Plant Growth and Development

Growth

Introduction

• In Chapter 5, we studied the structure of flowering plants.
• Have you wondered how plants grow roots, stems, leaves, flowers, fruits, and seeds in order?
• Terms like seed, seedling, plantlet, and mature plant are familiar.
• Trees grow in height and width, but their leaves, flowers, and fruits grow, fall off, and regrow periodically.
• Vegetative phase (leaf and stem growth) comes before flowering.
• Plant organs are made of tissues; their structure relates to their function.
• All plant cells come from the zygote but have different structures and functions.
• Development = Growth + Differentiation.
• Plants develop in a precise order from zygote to mature plant, producing roots, leaves, branches, flowers, fruits, and seeds.

Seed Germination

• First step of growth: Seed germination.
• Seeds germinate when conditions are right (water, temperature, soil).
• If conditions aren’t right, seeds go into rest.
• When conditions improve, seeds resume growth.

Factors Affecting Growth

• Developmental processes are controlled by:
  • Intrinsic Factors: Internal to the plant.
  • Extrinsic Factors: External environmental conditions.

Growth

• Growth: A fundamental feature of living beings.
• Definition: Permanent increase in size of an organ or cell.
• Growth involves metabolic processes (building up and breaking down) using energy.
• Example: Leaf expanding is growth.
• Question: Does swelling of wood in water count as growth? (Think about it!)

Key Points

• Plant growth is orderly and follows specific stages.
• Both internal and external factors influence growth.
• Growth involves energy-dependent processes.

1. Plant Growth is Indeterminate

• Indeterminate Growth: Plants can grow indefinitely due to meristems.
• Meristems: Special cells that divide and produce new cells.
  • Root Apical Meristem and Shoot Apical Meristem: Responsible for primary growth (length).
  • Lateral Meristems (Vascular Cambium and Cork Cambium): Responsible for secondary growth (girth).
• Open Form of Growth: New cells continuously added by meristems.
• Question: What if meristems stop dividing? (Think about it!)

2. Growth is Measurable

• Growth is the increase in protoplasm, but it’s hard to measure directly.
• Measured By:
  • Fresh weight
  • Dry weight
  • Length
  • Area
  • Volume
  • Cell number
• Examples:
  • Maize root apical meristem: Can produce 17,500 new cells per hour.
  • Watermelon cells: Can grow up to 3,50,000 times their original size.
  • Pollen tube growth: Measured by length.
  • Leaf growth: Measured by increase in surface area.

3. Phases of Growth

• Three Phases of Growth:
  • Meristematic Phase:
    • Found at root and shoot tips.
    • Cells are actively dividing, rich in protoplasm, have large nuclei, and thin cell walls.
  • Elongation Phase:
    • Located just after the meristematic zone.
    • Cells enlarge, vacuoles form, and new cell walls are deposited.
  • Maturation Phase:
    • Further from the tip, after the elongation zone.
    • Cells reach their final size with thickened walls and protoplasmic changes.

Key Points

• Plants grow indefinitely due to meristems.
• Growth can be measured in various ways.
• Growth happens in three main phases: meristematic, elongation, and maturation.

4. Growth Rates

• Growth Rate: Increase in growth per unit time.
• Arithmetic Growth:
  • One daughter cell divides, the other matures.
  • Example: Root elongating at a constant rate.
  • Formula:
    • 𝐿𝑡=𝐿0+𝑟𝑡Lt​=L0​+rt
    • 𝐿𝑡Lt​: Length at time ‘t’
    • 𝐿0L0​: Length at time ‘zero’
    • 𝑟r: Growth rate
• Geometrical Growth:
  • Initial slow growth (lag phase), then rapid exponential growth (log phase), followed by stationary phase due to limited nutrients.
  • Both daughter cells keep dividing.
  • Sigmoid (S-curve): Typical growth pattern in natural environments.
  • Formula:
    • 𝑊1=𝑊0𝑒𝑟𝑡W1​=W0​ert
    • 𝑊1W1​: Final size
    • 𝑊0W0​: Initial size
    • 𝑟r: Growth rate
    • 𝑡t: Time
    • 𝑒e: Base of natural logarithms
• Growth Rate Types:
  • Absolute Growth Rate: Total growth per unit time.
  • Relative Growth Rate: Growth per unit time per unit initial size.
• Example: Two leaves (A and B) grow to A1 and B1. One has a higher relative growth rate.

