🌿 Seed Germination

Seed germination is the restart of metabolic activity in a viable seed under favorable conditions. The sequence from imbibition to seedling emergence and the basis of seed dormancy are both high-yield exam topics.

---

The process of seed germination starts with the imbibition of water by seed coat and

emergence of growing root tip of embryo. The process ends with the development of embryo

into a seedling.

---

Physiological and biochemical changes during seed germination

1. Water uptake

Seed germination starts with the imbibition of water by dry seed coat. Due to

imbibition of water, the seed coats become 1) More permeable to O2 and water and 2) less

resistant to outward growth of embryo.

2. Respiration

Rapid increase in respiration rate of embryo occurs. Sucrose is probably the

respiratory substrate at this stage which is provided by endosperm.

3. Mobilization of reserve materials

As germination progresses, there is mobilization of reserve materials to provide.

1. building blocks for the development of embryo

2. energy for the biosynthetic process and

3. nucleic acids for control of protein synthesis and embryonic development

Changes in these components are as follows

i) Nucleic acids

In monocots, during imbibition, there is a rapid decrease of DNA and RNA content in

the endosperm with a simultaneous increase in the embryonic axis probably due to their

transportation as such. High concentration of RNA in the embryonic axis precedes cell

division. Due to more cell division, DNA content is increased.

ii) Carbohydrates

Insoluble carbohydrates like starch are the important reserve food of cereals in the

endosperm. During germination, starch is hydrolyzed first into maltose in the presence of α

amylase and β - amylase and then maltose is converted into glucose by maltase. The glucose

is further converted into soluble sucrose and transported to growing embryonic axis. During

germination, the embryonic axis secretes gibberellic acid into the aleurone layer which

causes synthesis of α-amylase.

3. Lipids

Plants like castor bean, peanut etc., store large amount of neutral lipids or fats as

reserve food in their seeds. During germination, the fats are hydrolyzed into fatty acids and

glycerol by lipase enzyme. Fatty acids are further converted into acetyl – CoA by the

process, β - oxidation. The acetyl CoA is further converted into sucrose via glyoxylate cycle

and is transported to the growing embryonic axis.

4. Proteins

Some plants store proteins as reserve food in their seeds in the form of aleurone

grains. Proteins are hydrolyzed into amino acids by peptidase enzyme. The amino acids

may either provide energy by oxidation after deamination (removal of amino group) or may

be utilized in the synthesis of new proteins.

5. Inorganic materials

A number of inorganic materials such as phosphate, calcium, magnesium and

potassium are also stored in seeds in the form of phytin. These stored materials are liberated

during germination due to the activity of various phosphatases including phytase.

---

Emergence of seedling out of the seed coat

All the changes described above gradually result in splitting of seed coat and

emergence of the growing seedling. The radical comes out first and grows downward, and

then plumule comes out and grows upward. Due to the continued growth of this seedling, the

plumule comes out of the soil, exposed to light and develops its own photosynthetic

apparatus.

Splitting of seed coat may take place either by imbibition pressure or by internal

pressure created by the growing primary root or by hydrolytic enzymes which act on cell

wall contents of seed coat and digest it (e.g. cellulose and pectinase). Sometimes the seed

coat may be extensively rotted by the activity of micro-organisms in the soil.

---

DORMANCY OF SEEDS

All the viable seeds have capacity to germinate if placed under suitable conditions

necessary for germination. But, some seeds fail to germinate sometimes even if placed under

the condition favourable for germination. This may be due to some internal factors or due to

specific requirement for some environmental factors. During this period, the growth of the

seed remains suspended and they are said to be in rest stage or dormant stage and this

phenomenon is called as dormancy of seeds.

---

Factors causing dormancy of seeds

1. Seed coats impermeable to water

The seeds of certain plants especially those belonging to the family’s leguminaceae,

solanaceae, malvaceae, etc. have very hard seed coats which are impermeable to water. The

seeds remain dormant until the impermeable layer decay by the action of soil micro

organisms.

2. Seeds coats impermeable to oxygen

In many plants such as cocklebur and many grasses, the seed dormancy is due to the

impermeability of the seed coat to oxygen. However, during the period of dormancy, the

seed coat gradually becomes more permeable to oxygen so that they may germinate.

3. Immaturity of the Embryo

In certain orchids, the seed dormancy is due to the immaturity of the embryos which

fail to develop fully by the time the seeds are shed. In such cases, the seeds germinate only

after a period or rest during which the development of embryo inside the seed is completed.

4. Germination Inhibitors

In certain seeds, the dormancy of the seeds is due to the presence of certain

germination inhibitors like coumarin, ferulic acid, abscissic acid, etc. These may be present

in endosperm, embryo, testa or juice or pulp of fruit.

5. Chilling or low temperature requirement

In certain plants such as apple, rose, peach etc, the seeds remain dormant after harvest

in the autumn as they have a low temperature or chilling requirement for germination. In

nature, this requirement is fulfilled by the winter temperatures. In such case the seeds remain

dormant throughout the winter season and germinate only in the following spring.

6. Light sensitive seeds

In many species, the germination of the seeds is affected by light resulting in seed

dormancy. Such light sensitive seeds are called photo blastic . Seeds of lettuce, tomato and

tobacco are positively photo blastic and germinate only after they have been exposed to light.

On the other hand, the seeds of certain plants are negatively photo blastic and their

germination is inhibited by light.

Advantages of dormancy

1. In temperature zones, the dormancy of seeds helps the plants to tide over the severe

colds which may be injurious for their vegetative and reproductive growth.

2. In tropical regions, the dormancy of seeds resulting from their impermeable seed

coats ensures good chances of survival.

3. Dormancy of seeds in many cereals is of utmost importance to mankind. If these

seeds germinate immediately after harvest in the field, they will become useless to

man for consumption as food.

---

Summary Cheat Sheet

Germination Essentials

• Germination begins with water uptake (imbibition).
• Enzyme activation mobilizes stored reserves for embryo growth.
• Radicle emergence precedes plumule emergence in normal seedling development.
• Dormancy prevents germination even when external conditions seem favorable.

Exam Traps

• Viability and dormancy are different: viable seeds may still remain dormant.
• Dormancy can arise from seed coat barriers, inhibitors, or embryo immaturity.
• Chilling and light requirements are species-specific dormancy controls.

References

1 source • [1]

[1]

Plant Physiology course notes - Seed Germination and Dormancy

Book