Class 11 Biology NCERT Notes- Chapter 13: Plant Growth and Development

Detailed Study Notes – Chapter 13: Plant Growth and Development (Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. Introduction to Plant Development

Development in a plant is the culmination of two fundamental processes: growth and differentiation. The entire life cycle of a plant, from a zygote (fertilized egg) to a mature organism, follows a highly ordered sequence of events. This process results in the formation of a complex body with structures like roots, leaves, branches, flowers, fruits, and seeds, eventually leading to senescence and death.

• Life Cycle Initiation: The first step in this process is seed germination, which occurs when environmental conditions are favorable.
• Dormancy: In the absence of favorable conditions, seeds enter a period of suspended growth or rest.
• Controlling Factors: Developmental processes are governed by both intrinsic (internal) and extrinsic (external) factors.

2. Growth

Growth is a fundamental characteristic of living organisms, defined as an irreversible permanent increase in the size of an organ, its parts, or an individual cell. It is an active process accompanied by metabolic activities (anabolic and catabolic) that require energy.

2.1. Plant Growth is Indeterminate

Unlike many animals, plants exhibit an “open form of growth,” meaning they have the capacity for unlimited growth throughout their lives. This is due to the presence of meristems, which are specific locations in the plant body containing cells that can continuously divide and self-perpetuate.

• Primary Growth: The root apical meristem and shoot apical meristem are responsible for primary growth, which is the elongation of the plant along its axis.
• Secondary Growth: In dicots and gymnosperms, lateral meristems (vascular cambium and cork-cambium) appear later and are responsible for increasing the girth of the organs, a process known as secondary growth.

2.2. Growth is Measurable

Growth is principally an increase in the amount of protoplasm. Since this is difficult to measure directly, growth is quantified using various parameters:

• Increase in fresh weight
• Increase in dry weight
• Increase in length, area, or volume
• Increase in cell number

Examples of Measurable Growth:

• A single maize root apical meristem can produce over 17,500 new cells per hour.
• A cell in a watermelon can increase in size by up to 350,000 times.
• Pollen tube growth is measured by length.
• Dorsiventral leaf growth is measured by surface area.

2.3. Phases of Growth

The period of growth is generally divided into three distinct phases, which can be observed at root and shoot tips:

1. Meristematic Phase:
   • Occurs at the root and shoot apices.
   • Characterized by constantly dividing cells.
   • Cells are rich in protoplasm, have large conspicuous nuclei, and thin, primary cellulosic walls with abundant plasmodesmatal connections.
2. Elongation Phase:
   • Located just proximal to the meristematic zone.
   • Characterized by increased vacuolation, cell enlargement, and new cell wall deposition.
3. Maturation Phase:
   • Located further from the apex, proximal to the elongation zone.
   • Cells attain their maximal size, with significant wall thickening and protoplasmic modifications.
   • Most specialized tissues and cell types are found in this phase.

2.4. Growth Rates

The growth rate is the increased growth per unit time. It can be expressed mathematically and occurs in two primary ways:

1. Arithmetic Growth:
   • Following mitotic division, one daughter cell continues to divide while the other differentiates and matures.
   • Results in a constant rate of elongation, producing a linear curve when plotted against time.
   • Formula: Lt = L0 + rt
     • Lt = length at time ‘t’
     • L0 = length at time ‘zero’
     • r = growth rate (elongation per unit time)
2. Geometric Growth:
   • Initially slow (lag phase), followed by a rapid exponential increase (log or exponential phase), and finally slows down due to limited nutrients (stationary phase).
   • Both progeny cells after mitosis retain the ability to divide.
   • When plotted, this growth pattern forms a characteristic sigmoid (S-curve), typical for living organisms in a natural environment.
   • Formula: W1 = W0 * e^(rt)
     • W1 = final size
     • W0 = initial size
     • r = relative growth rate (also called efficiency index)
     • t = time of growth
     • e = base of natural logarithms

Absolute vs. Relative Growth Rate:

• Absolute Growth Rate: The measurement and comparison of total growth per unit time.
• Relative Growth Rate: The growth of the system per unit time expressed on a common basis, such as per unit of the initial parameter. A smaller initial organ can have a higher relative growth rate even if its absolute growth is the same as a larger organ.

2.5. Conditions for Growth

Essential conditions required for plant growth include:

• Water: Necessary for cell enlargement, maintaining turgidity for extension growth, and as a medium for enzymatic activities.
• Oxygen: Required for releasing metabolic energy through respiration to fuel growth activities.
• Nutrients: Macro and micro essential elements are needed for the synthesis of protoplasm and as an energy source.
• Temperature: Plants have an optimum temperature range for growth; deviations can be detrimental.
• Light and Gravity: Environmental signals that affect certain phases and stages of growth.

3. Differentiation, Dedifferentiation, and Redifferentiation

These processes describe the changes in a cell’s structure and function throughout its life.

