Chemical Coordination in Plants - Question Paper

Section A: Multiple Choice Questions (MCQs) - 100 Questions

Instructions: Choose the correct option for each question.

Plants coordinate their activities through: a) Nervous system b) Chemical coordination c) Physical coordination d) None of the above
Plant hormones are also known as: a) Growth inhibitors b) Growth regulators c) Growth stimulants d) Growth factors
Which hormone promotes cell elongation? a) Cytokinin b) Auxin c) Gibberellin d) ABA
Auxins are primarily responsible for: a) Cell division b) Root formation c) Fruit ripening d) Dormancy
Which hormone promotes stem elongation? a) Auxin b) Gibberellin c) Cytokinin d) Ethylene
Gibberellins are involved in: a) Germination b) Flowering c) Stem elongation d) All of the above
Cell division is promoted by: a) Auxin b) Gibberellin c) Cytokinin d) ABA
Which hormone delays senescence? a) Ethylene b) ABA c) Cytokinin d) Auxin
Abscisic acid primarily: a) Promotes growth b) Inhibits growth c) Promotes flowering d) Promotes germination
ABA helps plants cope with: a) Light b) Gravity c) Stress d) Touch
Which hormone promotes fruit ripening? a) Auxin b) Gibberellin c) Cytokinin d) Ethylene
Ethylene is associated with: a) Cell elongation b) Senescence c) Cell division d) Dormancy
Tropisms are: a) Random movements b) Directional growth movements c) Chemical responses d) None of the above
Growth response to light is called: a) Geotropism b) Phototropism c) Hydrotropism d) Thigmotropism
Phototropism is triggered by: a) Gravity b) Water c) Light d) Touch
Growth in response to gravity is: a) Phototropism b) Geotropism c) Hydrotropism d) Chemotropism
Hydrotropism is growth response to: a) Light b) Gravity c) Water d) Chemicals
Touch response in plants is called: a) Phototropism b) Geotropism c) Thigmotropism d) Hydrotropism
Chemotropism is response to: a) Light b) Chemicals c) Water d) Gravity
Which of the following is NOT a tropism? a) Phototropism b) Metabolism c) Geotropism d) Thigmotropism
Root formation is primarily controlled by: a) Gibberellin b) Cytokinin c) Auxin d) Ethylene
Fruit growth is promoted by: a) ABA b) Ethylene c) Auxin d) Cytokinin
Which hormone is known as stress hormone in plants? a) Auxin b) Gibberellin c) ABA d) Cytokinin
Dormancy in plants is promoted by: a) Gibberellin b) Auxin c) ABA d) Cytokinin
Senescence is the process of: a) Growth b) Aging c) Germination d) Flowering
Which hormone counteracts senescence? a) Ethylene b) ABA c) Cytokinin d) Gibberellin
Germination of seeds is promoted by: a) ABA b) Ethylene c) Gibberellin d) Cytokinin
Flowering is induced by: a) Auxin b) Gibberellin c) Cytokinin d) ABA
Plants bend towards light due to: a) Geotropism b) Phototropism c) Hydrotropism d) Thigmotropism
Roots grow downward due to: a) Phototropism b) Positive geotropism c) Negative geotropism d) Hydrotropism
Shoots grow upward due to: a) Positive geotropism b) Negative geotropism c) Phototropism d) Thigmotropism
The stimulus in phototropism is: a) Unidirectional light b) Water gradient c) Gravity d) Chemical gradient
The stimulus in geotropism is: a) Light b) Gravity c) Water d) Touch
Hydrotropism helps plants in: a) Finding light b) Finding water c) Avoiding gravity d) Sensing touch
Thigmotropism is important for: a) Root growth b) Stem growth c) Climbing plants d) Fruit ripening
Which hormone regulates apical dominance? a) Cytokinin b) Auxin c) Gibberellin d) ABA
Cell wall loosening is caused by: a) Cytokinin b) ABA c) Auxin d) Ethylene
Which hormone promotes lateral bud growth? a) Auxin b) Cytokinin c) Gibberellin d) ABA
Leaf abscission is promoted by: a) Cytokinin b) Auxin c) Ethylene d) Gibberellin
Which hormone delays leaf yellowing? a) Ethylene b) ABA c) Cytokinin d) Auxin
Stomatal closure is promoted by: a) Auxin b) Gibberellin c) ABA d) Cytokinin
Which tropism involves directional stimulus? a) All tropisms b) Only phototropism c) Only geotropism d) None
The growth movement in tropism is: a) Reversible b) Irreversible c) Temporary d) Instantaneous
Auxin was first discovered in: a) Roots b) Leaves c) Coleoptiles d) Fruits
The natural auxin is: a) NAA b) IAA c) 2,4-D d) IBA
Gibberellins were first isolated from: a) Bacteria b) Fungi c) Algae d) Higher plants
Cytokinin was first discovered in: a) Coconut milk b) Root extract c) Leaf extract d) Stem extract
ABA is also known as: a) Growth hormone b) Stress hormone c) Flowering hormone d) Ripening hormone
Ethylene is a: a) Liquid hormone b) Solid hormone c) Gaseous hormone d) Crystalline hormone
Which hormone promotes bolting? a) Auxin b) Gibberellin c) Cytokinin d) ABA
Parthenocarpy is induced by: a) Ethylene b) ABA c) Auxin d) Cytokinin
Which hormone breaks seed dormancy? a) ABA b) Ethylene c) Gibberellin d) Cytokinin
