March 29, 2024

Germination is a process by which the embryo of a seed grows out of its testa and develops into a seedling.

In other words, Germination is the series of changes by which an embryo in a seed grows into seedling.

The embryo of developed seeds usually pass through a period, there is very little cell activity. The seeds can remain in this condition as long as they are dry and the condition for germination is not favourable. When conditions become favourable, the seeds undergo several changes to develop into seedling.

 

Conditions necessary for germination

The conditions which are necessary for the germination of the seed include: water or moisture, air or oxygen, warmth or suitable temperature, enzymes, energy or food and viable seeds.

  1. Water or moisture: The seed needs water to activate the cell. It is also needed to soften the testa or seed coat so that the radicle and plumule can come out of the seed with ease.
  2. Air or oxygen: The seed is a living organ. It needs oxygen to carry out respiration. During respiration, energy is released and it is used by the seed to grow.
  3. Warmth or suitable temperature: Some seeds require certain range of temperature to germinate. This range in temperature is called optimum temperature. Below or above this temperature, the seed may or will not germinate and may even die.
  4. Enzymes: Enzymes are organic catalyst which speed up the rate of reaction with the cells in the seed. Enzymes are required in the breaking down of food to release energy.
  5. Energy or food: There must be food within the seed from which it feeds. In dicotyledonous seed, the food is stored in the cotyledons or seed leaves while in a monocotyledonous seed, the food or energy source is stored in the endosperm.
  6. Viable seeds: For a seed to germinate, it must be viable or alive. Damaged seeds by insects, birds or man cannot germinate hence such seeds are not viable.

 

Dormancy

After the dispersal of seeds, the seeds are likely to germinate if the conditions required for germination are available. Seed cannot germinate unless they are viable. When a viable seed fails to germinate in the presence of all the conditions necessary for germination, such a seed is said to be in a state of dormancy.

Dormancy has been described as a short period of rest which precedes the germination of seeds. The dormant seeds are not resting, but are perhaps undergoing some other changes which will enable them to germinate.

 

Causes of dormancy

  1. The embryo may not be fully matured at the time the seed is released from the fruit. The seed will remain dormant to bring the embryo to maturity.
  2. The seed-coat may be too hard and may not be able to absorb water that is necessary for germination. In this case the seed-coat will have to decay before water can be absorbed.
  3. The seed-coat may absorb water but fails to swell to allow the emergence of the radicle. Again the seed-coat needs to decay.
  4. Sometimes the seed-embryo needs the presence of certain chemicals from the soil before it can germinate and unless these chemicals are available it will remain dormant.

 

Ways of Breaking Seed Dormancy

  1. Removal of testa.
  2. Treatment of seed with acid or dogestive enzymes or water.
  3. Exposure to alternating high and low temperature.
  4. Addition of dormancy -breaking hormones
  5. Exposure to fire.

 

Types of germination

There are two types of germination. These are epigeal germination and hypogeal germination.

1) Epigeal germination

Epigeal germination is defined as the type of germination in which the cotyledons or seed leaves are carried above the soil surface. Epigeal germination is associated with dicotyledonous plants, e.g. germination of cowpea, groundnut, melon and mango.

 

2) Hypogeal germination

Hypogeal germination is defined as the type of germination in which the cotyledons, seed leaves or endosperm remain below the soil surface.

Hypogeal germination is associated with monocotyledonous plants, e.g. germination of maize, oil palm, guinea corn, millet and wheat.

 

Differences between Epigeal germination and Hypogeal germination

 

Epigeal germination

  1. Testa splits.
  2. Hypocotyl emerges uncovered.
  3. Cotyledons are carried above the soil.
  4. Energy obtained from food is stored in the cotyledon.
  5. Hypocotyl elongates.
  6. Cotyledon is the first photosynthetic tissue.

 

Hypogeal germination

  1. Pericarp splits.
  2. Hypocotyl covered by the sheat/coleorrhiza.
  3. Cotyledons remain in the soil.
  4. Energy obtained from food is stored in the endosperm.
  5. Epicotyl elongates.
  6. Foliage leaves are the first photosynthetic tissue.

 

Germination of cowpea seed (epigeal germination)

Cowpea is a dicotyledonous non-endospermic seed with epigeal germination. The seed in the soil in the presence of moisture, air and suitable temperature, absorbs water and swells. Water is absorbed largely through the micropyle and also through the wet seed coat. The swelling of the seed causes the seed coat to rupture. After about 24 hours the radicle pushes out, grows downwards.

The radicle elongates and develops some root hairs to become a primary root. Within 48 hours the part of the embryo just below the cotyledons known as hypocotyl elongates and forms a loop above the soil. The hypocotyl later strengthens up, carrying the cotyledons into the air. The cotyledons which have been protecting the plumule open to expose the plumule. Both the cotyledons and plumule turn green in the presence of light. The plumule becomes the first foliage leaves. The cotyledons from which food is being absorbed into the growing embryo diminish in size. They later wither as the newly formed green leaves take over the function of food production by photosynthesis.

Meanwhile the primary root elongates deeper into the soil and develops into a tap root system. From it some lateral roots with root hairs develop. The root hairs absorb water. And so within 96 hours a cowpea seed by these changes becomes a young plant or seedling, the final result of sexual reproduction.