• Differentiation: The process by which cells derived from meristems (root apical, shoot apical, cambium) mature to perform specific functions. This involves major structural changes in cell walls and protoplasm. For example, to form a tracheary element for water transport, a cell loses its protoplasm and develops a strong, elastic, lignocellulosic secondary wall.
• Dedifferentiation: The phenomenon where living, differentiated cells that have lost the ability to divide regain this capacity under certain conditions. For example, fully differentiated parenchyma cells forming meristems like interfascicular cambium and cork cambium.
• Redifferentiation: The process where cells produced by dedifferentiated tissues (like cork cambium) once again lose the capacity to divide and mature to perform specific functions.

Open Differentiation: Like growth, differentiation in plants is “open.” This means cells arising from the same meristem can develop into different structures at maturity, often determined by their location. For example, cells positioned away from the root apical meristem become root-cap cells, while those pushed to the periphery become epidermal cells.

4. Development and Plasticity

Development encompasses all the changes an organism undergoes during its life cycle, from seed germination to senescence. It is the sum of growth and differentiation. The developmental process in a plant cell follows a sequence of plasmatic growth, differentiation, expansion, maturation, and finally senescence and death.

Plasticity is the ability of plants to follow different pathways in response to the environment or phases of life, leading to the formation of different kinds of structures.

• Heterophylly: A key example of plasticity.
  • Developmental Heterophylly: The leaves of a juvenile plant are different in shape from those of a mature plant (e.g., in cotton, coriander, larkspur).
  • Environmental Heterophylly: The shape of leaves produced in air is different from those produced in water (e.g., in buttercup).

5. Plant Growth Regulators (PGRs)

Also known as phytohormones or plant growth substances, PGRs are small, simple molecules of diverse chemical composition that control plant growth and development.

5.1. Characteristics and Groups

PGRs are broadly divided into two functional groups:

1. Plant Growth Promoters: Involved in activities like cell division, cell enlargement, flowering, fruiting, and seed formation.
   • Auxins
   • Gibberellins
   • Cytokinins
2. Plant Growth Inhibitors: Play a role in responses to stress and wounds, and in inhibiting activities like dormancy and abscission.
   • Abscisic Acid (ABA)
   • Ethylene (can fit in either group but is largely an inhibitor).

PGR Group	Chemical Nature Example
Auxins	Indole compounds (e.g., Indole-3-acetic acid, IAA)
Gibberellins	Terpenes (e.g., Gibberellic acid, GA₃)
Cytokinins	Adenine derivatives (e.g., N⁶-furfurylamino purine, Kinetin)
Abscisic Acid	Derivatives of carotenoids (e.g., ABA)
Ethylene	Gases (e.g., C₂H₄)

5.2. Discovery of PGRs

The discovery of each major PGR group was largely accidental:

• Auxin: Charles and Francis Darwin observed that the coleoptiles of canary grass bent towards a unilateral light source (phototropism). They concluded that a transmittable influence from the tip caused the bending. F.W. Went later isolated auxin from the tips of oat coleoptiles.
• Gibberellin: The “bakanae” (foolish seedling) disease in rice, caused by the fungus Gibberella fujikuroi, led to its discovery. E. Kurosawa found that sterile filtrates of the fungus could induce the same symptoms, and the active substance was identified as gibberellic acid.
• Cytokinin: F. Skoog and co-workers found that tobacco stem callus would only proliferate if the medium contained auxin plus extracts of vascular tissue, yeast, coconut milk, or DNA. Miller et al. later crystallized the active substance, which they named kinetin.
• Abscisic Acid (ABA): Discovered through three independent lines of research that identified different inhibitors (inhibitor-B, abscission II, and dormin), which were all later found to be the same chemical compound.
• Ethylene: H.H. Cousins confirmed that ripened oranges released a volatile substance that hastened the ripening of unripe bananas. This substance was later identified as ethylene.

5.3. Physiological Effects of PGRs

Auxins

• Production Sites: Growing apices of stems and roots.
• Types: Natural (IAA, IBA) and Synthetic (NAA, 2,4-D).
• Functions:
  • Initiate rooting in stem cuttings.
  • Promote flowering (e.g., pineapples).
  • Prevent early fruit and leaf drop but promote abscission of older leaves/fruits.
  • Apical Dominance: The growing apical bud inhibits the growth of lateral (axillary) buds. Removal of the shoot tip (decapitation) allows lateral buds to grow (used in tea plantations and hedge-making).
  • Induce parthenocarpy (fruit development without fertilization) in tomatoes.
  • Used as herbicides (e.g., 2,4-D kills dicot weeds).
  • Control xylem differentiation and help in cell division.