5. Conditions for Growth

• Essential Conditions:
  • Water: Needed for cell enlargement and enzymatic activities.
  • Oxygen: Releases energy for growth.
  • Nutrients: Macro and micro elements for protoplasm synthesis and energy.
• Other Factors:
  • Temperature: Optimum range needed; deviations can be harmful.
  • Environmental Signals: Light and gravity affect growth stages.

Key Points

• Growth rates can be arithmetic or geometrical.
• Arithmetic growth is linear; geometrical growth shows an S-curve.
• Growth requires water, oxygen, nutrients, and optimal temperature.
• Environmental factors like light and gravity influence growth.

Differentiation, Dedifferentiation, and Redifferentiation

Differentiation

• What is it?
  Differentiation is when cells from root apical, shoot apical meristems, and cambium mature to perform specific tasks.
• Changes in Cells:
  During differentiation, cells change a lot:
  • Cell walls and protoplasm undergo structural changes.
  • Example: Tracheary elements lose their protoplasm and develop strong, elastic, lignocellulosic secondary cell walls to carry water over long distances.

Dedifferentiation

• What is it?
  Dedifferentiation happens when mature cells regain the ability to divide under certain conditions.
• Example:
  Parenchyma cells forming meristems like interfascicular cambium and cork cambium.

Redifferentiation

• What is it?
  Redifferentiation is when dedifferentiated cells divide and then mature again to perform specific functions.
• Example:
  Tissues like secondary xylem and phloem in a woody dicot plant are products of redifferentiation.

Open Differentiation

• What does it mean?
  Just like growth, differentiation in plants is “open,” meaning cells from the same meristem can develop differently based on their location.
• Examples:
  • Cells away from the root apical meristem become root-cap cells.
  • Cells at the periphery become epidermis cells.

Thought Questions/Topic

• Tumours:
  A tumour is an uncontrolled growth of cells.
• Parenchyma Cells in Tissue Culture:
  These are cells made to divide in controlled lab conditions.

Examples of Open Differentiation

• Position Matters:
  The final structure of a cell depends on its position. For instance:
  • Root-cap cells form at the root tip.
  • Epidermal cells form on the surface of roots.

Remember, plant cells can change and adapt based on where they are and what they need to do!

Development in Plants

What is Development?

• Definition:
  Development includes all the changes a plant goes through during its life, from seed germination to old age (senescence).

Key Processes in Development

• Sequence of Changes:
  Development involves a sequence of processes that can be shown in diagrams (like Figure 15.8 in the book).
• Includes:
  • Growth: Increase in size and number of cells.
  • Differentiation: Cells becoming specialized for specific functions.

Plasticity in Plants

• What is Plasticity?
  Plasticity is the plant’s ability to change its structure in response to the environment or different life phases.
• Examples of Plasticity:
  • Heterophylly:
    • Cotton, Coriander, Larkspur: Juvenile plants have leaves of different shapes than mature plants.
    • Buttercup: Leaves have different shapes when grown in air versus water.

Development: A Combination of Growth and Differentiation

• Interrelated Events:
  Growth, differentiation, and development are closely linked in a plant’s life.
• Factors Influencing Development:
  • Intrinsic Factors:
    • Genetic: Inside cells.
    • Intercellular Chemicals: Plant growth regulators.
  • Extrinsic Factors:
    • Environmental: Light, temperature, water, oxygen, and nutrition.

Summary

• Development is the sum of growth and differentiation.
• Plasticity shows how plants adapt their structures based on their environment.
• Both internal and external factors control plant development.

Plant Growth Regulators (PGRs)

Characteristics of PGRs

• What are they?
  Small, simple molecules with diverse chemical compositions.
• Types of PGRs:
  • Indole Compounds: Indole-3-acetic acid (IAA)
  • Adenine Derivatives: Kinetin
  • Carotenoid Derivatives: Abscisic acid (ABA)
  • Terpenes: Gibberellic acid (GA3)
  • Gases: Ethylene (C2H4)
• Other Names:
  Also known as plant growth substances, plant hormones, or phytohormones.
• Two Main Groups Based on Function:
  • Growth Promoters:
    • Activities: Cell division, enlargement, pattern formation, tropic growth, flowering, fruiting, seed formation.
    • Examples: Auxins, Gibberellins, Cytokinins.
  • Growth Inhibitors:
    • Activities: Response to wounds and stresses, dormancy, abscission.
    • Examples: Abscisic acid (ABA), Ethylene (mostly an inhibitor).

