Plant Growth and Development

Growth

Introduction

• Plants show a definite and orderly pattern of growth and development.
• Trees continue to increase in height and girth throughout their life, whereas their leaves, flowers and fruits appear, grow, fall off and are replaced periodically.
• Development in plants refers to the sequence of changes that occur from the zygote (fertilised egg) to a mature plant body producing roots, stems, leaves, flowers, fruits and seeds.
• Development is the sum of two processes: growth and differentiation.
• Although all plant cells originate from a single zygote, they acquire different structures and functions due to differentiation.
• Developmental processes are controlled by:
  • Intrinsic Factors: Internal to the plant.
  • Extrinsic Factors: External environmental conditions.

Development = Growth + Differentiation.

Seed Germination

• Seed germination is the first visible event of growth and development.
• It involves the activation and growth (rejuvenation) of the embryo into a seedling under favourable conditions.
• These conditions include the presence of water, oxygen and suitable temperature, while internal factors include food reserves, growth regulators, completion of rest period and seed viability.
• Germination may be epigeal, where cotyledons come above the soil, or hypogeal, where cotyledons remain below the soil.
• Seeds that require light for germination are called photoblastic seeds.
• If favourable conditions are absent, seeds remain dormant and resume growth when conditions improve.

Growth

• Growth: One of the most fundamental characteristics of living organisms.
• Definition: An irreversible and permanent increase in size of an organ, its parts or even an individual cell.
• Growth involves metabolic activities, both anabolic and catabolic, and requires energy.
  • Example: The expansion of a leaf represents true growth.

Question: Does swelling of wood in water count as growth? (Think about it!)

• Physical changes such as swelling of wood due to water absorption do not represent growth because they are reversible and not associated with metabolism.

1. Plant Growth is Indeterminate

• Indeterminate Growth: meaning plants continue to grow throughout their life.
  • This is due to the presence of meristems.
• Meristems: Specialised regions of actively dividing cells.
  • Root Apical Meristem and Shoot Apical Meristem: Responsible for primary growth (increase in length).
  • Lateral Meristems (Vascular Cambium and Cork Cambium): Responsible for secondary growth (increase in girth).

This continuous addition of new cells by meristems is known as the open form of growth.

2. Growth is Measurable

• Growth is essentially an increase in protoplasm, but it’s hard to measure directly.
• Growth measurable parameters:
  • Increase in fresh weight
  • Increase in dry weight
  • Increase in length
  • Increase in area
  • Increase in volume
  • Increase in 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:
  1. Meristematic Phase:
     • Found at root and shoot tips (apices).
     • Cells are actively dividing, rich in protoplasm, have large nuclei, and thin primary cell walls.
     • Due to intense biosynthetic activity, the respiration rate in these cells is very high.
  2. Elongation Phase:
     • Located just after the meristematic zone.
     • Cells enlarge, due to vacuoles formation, and new cell wall materials are deposited.
     • Cell enlargement may occur in all directions or predominantly in one direction, leading to increase in length.
  3. Maturation Phase:
     • Further away from the tip, after the elongation zone.
     • Cells attain their maximum size and undergo structural and physiological differentiation to become specialised tissues.
     • Cell walls thicken and protoplasmic modifications occur according to function.

4. Growth Rates

• Growth Rate: Increase in growth per unit time.
• It expresses how fast an organism, organ or tissue grows.
• Growth rates may be arithmetic or geometrical, depending on the pattern of cell division and enlargement.