Gibberellins (GAs)

• Characteristics: Over 100 types (GA₁, GA₂, GA₃, etc.); all are acidic. GA₃ is the most studied.
• Functions:
  • Increase the length of the axis (used to lengthen grape stalks).
  • Cause fruits like apples to elongate and improve shape.
  • Delay senescence, extending the market period for fruits.
  • Speed up the malting process in the brewing industry.
  • Increase sugarcane yield (up to 20 tonnes per acre) by increasing stem length.
  • Hasten maturity in juvenile conifers for early seed production.
  • Promote bolting (internode elongation before flowering) in rosette plants like beet and cabbage.

Cytokinins

• Discovery: Discovered as kinetin from autoclaved herring sperm DNA. Zeatin is a naturally occurring cytokinin isolated from corn kernels and coconut milk.
• Production Sites: Regions of rapid cell division (root apices, developing shoot buds, young fruits).
• Functions:
  • Promote cytokinesis (cell division).
  • Help produce new leaves and chloroplasts.
  • Promote lateral shoot growth and adventitious shoot formation.
  • Help overcome apical dominance.
  • Promote nutrient mobilization, which delays leaf senescence.

Ethylene (C₂H₄)

• Characteristics: A simple gaseous PGR.
• Production Sites: Tissues undergoing senescence and ripening fruits.
• Functions:
  • Promotes senescence and abscission of leaves and flowers.
  • Highly effective in fruit ripening and enhances respiration rate during this process (respiratory climactic).
  • Breaks seed and bud dormancy (initiates germination in peanut seeds, sprouting in potato tubers).
  • Promotes rapid internode/petiole elongation in deep-water rice plants.
  • Promotes root growth and root hair formation, increasing absorption surface area.
  • Used to initiate flowering and synchronize fruit-set in pineapples and mangoes.
  • Ethephon: A compound widely used in agriculture that releases ethylene slowly. It hastens ripening (tomatoes, apples), accelerates abscission (cotton, cherry), and promotes female flowers in cucumbers.

Abscisic Acid (ABA)

• Characteristics: A general plant growth inhibitor and an inhibitor of plant metabolism.
• Functions:
  • Regulates abscission and dormancy.
  • Inhibits seed germination.
  • Stimulates the closure of stomata, increasing plant tolerance to various stresses. It is known as the stress hormone.
  • Plays a key role in seed development, maturation, and dormancy, helping seeds withstand desiccation.
  • Acts as an antagonist to gibberellins (GAs) in most situations.

#################################################################

Review Quiz

Short-Answer Questions

1. Define the term “growth” as it applies to living beings.
2. What are the two processes that together constitute “development” in a plant?
3. Why is plant growth described as having an “open form”?
4. Name the two primary meristems responsible for the elongation of a plant’s axis.
5. What is secondary growth, and which meristems are responsible for it?
6. List three parameters used to measure plant growth.
7. Describe the key characteristics of cells in the meristematic phase of growth.
8. What cellular changes occur during the phase of elongation?
9. Explain the difference between arithmetic and geometric growth in terms of daughter cell division.
10. What is a sigmoid growth curve, and what are its three phases?
11. Differentiate between absolute growth rate and relative growth rate.
12. List three essential extrinsic conditions required for plant growth.
13. What is differentiation in the context of plant cells?
14. Explain the phenomenon of dedifferentiation using an example.
15. Define “plasticity” in plants and provide an example of environmental heterophylly.
16. Name the five principal groups of plant growth regulators (PGRs).
17. What is the chemical nature of auxins and gibberellins?
18. What observation did Charles and Francis Darwin make that led to the discovery of auxins?
19. What is the “bakanae” disease, and which PGR was discovered through its study?
20. Explain the phenomenon of apical dominance and how it can be overcome.
21. What is “bolting,” and which PGR is used to promote it in rosette plants?
22. Name a naturally occurring cytokinin and a synthetic one.
23. What is the “respiratory climactic,” and which gaseous PGR is associated with it?
24. Why is abscisic acid (ABA) referred to as the “stress hormone”?
25. Explain how ABA and GAs act as antagonists to each other.

Multiple-Choice Questions (MCQs)

