RBSE Solutions for Class 12 Biology Chapter 13 Plant Growth

RBSE Solutions for Class 12 Biology Chapter 13 Plant Growth

Rajasthan Board RBSE Class 12 Biology Chapter 13 Plant Growth

RBSE Class 12 Biology Chapter 13 Multiple Choice Questions

Question 1.
Which plant hormone was first of all discovered?
(a) Auxin
(b) Gibberellin
(c) Ethylene
(d) Cytokinin
Answer:
(a) Auxin

Question 2.
Which hormone is sprayed on rosette plant of Cauliflower to convert it into a long shoot?
(a) IAA
(b) ABA
(c) GA
(d) Ethylene
Answer:
(c) GA

 

Question 3.
A hormone found in a gaseous state is?
(a) Auxin
(b) Gibberellin
(c) Cytokinin
(d) Ethylene
Answer:
(d) Ethylene

Question 4.
Which hormone is most active in plants at the time of leaf shedding (in autumn season)?
(a) IAA
(b) ABA
(c) GA
(d) All the above
Answer:
(b) ABA

Question 5.
Apical dominance is found due to?
(a) Auxin
(b) Gibberellin
(c) Cytokinin
(d) Ethylene
Answer:
(a) Auxin

Question 6.
Which of the plant hormone is still not isolated?
(a) Auxin
(b) Florigen
(c) Cytokinin
(d) Gibberellin
Answer:
(b) Florigen

 

Question 7.
Which hormone promotes the growth of apical part?
(a) Cytokinin
(b) Auxin
(c) Gibberellin
(d) All of the above
Answer:
(b) Auxin

Question 8.
Which hormone is used to control dicotyledonous weeds in the field?
(a) IAA
(b) GA
(c) IBA
(d) 2 – 4D
Answer:
(d) 2 – 4D

Question 9.
Which growth regulator is known as a stress hormone in plants?
(a) IAA
(b) ABA
(C) IBA
(d) NAA
Answer:
(b) ABA

Question 10.
Who discovered phytochrome?
(a) Borthwick and Hendricks
(b) Boysen – Jensen
(c) Garner – Allard
(d) Darwin – Went
Answer:
(a) Borthwick and Hendricks

 

RBSE Class 12 Biology Chapter 13 Very Short Answer Questions

Question 1.
What do you understand by the term grand period of growth (GPG)?
Answer:
The total time period during which maximum growth takes place for completing all the phases of growth is called a grand period of growth.

Question 2.
Which substance is used for artificial ripening of fruits?
Answer:
Ethaphone (2-Chloro ethyl phosphoric acid).

Question 3.
What is the optimum temperature for chilling (Cold) treatment?
Answer:
Temperature between 0°C – 5°C.

Question 4.
What is Zeatin?
Answer:
Zeatin is the first naturally occurring cytokinin. Letham and Miller (1964) isolated this from the endosperm of maize.

Question 5.
What is Vernalin?
Answer:
Vernalin is a hormone-like substance produced in leaves during chilling (Cold) treatment. It is believed that it acts as a precursor of florigen hormone which induces flowering.

 

Question 6.
Name the hormone which causes senescence and results in the closing of stomata.
Answer:
Abscisic acid (ABA).

Question 7.
Write the chemical name of kinetin.
Answer:
Kinetin is 6- furfuryl amino purine.

Question 8.
Name two artificially synthesized auxins.
Answer:

• NAA (Naphthalene Acetic Acid).
• IBA (Indole -3 Butyric Acid).

Question 9.
Which hormone is used to promote parthenocarpy?
Answer:
Auxin and Gibberellin treatment induce parthenocarpy.

Question 10.
What is dormancy?
Answer:
Dormancy is a state of the temporary arrest of germination of seeds in spite of the presence of favourable conditions.

Question 11.
Which seeds are called photoelastic?
Answer:
Germination in some seeds is influenced by photoperiod. Such seeds are called photoelastic.

 

RBSE Class 12 Biology Chapter 13 Short Answer Questions

Question 1.
What is sigmoid growth curve?
Answer:
When a graph is plotted between the rate of growth and time, an S-shaped curve is formed. This is called the sigmoid growth curve.
This growth curve can be divided into four parts.

• Lag phase or lag period.
• Log phase or log period.
• Decline growth period
• Stationary phase or stationary period.

Question 2.
Differentiate between free and bound auxin.
Answer:
The auxins which can be easily isolated from cells are called free auxins and those which can be isolated with difficulty with the help of organic solvents are called bound auxins.