The concentration of auxin is highest in: a) Roots b) Stems c) Growing tips d) Leaves
Positive phototropism is shown by: a) Roots b) Shoots c) Both roots and shoots d) Neither roots nor shoots
Negative geotropism is shown by: a) Roots b) Shoots c) Leaves d) Flowers
Positive geotropism is shown by: a) Shoots b) Roots c) Leaves d) Flowers
The site of auxin synthesis is: a) Root tips b) Shoot apex c) Leaves d) Stem
Auxin transport is: a) Bidirectional b) Unidirectional c) Random d) Multidirectional
The direction of auxin transport is: a) Upward only b) Downward only c) Basipetal d) Acropetal
Which hormone induces rooting in cuttings? a) Gibberellin b) Cytokinin c) Auxin d) ABA
Internodal elongation is promoted by: a) Auxin b) Gibberellin c) Cytokinin d) Ethylene
Which hormone promotes cambial activity? a) ABA b) Ethylene c) Auxin d) Gibberellin
Fruit drop is prevented by: a) Ethylene b) ABA c) Auxin d) Gibberellin
Which hormone is involved in vernalization response? a) Auxin b) Gibberellin c) Cytokinin d) ABA
The bending in phototropism is due to: a) Equal growth on both sides b) Unequal growth on both sides c) No growth d) Death of cells
In phototropism, auxin accumulates on: a) Illuminated side b) Shaded side c) Both sides equally d) Neither side
The curvature in geotropism is due to: a) Gravity directly b) Unequal distribution of auxin c) Light effect d) Temperature difference
Which part of plant shows hydrotropism? a) Shoots b) Roots c) Leaves d) Flowers
Thigmotropism is best observed in: a) Sunflower b) Mango tree c) Grape vine d) Rose plant
Chemical coordination in plants is: a) Faster than animals b) Slower than animals c) Same as animals d) Not comparable
Plant hormones act in: a) High concentrations b) Very low concentrations c) Moderate concentrations d) Variable concentrations
The effect of plant hormones is: a) Immediate b) Delayed c) Instantaneous d) None of the above
Multiple hormones can affect: a) Different processes only b) Same process c) No process d) Metabolic activities only
Hormone interaction in plants is: a) Always synergistic b) Always antagonistic c) Can be both d) Neither synergistic nor antagonistic
Which hormone shows apical dominance? a) Cytokinin b) Auxin c) Gibberellin d) ABA
Synthetic auxins are used for: a) Rooting cuttings b) Preventing fruit drop c) Inducing parthenocarpy d) All of the above
Which hormone is used as herbicide? a) IAA b) 2,4-D c) NAA d) IBA
Gibberellins increase: a) Cell number only b) Cell size only c) Both cell number and size d) Neither cell number nor size
Cytokinins are derivatives of: a) Tryptophan b) Adenine c) Tyrosine d) Glycine
ABA is synthesized from: a) Amino acids b) Carotenoids c) Fatty acids d) Sugars
Which hormone regulates transpiration? a) Auxin b) Gibberellin c) ABA d) Cytokinin
Ethylene promotes: a) Cell elongation b) Cell division c) Cell maturation d) Cell differentiation
The gaseous hormone among the following is: a) Auxin b) Gibberellin c) Ethylene d) ABA
Which hormone causes epinasty? a) Auxin b) Gibberellin c) Ethylene d) Cytokinin
Tropisms occur due to: a) Equal growth b) Differential growth c) No growth d) Death of tissues
The term tropism was coined by: a) Darwin b) Sachs c) De Candolle d) Went
Phototropic response is first observed in: a) Roots b) Coleoptiles c) Leaves d) Stem
The photoreceptor for phototropism is: a) Chlorophyll b) Phytochrome c) Phototropin d) Carotenoids
Gravitropism perception occurs in: a) Shoot tip b) Root tip c) Root cap d) Both b and c
Statoliths are involved in perception of: a) Light b) Gravity c) Water d) Touch
Which cells contain statoliths? a) Parenchyma b) Collenchyma c) Sclerenchyma d) Statocytes
Hydrotropism is exhibited by: a) All plant parts b) Only roots c) Only shoots d) Only leaves
Thigmotropism involves: a) Mechanical stimulus b) Chemical stimulus c) Light stimulus d) Gravitational stimulus
Which plant part shows negative phototropism? a) Shoot b) Root c) Leaf d) Flower
Nastic movements differ from tropisms in being: a) Directional b) Non-directional c) Permanent d) Chemical
The primary site of hormone action is: a) Cell wall b) Cell membrane c) Cytoplasm d) All of the above
Hormone receptors in plants are located in: a) Cell wall only b) Cell membrane only c) Cytoplasm only d) All cellular compartments
Signal transduction in plants involves: a) Primary messenger only b) Secondary messenger only c) Both primary and secondary messengers d) No messengers
The ultimate response to hormones is: a) Gene expression b) Enzyme activation c) Metabolic changes d) All of the above
Chemical coordination in plants evolved: a) After nervous system b) Before nervous system c) Simultaneously with nervous system d) Independent of nervous system