1. Growth is defined as an irreversible increase in… a) Water content b) Size, accompanied by metabolic processes c) Cell turgidity d) The number of dormant cells
2. The ability of plants to have unlimited growth throughout their life is due to… a) The presence of differentiated cells b) The presence of meristems c) Secondary growth only d) Favorable environmental conditions
3. The increase in the girth of a dicot plant is a result of… a) Primary growth b) Apical meristem activity c) Secondary growth d) The elongation phase
4. A sigmoid (S-curve) is characteristic of which type of growth? a) Arithmetic growth b) Logarithmic decline c) Geometric growth d) Linear growth
5. The mathematical expression for exponential growth is… a) Lt = L0 + rt b) W1 = W0 * e^(rt) c) r = Lt – L0 d) W1 = W0 + rt
6. The process where mature, differentiated cells regain the capacity to divide is called… a) Redifferentiation b) Dedifferentiation c) Differentiation d) Senescence
7. Heterophylly in buttercup, where leaf shapes differ between water and air, is an example of… a) Apical dominance b) Senescence c) Plasticity d) Phototropism
8. Which of the following PGRs is an adenine derivative? a) Indole-3-acetic acid (IAA) b) Abscisic acid (ABA) c) Gibberellic acid (GA₃) d) Kinetin
9. The experiment on canary grass coleoptiles by the Darwins demonstrated… a) Geotropism b) The effect of ethylene c) Phototropism and the role of the tip d) The function of cytokinins
10. Which PGR is widely used to kill dicotyledonous weeds in lawns? a) IAA b) GA₃ c) 2,4-D d) ABA
11. The application of which hormone can increase the yield of sugarcane by increasing stem length? a) Auxin b) Ethylene c) Gibberellin d) Cytokinin
12. Cytokinins are primarily synthesized in regions of… a) Mature leaves b) Rapid cell division c) Food storage d) Senescing tissue
13. The gaseous hormone responsible for fruit ripening is… a) Abscisic acid b) Gibberellin c) Auxin d) Ethylene
14. The “stress hormone” that causes stomatal closure during water stress is… a) Cytokinin b) Abscisic acid c) Ethylene d) Gibberellic acid
15. Removal of the shoot tip (decapitation) is a common practice in tea plantations to… a) Induce flowering b) Promote the growth of lateral buds c) Increase the length of the main stem d) Delay senescence
16. The phenomenon of internode elongation just prior to flowering in rosette plants is known as… a) Abscission b) Dormancy c) Bolting d) Parthenocarpy
17. Ethephon is a compound used in agriculture because it releases… a) Auxin b) Cytokinin c) Ethylene d) ABA
18. Which PGR was first isolated from human urine? a) Gibberellin b) Kinetin c) Auxin d) Zeatin
19. In most situations, Abscisic Acid (ABA) acts as an antagonist to… a) Auxins b) Ethylene c) Cytokinins d) Gibberellins (GAs)
20. A callus of undifferentiated cells was observed to proliferate only when the medium was supplemented with auxin and coconut milk. This led to the discovery of… a) Gibberellins b) Auxins c) Cytokinins d) Abscisic acid

#################################################################

Answer Key

Short-Answer Questions Key

1. Growth is an irreversible permanent increase in the size of an organ, its parts, or an individual cell. It is accompanied by metabolic processes and requires energy.
2. The two processes that constitute development are growth and differentiation. Development is considered the sum of these two events.
3. Plant growth is described as having an “open form” because plants retain the capacity for unlimited growth throughout their lives. This is due to the activity of meristems, which continuously add new cells to the plant body.
4. The two primary meristems are the root apical meristem and the shoot apical meristem. They contribute to the primary growth (elongation) of the plant.
5. Secondary growth is the increase in the girth of plant organs. It is caused by the activity of lateral meristems: the vascular cambium and the cork-cambium.
6. Three parameters are: increase in fresh/dry weight, increase in length/area/volume, and increase in cell number.
7. Cells in the meristematic phase are rich in protoplasm, possess large conspicuous nuclei, and have thin, primary cellulosic walls with abundant plasmodesmatal connections. They are constantly dividing.
8. During the phase of elongation, cells undergo increased vacuolation, cell enlargement, and deposition of new cell wall material.
9. In arithmetic growth, only one daughter cell continues to divide while the other differentiates. In geometric growth, both progeny cells following mitosis retain the ability to divide.
10. A sigmoid growth curve is a characteristic S-shaped curve of organisms growing in a natural environment. Its three phases are the initial slow lag phase, the rapid log (or exponential) phase, and the final stationary phase where growth slows.
11. Absolute growth rate is the total growth per unit time. Relative growth rate is the growth per unit time expressed relative to the initial size or parameter.
12. Three essential extrinsic conditions are water, oxygen, and an optimum temperature range. Light and gravity are also important environmental signals.
13. Differentiation is the process where cells from meristems mature to perform specific functions. This involves significant structural changes to both their cell walls and protoplasm.
14. Dedifferentiation is when living, differentiated cells regain the capacity to divide. An example is the formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
15. Plasticity is a plant’s ability to form different structures in response to its environment or life phases. Environmental heterophylly in buttercup shows this, where leaves produced in water have a different shape from those produced in air.
16. The five principal groups are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
17. Auxins are indole compounds (like indole-3-acetic acid). Gibberellins are terpenes (like gibberellic acid).
18. Charles and Francis Darwin observed that the coleoptiles of canary grass bent towards a unilateral light source (phototropism), concluding that a transmittable influence from the tip was responsible.
19. The “bakanae” (foolish seedling) disease of rice is caused by the fungus Gibberella fujikuroi. Study of this disease led to the identification of the active substance, gibberellic acid.
20. Apical dominance is the phenomenon where the growing apical bud inhibits the growth of lateral buds. It can be overcome by removing the shoot tip (decapitation) or by the application of cytokinins.
21. Bolting is the rapid elongation of the internode just prior to flowering. Gibberellins are used to promote this in rosette plants like beet and cabbage.
22. A naturally occurring cytokinin is zeatin (from corn kernels). A synthetic one is kinetin (derived from autoclaved herring sperm DNA).
23. The “respiratory climactic” is the rise in the rate of respiration during fruit ripening. It is associated with the gaseous PGR, ethylene.
24. Abscisic acid (ABA) is called the “stress hormone” because it stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses, such as water scarcity.
25. ABA acts as an antagonist to GAs by inhibiting processes that GAs promote. For example, ABA inhibits seed germination and induces dormancy, while GAs promote germination.