Question 3.
What is “Baknae” disease?
Answer:
In the crop field of paddy in Japan, some plants showed excessive growth in length and failed to flower. These plants were found infected with a fungus named as Gibberella fuji-Kuro. Hori (1898) studied this disease and named it a Baknae disease. These plants were commonly named as foolish seedling because of their unusual appearance in the crop field. Baknae in Japanese means foolish. Hence this disease was named as Baknae.

 

Question 4.
What do you understand by the term apical dominance?
Answer:

• In some plants, growth of lateral and axillary buds remain suppressed partially or completely under the influence of the apical bud. This effect of apical bud is called as apical dominance.
• In such cases, when the apical bud is removed the lateral and the axillary buds become active and begin to grow and the plant becomes bushy.

Question 5.
What is meant by bolting effect?
Answer:
In some plants, leaves are borne on the axis in the crowded form on short internodes. Such form of growth is called rosette. It has been observed that such plants show excessive internode elongation just before flowering. This stimulation of internode elongation just before flowering is called bolting. Gibberellins spray on such plants during the rosette stage results in bolting.

Question 6.
Why there is a decrease in dry-weight of a seedling at the end of germination?
Answer:
During germination of a seed, the reserve food material stored in the seed is used up in the formation of seedling hence the dry weight decreases.

Question 7.
Why the gardeners often cut the top or apical part of Hina (“Mehandi”) bush?
Answer:
By cutting of apical bud, the lateral and axillary buds become active and on account of this, the bush becomes dense. Hence to discourage apical growth and promote the growth of lateral buds, the top of the hedge is trimmed.

Question 8.
What is phytochrome and what is its significance?
Answer:
Phytochrome is bright blue or bluish-green protein found in the plant and acts as a photoreceptor pigment. It occurs in two forms. These are phytochrome PFR p700 (Phytochrome far red) and p660 (Phytochrome red pr.) These two forms are interconvertible. It is believed that these two forms control the formation of the hormone necessary for flowering in plants.

Question 9.
Why abscisic acid is called stress hormone?
Answer:
Abscisic acid helps the plant to survive during unfavourable conditions of the environment. It is observed that when there are a deficiency of water in leaves, ABA concentration increases and it promotes closure of stomata hence it is called stress hormone.

 

Question 10.
Write short notes on the following.

1. Dormancy.
2. After the ripening period.
3. Photoperiodism.

Answer:
1. Dormancy:
Seeds of some plants do not germinate just after their development on the plant, even if all conditions are favourable. This state of seed in which germination is temporarily suspended is called dormant stage and this condition is called dormancy. Seed dormancy may be due to internal factors or external factors.

2. After Ripening Period:
Seeds of some plants do not germinate immediately after their formation and enter in the resting phase. During the resting phase, some internal changes occur in the seeds and after this only they acquire the capacity to germinate. This resting period is called after the ripening period. Such seeds germinate after completing this period.

3. Photoperiodism:
The phenomenon of developmental responses of plants, in terms of transfer of plant from the vegetative phase to reproductive phase, to the relative light and dark period is called photoperiodism. Some plants enter flowering when exposed to uninterrupted light for more than a critical period. These are called long-day plants. On the other hand, some plants enter flowering when the day length is shorter than a critical time period. These plants are called short-day plants.

RBSE Class 12 Biology Chapter 13 Essay Type Questions

Question 1.
Write notes on the following:

1. Phases of Growth.
2. Growth Kinetics.

Answer:
1. Phases of Growth:
Plant growth is completed in three phases.

• The phase of Cell Division: This is the initial phase. The meristematic cells divide mitotically and add new cells continuously. This is also called a phase of formation.
• The phase of cell elongation: In this phase, the newly formed cells elongate in a particular direction. The thin and elastic cell wall stretches and a large vacuole begins to develop in the centre of the cell. New protoplasm is synthesized & dry weight of cell increases. This phase is also called a phase of enlargement.
• The phase of Cell differentiation: In this phase the cells mature, secondary wall material is deposited and cells stop dividing. Depending upon the function they are to perform, the cells undergo physiological and biochemical changes. This phase is also called a phase of maturation.

Phases of Growth:

• Meristematic cells divide mitotically to form new cells.
• Some of the newly formed cells continue to divided and add new cells whereas some begin to elongate and undergo differentiation to form permanent tissue and become part of mature tissue.
• The whole process of growth may be divided into the following three phases.

 

1. A phase of Cell Division: This is the first step of growth. In this phase, the number of cells increases by the division of meristematic cells.

