seed maturation

Takes place in the fruit on the parent plant.

Endospermous seeds: Retain the endosperm tissue,which eventually dies but it is surrounded by a layer of living cells, the aleurone layer.

Non-endospermous seeds: The endosperm tissue is absorbed by the cotyledons. These then become the food reserve for the seed.

The development of the cotyledons during seed maturation

• They grow and dominate the non-endospermous seed.
• Cells highly vacuolated.
• Mitochondria, ribosomes and ER numerous.
• Large nuceoli but DNA : Cell Volume ratio remains constant. The cells are growing.
• Numerous plastids for food storage (amylopasts for starch, aleuroneplasts for protein, elaioplasts for oil).
• Increase in size of cotyledons to maximum capacity.

Then everything stops! The seed becomes dormant

• Metabolism falls
• Number of organelles falls
• Dehydration – water content falls
• Vacuoles deflate
• Food reserves become dense crystalline bodies.

Maintaining dormancy

Physical barriers – the seed coat (testa) is waxy = waterproof and impermeable to oxygen.
Physical state – dehydrated
Chemical inhibitors present e.g. salts, mustard oils, organic acids, alkaloids
Growth promoters absent

Seed viability

Viability: When a seed is capable of germinating after all the necessary environmental conditions are met.
If it does not germinate in these optimum conditions it is dead!

Average life span of a seed 10 to 15 years.
Some are very short-lived e.g. sugar maple, rice and willow (< 1 week)
Some are very long-lived chrysanthemum 30 years, red clover 100 years, mimosa 221 years

The record longevity  Lupinus arctinus 10 000 years found in permafrost in central Yukon

Conditions are very important for longevity
Cold, dry, anaerobic conditions
These are the conditions which are maintained in seed banks.

Germination: The breaking of dormancy

The growth of the embryo and its penetration of the seed coat.

Rehydration

Rehydration is reversible up to a point (about 45 hours).
Once DNA synthesis starts and leads to mitosis it becomes irreversible.

Steps leading to DNA synthesis

The control of food reserve hydrolysis

1. Through growth promotors such as gibberellin and growth inhibitors such as abscisic acid.
   These directly affect the genes for enzyme synthesis or the activity of the enzymes themselves.
   The growth substances are affected by environmental factors (e.g. light, temperature, humidity).
   
    

2. Negative feedback control of enzymes
    

    

3. The action of the enzyme limited by substrate.
   
   Once all the starch in an amyloplast is hydrolysed the enzyme stops work.

The mobilisation of food reserves

The food reserves are stored as large insoluble macromolecules.
They are hydrolysed using enzymes to smaller soluble molecules for transport.

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© Paul Billiet 2006