All germinations begin with the emergence of the radicle. The difference which occur in different seeds depends on whether the radicle develops into a tap root or a fibrous root system; whether the cotyledons are carried above (epigeal) or left in the soil (hypogeal) or whether the growing embryo gets its nourishment from food stored in the cotyledons or in the endosperm.

 

Germination of maize grain (hypogeal germination)

The maize grain is a monocotyledonous endospermic seed with hypogeal germination. The seed in the soil absorbs water and swells.

The radicle is stimulated into growth. The radicle sheath elongates and pushes out through the fused pericarp and testa. Later the sheath breaks at its end due to the pressure of the elongating radicle which grows out and develops into a primary root. Similarly the plumule sheath grows up due to the elongation of the epicotyl (part of the embryo above the cotyledon). The plumule sheath comes above the soil, breaks and releases the plumule which later turns green to become the first foliage leaf. The cotyledon remains within the grain in the soil absorbing food from the endosperm into the growing embryo. Meanwhile the primary root stops growing.

 

This is because some secondary roots develop from the base of the plumule sheath to form a fibrous system.

 

Note

A tap root system is characteristic of the dicotyledonous plants while a fibrous root system is characteristic of the monocotyledonous plants.

 

Germination of African yam bean seed (hypogeal germination)

This is a dicotyledonous non-endospermic seed with hypogeal germination. Its germination is similar to that of cowpea, except that the cotyledons are left in the soil.

Germination begins with the absorption of water from the soil and subsequent swelling of the seed. The seed coat breaks and the radicle emerges and grows downwards. The epicotyl grows and loops above the soil. The looping of the epicotyl is important because it offers enough protection to the plumule by pushing through the soil before it. The epicotyl straightens and carries the plumule into the air. The plumule opens, turns green in the presence of light to become the first foliage leaves. The radicle develops into a tap root system.

 

How the developing embryo obtains energy during germination

It has earlier been pointed out that for a viable seed to germinate, certain conditions are necessary. These are water, oxygen and suitable temperature. Water is necessary to reactivate the enzymes which hydrolyse the complex food materials stored either in the cotyledons or in the endosperm of the seed to simple soluble products which can be passed to the embryo for respiration and growth. It is for this reason that germination starts with the absorption of water. During the enzyme hydrolysis, starch is hydrolysed to simple sugar, proteins are hydrolysed to amino acids and fats and oils are hydrolysed to fatty acid and glycerol. The oxygen of the air is required to oxidize the simple sugar to carbon (IV) oxide, water and energy, a process known as aerobic respiration. This is expressed in the equation.

 

This reaction takes place in the growing cells of the embryo (radicle + plumule) thus providing the energy required for growth.

 

The importance of suitable temperature

Seeds germinate over a range of temperature. But for a particular seed, there is a certain maximum temperature which is most suitable for its germination. This is called the optimum temperature. Any temperature far below the optimum temperature retards the rate of germination. Also temperatures above the optimum reduce the rate of germination and when the temperatures become too high or too low, seeds fail to germinate.

The reason is that enzymes responsible for hydrolysis in the seed are very sensitive to changes in temperature. They act best at the optimum temperature. At a lower temperature they are less active while at a very high temperature they are destroyed hence no germination occurs even if all other necessary conditions are available. Seeds therefore require a suitable temperature to germinate.

 

Conditions necessary for germination

For seeds to germinate the following conditions are necessary: water, suitable temperature, oxygen and viable seeds.

 

Experiment

Aim: To demostrate that water, oxygen and suitable temperature are necessary for germination.

Method: Into each of four boiling beakers, a piece of cotton wool is placed in the bottom. The boiling tubes are labelled with letters A,B,C,D. In boiling beaker A are placed dry broad bean seeds and placed in suitable temperature and air. There is no water.

Into beaker B, the cotton wools is moistened with water. Broad beans seeds are placed inside. The beaker and its contents are placed in a refrigerator regulated at 4°C. The seeds receive oxygen and water.

In beaker tube C, are placed broad bean seeds and the beaker is filled with water which has been boiled and cooled and oil is placed on the top of the water to prevent air from dissolving in the water. The beaker is placed in a suitable temperature.

In beaker D, the cotton wool inside is moistened, and the beaker and its content are placed in a suitable temperature and well provided with air. This is the “control experiment”.

The seeds are left for 4 days.

 

Observation: The seeds in beaker A,B,C. fail to germinate while those in D germinate.

Conclusion: The seeds in beaker ‘A’ fail to germinate because there is no water. Water is needed by germinating seeds to speed up biochemical activities of the seeds and also to soften the testa.

No germination in beaker ‘B’ because of unsuitable temperature. At this temperature, the enzymes necessary to digest the food will not work effectively.

No germination in beaker C because of the absence of air (oxygen) which is necessary for the breakdown of stored food.

In D where all conditions necessary are present, the seeds germinate.

Therefore water, suitable temperature and oxygen are necessary for germination.

 

Viable seed

A viable seed is a seed which is living and healthy.

It can be shown that only viable seeds germinate, by boiling some of the seeds used in the above experiments, and exposing them to the three conditions necessary for germination. They will not germinate because boiling kills the living cells of the seed as well as destroying their enzymes.

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