Multiple-Choice Questions (MCQs) Key

1. b) Size, accompanied by metabolic processes
2. b) The presence of meristems
3. c) Secondary growth
4. c) Geometric growth
5. b) W1 = W0 * e^(rt)
6. b) Dedifferentiation
7. c) Plasticity
8. d) Kinetin
9. c) Phototropism and the role of the tip
10. c) 2,4-D
11. c) Gibberellin
12. b) Rapid cell division
13. d) Ethylene
14. b) Abscisic acid
15. b) Promote the growth of lateral buds
16. c) Bolting
17. c) Ethylene
18. c) Auxin
19. d) Gibberellins (GAs)
20. c) Cytokinins

#################################################################

Essay Questions and Answers

1. Explain the concepts of “open growth” and “open differentiation” in plants. How do meristems contribute to these phenomena?

“Open growth” refers to the indeterminate nature of plant growth, where plants retain the capacity for unlimited growth throughout their lives. This is made possible by the presence of meristems—localized regions of dividing cells—at certain locations in the plant body, such as the root and shoot apices. These meristematic cells continuously divide and self-perpetuate, constantly adding new cells to the plant body. While some of these new cells lose the capacity to divide and form the permanent plant body, the meristem itself remains active, allowing for continuous growth.

“Open differentiation” is a related concept which states that the developmental fate of a cell is not fixed and is often determined by its location within the plant body. Cells arising from the same meristem can differentiate into various types of tissues and structures depending on their final position. For example, cells produced by the root apical meristem that are positioned away from the tip will differentiate into root-cap cells for protection, while those pushed to the outer layer will mature into the epidermis. This flexibility in differentiation, where the final structure is determined by location, is what makes it “open.” Meristems are central to this because they are the source of these undifferentiated cells whose fate is yet to be determined by positional cues.

2. Describe the three phases of growth in plants and the key cellular characteristics of each phase.

The period of plant growth is typically divided into three distinct phases: meristematic, elongation, and maturation.

1. Meristematic Phase: This phase occurs at the apices of roots and shoots, where meristematic tissues are located. The cells in this zone are characterized by being in a state of constant division. They are rich in protoplasm, possess large and conspicuous nuclei, and have thin, primary cell walls made of cellulose. These cells are interconnected by abundant plasmodesmatal connections, facilitating communication.
2. Elongation Phase: This phase is found in the region just proximal to (behind) the meristematic zone. The primary characteristic of cells in this phase is enlargement. This is achieved through increased vacuolation (the central vacuole fills with water, creating turgor pressure) and the deposition of new cell wall material, which allows the cell to expand in size.
3. Maturation Phase: Located further away from the apex, proximal to the zone of elongation, this is where cells reach their final, mature state. The cells in this zone attain their maximal size and undergo significant protoplasmic modifications and cell wall thickening to become specialized for their specific functions. The various tissues like xylem, phloem, parenchyma, and epidermis that have been studied are representative of this phase.

3. Compare and contrast arithmetic and geometric growth. Provide the mathematical formula for each and describe the resulting growth curve.

Arithmetic and geometric growth are two patterns by which an organism or its parts can increase in cell number and size.

Arithmetic growth occurs when, following a mitotic cell division, only one of the two daughter cells continues to divide, while the other differentiates and loses the capacity for division. This results in a constant increase in size or length over time. When plotted on a graph with the growth parameter against time, it produces a straight, linear curve. This pattern is exemplified by a root elongating at a constant rate. The mathematical expression is Lt = L0 + rt, where Lt is the length at time ‘t’, L0 is the initial length, and ‘r’ is the constant growth rate.

Geometric growth, in contrast, occurs when both daughter cells from a mitotic division retain the ability to divide. This leads to an exponential increase in cell number. The initial growth is slow (lag phase), followed by a very rapid exponential increase (log phase). However, as resources like nutrients become limited, the growth rate slows down and eventually stops, leading to a stationary phase. When plotted against time, this pattern produces a characteristic sigmoid or S-shaped curve. This type of growth is typical for most living organisms, tissues, and cells growing in a natural environment. The mathematical expression is W1 = W0 * e^(rt), where W1 is the final size, W0 is the initial size, ‘r’ is the relative growth rate, and ‘t’ is time.