Section B: One Mark Questions - 100 Questions

Instructions: Answer in one word or one sentence.

What is the alternative name for plant hormones?
Which hormone promotes cell elongation?
Name the hormone that delays senescence.
What does ABA stand for?
Which hormone is gaseous in nature?
Define tropism.
What is phototropism?
Name the tropism related to gravity.
Which tropism involves response to water?
What is thigmotropism?
Which hormone promotes root formation?
Name the hormone that inhibits growth.
What promotes fruit ripening?
Which hormone helps in germination?
What is chemotropism?
Name the natural auxin.
Which hormone promotes stem elongation?
What is the stress hormone in plants?
Which hormone promotes cell division?
What causes fruit growth?
Name the process of aging in plants.
Which hormone breaks seed dormancy?
What promotes flowering?
Which hormone causes leaf fall?
Name the hormone that regulates transpiration.
What is apical dominance?
Which hormone prevents fruit drop?
What is parthenocarpy?
Which hormone is used as herbicide?
Name the photoreceptor for phototropism.
What are statoliths?
Where are statocytes found?
Which part shows positive geotropism?
What shows negative geotropism?
Which hormone promotes cambial activity?
What is bolting?
Which hormone causes epinasty?
What is vernalization?
Name a synthetic auxin.
Which hormone is derived from adenine?
From what is ABA synthesized?
What promotes lateral bud growth?
Which hormone closes stomata?
What is the site of auxin synthesis?
How is auxin transported?
Which direction does auxin move?
What induces rooting in cuttings?
Which hormone shows antagonistic effect to auxin?
What type of movement are tropisms?
Who coined the term tropism?
Which plant part perceives gravity?
What contains phototropin?
Which cells perceive touch stimulus?
What type of stimulus triggers thigmotropism?
Which hormone regulates fruit abscission?
What is the concentration range of plant hormones?
Are plant hormone effects immediate?
Can multiple hormones affect the same process?
What type of interactions do plant hormones show?
Which hormone maintains apical dominance?
What are synthetic auxins used for?
Which hormone acts as growth retardant?
What promotes internodal elongation?
Which hormone is involved in tissue culture?
What causes bending in phototropism?
Where does auxin accumulate in phototropism?
What causes curvature in geotropism?
Which plant part shows hydrotropism mainly?
Where is thigmotropism best observed?
How does chemical coordination compare to nervous coordination in speed?
What is the primary site of hormone perception?
Where are hormone receptors located?
What does signal transduction involve?
What is the ultimate response to hormones?
When did chemical coordination evolve relative to nervous system?
Which hormone prevents premature fruit drop?
What is the effect of cytokinin on leaves?
Which hormone promotes shoot elongation?
What inhibits seed germination?
Which hormone accelerates ripening?
What is the primary function of gibberellins?
Which hormone is anti-senescence?
What promotes root hair formation?
Which hormone causes chlorophyll breakdown?
What regulates opening and closing of stomata?
Which hormone promotes cell wall loosening?
What causes shoot curvature towards light?
Which hormone inhibits lateral bud growth?
What promotes adventitious root formation?
Which hormone delays leaf abscission?
What is responsible for gravitropic response?
Which hormone promotes male flower formation?
What causes triple response in seedlings?
Which hormone breaks apical dominance?
What promotes secondary growth?
Which hormone is involved in photoperiodism?
What causes fruit set without fertilization?
Which hormone promotes enzyme synthesis during germination?
What regulates phloem transport?
Which hormone is involved in defense responses?