• The meristematic cells continuously divide by mitosis and form new cells exactly similar to parent cells.
• The meristematic cells have dense cytoplasm, a conspicuous nucleus placed in the centre with other cell organelles and have very small vacuoles.
• The cell wall is thin and elastic and is made up of cellulose.
• These cells are metabolically highly active.
• This phase of growth is also called a phase of formation.

2. A phase of cell Elongation: This is the second phase of growth in which the cells stop dividing and begin to elongate in size.

• The increase in size is accompanied by the formation of a large central vacuole.
• The cytoplasm is pushed as a thin layer between the cell wall and the periphery of the vacuole.
• The nucleus is now situated in the cytoplasm near the cell wall.
• The new cell wall material is synthesized to cope up with the enlargement.
• In this phase, there is an irreversible increase in size and dry weight of the cell. This phase is also called a phase of enlargement.

3. A phase of Differentiation: In this phase, the cells differentiate and develop into specified permanent tissue.

• The cells begin to differentiate according to the function performed by these and according to their shape and size, they undergo conspicuous changes.
• The cell wall of the cells of mechanical tissue becomes highly thickened.
• Cell wall shows deposition of cellulose, suberin, lignin etc. and the cells mature, become strong and form permanent tissue.
• Xylem and phloem tissues are differentiated and cells undergo differentiation to form the components tissue of xylem & phloem.
• The cells show structural, physiological and biochemical differentiation by changes in their physiological and biochemical reactions.

 

2. Growth Kinetics:
Growth of an organism or part of an organism is expressed in the form of growth rate, for example – increase in a number of cells per unit time or increase in dry weight per unit time. The growth rate of an organism may be expressed arithmetically or geometrically.

• The growth rate is defined as an increase in the growth of an organism or a part of the organism per unit time. For example, an increase in the number of cells per unit time or increase in dry weight per unit time.
• Growth rate can be expressed arithmetically or geometrically.
• Geometrically, the two daughter cells formed by one cell again divide and hence after each division the number of cells becomes double. Example: 1-2-4-8-16 and so on.
• This geometric growth is seen in initial divisions of a zygote.
• During arithmetic growth, out of the two cells formed by each division, one differentiates into permanent tissue and one continues to divide.
• Hence in arithmetic growth, new cells are continuously added to the body resulting in the continuous growth. This can be expressed as 1-1-1-1.
• Such divisions occur in root and shoot apex.
  

Growth Curve:

• Keeping all the factors controlling growth at optimum, the pattern of growth of the cell or organ or the whole plant is not the same at all times.
• When a graph is plotted taking growth rate on one axis and time on the other, an ‘S’ shaped or sigmoid curve is obtained. This is called a growth curve.
• The whole growth curve can be divided into four parts.

1. Lag Phase: This is the initial phase or period of growth. In this period, the cell undergoes internal changes and stored food material is used resulting in a loss in dry weight. New cells are added due to repeated cell division and the size increases gradually.

2. Log Phase: In this phase growth rate gradually becomes rapid and the size increases exponentially with time. It means the growth rate is slow at first but it increases continuously. It is also known as the exponential phase.

3. Decline Phase: The growth rate becomes again slow because the cells mature in this phase. The metabolic activities also become slow.

4. Stationary or Steady phase: In this stage cells become fully mature and the rate of growth becomes almost steady. According to Sachs (1882), the time period during which maximum growth takes place is called a grand period of growth (GPG)

 

Question 2.
How growth is measured? Describe different factors affecting growth.
Answer:
Measurement of Growth:
Growth in plants can be measured by one of the following methods.

• By the increase in a number of cells produced.
• By measuring the increase in the size of cells, tissues and plant parts such as leaves, flowers & fruits.
• By an increase in the dry weight.
• By the increase in length and girth of stem and root.
• By linear measurement.

This can be done by two methods.

1. Simple method: By direct measurement with scale.
2. By auxanometer.

1. Simple or Direct Method:

• The most simple method to measure the growth in length of the plant or any organ of a plant is, where the initial length is measured by a scale.
• After a particular interval of time, the length is again measured by the scale.
• The increase in length denotes the growth in that time period.

2. By Auxanometer:

• Normally plant growth is measured by linear growth. For this purpose, Auxanometer instrument is used.
• Here we will learn about a simple arc auxanometer.