Discovery of PGRs

• Auxins:
  • Discovered by Charles and Francis Darwin through experiments with canary grass coleoptiles bending towards light (phototropism).
  • F.W. Went isolated auxin from oat seedling tips.
• Gibberellins:
  • Discovered by E. Kurosawa when rice seedlings showed disease symptoms after being treated with sterile filtrates of the fungus Gibberella fujikuroi.
  • The active substance was identified as gibberellic acid.
• Cytokinins:
  • F. Skoog and co-workers found that callus from tobacco stems grew only with auxins and additional supplements like vascular tissue extracts, yeast extract, coconut milk, or DNA.
  • Miller et al. identified and crystallized kinetin.
• Abscisic Acid (ABA):
  • Mid-1960s: Three different inhibitors (inhibitor-B, abscission II, dormin) were found to be chemically identical.
  • Named abscisic acid.
• Ethylene:
  • H.H. Cousins discovered that a volatile substance from ripened oranges accelerated ripening in stored bananas.
  • Identified as ethylene.

Summary

• PGRs are essential for:
  Growth promotion and inhibition in plants.
• Discovery of PGRs:
  Often accidental, involving observations and experiments with plants and fungi.
• Next Steps:
  Study the physiological effects of these five categories of PGRs.

Physiological Effects of Plant Growth Regulators

a. Auxins

• What are Auxins?
  Auxins, derived from the Greek word ‘auxein’ (to grow), include natural compounds like indole-3-acetic acid (IAA) and indole butyric acid (IBA), as well as synthetic ones like NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic acid).
• Production and Movement:
  Produced by the growing tips of stems and roots, then move to action sites.
• Uses in Agriculture and Horticulture:
  • Rooting: Initiate rooting in stem cuttings for plant propagation.
  • Flowering: Promote flowering, e.g., in pineapples.
  • Prevent Drop: Prevent early fruit and leaf drop; promote shedding of mature leaves and fruits.
  • Apical Dominance: Inhibit growth of lateral buds; removal of shoot tips (decapitation) promotes lateral growth. Used in tea plantations and hedge-making.
  • Parthenocarpy: Induce seedless fruit formation in tomatoes.
  • Herbicides: 2, 4-D kills dicot weeds but not monocot plants; used for weed-free lawns.
  • Other Functions: Control xylem differentiation and promote cell division.

b. Gibberellins

• What are Gibberellins?
  Gibberellins (GAs) are growth-promoting regulators. Over 100 types exist, labeled GA1, GA2, GA3, etc. GA3 (Gibberellic acid) is well-studied.
• Functions and Uses:
  • Increase Length: Used to elongate grape stalks.
  • Fruit Improvement: Elongate and improve the shape of apples.
  • Delay Senescence: Extend the market period by keeping fruits on trees longer.
  • Malting Process: GA3 speeds up malting in brewing.
  • Sugarcane Yield: Spraying increases stem length and sugar yield (up to 20 tonnes/acre).
  • Early Seed Production: Spraying juvenile conifers with GAs speeds up maturity.
  • Bolting: Promote internode elongation before flowering in plants like beet and cabbages.

c. Cytokinins

• What are Cytokinins?
  Cytokinins promote cell division (cytokinesis).
• Discovery:
  • First discovered as kinetin from autoclaved herring sperm DNA (not natural in plants).
  • Natural cytokinin, zeatin, was found in corn kernels and coconut milk.
• Functions:
  • Produced in areas of rapid cell division (root tips, shoot buds, young fruits).
  • Promote new leaf and chloroplast production.
  • Encourage lateral shoot growth and adventitious shoot formation.
  • Overcome apical dominance.
  • Mobilize nutrients and delay leaf aging (senescence).