Section C: Two Marks Questions - 100 Questions

Instructions: Answer each question in 2-3 sentences.

Explain why plants need chemical coordination despite lacking a nervous system.
Differentiate between plant hormones and animal hormones.
Describe the role of auxins in plant growth and development.
How do gibberellins affect plant growth? Give two examples.
Explain the importance of cytokinins in plant development.
Describe the role of ABA as a stress hormone in plants.
How does ethylene affect fruit ripening and senescence?
Define tropism and explain why it is important for plants.
Compare positive and negative phototropism with examples.
Explain how geotropism helps plants in their survival.
Describe the significance of hydrotropism in plant life.
How does thigmotropism benefit climbing plants?
Explain the mechanism of phototropic response in shoots.
Describe how roots show positive geotropic response.
What is the role of auxin in apical dominance?
Explain how cytokinins delay senescence in plants.
Describe the antagonistic relationship between auxin and cytokinin.
How does ABA help plants during water stress?
Explain the role of ethylene in leaf abscission.
Describe the importance of gibberellins in seed germination.
What is parthenocarpy and how is it induced?
Explain the concept of hormone interaction in plants.
How do synthetic auxins benefit agriculture?
Describe the role of hormones in tissue culture.
Explain how plant hormones work at cellular level.
What is the significance of hormone transport in plants?
Describe the role of auxin in root formation.
How do gibberellins overcome genetic dwarfism?
Explain the anti-aging effect of cytokinins.
Describe how ABA regulates stomatal movement.
What is the triple response caused by ethylene?
Explain the difference between tropism and nastic movement.
How does the direction of light affect phototropic response?
Describe the role of gravity in plant orientation.
What are statoliths and their function in geotropism?
Explain how plants perceive touch stimulus.
Describe the adaptive significance of chemotropism.
How do environmental factors affect hormonal responses?
Explain the role of photoreceptors in phototropism.
What is the significance of hormone concentration in plant responses?
Describe the temporal aspects of plant hormone action.
How do plant hormones regulate gene expression?
Explain the role of second messengers in hormone action.
What is signal transduction in the context of plant hormones?
Describe the evolution of chemical coordination in plants.
How do plant hormones coordinate growth and development?
Explain the role of hormones in plant reproduction.
What is the significance of dormancy and how is it regulated?
Describe how hormones regulate flowering in plants.
How do plant hormones respond to seasonal changes?
Explain the role of auxin in fruit development.
How does gibberellin affect stem elongation?
Describe the cytokinins effect on lateral bud growth.
What is the mechanism of ABA action during drought?
Explain how ethylene accelerates fruit ripening.
Describe the phototropic bending mechanism in coleoptiles.
How do roots perceive and respond to gravity?
What is the role of root cap in geotropism?
Explain how plants exhibit hydrotropic response.
Describe the mechanosensitive response in thigmotropism.
How do hormones regulate cambial activity?
Explain the role of auxin in vascular differentiation.
What is the effect of gibberellins on alpha-amylase production?
How do cytokinins affect chloroplast development?
Describe the role of ABA in seed development.
Explain how ethylene affects sex expression in plants.
What is the significance of hormonal balance in plant growth?
How do external stimuli trigger hormonal responses?
Describe the feedback mechanisms in hormonal control.
What is the role of plant hormones in stress tolerance?
Explain how hormones coordinate source-sink relationships.
How do plant hormones affect photosynthesis?
Describe the role of hormones in senescence and abscission.
What is the importance of timing in hormonal responses?
Explain how plant hormones integrate environmental signals.
How do hormones regulate resource allocation in plants?
Describe the role of plant hormones in defense mechanisms.
What is the significance of hormone gradients in development?
Explain how plant hormones coordinate organ formation.
How do hormones regulate the transition from vegetative to reproductive growth?
Describe the role of auxin transport in plant development.
What is the effect of light quality on hormonal responses?
How do temperature changes affect plant hormone action?
Explain the role of hormones in circadian rhythms.
What is the significance of hormone receptors in plant cells?
How do plant hormones regulate water relations?
Describe the role of hormones in root-shoot communication.
What is the effect of mechanical stress on hormone production?
Explain how plant hormones coordinate responses to biotic stress.
How do hormones regulate leaf development and morphology?
Describe the role of plant hormones in seed dispersal.
What is the significance of hormone metabolism in plants?
How do plant hormones affect mineral uptake?
Explain the role of hormones in plant architecture.
What is the effect of hormone application timing on plant responses?
How do plant hormones coordinate cellular division and expansion?
Describe the role of hormones in programmed cell death.
What is the significance of hormone crosstalk in plant development?
How do environmental stresses modify hormonal responses?
Explain the future prospects of plant hormone research in agriculture.