• Increase in the length of the plant is measured by arc auxanometer.
• One end of a silk thread is tied to the stem tip of a potted plant.
• This thread is hung on the pully of auxanometer.
• The other end of the thread is kept stretched by putting a suitable weight with it.
• The pulley is fixed with a long pointer which slides over a graduated arc. An initial reading of pointer is noted.
• As the stem grows in length, the weight pulls the thread down.
• This results in the movement of pully and the pointer moves on the graduated arc. The final reading of pointer is now noted & the growth is calculated by the following formulae.

Factors Affecting Growth:
The plant growth is influenced by a variety of external and internal factors. These factors may be external or environmental factors and internal or physiological factors.

 

External or environmental factors include seasonal and soil-related factors and Internal or Physiological factors include absorption of water and nutrients, photosynthesis, respiration etc.
Following are some of the important factors affecting plant growth.
1. Light:
Light affects growth in many ways such as:

• Light Intensity: High intensity of light slows down the growth. It induces dwarf-ness in a plant as it reduces the size of nodes/internodes and size of leaves become small.
• Quality of Light: Pigments found in plants absorb a specific wavelength of light. The red part of the light is most appropriate for growth, whereas ultraviolet light rays diminish plant growth.
• Duration of Light: Duration of light has a remarkable impact on the growth of vegetative or reproductive structures. For flowering to occur in the plant, they require a fixed period of light known as photoperiod. In the absence of adequate photoperiod, the flowering process stops.

2. Water:
Availability of water has a direct correlation with the growth rate. The amount of water available to plants influences plant growth as well as its development. All biological processes of plants such as absorption, osmosis, transpiration, photosynthesis, respiration, germination etc. depend on water.

3. Temperature:
Plant growth is the outcome of cellular metabolism. Since the metabolic activity of plants is directly affected by variation in temperature, the growth rate is also influenced. So it affects the growth directly or indirectly.

Generally, there is normal growth of plants between 5°C to 35°C. Increase in temperature above 35°C leads to, damage to plants because of heating effect and below 0°C temperature leads to freezing of water and inactivity of protoplasm and ultimately cells die.

4. Oxygen Supply:
Quantity of oxygen increases growth in plants because along with growth, oxygen takes part in other biological processes, by helping in respiration process to convert potential energy into kinetic energy.

5. Mineral Salts:
Deficiency of mineral nutrients may lead to deficiency disorders or diseases and leads to decreased or stunted growth.

6. Plant Hormones:
Growth of plants is regulated by some organic compounds which are found in very minute quantity. These compounds are called Plant hormones or Growth hormones/regulators. Deficiency of these leads to stoppage in the growth of plants.

 

Question 3.
What is auxin? Critically analyse the physiological effects of auxin on growth.
Answer:
Auxin word originated from the Greek word Auxin which means “To grow” or increase in growth. Presently auxin term represents the group of plant hormones which promote the growth of stem or coleoptile. Indole Acetic Acid (IAA) and those naturally occurring and artificially synthesized products which have some properties as IAA are termed as Auxins.
Physiological Effects of Auxins on Plant growth are as follows:

1. Apical dominance.
2. Cell elongation.
3. Root initiation.
4. Parthenocarpy.
5. Prevention of lodging.
6. Control of dormancy.
7. Thinning of flowers.
8. Effect on abscission.
9. Eradication of weeds.
10. Shortening of Internodes.
11. Tissue Culture.

1. Apical Dominance:

• It is commonly observed that if the apical bud remains intact and growing, the lateral buds remain suppressed. This is called apical dominance.
• Removal of apical bud causes fast growth of lateral buds, and the plant becomes bush-like.
• It is because of this the hedge is regularly cut and trimmed to remove apical buds so that it becomes dense.

2.Cell Elongation:

• The main function of auxin is to induce elongation of the cells which are formed by division of cells of the apical meristem.
• The high concentration of auxin in apical region promotes cell elongation.
• Because of this, the shoot apex is negatively geotropic and positively phototropic.

3. Root Initiation:

• Auxins promote the initiation of adventitious roots from the nodes or basal region of stem.
• Plant growers use BA and NAA to induce root formation in the stem.
• Auxins promote root growth at extremely low concentration (i.e. 10^–7 to 10^–13 M concentration).
• If the lower end of stem cutting is dipped in auxin and then planted, roots are rapidly developed. Example: Rose, Bougainvillea, Citrus, etc.

4. Parthenocarpy:

• The process of development of ovary into the fruit without fertilization is called parthenocarpy.
• Fruits formed by parthenocarpy are called parthenocarpic or seedless fruits.
• For inducing parthenocarpy, the stamens are removed in the bud stage and auxin is sprayed on the stigma.
• Seedless fruits are developed by external application of auxin in tomatoes, apples, cucumbers, orange, lemon, brinjal and grapes etc.