1. Arithmetic Growth:

• After following mitotic cell division, only one of the two daughter cells continues to divide, while the other differentiates and matures.
• As a result, growth proceeds at a constant rate.
• Example: Elongation of a root at a constant rate.
• When length is plotted against time, a straight line is obtained, indicating linear growth.
• Formula: Lt = L0 + rt Where:
  • Lt = length at time t
  • L0 = length at time zero
  • r = growth rate per unit time
  • t = time

2. Geometrical Growth:

• Both daughter cells produced by mitosis retain the capacity to divide and continue dividing.
• Initially, growth is slow (lag phase), followed by a rapid increase at an exponential rate (log or exponential phase).
• As nutrients become limiting, growth slows down and finally reaches a stationary phase.
• Sigmoid (S-curve): When growth is plotted against time, a sigmoid or S-shaped curve is obtained.
  • This sigmoid curve is characteristic of growth in living organisms under natural conditions and is typical of cells, tissues and organs.
• Formula: 𝑊1 = 𝑊0 𝑒^𝑟𝑡 Where:
  • W1 = final size (weight, height, number, etc.)
  • W0 = initial size
  • r = relative growth rate
  • t = time
  • e = base of natural logarithms
• Here, the relative growth rate represents the efficiency of the plant in producing new plant material. The final size of an organism depends upon its initial size and growth rate.

• Types of Growth Rates:
  1. Absolute Growth Rate: Total increase in growth per unit time.
  2. Relative Growth Rate: Growth per unit time per unit initial size.
• Example: Two leaves may show the same absolute increase in area, but the smaller leaf will have a higher relative growth rate.

5. Conditions Necessary for Growth

• Essential Conditions:
  1. Water: Needed for cell enlargement, maintains cell turgidity, and a medium for enzymatic activities.
  2. Oxygen: For aerobic respiration, which releases the energy necessary for growth-related metabolic activities.
  3. Nutrients: Serve as raw materials for the synthesis of protoplasm and also act as sources of energy.
     • Both macronutrients and micronutrients are essential for proper growth.
  4. Temperature: Growth occurs optimally within a specific temperature range, generally around 28–30°C for most plants.
     • Low temperatures reduce enzyme activity, while high temperatures may denature enzymes and inhibit growth.
  5. Light: Light influences tissue differentiation, synthesis of chlorophyll and other photosynthetic pigments, and photoperiodic responses such as flowering.
  6. Gravity: Gravity determines the direction of growth of roots and shoots and plays an important role in plant orientation.
• Other Factors:
  • Excess salts, mineral deficiencies and environmental stress conditions adversely affect growth.

Differentiation, Dedifferentiation, and Redifferentiation

Differentiation

• What is it?
  Permanent qualitative changes in the structure, chemistry and physiology of cells, tissues and organs.
• Differentiation occurs when cells derived from root apical, shoot apical meristems, and cambium mature to perform specific functions.
• Changes in Cells:
  During differentiation, cells change a lot:
  • Cell walls and protoplasm undergo structural modifications due to selective activation and repression of genes.
  • Example: Tracheary elements lose their protoplasm and develop strong, elastic, lignocellulosic secondary cell walls, enabling them to transport water over long distances even under high tension.

Dedifferentiation

• What is it?
  Dedifferentiation happens when mature, differentiated living cells regain the capacity to divide under specific conditions.
  • These cells become meristematic again.
• Example:
  Formation of interfascicular vascular cambium, cork cambium and wound cambium from mature parenchyma cells.
  • In tissue culture experiments, parenchyma cells dedifferentiate to form a mass of actively dividing cells called callus.

Callus = mass of undifferentiated cells.

Redifferentiation

• What is it?
  Redifferentiation is when dedifferentiated cells undergo structural, chemical and physiological specialization again to perform specific functions.
  • It is similar to differentiation occurring from primary meristems.
• Example:
  Tissues like secondary xylem, secondary phloem, cork and secondary cortex in woody dicot plants are products of redifferentiation.

Open Differentiation and Determination

• 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.
  • In plants, differentiation is open, meaning cells derived from the same meristem can differentiate into different cell types depending on their position and signals received. This property is known as determination or commitment.
• Examples:
  • Cells away from the root apical meristem differentiate into root cap cells.
  • While cells at the periphery differentiate into epidermal cells, followed by cortex, endodermis, pericycle and vascular tissues.

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

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).
• Development is the sequence of changes that occur in the structure and functioning of an organism, organ, tissue or cell throughout its life.
• It includes formation, growth, differentiation, maturation, reproduction, senescence and death.