Section D: Three Marks Broad Questions - 50 Questions

Instructions: Answer each question in detail with proper explanations and examples.

Discuss in detail the structure, functions, and applications of auxins in plant biology. Include their role in various developmental processes and commercial uses.
Elaborate on the multiple roles of gibberellins in plant growth and development. Explain their mechanism of action and significance in overcoming dormancy and dwarfism.
Analyze the comprehensive role of cytokinins in plant development. Discuss their interaction with auxins and their applications in tissue culture and anti-aging treatments.
Examine the role of Abscisic Acid (ABA) as a stress hormone. Explain its functions in drought tolerance, seed dormancy, and stomatal regulation with detailed mechanisms.
Investigate the diverse functions of ethylene in plant physiology. Discuss its role in fruit ripening, senescence, abscission, and response to mechanical stress.
Compare and contrast the five major plant hormones in terms of their synthesis, transport, functions, and interactions. Provide a comprehensive analysis of their coordinated effects.
Explain the concept of phototropism in detail. Discuss the molecular mechanism, the role of auxin redistribution, and the adaptive significance of this response.
Analyze geotropism in both roots and shoots. Explain the perception mechanism, the role of statoliths, and how differential auxin distribution causes gravitropic bending.
Discuss the phenomenon of hydrotropism and its ecological significance. Explain how plants perceive water gradients and orient their growth accordingly.
Elaborate on thigmotropism with special reference to climbing plants. Discuss the mechanosensitive responses and their adaptive advantages in different ecological niches.
Examine the concept of chemical coordination in plants and compare it with nervous coordination in animals. Discuss the advantages and limitations of each system.
Analyze the interactions between different plant hormones. Discuss synergistic and antagonistic effects with specific examples from plant development.
Investigate the role of plant hormones in regulating the plant life cycle from seed germination to senescence. Provide a comprehensive overview of hormonal control at each stage.
Discuss the applications of plant hormones in modern agriculture and horticulture. Include examples of how synthetic hormones are used to improve crop production and quality.
Examine the molecular mechanisms of hormone action in plants. Discuss signal perception, transduction, and the ultimate cellular responses including gene expression changes.
Analyze the role of environmental factors in modulating plant hormone responses. Discuss how light, temperature, water availability, and stress conditions affect hormonal coordination.
Investigate the evolution of chemical coordination in plants. Compare the hormonal systems in different plant groups and discuss their adaptive significance.
Discuss the concept of apical dominance and its hormonal regulation. Explain how the balance between auxins and cytokinins controls branching patterns in plants.
Examine the role of plant hormones in reproductive development. Discuss their involvement in flower initiation, fruit development, and seed formation.
Analyze the hormonal control of stomatal behavior. Explain how ABA and other hormones regulate gas exchange and water relations in response to environmental conditions.
Investigate the role of plant hormones in root development and architecture. Discuss how different hormones coordinate primary and lateral root formation.
Discuss the phenomenon of senescence and its hormonal regulation. Explain the roles of different hormones in aging processes and programmed cell death.
Examine the hormonal regulation of seed germination and dormancy. Discuss the balance between promoting and inhibiting factors and their ecological significance.
Analyze the role of plant hormones in stress responses. Discuss how hormonal coordination helps plants survive biotic and abiotic stresses.
Investigate the concept of photoperiodism and its hormonal basis. Explain how plants perceive day length and coordinate flowering responses.