5. Prevention of Lodging:

• In many crop plants such as wheat, the thin and weak stem bends near the base due to strong winds and falls on the ground. This is called lodging.
• If the solution of auxin is sprayed on small young plants, the basal portion of the plants become strong and chances of falling in the strong wind are reduced.
• Hence use of auxin prevents lodging.

 

6. Control of Dormancy:

• Auxin maintains dormancy in seed and vegetative propagules (Rhizome etc.).
• Auxin prevents germination of seeds and sprouting of buds so that these can be stored for a long duration.
• Potato tubers sprayed with NAA can be stored for about three (3) years.

7. Thinning of Flowers:

• In some plants such as some varieties of mango, excessive flowering occurs in some years.
• As a result, more fruits are formed, but they are small-sized. A spray of NAA can help in preventing excessive flowering.

8. Effect on Abscission:

• Premature fall of leaves, flowers and fruits is called abscission.
• This occurs due to the formation of the abscission layer on account of deficiency of auxin.
• Auxin prevents the formation of abscission layer.
• A spray of NAA, 2,4-D, BA etc. can help in preventing or delaying premature fall of leaves, flowers and fruits.

9. Eradication of Weeds:

• Unwanted plants growing in a crop field along with crop are called weeds.
• They compete with the crop for water, minerals, light, and space etc. and do not allow proper growth of the crop.
• By use of auxin, we can get rid of these weeds.
• Weeds with broad leaves can be destroyed by spraying 2, 4-D (2, 4-Dichlorophenoxy acetic acid) and grass weeds can be destroyed by the use of a synthetic hormone “Delapen” (2, 2-Dichloropropionic acid.)

10. Shortening of Internodes:
In some plants such as apple and pear etc. fruits are formed on short branches. In such plants by the spray of NAA, the internodes of long branches are shortened and the number of short branches is increased and hence the number of fruits is increased.

11. Tissue Culture:

• Tissues and organs are extensively cultured artificially by tissue culture technique.
• Auxins play an important role in root formation and differentiation of callus.
• A spray of these auxins also helps in an increasing number of female flowers in cucurbits.

 

Question 4.
Describe main experiments conducted by a different scientist in context with the discovery of auxins.
Answer:
The following scientist carried out important experiments in the discovery of auxins.

1. Charles Darwin (1881).
2. Boysen Jensen (1910 – 1913).
3. Paal (1919).
4. F.W. Went (1928).

In the case of root apex, auxin retards the growth and hence dut to decreased growth on the lower side, the root tip bends downwards, causing downward curvature of the radicle.

Definition of Auxins:
The organic compounds capable of inducing elongation of shoot and inhibition of elongation of the root at low concentration (below 10^–3M or 0.001M) are known as auxins.

Discovery of Auxins:

• Charles Darwin (1881) known for the theory of evolution was first to realise the presence of a growth-promoting substance at the apex of the plant.
• Charles Darwin and Franci’s Darwin described their work, done on canary grass (Phalaris canariensis) in their book “The power of movements in plants”.
• According to Darwin, coleoptile shows bending towards a source of light when it is illuminated from one side.
• On the basis of this experiment they concluded that when coleoptile is given unidirectional light, some substances are formed at the tip which is transported downwards and influences differential growth in the nonilluminated side to cause curvature towards the light.
• The experiments of Darwin were carried forward by Boysen Jensen (1910 – 13) and Paal (1919).
• Boysen Jensen – (1910 – 1913) – Experimented on oat (Avena sativa coleoptile) and found that when the coleoptile is decapitated (tip of coleoptile is removed), the coleoptile does not show curvature towards the light.
• It was seen that if the decapitated tip is replaced on the cut end, the coleoptile bends towards light.
• It was also found that if a thin mica sheet is inserted between the decapitated tip and the stump, curvature does not take place.
• But if in place of mica a block of gelatin is placed between the stump and decapitated tip curvature resulted in the usual way.