d. Ethylene

• What is Ethylene?
  A simple gaseous PGR produced in large amounts by senescing tissues and ripening fruits.
• Functions:
  • Seedlings: Causes horizontal growth, axis swelling, and apical hook formation in dicot seedlings.
  • Senescence and Abscission: Promotes aging and shedding of leaves and flowers.
  • Fruit Ripening: Highly effective in ripening fruits; increases respiration rate during ripening (respiratory climactic).
  • Breaking Dormancy: Breaks seed and bud dormancy, initiates germination in peanut seeds, and sprouting in potato tubers.
  • Elongation in Water: Promotes internode and petiole elongation in deep water rice plants, keeping leaves above water.
  • Root Growth: Enhances root growth and root hair formation, increasing absorption surface.
• Agricultural Uses:
  • Flowering: Initiates and synchronizes flowering in pineapples, induces flowering in mango.
  • Ripening and Abscission: Ethephon, a source of ethylene, is used to ripen tomatoes and apples, accelerate flower and fruit drop (thinning) in cotton, cherry, and walnut.
  • Yield Improvement: Promotes female flowers in cucumbers, increasing yield.

e. Abscisic Acid (ABA)

• What is ABA?
  Abscisic acid (ABA) is known for regulating abscission (dropping of leaves, fruits, etc.) and dormancy in plants.
• Functions:
  • Growth Inhibition: Acts as a general inhibitor of plant growth and metabolism.
  • Seed Germination: Inhibits seed germination.
  • Stress Response: Closes stomata, increasing plant tolerance to stress (hence called the stress hormone).
  • Seed Development: Important in seed development, maturation, and dormancy.
  • Dormancy: Helps seeds withstand desiccation and other unfavorable conditions by inducing dormancy.
  • Antagonist to Gibberellins (GAs): Often works in opposition to gibberellins.

Summary

1. Auxins: Help in rooting, flowering, preventing early drop, promoting apical dominance, inducing parthenocarpy, acting as herbicides, and controlling cell processes.
2. Gibberellins: Used for increasing length of fruits and stems, delaying aging, aiding brewing, boosting crop yields, speeding up seed production, and promoting flowering.
3. Cytokinins: Help in cell division, leaf production, shoot growth, overcoming apical dominance, nutrient mobilization, and delaying leaf aging.
4. Ethylene: Involved in fruit ripening, seed and bud dormancy breaking, promoting elongation in water, root growth, flowering initiation, and agricultural uses like ripening and thinning of crops.

• Plant Growth Regulators (PGRs):
  • PGRs play crucial roles in every phase of plant growth, differentiation, and development.
  • Their roles can be complementary or antagonistic, individualistic or synergistic.
  • Multiple PGRs often interact in events like seed dormancy, bud dormancy, abscission, senescence, and apical dominance.
• Control of Plant Growth:
  • PGRs are one type of intrinsic control, along with genomic control and extrinsic factors (like temperature and light).
  • Extrinsic factors influence plant growth and development through PGRs.
  • Examples of events influenced by extrinsic factors: vernalization, flowering, dormancy, seed germination, and plant movements.

By understanding these regulators, we get a clear picture of how plants grow, develop, and respond to their environment.

Comparative Table of Plant Growth Regulators (PGRs)

Function/Aspect	Auxins	Gibberellins	Cytokinins	Ethylene	Abscisic Acid (ABA)
Promote Cell Division	Yes	Yes	Yes	No	No
Promote Growth	Yes	Yes	Yes	No (except in certain conditions)	No
Root Development	Promote rooting in stem cuttings	Enhance root growth in some conditions	Promote adventitious root formation	Promote root hair formation	Inhibit root growth
Stem Elongation	Promote cell elongation	Strongly promote stem elongation	No	Can promote in deep-water rice	No
Leaf Senescence (Aging)	Delay (inhibit abscission)	Delay senescence	Delay senescence	Promote senescence	Promote senescence
Flowering	Promote in some plants (e.g., pineapples)	Promote bolting (flowering stalk elongation)	No specific role	Initiate flowering in some plants (e.g., mango)	No direct role
Fruit Development and Ripening	Induce parthenocarpy (seedless fruit)	Improve fruit size and shape	No direct role	Promote ripening (e.g., ethephon in tomatoes)	No direct role
Seed Dormancy and Germination	Break dormancy in some conditions	Break dormancy, promote germination	No direct role	Break dormancy	Induce and maintain dormancy, inhibit germination
Stress Response	No specific role	No specific role	No specific role	Increase stress tolerance (e.g., water stress)	Increase stress tolerance (called stress hormone)
Apical Dominance	Promote apical dominance (inhibit lateral buds)	No specific role	Overcome apical dominance	No direct role	No direct role
Interaction	– Work with cytokinins for cell division<br>- Oppose ABA for dormancy and senescence	– Work with auxins and cytokinins for growth<br>- Oppose ABA in seed dormancy	– Work with auxins for cell division<br>- Oppose auxins in apical dominance	– Oppose auxins in senescence<br>- Work with ABA in stress response	– Oppose auxins and gibberellins in dormancy and germination<br>- Work with ethylene in stress response