4. Discuss the discovery and key physiological functions of auxins in plants. How are they applied in agriculture and horticulture?

The discovery of auxins began with observations by Charles and Francis Darwin on phototropism in canary grass coleoptiles. They concluded that a transmittable influence from the tip caused the coleoptile to bend towards light. This substance, named auxin, was later isolated from the tips of oat seedlings by F.W. Went. Auxin was the first PGR to be discovered and was initially isolated from human urine.

The term ‘auxin’ applies to indole-3-acetic acid (IAA) and other natural and synthetic compounds with similar properties. They are primarily produced in the growing apices of stems and roots.

Physiological Functions:

• Apical Dominance: Auxins produced by the apical bud inhibit the growth of lateral (axillary) buds.
• Cell Division and Differentiation: They control xylem differentiation and help in cell division.
• Flowering and Fruiting: Auxins promote flowering in plants like pineapples and can prevent the premature drop of fruits and leaves. Conversely, they promote the abscission of older, mature leaves and fruits.
• Parthenocarpy: They can induce the development of fruit without fertilization, for example, in tomatoes.

Agricultural and Horticultural Applications:

• Plant Propagation: Auxins are widely used to initiate rooting in stem cuttings.
• Weed Control: Synthetic auxins like 2,4-D are used as selective herbicides to kill broad-leaf (dicotyledonous) weeds without harming mature monocotyledonous plants, making them ideal for creating weed-free lawns.
• Hedge-making and Tea Plantations: The principle of apical dominance is exploited by removing shoot tips (decapitation), which encourages the growth of lateral buds, leading to bushier plants.

5. Explain the roles of ethylene as a plant growth regulator. Why can it be considered both a promoter and an inhibitor, and how is it used commercially?

Ethylene is a simple gaseous PGR that plays a diverse role in plant life, acting as both a growth promoter and an inhibitor depending on the context. It is synthesized by tissues undergoing senescence and by ripening fruits.

Inhibitory and Senescence-Related Roles:

• It is largely considered an inhibitor because it promotes the senescence (aging) and abscission (shedding) of plant organs, especially leaves and flowers.
• It is highly effective in fruit ripening, a process that leads to the eventual decay of the fruit. This is accompanied by a rise in respiration known as the “respiratory climactic.”

Promotive Roles:

• Ethylene breaks seed and bud dormancy, initiating germination in peanut seeds and sprouting in potato tubers.
• It promotes rapid internode and petiole elongation in deep-water rice plants, helping the upper parts of the plant stay above water.
• It also promotes root growth and the formation of root hairs, which increases the plant’s surface area for water and nutrient absorption.

Commercial Use: Because it regulates so many processes, ethylene is one of the most widely used PGRs in agriculture. The compound ethephon is often used as a source. Ethephon is absorbed by the plant in an aqueous solution and slowly releases ethylene. Its applications include:

• Hastening fruit ripening in tomatoes and apples.
• Accelerating abscission for thinning fruits like cotton, cherry, and walnut.
• Promoting female flowers in cucumbers, thereby increasing the final yield.
• Initiating flowering and synchronizing fruit-set in pineapples and mangoes.

6. Describe the phenomenon of plasticity in plants, using the concept of heterophylly as a key example.

Plasticity is the ability of a plant to follow different developmental pathways and produce different kinds of structures in response to various environmental conditions or phases of its life. This adaptability is a crucial survival mechanism.

A prime example of plasticity is heterophylly, which is the occurrence of different forms of leaves on the same plant. There are two main types:

1. Developmental Heterophylly: This is related to the life phase of the plant. The leaves of a juvenile plant are different in shape from those found on the mature plant. Examples include cotton, coriander, and larkspur, where the early leaves look significantly different from the later ones.
2. Environmental Heterophylly: This is a direct response to environmental conditions. A classic example is the buttercup plant. The leaves that are produced while submerged in water are finely dissected, whereas the leaves produced above the water in the air are broad and lobed. This difference in leaf shape is a direct adaptation to the different physical environments (water vs. air).

This ability to alter leaf morphology demonstrates how a plant’s development is flexible and can be modified by both internal (life phase) and external (environmental) factors.

7. Why is Abscisic Acid (ABA) known as the “stress hormone”? Detail its major physiological effects, including its antagonistic relationship with gibberellins.

Abscisic Acid (ABA) is known as the “stress hormone” because it plays a crucial role in helping plants cope with various environmental stresses, particularly water scarcity (drought). Its primary stress-response mechanism is to stimulate the closure of stomata on the leaf surface. This action reduces water loss through transpiration, conserving water within the plant during stressful conditions. By increasing the plant’s tolerance to these stresses, ABA is vital for its survival.

