What is Seed Germination?

• Is the process where growth begins from a resting stage.

• Is an irreversible process, include the change that take place from the time the dry seed is provided with suitable conditions to when the seedlings become established as independent pant

Factors affecting Seed Germination?

• Environmental factors- availabilty of water, oxygen, and light as well as temperature.

• Intrinsic factor- dormancy, permeability of the testa to water, oxygen and obstruction of radicle emergence by the endosperm.

The cellular and metabolic events triggered by water uptake during seed germination.

Germination process

A rapid imbibitions phase (phase 1) launches the resumption of basic metabolism. During this phase, known as ‘physical’ imbibitions, a step-by-step activation of metabolic pathways results from the gradual increase in hydration (arrows).

When the level of hydration exceeds 60%, the rate of hydration slows (phase 2) and new physiological mechanisms prepare cell expansion in the embryonic axes, culminating in the start of cell elongation. Osmotically active substances (solutes, such as sugars, amino acids, and potassium ions) are accumulated and acidification of the cell wall leads to a loosening of the bonds between cell-wall polymers. These events coincide with the activation of the H+ ATPase in the plasmalemma, which results in a further increase in water uptake that may coincide with weakening of the surrounding tissues (the endosperm) as the embryonic axes elongate and germination is completed.

Completion of seed germination can be temporarily blocked by dormancy, which is in turn released by antagonistic interactions between the endogenous plant growth factors abscisic acid (ABA) and gibberellins (GAs). Storage nutrients (lipids, proteins or starch) accumulated in the embryo’s cotyledon and/or endosperm start to be mobilized before completion of germination and are used in the post-germination steps to sustain the young plant in its early growth stages, before it becomes autotrophic. If the cell cycle resumes during germination, the first cell division (mitosis) occurs in the postgerminative phase. The arrows indicate the particular hydration levels that are known to correlate with individual metabolic events.

• A typical seed stores carbohydrates, lipids and proteins  for germination.• As a result of imbibation  and osmosis the embryo becomes  hydrated which activates

the enzymes for respiration.• Basically the storage centre (food) and the growth centre(embryo) are the main

regions’ of activity.• Digestion of food on storage centre occurs  by hydrolysis  and amino acids, sugars and

fatty acids and glygerol arte produced• Soluble products are translocated to the growing regions of the embryo• The majority of glucose is used for cellulose synthesis and other cell wall materials.

amino acids are used for enzymes and structural components. • The net loss of mass of the seed occurs due to change of sugars to  Co2  and water

where C02 is lost as gas than O2  taken up by aerobic respiration.• Within the embryo growth occurs by cell division and amounts of proteins, cellulose,

nucleic acids steadily increase in the growing regions of the seed.• The first visible sign of growth is the radicle, that emerges from the micropyle and is

positively geotropic in nature. It forms the root system of the plant • The next structure that emerges is the plumule, that is positively phototropic and

forms the shoot system of the plant.                             

Physiology of Germination

Determines its genetic composition, which, in turn, is the deciding factor to develop suitable breeding and selection methods. Knowledge of mode of reproduction is also essential for its artificial manipulation to breed improved types. Only those breeding and selection methods are suitable for a crop which does not interfere with its natural state or ensure the maintenance of such a state. It is due to such reasons that imposition of self-fertilization on cross-pollinating crops leads to drastic reduction in their performance.

Importance of mode of reproduction

For teaching purpose, plant breeding is presented as four categories: Line breeding (autogamous crops), population breeding (allogamous crops), hybrid breeding (mostly allogamous crops, some autogamous crops), clone breeding (vegetatively propagated crops).