In plants, development proceeds through well-defined stages such as seed germination, seedling stage, juvenile phase, maturation, flowering, seed formation and senescence. Conversion of one phase into another, such as vegetative phase to flowering or leaf initiation to leaf expansion, is also considered development.

Development may also occur at the sub-cellular level. For example, chloroplasts appear in cells exposed to sunlight.

Development is the sum of growth and differentiation, and these processes are closely interrelated.

Plasticity in Plants

• What is Plasticity?
  Plasticity is the plant’s ability to alter their structure or development in response to environmental conditions or different phases of life.
• Examples of Plasticity:
  • Heterophylly:
    • Cotton, Coriander, Larkspur: juvenile plants have leaves of different shapes compared to mature plants.
    • Buttercup: leaves formed in water differ in shape from those formed in air, showing environmentally induced plasticity.

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: Genetic makeup 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.
  • They are effective even in very low concentrations and regulate growth, differentiation and development in plants.
• Types of PGRs based on chemical nature:
  • 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.

“Phytohormones are chemical substances produced naturally in plants, other than nutrients, and are capable of being transported to other parts to regulate physiological activities even in very low concentrations.”

• Two Main Groups based on Function:
  • Growth Promoters:
    • Promote Activities: such as cell division, cell enlargement, pattern formation, tropic movements, flowering, fruiting and seed formation.
    • Examples: Auxins, Gibberellins, Cytokinins.
  • Growth Inhibitors:
    • Activities: Response to wounds and stresses, induce dormancy and abscission.
    • Examples: Abscisic acid (ABA), Ethylene (mostly inhibitory).

Discovery of PGRs

• Auxins:
  • Charles Darwin and Francis Darwin demonstrated that the tip of coleoptile perceives light during phototropism in canary grass.
  • F.W. Went isolated the growth-promoting substance from oat coleoptile tips using agar blocks and named it auxin.
  • “Later, indole-3-acetic acid (IAA) was identified as the naturally occurring auxin, synthesized from the amino acid tryptophan.”
• Gibberellins:
  • E. Kurosawa observed excessive elongation in rice seedlings 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 tobacco stem callus required auxin along with substances like coconut milk or yeast extract for growth.
  • Miller and co-workers later isolated and crystallized kinetin.
• Abscisic Acid (ABA):
  • In the mid-1960s, three inhibitors named inhibitor-B, abscission II and dormin were discovered.
  • “They were later found to be chemically identical and named abscisic acid.”
• Ethylene:
  • H.H. Cousins observed that ripened oranges released a volatile substance that accelerated ripening of stored bananas.
  • This substance was identified as ethylene.

Summary

• Plant growth regulators control and coordinate growth, development and responses in plants.
• They act in very low concentrations and may promote or inhibit growth depending on type and concentration.
• The five major categories of PGRs are auxins, gibberellins, cytokinins, abscisic acid and ethylene.
• Understanding their discovery and physiological roles helps explain plant growth patterns and agricultural applications.

Physiological Effects of Plant Growth Regulators

a. Auxins

• What are Auxins?
  Auxins are growth-promoting substances derived from the Greek word “auxein”, meaning to grow.
• “Auxins are weakly acidic compounds with an unsaturated ring structure.”
  • Natural auxins include indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA).
  • Synthetic auxins include NAA, 2,4-D and MCPA.
• Production and Movement:
  Auxins are produced mainly in the growing tips of stems and roots and are transported to sites of action.
  • “They show basipetal transport, moving from the apex towards the base.”
• Physiological Functions of Auxins:
  • Promote cell elongation in shoots and cell division in cambium.
  • Induce early differentiation of xylem and phloem
  • Rooting: Initiate rooting in stem cuttings (low concentration).
    • Inhibit root growth at high concentration
  • Apical Dominance: Maintain apical dominance by inhibiting lateral buds
    • Removal of apical bud/shoot tips promotes lateral branching (used in tea plantations and hedge making)
  • Flowering: Promote flowering, e.g., in pineapples.
  • Prevent Drop: Delay early leaf and fruit drop but promote abscission of mature leaves and fruits
  • Parthenocarpy: Induce parthenocarpy (seedless fruits) in tomato
  • Herbicides: Used as selective herbicides: 2,4-D kills dicot weeds but not monocots
  • Other Functions: Control xylem differentiation and promote cell division.