Major Physiological Effects:

• Growth Inhibition: ABA generally acts as a plant growth inhibitor and an inhibitor of plant metabolism.
• Dormancy and Germination: It regulates abscission and is a key player in inducing and maintaining dormancy in seeds and buds. By inducing dormancy, it helps seeds withstand desiccation and other unfavorable growth conditions. It also inhibits seed germination.
• Seed Development: ABA plays an important role in seed development and maturation.

Antagonistic Relationship with Gibberellins (GAs): In most situations, ABA and GAs have opposing effects. This antagonistic relationship is a critical control mechanism in plant development.

• Germination vs. Dormancy: GAs are strong promoters of seed germination, while ABA is a strong inhibitor of germination and a promoter of dormancy. The balance between these two hormones often determines whether a seed will sprout or remain dormant.
• Growth Promotion vs. Inhibition: GAs generally promote growth, such as stem elongation, while ABA inhibits overall growth.

This balance between a promoter (GA) and an inhibitor (ABA) allows the plant to finely tune its growth and development in response to environmental cues.

8. Outline the discoveries of gibberellins and cytokinins. What distinct roles do they play in plant development?

The discoveries of gibberellins and cytokinins were both accidental and stemmed from observations of unusual growth patterns.

Discovery of Gibberellins: Gibberellins were discovered through the investigation of the “bakanae” or “foolish seedling” disease in rice. In 1926, Japanese scientist E. Kurosawa found that rice seedlings infected with the fungus Gibberella fujikuroi grew abnormally tall. He demonstrated that applying sterile filtrates from the fungus to healthy seedlings could reproduce the disease symptoms. The active substance responsible for this excessive stem elongation was later isolated and named gibberellic acid.

Discovery of Cytokinins: The discovery of cytokinins came from tissue culture experiments. F. Skoog and his colleagues observed that callus (a mass of undifferentiated cells) from tobacco stems would only proliferate if the nutrient medium, in addition to auxin, was supplemented with substances like yeast extract, coconut milk, or DNA. Later, Miller et al. (1955) identified and crystallized a specific cell division-promoting (cytokinesis-promoting) substance from autoclaved herring sperm DNA, which they named kinetin. Though kinetin is not found naturally in plants, the search for similar natural substances led to the isolation of zeatin from corn kernels and coconut milk.

Distinct Roles:

• Gibberellins are primarily promotory PGRs associated with elongation. Their key roles include causing an increase in the length of the plant axis (stem elongation in sugarcane, increasing grape stalk length), promoting bolting in rosette plants, delaying senescence, and breaking dormancy.
• Cytokinins are primarily promotory PGRs associated with cell division (cytokinesis). Their main functions include promoting the formation of new leaves and chloroplasts, stimulating lateral shoot growth (helping to overcome apical dominance), promoting adventitious shoot formation, and delaying leaf senescence by promoting nutrient mobilization.

9. Explain the interaction of intrinsic and extrinsic factors in controlling plant development. How do PGRs mediate the effects of external factors like light and temperature?

Plant development is a complex process controlled by a combination of intrinsic (internal) and extrinsic (external) factors.

Intrinsic factors include both intracellular genetic controls (the plant’s genome) and intercellular chemical factors, which are the plant growth regulators (PGRs). The genetic blueprint determines the potential for growth and the basic body plan, while PGRs act as chemical messengers that coordinate various developmental events like cell division, elongation, flowering, and dormancy.

Extrinsic factors are environmental cues such as light, temperature, water, oxygen, and nutrition. These external signals provide the necessary conditions for growth and also trigger specific developmental changes.

The role of PGRs is crucial as they often act as intermediates that translate the external environmental signals into an internal physiological response. Many extrinsic factors control plant growth and development via their effect on PGRs. For example:

• Light: Phototropism (bending towards light) is mediated by the redistribution of auxin. Light can also influence the synthesis or sensitivity of other hormones that control flowering.
• Temperature: Temperature changes can affect the balance of PGRs, influencing events like seed germination, dormancy, and vernalisation (the promotion of flowering by a period of low temperature).

Therefore, development is not governed by a single factor but by the intricate interplay between the plant’s genetic makeup, its internal hormonal state (PGRs), and the surrounding environmental conditions. PGRs provide the link that allows a plant to respond and adapt its growth and development to a changing environment.

10. Describe the five major groups of PGRs based on their chemical composition and classify them as either primarily growth promoters or inhibitors.

The five major groups of Plant Growth Regulators (PGRs) are distinguished by their diverse chemical compositions and functions. They can be broadly classified as growth promoters or growth inhibitors.

