SEED GERMINATION

GERMINATION

Germination is the process by which seeds undergo initial stages of growth and developing into a seedling. It is the change of a seed into a seedling given the right conditions.In plants, growth and development begins with germination of a seed. 

       CONDITIONS NECESSARY FOR SEED GERMINATION

1.Water

It is required for enzymes activities. It enters the seed through the micropyle. It soften the testa allowing water to enter in. i

The role of water in germination;

• Provide medium for functioning of enzymes
• Hydrolysis of stored food substances for respirations
• For dissolving food substances for easy transport
• Development of cell sap vacuoles.

2.Oxygen.

It diffuses into the seed through the micropyle. When testa is raptured, oxygen diffuse directly into the tissues.

Oxygen is used during respiration for production of energy for processes like cell division and transport.

3.Optimum temperature.

Enzymes which assist in germination needs the right temperature to function. High temperature denatures enzymes, while very low temperature deactivates them.

TYPES OF GERMINATION 

basically we have two types of germination; hypogeal germination and epigeal germination.

(a )Hypogeal germination- in this type of germination, the seed cotyledon remains underground. Most in monocot plants such as Maize and rice plants 

(b)Epigeal germination- is a type of germination in which cotyledons are brought on the surface of the soil.

This type of germination occur in dicot plants such as beans and peas. 

Figure 1.11. Epigeal Germination.

    The process of germination

The seed absorbs water and softens the testa

The cotyledon swell and rapture the testa

The radicle elongates and emerges through the seed coat, thus roots develop from the radicle.

The hypocotyl elongates and develop a curvature. The curved part emerges above the soil. The hypocotyl eventually straightens raising the cotyledons above the soil surface. The cotyledons are also called the seed leave.

The cotyledon shriven as the stored food is exhausted. The first foliage leaves enlarge and start carrying out photosynthesis. The shriveled cotyledon falls off.

       SEED  VIABILITY 

A viable seed is one which retains its ability to germinate

     Factors affecting seed viability.

Viability is the ability of the seed to stay alive so as to germinate and grow into an adult plant. The following are some of the factors that affect seed viability.

• Seed marturity.

A mature seed is more viable than immature seed

• Temperature and moisture

Too dry conditions do not allow germination to take place. In the same way, too high temperature, or too low temperature does not allow the germination of seed.

• Storage conditions.

Storing seeds in areas where it is too hot or too cold kills the embryo. Seeds need an optimum temperature of 25Oc.

• Food stores

For germination to take place, the seed will need to have adequate food stored. This is marked by large endosperm and cotyledon.

• Testa.

When the testa is too thin, the seed dries faster and it dies, but when the testa is too thick, it prevents the entry of water and thus inhibiting germination.

• Seed dormancy

Seed dormancy is the inability of the seed to germinate. It is a state of relative inactivity where growth and development stops.

      Causes of seed dormancy.

• Temperature.

Very low temperature prevents germination because it interferes with enzyme activities, a very high temperature will also not allow germination because the enzymes will be denatured. For seed to germinate provide the right temperature

• Thick testa

The testa prevents the seed from dehydration because it is water proof. A very thick testa however will prevent germination because it not allow water to enter the seed.

• Immature embryo

If the embryo is immature, germination cannot take place. To break this dormancy, allow the embryo to mature.

• Growth inhibitors

Presence of chemical substances prevents the germination of seeds soon after dispersal.

These substances are called growth inhibitors and a good example is abscisic acid.

• Light.

Some seeds such as lettuce, capsicum and tobacco need light to germinate. Seeds which don't require light to germinate are planted deeper into the soil.

          Importance of seed dormancy

• Help an organism to withstand unfavourable climate such as drought which could kill the plant if it were to germinate
• It enables an organism to survive for very long period of time or even years without exhausting its food reserves.
• It allows time for dispersal of seeds by agents such as water and wind