• It was further seen that if a horizontal slit is made in the non-illuminated side (dark side) and a mica sheet is inserted, the coleoptile does not bend towards the light.
• But if the mica sheet is inserted in the slit towards the illuminated side, the coleoptile shows curvature towards the light.
• From these results, Boysen Jensen concluded that some substance responsible for curvature is synthesized in the tip of coleoptile and migrates down towards the dark side (nonilluminated side).
• Paal (1919) also conducted similar experiments and proved that there are certain chemicals present at the apex, which promote growth. These chemicals are synthesized in the apex and are water-soluble.
• F.W. Went (1926 – 1928) is credited for the discovery of auxins, as he isolated this growth-regulating substance. He provided final proof of the presence of some substance responsible for the growth and is produced by the apex.
• Went placed several decapitated tips of Avena coleoptile on a thin block of agar-agar (a polysaccharide which becomes gelatin-like when dissolved in hot water and then cooled for some time) and then removed the complete tips after some time.
• This agar block was then cut into small pieces and each block was placed eccentrically on the cut end of coleoptile (stump).
• It was observed that characteristic bending occurred although the experiment was performed in darkness.
• It was also found that by placing ordinary untreated agar block, there was no curvature.
• This clearly established that some substance leached out (moved downwards) in the agar block from the coleoptile tip and then it migrated downward in the coleoptile where it triggered longitudinal growth on one side.
• In his experiments Went observed that when coleoptile is illuminated from one side, the amount of auxin is less in the illuminated side (35%) and it migrates towards the nonilluminated side (65%).
• Displacement of the hormone on nonilluminated (shaded) side of the tip causes an increase in the amount of hormone resulting into growth on the shaded side and hence the curvature towards the direction of light.
• Experiments conducted by several other scientists proved that when plumule (shoot apex) is kept in the horizontal position, hormone migrates towards the lower side due to gravitational force.
• Due to an increase in the concentration of auxin in the lower cells, these cells divide and the short apex bends towards the upper side. This causes upward curvature of the shoot apex.
• In the case of root apex, auxin retards the growth and hence dut to decreased growth on the lower side, the root tip bends downwards, causing downward curvature of the radicle.

 

Question 5.
What is seed dormancy? Describe various reasons for dormancy and various methods of breaking seed dormancy.
Answer:
Normally a fully developed seed must germinate if conditions are favourable for germination. But in some plants fully formed seeds do not germinate immediately even if favourable conditions are provided. This state of temporary suspension of germination of seeds is called dormancy. This period of suspension many varies in plants.
The dormancy may be on account of the following:

1. Hard seed coat
2. Immature embryo.
3. The requirement of the after-ripening period.
4. A requirement of specific temperature and light.
5. Presence of germination inhibitor.

Depending upon the reason, the following methods are used for breaking seed dormancy.

1. Scarification.
2. Chilling treatment.
3. Exposure of specific light.
4. Exposure of alternate temperature.

Dormancy:

• When seeds are fully developed their further growth stops.
• In some plants, the fully developed seeds germinate immediately after their formation, if favourable conditions are available.
• But in some other plants, the fully formed seeds do not germinate even when all conditions for germination are favourable.
• This indicates that these seeds are functionally inactive.
• This state of seeds is called dormant state and the condition is called dormancy.
• In dormant seeds germination is temporarily suspended for some time and these seeds remain viable for a long duration.
• The dormancy period of seeds of some plants is given in the following table.
  

  Name of Plant	Suspended Period/Dormancy
  Rhizophora, Pea, Pulses	Absent
  Most crop Plant	2-5 years
  Desert Plants (Acacia, Grasses)	5-10 years
  Lupin (In arctic region Tundra)	100 years

Factors affecting dormancy:
Seed dormancy is due to many external or internal factors as described below:
1. Hard seed coat:
In some seeds, the seed coat is very hard. It is impermeable to water (Gram, Pea) and oxygen (Xanthium) required at the time of germination. Sometimes it acts as a mechanical barrier and resists the development of an embryo.
Example: Amaranthus.

2. Immaturity of an embryo:
In some plants, seeds are dispersed (dehiscence) even before the maturity of the embryo. So germination of seed will not occur until unless the embryo fully matures.
Example: Ginkgo
biloba, Gnetum anemone.

3. A requirement of “after-ripening period”:
Seeds of some plants do not germinate immediately after formation. But germinate after completing a certain resting phase. During the resting stage, they acquire the capacity of germination. This resting period is “after-ripening period”. It varies from weeks to months in different plants.
Example: wheat, Sorghum, oats.

 

4. A requirement of specific temperature and light:
seeds of some plants require a cold treatment before germination. Until unless these seeds pass through specific cold temperature treatment, they do not germinate. In the winter season, these seeds germinate naturally. The optimum temperature for cold treatment is 0°C to 5°C.
Example: Cherry, Oak.
In similar way germination of some seeds depend on photoperiod, quality and quantity of light. These seeds are sensitive to light factors and are called Photoblastic.
Example: Tobacco, Capsella etc.

