Seeds and their Germination
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
Number of organelles falls
Dehydration - water content falls
Food reserves become dense crystalline bodies.
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
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
0 - 50% water
High matrix potential of dry seed.
Passive (temperature insensitive)
70 - 80% water
Active (temperature sensitive)
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
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).
Negative feedback control of enzymes
The action of the enzyme limited by substrate.
Once all the starch in an amyloplast is hydrolysed the enzyme stops work.
Therefore the release of the stored food is adjusted to suite the demand.
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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