1. Faculty of Science, School of Sciences, Natabua Campus Lautoka
BIO706 Embryology Lecture 30: Seeds and Seed Germination

2. Seeds and Seed Germination

3. Seed structure
Cotyledon
Plumule
Radicle
Micropyle
Seed coat or testa

4. 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.

5. Dormancy Metabolism falls Number of organelles per cell falls
Dehydration – water content falls
Vacuoles in cells deflate
Food reserves become dense crystalline bodies

6. 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

7. Seed viability
Viability: When a seed is capable of germinating after all the necessary environmental conditions are met.
Average life span of a seed 10 to 15 years.
Some are very short-lived e.g. willow (< 1 week)
Some are very long-lived e.g. mimosa 221 years
Conditions are very important for longevity
Cold, dry, anaerobic conditions
These are the conditions which are maintained in seed banks

8. Germination: The breaking of dormancy
The growth of the embryo and its penetration of the seed coat
Break down of barriers
Abrasion of seed coat (soil particles)
Decomposition of seed coat (soil microbes, gut enzymes)
Cracking of seed coat (fire)
Change in physical state - rehydration
Destruction and dilution of inhibitors
Light, temperature, water
Production of growth promoters

9. Germination STAGE EVENTS PREGERMINATION
Rehydration – imbibition of water.
RNA & protein synthesis stimulated.
Increased metabolism – increased respiration.
Hydrolysis (digestion) of food reserves by enzymes.
Changes in cell ultrastructure.
Induction of cell division & cell growth.
GERMINATION
Rupture of seed coat.
Emergence of seedling, usually radicle first.
POST GERMINATION
Controlled growth of root and shoot axis.
Controlled transport of materials from food stores to growing axis.
Senescence (aging) of food storage tissues.

10. Stages leading to cell division
Respiration
Initially anaerobic
Later aerobic
Mitchondria reconstituted
Soluble sugars
ATP
RNA activated
Polysomes
Protein synthesis (0.5h)
Enzymes (proteins)
DNA synthesis (45h)
Mitosis (70h)

11. The control of food reserve hydrolysis
Control by growth promoters 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)

12. The control of food reserve hydrolysis
Negative feedback control of enzymes
The action of the enzyme also 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
Starch + H20
Maltose
 - amylase
Negative feedback

13. The mobilisation of food reserves
Carbohydrates
Starches (amylopectin & amylose)
Amylases
Maltose and glucose
Proteins
e.g. Zein
Proteases
Amino acids
Lipids
Oils
Lipases
Fatty acids & glycerol
The food reserves are stored as large insoluble macromolecules
They are hydrolysed using enzymes to smaller soluble molecules for transport

14. Germination of seeds depends on imbibition, the uptake of water due to the low water potential of the dry seed.
This causes the expanding seed to rupture its seed coat and triggers metabolic changes in the embryo that enable it to resume growth.
Enzymes begin digesting the storage materials of endosperm or cotyledons, and the nutrients are transferred to the growing regions of the embryo.

16. The first organ to emerge from the germinating seed is the radicle, the embryonic root.
Next, the shoot tip must break through the soil surface.
In garden beans and many other dicots, a hook forms in the hypocotyl, and growth pushes it aboveground.
Stimulated by light, the hypocotyl straightens, raising the cotyledons and epicotyl.

18. As it rises into the air, the epicotyl spreads its first foliage leaves (true leaves).
These foliage leaves expand, become green, and begin making food for photosynthesis.
After the cotyledons have transferred all their nutrients to the developing plant, they shrivel and fall off the seedling.

19. Light seems to be main cue that tells the seedling that it has broken ground.
A seedling that germinates in darkness will extend an exaggerated hypocotyl with a hook at its tip, and the foliage leaves fail to green.
After it exhausts its food reserves, the spindly seedling stops growing and dies.

20. Peas, though in the same family as beans, have a different style of germinating.
A hook forms in the epicotyl rather than the hypocotyl, and the shoot tip is lifted gently out of the soil by elongation of the epicotyl and straightening the hook.
Pea cotyledons, unlike those of beans, remain behind underground.

22. Corn and other grasses, which are monocots, use yet a different method for breaking ground when they germinate.
The coleoptile pushes upward through the soil and into the air.
The shoot tip then grows straight up through the tunnel provided by the tubular coleoptile.

24. The tough seed gives rise to a fragile seedling that will be exposed to predators, parasites, wind, and other hazards.
Because only a small fraction of seedlings endure long enough to become parents, plants must produce enormous numbers of seeds to compensate for low individual survival.

25. This provides ample genetic variation for natural selection to screen.
However, flowering and fruiting in sexual reproduction is an expensive way of plant propagation especially when compared to asexual reproduction.

26. Courtesy: Pearson Education, Inc
Courtesy: Pearson Education, Inc., publishing as Benjamin Cummings, relevant books and Internet sources
Questions are welcome