• Auxin (IAA) was first isolated in the early 1930s by Dutch chemists including Fritz Kögl.
• It was extracted from large volumes of human urine due to its low level in plants.
• It appears in urine as a harmless product of tryptophan metabolism from plant-based foods/diet.

“Auxins also induce phototropism and geotropism due to their unequal distribution in plant organs”

b. Gibberellins (GAs)

• What are Gibberellins?
  Gibberellins (GAs) are growth-promoting plant growth regulators.
• “Gibberellins are weakly acidic hormones having a gibbane ring structure.”
  • More than 100 different gibberellins have been identified and are named GA₁, GA₂, GA₃, etc.
  • Among them, GA₃ (gibberellic acid) is the most extensively studied and widely used.
• Functions and Uses:
  • Increase Length: Gibberellins promote elongation of stems and internodes.
    • They are used to elongate grape stalks.
  • Fruit Improvement: They elongate fruits and improve fruit shape, especially in apples.
  • Delay Senescence: GAs delay senescence and fruit drop, thereby extending the market period by keeping fruits attached to plants for a longer time.
  • Malting Process: GA₃ speeds up the malting process in the brewing industry by stimulating enzyme synthesis.
  • Sugarcane Yield: Spraying GAs on sugarcane increases stem length and sugar yield (up to 20 tonnes/acre).
  • Early Seed Production: Spraying juvenile conifers with GAs speed up maturity and leads to early seed production.
  • Bolting: GAs promote internode elongation before flowering in rosette plants like beet and cabbage.
    • This rapid stem elongation is called bolting.

Malting is the controlled germination of cereal grains (usually barley) to activate enzymes that convert stored starch into fermentable sugars for brewing.

Bolting is the rapid elongation of the stem just before flowering in plants or simply internodal stem length increase is Bolting.

Additional uses:

• “Gibberellins break seed dormancy and promote germination by stimulating synthesis of α-amylase.”
• “They can overcome genetic dwarfism and make dwarf plants tall.”
• “Gibberellins promote maleness by inducing male flower formation in certain plants.”
• “They can induce parthenocarpy in some plants.”

c. Cytokinins

• What are Cytokinins?
  Cytokinins are plant growth regulators that promote cell division (cytokinesis).
• “Chemically, cytokinins are basic compounds derived from aminopurines or phenyl urea.”
• Discovery:
  • Cytokinins were first discovered as kinetin from autoclaved herring sperm DNA.
    • Kinetin does not occur naturally in plants.
  • The first naturally occurring cytokinin, zeatin, was later isolated from immature maize grains and coconut milk.
• Functions of Cytokinins:
  • Production Sites: Produced in areas of rapid cell division (root tips, shoot buds, young fruits).
  • Cell Division and Growth: They promote cytokinesis and increase seedling growth, particularly in width.
  • Leaf and Chloroplast Development: Promote new leaf and chloroplast production.
  • Lateral Shoot Growth: Encourage lateral bud/shoot growth and adventitious shoot formation.
  • Overcoming Apical Dominance: Cytokinins counteract apical dominance and promote branching.
  • Delay Senescence: They delay leaf aging and degradation of chlorophyll, known as the Richmond–Lang effect.
  • Mobilisation of Nutrients: Cytokinins mobilize nutrients toward young tissues.