1. Auxins:
   • Chemical Nature: These are indole compounds. The primary natural auxin is indole-3-acetic acid (IAA).
   • Classification: Primarily growth promoters. They are involved in cell enlargement, rooting, and apical dominance.
2. Gibberellins:
   • Chemical Nature: These are terpenes. A well-known example is gibberellic acid (GA₃).
   • Classification: Primarily growth promoters. They are crucial for stem elongation, seed germination, and bolting.
3. Cytokinins:
   • Chemical Nature: These are adenine derivatives. A synthetic example is kinetin (N⁶-furfurylamino purine), and a natural example is zeatin.
   • Classification: Primarily growth promoters. Their main role is in promoting cell division (cytokinesis) and overcoming apical dominance.
4. Abscisic Acid (ABA):
   • Chemical Nature: This is a derivative of carotenoids.
   • Classification: Primarily a growth inhibitor. It is involved in inducing dormancy, promoting abscission, and mediating stress responses like stomatal closure.
5. Ethylene:
   • Chemical Nature: This is a simple gas (C₂H₄).
   • Classification: Fits into both groups but is largely an inhibitor. While it can promote processes like root hair formation, it is primarily known for inhibiting growth activities and promoting senescence, abscission, and fruit ripening.

#################################################################

Glossary of Key Terms

• Absolute Growth Rate: The measurement and comparison of total growth per unit time.
• Abscisic Acid (ABA): A plant growth inhibitor involved in regulating abscission, dormancy, and stress responses; known as the stress hormone.
• Apical Dominance: A phenomenon where the growing apical bud inhibits the growth of the lateral (axillary) buds.
• Arithmetic Growth: A growth pattern where only one daughter cell from a division continues to divide, resulting in a linear increase over time.
• Auxins: A group of plant growth promoters (indole compounds) involved in cell enlargement, rooting, and apical dominance.
• Bolting: The rapid elongation of an internode just prior to flowering, especially in plants with a rosette habit.
• Cytokinins: A group of plant growth promoters (adenine derivatives) that have specific effects on cell division (cytokinesis).
• Decapitation: The removal of shoot tips, a practice used to overcome apical dominance.
• Dedifferentiation: The process by which living, differentiated cells that have lost the capacity to divide regain this ability under certain conditions.
• Development: The sum of all changes an organism goes through during its life cycle, from germination to senescence. It includes both growth and differentiation.
• Differentiation: The process leading to maturation, where cells derived from meristems undergo structural changes to perform specific functions.
• Dormancy: A period of suspended growth or rest, often seen in seeds.
• Efficiency Index: Another term for relative growth rate (r), representing the ability of a plant to produce new plant material.
• Ethephon: A compound widely used in agriculture as a source of ethylene.
• Ethylene: A simple gaseous plant growth regulator involved in fruit ripening, senescence, and abscission.
• Extrinsic Factors: External factors that control developmental processes, such as light, temperature, water, and oxygen.
• Geometric Growth: A growth pattern where both progeny cells from a division retain the ability to divide, leading to an exponential increase and a sigmoid (S-curve).
• Gibberellins (GAs): A group of promotory PGRs (terpenes) that cause a wide range of physiological responses, notably stem elongation.
• Growth: An irreversible permanent increase in the size of an organ, its parts, or an individual cell, accompanied by metabolic processes.
• Growth Rate: The increased growth per unit time.
• Heterophylly: A phenomenon where a plant has different forms of leaves at different stages of life or in different environments; an example of plasticity.
• Indeterminate Growth: The capacity for unlimited or prolonged growth, characteristic of plants due to meristems.
• Intrinsic Factors: Internal factors that control developmental processes, including genetic factors and chemical factors like PGRs.
• Kinetin: A synthetic cytokinin (a modified form of adenine) discovered from autoclaved herring sperm DNA.
• Meristems: Localized regions in a plant body containing cells that have the capacity to divide and self-perpetuate, responsible for growth.
• Open Form of Growth: A form of growth where new cells are always being added to the plant body by the activity of a meristem.
• Parthenocarpy: The development of fruit without prior fertilization.
• Phytohormones: Another term for plant growth regulators.
• Plasticity: The ability of plants to follow different developmental pathways to form different kinds of structures in response to the environment or phases of life.
• Redifferentiation: The process where cells that have dedifferentiated and divided once again lose the capacity to divide and mature to perform specific functions.
• Relative Growth Rate: The growth of a system per unit time expressed on a common basis, such as per unit initial parameter.
• Respiratory Climactic: The rise in the rate of respiration that occurs during the ripening of fruits, enhanced by ethylene.
• Senescence: The process of aging in plants, leading to death of cells, tissues, or the whole organism.
• Sigmoid Curve (S-curve): The characteristic S-shaped curve produced when plotting geometric growth against time.
• Stress Hormone: A term for Abscisic Acid (ABA) due to its role in helping plants tolerate various kinds of stresses.
• Zeatin: A naturally occurring cytokinin isolated from corn-kernels and coconut milk.