5. Presence of germination inhibitors:
There are certain substances present in fleshy fruits, which inhibit germination. It includes abscisic acid (ABA). coumarin, para – ascorbic acid, phenolic acid etc. When these seeds remain in the soil, these substances are gradually passed away (filtered out) and then germination can occur. The inhibitory effect of these substances can be neutralised by application gibberellin.

Methods of breaking seed dormancy:
Various methods are applied for breaking see dormancy. These depend on dormancy factor a plant species. Some of the methods to break seed dormancy are given below:
1. Scarification:
The hard seed coat is weakened either t breaking or scratching so that the seeds can absorb water easily. Seeds are also made soft either t soaking them in dilute sulphuric acid (H₂SO₄), hot water or fat solvents.

2. Chilling Treatment:
Those seeds which require chilling treatment for breaking dormancy an exposed to artificial chilling treatment to break the dormancy.

3. Exposure to alternate temperature:
Seed dormancy in some can be broken by subjecting the to alternate high and low-temperature exposure.

4. Light:
Exposure of positive photoelastic seed with red light can increase the efficiency of seed germination.

5. Pressure:
Some seeds if kept on 2000 atmospheric pressure at 18°C to 20°C show increase see germination capacity because of softening of see the coat and increase in permeability.

6. Use of growth regulators:
Dormancy due to chilling treatment, after-ripening period, presence germination inhibitors, a requirement of specific light can be broken by the use of growth regulators.
Example: Gibberellin, Ethylene, Chlorohydrin and Thiourea.

Question 6.
Write Essay on Following:

1. Gibberellin and Cytokinin.
2. Growth inhibitor substances.
3. Photoperiodism.
4. Senescence and Abscission.
5. Vernalization.

Answer:
1. Gibberellins:

• Gibberellins were discovered in Japan and the discovery is related to a disease in Rice (Paddy) plant.
• In 1890 a disease occurred in the Rice fields in japan where some Rice plants grew extraordinary tall and remained flowerless.
• This disease was named as “Bakanae disease” (In Japanese language Bakanae means foolish) Hori (1898) Studied this disease in detail and found that these plants were infected with a fungus Gibberella fuji-Kuro (perfect state of an imperfect fungus Fusarium moniliform).
• The infected seedlings grew extra tall, and thin and do not bear flowers, fruits and seeds. So these were called “foolish seedling” and the disease as a foolish seedling disease.
• Kurosawa (1926) proved that secretion by the fungus sprayed on the healthy plant results in this disease.
• Yobuta and Hayashi (1939), isolated the substance in pure crystalline form from the culture filtrate of Gibberella fuji-Kuro and named it Gibberellin.
• Brian et. al (1954) isolated pure form of single gibberellin and named if Gibberellic acid.
• About 100 different gibberellins have been obtained from different fungi and higher plants. These are known as GA₁, GA₂, GA₃, G₄……… GA₁₀₀. Among this GA₃ is the first discovered and commonly found gibberellin.

 

2. Cytokinins:

• G Haberland (1913) First observed that in the phloem of some plants, some substances are found which stimulate cell division.
• J. Van Overbeek (1941) showed that there are certain substances in coconut milk which promote cell division.
• Skoog and Miller (1955) isolated a very useful substance from DNA of yeast which promotes cell division and named it as kinetin.
• Letham (1963) named Kinetin as Cytokinin.
• Letham and Miller (1964) isolated a substance, similar to cytokinin, from the endosperm, of maize and called it Zeatin.
• Zeatin is the first naturally occurring cytokinin.

3. Growth inhibitor substance:

• Those hormones or substances which inhibit the rate of growth are called growth inhibitors.
• These substances are required for controlling and balancing growth and development.
• Abscisic acid is the most important growth-inhibiting hormone.

Abscisic Acid (ABA):

• This is the most important naturally occurring growth inhibitor found in plants.
• It helps the plant to successfully survive adverse environmental conditions.
• Hence it is also called stress hormone.
• Wareing (1963) isolated one growth-inhibiting substance from the leaves of Acer plant and named it as Dormin.
• Addicott et. al. (1963) isolated one substance from the floral buds of cotton and named it as Abscisin.
• Later on, it was proved that domain and abscision are the same substance and these were named as Abscisic Acid (ABA).
• The chemical formula of ABA is C₁₅H₂₀O₄.
• It is made up of 5 carbon-containing three isoprene units.
• It contains one carboxylic group (COOH).
• ABA is synthesized by the degradation of carotenoids in chloroplast on account of deficiency of water.