Additional uses:

• “Cytokinins retard abscission of leaves and fruits.”
• “They prolong the life of cut flowers and shoots.”
• “Application of cytokinins keeps harvested vegetables fresh for several days.”
• “They help in morphogenesis by controlling organ formation in tissue culture.”

d. Ethylene

• What is Ethylene?
  A simple gaseous PGR produced in large amounts by ripening fruits and senescing tissues.
• “Ethylene is the only gaseous phytohormone and is synthesised from the amino acid methionine.”
• Functions:
  • Seedlings (Triple Response): Ethylene causes horizontal growth, axis swelling, and apical hook formation in dicot seedlings.
    • “This characteristic effect of ethylene on etiolated seedlings is called the triple response.”
  • Senescence and Abscission: Promotes aging (senescence) and shedding (abscission) of leaves, flowers, and fruits.
  • Fruit Ripening: Ethylene is highly effective in fruit ripening; increases respiration rate during ripening.
    • “The sudden rise in respiration during fruit ripening due to ethylene is called respiratory climacteric.”
  • Breaking Dormancy: Ethylene 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, helping leaves remain above water.
  • Root Growth: Ethylene enhances root growth and root hair formation, increasing the surface area for absorption.
    • “Ethylene formed under waterlogged conditions induces adventitious root formation.”
• Agricultural Uses of Ethylene:
  • Flowering: Initiates and synchronizes flowering in pineapples and induces flowering in mango.
  • Ripening and Abscission: Ethephon, a chemical source of ethylene, is used to ripen tomatoes and apples.
    • “Ethephon is readily absorbed in aqueous solution and releases ethylene inside plant tissues.”
    • It is also used to accelerate flower and fruit drop (thinning) in crops like cotton, cherry, and walnut.
  • Yield Improvement: Ethylene promotes the formation of female flowers in cucumbers, thereby increasing yield.

e. Abscisic Acid (ABA)

• What is ABA?
  Abscisic acid (ABA) is a plant growth regulator mainly associated with abscission, dormancy, and stress responses.
• “ABA is a mildly acidic hormone that acts as a general growth inhibitor.”
• “It is also known as the stress hormone because its production increases under adverse conditions like drought and waterlogging.”
• Functions:
  • Growth Inhibition: ABA inhibits overall plant growth and metabolic activities.
  • Seed Germination: ABA inhibits seed germination and induces dormancy.
  • Stress Response: ABA causes partial closure of stomata during water stress, reducing transpiration and increasing stress tolerance.
  • Seed Development: ABA plays an important role 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.
    • “ABA counteracts many growth-promoting effects of gibberellins.”

Summary

1. Auxins: Help in rooting, flowering, prevention of early drop, promotion of apical dominance, induction of parthenocarpy, herbicidal action, and regulation of cell differentiation.
2. Gibberellins: Increase stem and fruit length, delay senescence, aid brewing, increase crop yield, speed up seed production, and promote bolting and flowering.
3. Cytokinins: Promote cell division, leaf and shoot formation, overcome apical dominance, mobilise nutrients, and delay senescence.
4. Ethylene: Regulates fruit ripening, senescence, abscission, dormancy breaking, elongation under water, root growth, flowering initiation, and crop thinning.
5. Abscisic Acid: Acts as a growth inhibitor, induces dormancy, closes stomata under stress, promotes senescence, and antagonises growth promoters.

• Plant Growth Regulators (PGRs) – Overall Concept
  • PGRs control growth, differentiation, and development throughout the plant life cycle.
  • 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.
  • Intrinsic Control of Growth: PGRs act as intrinsic regulators along with genetic control.
  • Extrinsic Factors: Environmental factors such as light, temperature, and water influence plant growth indirectly through PGRs.
  • “Processes like vernalization, flowering, dormancy, seed germination, and plant movements are mediated through plant growth regulators.”

