Major events during germination

a. Imbibition of water:

• If the seed is kept in a moist or humid medium, water is absorbed through the natural openings in the seed coat and diffuses through the seed tissues, this process is known as imbibition.
• The water causes the cells to become turgid and the entire seed grows in volume and the seed coat becomes more permeable to oxygen and carbon dioxide.
• Hydrophilic groups (-NH2, -OH, -COOH) of proteins and CHO in seed coat attract dipolar water molecules and form hydrated cells around them resulting in the swelling of these substances.
• When swelling occurs, the seed coat often ruptures, facilitating both water and gas uptake and the emergence of the growing points.
• Respiratory activities are initiated and some dry weight losses occur.

b. Enzyme activation:

• Enzyme activation starts when the seed is hydrated at 30- 50% water content.
• After imbibition, hydrolytic enzyme formation takes place.
• These hydrolytic enzymes are protease, α-amylase, β- amylase, lipase, etc, which helps:

– To breakdown the stored tissues

– To aid in the transfer of nutrients from the storage area of cotyledon and endosperm to growing points

– To trigger chemical reactions that use breakdown products to synthesize new materials

• After enzyme activation, the macromolecules of protein, carbohydrate, and lipids are converted into micro molecules and transported to the growing points (root and shoot tip) and utilized as the source of energy.

c. Initiation of embryo growth:

• Embryo increases in size by two ways i.e. cell elongation and cell division or in combination of both, but cell elongation is first.
• Growth of the root shoot axis occurs at the expense of the storage tissues, which gradually decrease as the food reserves are depleted.
• By the time the young seedlings can synthesize their food, most of the storage tissues are exhausted.

d. Rupture of seed coat and emergence of seedlings:

• When imbibition pressure is as high as 200 atmospheres, there is rupturing of the seed coat, and the first part coming out is the radicle and then the plumule.
• Ordinarily, the primary root (radicle) is the first structure to emerge, but in some species, the shoot (plumule) emerges first.

e. Seedling establishment:

• During the initial phases of seedling establishment, it undergoes a transition stage during which it begins to produce some of its food but it is still dependent on food breakdown from the reserve storage tissues.
• As the seedlings become firmly established in the soil, they begin water uptake and manufacture most of their food (photosynthesis), this is called seedling establishment.
• It is entirely heterotrophic in the initial stage, then transitional, and becomes autotrophic eventually. Then the germination process is complete.