4. Photoperiodism:

• In flowering plants, the transformation from the vegetative phase to reproductive phase is controlled by several factors and duration of light (Photoperiod) is one among these.
• The role of duration of light in inducing flowering in plants is called photoperiodism.
• In 1920, W.W. Garner and H.A. Allard of the U.S. Department of Agriculture first demonstrated the role of photoperiod in the control of flowering.
• These workers observed that the Maryland mammoth variety of tobacco (Nicotiana tabacum) sown in spring despite rich and profuse vegetative growth failed to flower during summer.
• When the seeds of the same variety were grown in a greenhouse during winter and the seedlings were transferred to the field, produced flowers and fruits at the beginning of summer.
• On the basis of the results of these experiments, these scientists concluded that in these plants a critical photoperiod is necessary for flowering. In this variety of tobacco, flowering is controlled by a critical factor and it was the short day length which controls flowering.
• They called this critical factor as photoperiod and defined photoperiodism as “Length of the day which is favourable for flowering and response of plant towards adequate day length or adequate photoperiod is called photoperiodism”.
• According to Hillman (1969) – Developmental response of plants to the relative lengths of light and dark period is called photoperiodism.
• It is a physiological response of a plant to the length of day or night.
• On the basis of photoperiodic response, plants are classified as follows.
  1. Short Day Plants
  2. Long Day Plants
  3. Day Neutral Plants

 

5. Senescence:

• No organism is immortal and every organism has borne on earth, after completing some time (it’s life span), dies. The duration between the birth and natural death of an organism is called its life span.
• When an organism attains maturity during its life span, such a deteriorative process begins which leads to the death of the organism. Hence the period between the maturity of an organism and its death is called senescence.
• During senescence the organism becomes weak, and its functional capacity decreases.
• Metabolic substances are stored and the dry weight decreases.
• A.C. Leopold (1961) has described four types of senescence.

1. Whole plant senescence: In most annual plants, after fruit formation, the whole plant becomes yellowish and finally dies. This is called whole plant senescence.
Example: Wheat, Grain, Mustard, Tomato etc.

2. Top or shoot senescence: In this type of senescence, the above-ground part dies every year after the end of the growing season and the underground stem and roots remain living. In the next growing season, buds develop in the underground stem and aerial parts are again formed.
Example: Potato, Ginger, Onion etc.

3. Leaf Senescence: In most perennial trees, leaves die in autumn and fall off. The stem and roots survive and remain alive. On return of favourable conditions, new leaves are again formed.

4. Gradual senescence: In the majority of annual plants during the normal life span, the old leaves die first and fall off. Gradually other leaves, stem and root also undergo senescence and die. In this type of senescence, the whole plant disappears gradually. This is called sequential senescences.

• ABA mainly promotes senescence in leaves and ethylene promotes senescence in fruits.
• Auxins, gibberellins and cytokinin, in most cases, counter or delay the process of senescence.

6. Abscission:

• Shedding of leaves, flowers and fruits from the mother plant by a natural process is called abscission.
• It is a biological process which is initiated due to changes in the cells of the basal portion of the parts which fall off.
  The middle lamella and the cell wall of the cells in the basal portion are digested by pectinase and cellulase enzymes.
• As a result of breaking of middle lamella and cell-wall, cells in a definite region begin to separate from each other.
• The tissues in this area become weak and soft and a layer is formed which is called an abscission layer.
• The cells situated just below the abscission layer become meristematic and form cork cells. This cork forms a protective layer or abscission zone.
• Due to strong wind or rain, the plant part separates from the region of abscission and fall off.
• Abscission is due to a change in the hormonal balance. Abscisic acid plays a major role in the process of abscission.

 

7. Vernalization:

• In the majority of plants, besides photoperiod temperature also has an important role in growth and development.
• The response of plants towards temperature relating to growth and development was first observed by a Russian scientist Lysenko (1928) while working on winter and spring varieties of rice.
• In annual plants, light has a major effect on flowering and the effect of temperature is secondary.
• In biennial plants the situation is different. In these plants, there is only vegetative growth in the first year and flowering is completed in the second year.
• Before flowering cold temperature exposure of winter is necessary for these plants, and in absence of cold treatment reproductive stage does not begin and they remain in the vegetative stage.
• The method of inducing flowering by chilling (cold) treatment is called vernalization.
• According to Choird (1960), vernalization can be defined as “Inducing ability of flowering by chilling treatment artificially is called vernalization”.

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