Comparative Table of Plant Growth Regulators (PGRs)

Function/Aspect	Auxins	Gibberellins	Cytokinins	Ethylene	Abscisic Acid (ABA)
Promote Cell Division	Yes (with cytokinins)	Yes	Yes	No	No
Promote Growth	Yes	Yes	Yes	No (except in certain conditions)	No
Root Development	Promote rooting in stem cuttings	Limited or indirect role	Promote root hair formation	Promote adventitious roots	Inhibits root growth
Stem Elongation	Promote cell elongation	Strongly promote internode elongation	Little direct effect	Promotes elongation in deep-water rice	Inhibits growth
Leaf Senescence (Aging)	Delay (inhibit abscission)	Delay senescence	Delay senescence	Promote senescence	Promote senescence
Flowering	Promote in some plants (e.g., pineapples)	Promote bolting and flowering	No major direct role	Initiate flowering in some plants (e.g., pineapple, mango)	Inhibits flowering in many plants
Fruit Development and Ripening	Induce parthenocarpy (seedless fruit)	Increase fruit size and shape	No direct role	Promote ripening (e.g., ethephon in tomatoes)	No direct role
Seed Dormancy and Germination	Help break dormancy	Break dormancy, promote germination	No direct role	Break dormancy in some seeds	Induce and maintain dormancy
Stress Response	No major role	No major role	No major role	Works during stress responses	Major stress hormone (closes stomata)
Apical Dominance	Promote apical dominance (inhibit lateral buds)	No direct role	Overcome apical dominance	No direct role	No direct role

KEY PAIRINGS AND INTERACTIONS (EXAM-IMPORTANT)

• Similar (Synergistic) Actions:
  • Auxins + Gibberellins + Cytokinins → promote growth and cell division
  • Auxins + Cytokinins → tissue culture growth and organ formation
• Opposite (Antagonistic) Actions:
  • Auxins vs Cytokinins → apical dominance vs lateral bud growth
  • Auxins & Gibberellins vs ABA → germination vs dormancy
  • Auxins vs Ethylene → delay vs promote senescence
• Unique Roles:
  • Auxins → rooting, parthenocarpy, herbicides
  • Gibberellins → stem elongation, bolting, malting
  • Cytokinins → nutrient mobilization, delay aging
  • Ethylene → fruit ripening, gaseous hormone
  • ABA → stress tolerance, stomatal closure, dormancy

Comparative Table -2 of Plant Growth Regulators (PGRs)

PGR	Target Area	Unique Features	Similar/Common Features
Auxins	Growing tips of shoots and roots	Growing tips of shoots and roots Promote apical dominance, rooting in cuttings, parthenocarpy, selective herbicides like 2,4-D	Promote growth and cell division, widely used in agriculture
Gibberellins	Young leaves, roots, seeds, stems	Strong stem elongation, bolting, malting in brewing, increased sugarcane yield	Promote growth, improve crop size and yield
Cytokinins	Root tips, shoot buds, young fruits	Promote lateral buds, delay leaf aging, mobilise nutrients, new leaf and chloroplast formation	Promote cell division and delay senescence
Ethylene	Ripening fruits, senescing (old) tissues	Only gaseous hormone, fruit ripening, apical hook formation, ethephon releases ethylene	Controls senescence, abscission, used in agriculture
Abscisic Acid (ABA)	Leaves, seeds, stressed tissues	Stress hormone, stomatal closure, dormancy induction, growth inhibition	Regulates stress responses and developmental arrest

Summary

• Auxins, gibberellins, and cytokinins mainly act as growth promoters.
• Ethylene and abscisic acid mainly regulate stress, senescence, and dormancy.
• Plant growth regulators do not act alone; they interact synergistically or antagonistically depending on the process.
• Plant development is controlled by the balance between different PGRs rather than a single hormone.

Photoperiodism

What is Photoperiodism?

• Some plants require a specific duration of light exposure to initiate flowering.
• Plants can measure the length of light and dark periods.
• “Photoperiodism is the effect of photoperiods or daily duration of light and dark periods on the growth and development of plants, especially flowering.”
• “Photoperiodism was first studied by Garner and Allard (1920).”