Key Pairings

• Similar Functions:
  • Auxins, Gibberellins, Cytokinins: Promote cell division and growth.
  • Auxins, Gibberellins: Delay senescence (aging), enhance fruit development.
• Opposite Functions (Antagonistic):
  • Auxins vs. Cytokinins: Auxins promote apical dominance, cytokinins overcome it.
  • Auxins, Gibberellins vs. Abscisic Acid: Auxins and gibberellins break dormancy, ABA induces and maintains dormancy.
  • Ethylene vs. Auxins: Ethylene promotes senescence, auxins inhibit it.
• Unique Functions:
  • Auxins: Promote rooting in cuttings, induce parthenocarpy, used as herbicides.
  • Gibberellins: Strongly promote stem elongation, used in brewing, increase sugarcane yield.
  • Cytokinins: Promote nutrient mobilization, produce new leaves and chloroplasts.
  • Ethylene: Gaseous hormone, promotes fruit ripening, used agriculturally as ethephon.
  • Abscisic Acid: Stress hormone, inhibits seed germination, induces stomatal closure.

Summary

• Auxins, Gibberellins, Cytokinins are primarily growth promoters.
• Ethylene and Abscisic Acid often act as growth inhibitors or stress responders.
• Interactions: PGRs can work synergistically or antagonistically depending on the plant process. For instance, ABA and ethylene promote senescence, whereas auxins inhibit it.

Comparative Table -2 of Plant Growth Regulators (PGRs)

PGR	Target Area	Unique Features	Similar Features
Auxins	Growing tips of stems and roots	– First isolated from human urine<br>- Promotes apical dominance<br>- Induces parthenocarpy (seedless fruits)<br>- Used as herbicides (e.g., 2, 4-D)	– Promote cell division and growth<br>- Influence root development<br>- Used in agriculture and horticulture
Gibberellins	Various plant tissues	– Over 100 types identified (e.g., GA3)<br>- Increase stem and fruit length<br>- Delay senescence (aging)<br>- Speed up malting in brewing<br>- Promote bolting in certain plants	– Promote growth and development<br>- Used to improve crop yields<br>- Enhance plant size and shape
Cytokinins	Root tips, shoot buds, young fruits	– First discovered as kinetin (not natural)<br>- Promote nutrient mobilization<br>- Help produce new leaves and chloroplasts<br>- Delay leaf senescence	– Promote cell division and growth<br>- Involved in delaying aging<br>- Used in agriculture and horticulture
Ethylene	Ripening fruits, senescing tissues	– A gaseous hormone<br>- Influences horizontal growth and apical hook formation<br>- Breaks seed and bud dormancy<br>- Promotes root hair formation<br>- Ethephon used to release ethylene	– Involved in senescence and abscission<br>- Promotes fruit ripening<br>- Used in agriculture to regulate growth

Key Points

Key Points

• Auxins:
  • Target Area: Growing tips of stems and roots.
  • Unique Features: Promotes apical dominance, induces parthenocarpy, used as herbicides.
  • Similar Features: Promote cell division and growth, used in agriculture and horticulture.
• Gibberellins:
  • Target Area: Various plant tissues.
  • Unique Features: Increase stem and fruit length, delay senescence, speed up malting, promote bolting.
  • Similar Features: Promote growth and development, improve crop yields, enhance plant size and shape.
• Cytokinins:
  • Target Area: Root tips, shoot buds, young fruits.
  • Unique Features: Promote nutrient mobilization, help produce new leaves and chloroplasts, delay leaf senescence.
  • Similar Features: Promote cell division and growth, involved in delaying aging, used in agriculture and horticulture.
• Ethylene:
  • Target Area: Ripening fruits, senescing tissues.
  • Unique Features: A gaseous hormone, breaks seed and bud dormancy, promotes root hair formation, ethephon used to release ethylene.
  • Similar Features: Involved in senescence and abscission, promotes fruit ripening, used in agriculture to regulate growth.