Types of Plants Based on Light Exposure:

• Long Day Plants: Need more light than a critical duration to flower.
  • Examples: wheat and spinach.
• Short Day Plants: Need less light than a critical duration to flower.
  • Examples: tobacco, soybean, and Xanthium.
• Day-Neutral Plants: Flowering is not affected by light duration.
  • Examples: cotton, cucumber, and pea.

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 chemical pigment called phytochrome present in leaves perceives the photoperiodic stimulus.”
• A flowering signal (hormone) is produced in the leaves and transported to the shoot apical meristem to induce flowering under appropriate photoperiod.

Vernalisation

What is Vernalisation?

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

“Vernalisation is the process of shortening of the juvenile or vegetative phase and hastening flowering by a precocious cold treatment.”

Purpose of Vernalisation

• It prevents premature flowering and ensures proper maturation of the plant before reproduction.

Examples of Vernalisation:

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

“In vernalisation, winter varieties can be converted into spring varieties by cold treatment.”

“Site of vernalisation is the apical meristem or other meristematic tissues such as embryo tips and root apices.”

Biennial Plants:

• Biennials flower and die in the second season.
• Examples: Sugarbeet, cabbages, carrots.
• “Cold treatment stimulates flowering in biennial plants.”

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 (internal factors), not the environment.

“This blocking of germination despite favourable conditions is called seed dormancy.”

Causes of Seed Dormancy:

• Hard Seed Coat: Seed coats may be hard or impermeable to water and oxygen.
• Chemical Inhibitors: Presence of substances like abscissic acids, phenolic acids, and para-ascorbic acid prevents germination.
• Immature Embryos: Embryos inside some seeds are not fully developed at the time of seed dispersal.

“Other causes include embryo requiring after-ripening and impermeability of seed coat to gases.”

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 (Scarification): Using knives, sandpaper, or vigorous shaking to break the seed coat.
• Chilling: Exposing seeds to low temperature.
• Chemical Treatments: Applying chemicals like gibberellic acid and nitrates.
• Environmental Changes: Altering light and temperature conditions.

Summary

• Photoperiodism regulates flowering based on the duration of light and dark periods.
• Vernalisation promotes flowering through cold exposure.
• Seed dormancy ensures survival by preventing germination under unsuitable conditions.
• Together, these mechanisms help plants synchronize growth and reproduction with favourable environmental conditions.

Chapter Summary:

• Growth is an essential characteristic of all living organisms.
• It is defined as an irreversible increase in size, area, length, height, volume, cell number, or mass, and is always associated with an increase in protoplasmic material.
• In plants, growth occurs in specific regions called meristems.
• Root apical meristems and shoot apical meristems, and in some plants intercalary meristems, are responsible for elongation of plant axes.
• Growth in higher plants is indeterminate because meristems remain active throughout life.

• After cell division in root and shoot apical meristems, growth may follow either arithmetic growth, where only one daughter cell continues to divide, or geometrical growth, where both daughter cells divide.
• Growth does not remain rapid throughout the entire life of a cell, tissue, organ, or organism.
• It shows different phases, namely lag phase, log (exponential) phase, and senescent phase.

• When cells stop dividing, they undergo differentiation, during which they develop structural and functional specialisation.
• Differentiated cells may regain the capacity to divide through dedifferentiation, and the newly formed cells may again mature through redifferentiation.
• Thus, plant development is the sum of growth and differentiation and shows flexibility depending on conditions.

• Plant growth and development are regulated by intrinsic and extrinsic factors.
• Intrinsic factors include chemical substances known as plant growth regulators (PGRs).
• The major PGRs are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
• These regulators are synthesised in different parts of the plant and control various growth and developmental processes.
• Each PGR has multiple physiological effects, and they may act synergistically or antagonistically with one another.

• Extrinsic factors affecting plant growth and development include light, temperature, nutrition, oxygen status, and gravity.
• Flowering in many plants is influenced by photoperiod, which is the duration of light exposure.
• Based on photoperiodic response, plants are classified as short day plants, long day plants, or day-neutral plants.
• In some plants, exposure to low temperature is required to hasten flowering, a phenomenon known as vernalisation.