Photoperiodism

What is Photoperiodism?

• Some plants need a specific amount of light exposure to flower.
• They can measure how long they are exposed to light.

Types of Plants Based on Light Exposure:

• Long Day Plants: Need more light than a critical duration to flower.
• Short Day Plants: Need less light than a critical duration to flower.
• Day-Neutral Plants: Flowering is not affected by light duration.

Importance of Dark Period:

• Both light and dark periods are important for flowering.
• The duration of darkness matters as much as the duration of light.

How Do Plants Perceive Light/Dark?

• Leaves detect light/dark durations, not the shoot apices.
• A hormone moves from the leaves to the shoot apices to induce flowering when exposed to the right photoperiod.

Vernalisation

What is Vernalisation?

• Some plants need exposure to low temperatures to flower.
• This helps plants avoid early reproductive development and ensures they mature properly.

Examples of Vernalisation:

• Wheat, Barley, Rye: Have winter and spring varieties.
  • Spring Varieties: Planted in spring, flower, and produce grain within the season.
  • Winter Varieties: Planted in autumn, grow as seedlings in winter, and flower in spring.

Biennial Plants:

• Flower and die in the second season.
• Examples: Sugarbeet, cabbages, carrots.
• Cold treatment stimulates flowering.

By understanding photoperiodism and vernalisation, we can better appreciate how plants grow and flower based on light and temperature!

Seed Dormancy

What is Seed Dormancy?

• Some seeds don’t germinate even when conditions are good.
• This is called dormancy and is controlled by the seed itself, not the environment.

Causes of Seed Dormancy:

• Hard Seed Coat: Seed coats are too hard or impermeable.
• Chemical Inhibitors: Presence of substances like abscissic acids, phenolic acids, and para-ascorbic acid.
• Immature Embryos: Embryos inside the seeds are not fully developed.

Overcoming Seed Dormancy:

Natural Methods:

• Mechanical Abrasions: Nature breaks the seed coat through microbial action or when seeds pass through animal digestive tracts.

Man-Made Methods:

• Mechanical Tools: Using knives, sandpaper, or vigorous shaking to break the seed coat.
• Chilling: Subjecting seeds to cold temperatures.
• Chemical Treatments: Applying chemicals like gibberellic acid and nitrates.
• Environmental Changes: Changing light and temperature conditions.

Understanding seed dormancy helps us find ways to make seeds germinate when we want them to!

Chapter Summary:

• Growth is an important event in living organisms.
• It is an irreversible increase in size, area, length, height, volume, cell number, etc.
• Growth involves increased protoplasmic material.
• In plants, growth occurs at meristems.
• Root and shoot apical meristems, and sometimes intercalary meristems, help in elongation of plant axes.
• Growth in higher plants is indeterminate.
• Growth after cell division in root and shoot apical meristem cells can be arithmetic or geometrical.
• Growth is not always sustained at a high rate throughout the life of a cell/tissue/organ/organism.
• There are three main phases of growth: the lag, log, and senescent phases.
• When a cell stops dividing, it differentiates.
• Differentiation leads to the development of structures suited for specific functions.
• Differentiated cells can dedifferentiate and then redifferentiate.
• Plant development is the sum of growth and differentiation and can be flexible.
• Plant growth and development are controlled by intrinsic and extrinsic factors.
• Intrinsic factors are chemical substances called plant growth regulators (PGR).
• PGRs include auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
• PGRs are made in various parts of the plant and control different growth and development events.
• Each PGR has diverse physiological effects on plants.
• PGRs can work together (synergistically) or against each other (antagonistically).
• External factors affecting plant growth and development include light, temperature, nutrition, oxygen status, and gravity.
• Flowering in some plants is influenced by photoperiod (duration of light exposure).
• Plants can be short day, long day, or day-neutral based on their photoperiod requirements.
• Some plants need low temperature exposure to hasten flowering, known as vernalisation.