Discuss the hormonal control of cambial activity and secondary growth. Explain how auxin and other hormones regulate wood formation and vascular development.
Examine the role of plant hormones in fruit development and ripening. Discuss the coordinated action of different hormones in fruit physiology.
Analyze the hormonal regulation of leaf development and morphology. Discuss how hormones control leaf initiation, expansion, and differentiation.
Investigate the role of plant hormones in tissue culture and biotechnology applications. Discuss how hormonal manipulation enables plant propagation and genetic transformation.
Discuss the concept of hormone transport and its significance in plant development. Explain polar auxin transport and the distribution of other hormones.
Examine the hormonal basis of plant responses to mechanical stimuli. Discuss thigmomorphogenesis and its role in plant adaptation to environmental challenges.
Analyze the role of plant hormones in coordinating source-sink relationships. Discuss how hormones regulate resource allocation and translocation.
Investigate the hormonal control of flower sex determination. Explain how ethylene and other hormones influence male and female flower development.
Discuss the role of plant hormones in circadian rhythms and biological clocks. Explain how hormonal oscillations coordinate daily physiological processes.
Examine the concept of hormone gradients and their role in plant development. Discuss how concentration differences direct morphogenesis and organogenesis.
Analyze the hormonal regulation of abscission processes. Explain how plants control the shedding of leaves, fruits, and other organs.
Investigate the role of plant hormones in pathogen defense responses. Discuss how hormonal coordination enhances plant immunity and resistance.
Discuss the hormonal control of plant architecture and form. Explain how different hormones shape plant morphology and branching patterns.
Examine the role of plant hormones in vernalization responses. Discuss how cold treatment affects hormonal status and flowering competence.
Analyze the concept of hormone homeostasis in plants. Discuss the mechanisms that maintain appropriate hormone levels and responses.
Investigate the role of plant hormones in coordinating development with nutrition. Discuss how hormonal signals integrate nutrient availability with growth processes.
Discuss the temporal regulation of plant hormone action. Explain how the timing of hormonal responses coordinates developmental programs and environmental adaptations.
Examine the role of plant hormones in cell fate determination. Discuss how hormonal gradients and interactions specify different cell types during development.
Analyze the hormonal control of plant responses to flooding and waterlogging. Explain how plants coordinate physiological and morphological adaptations to anaerobic conditions.
Investigate the role of plant hormones in coordinating responses to salt stress. Discuss the hormonal mechanisms that enable plants to maintain ion homeostasis and osmotic balance.
Discuss the concept of hormonal priming in plants. Explain how previous hormone exposure can modify future responses and enhance stress tolerance.
Examine the role of plant hormones in regulating photosynthetic efficiency. Discuss how hormonal coordination optimizes carbon fixation under varying environmental conditions.
Analyze the hormonal basis of plant movements beyond tropisms. Discuss nastic movements, sleep movements, and other hormone-mediated responses.
Investigate the role of plant hormones in coordinating reproductive timing. Discuss how hormonal signals ensure optimal conditions for reproduction and offspring success.
Discuss the future directions and emerging concepts in plant hormone research. Examine new discoveries, technological advances, and their potential applications in agriculture and biotechnology.

Answer Key Guidelines

Chemical Coordination in Plants - Answer Script

Section A: Multiple Choice Questions (MCQs)

b) Chemical coordination
b) Growth regulators
b) Auxin
b) Root formation
b) Gibberellin
d) All of the above
c) Cytokinin
c) Cytokinin
b) Inhibits growth
c) Stress
d) Ethylene
b) Senescence
b) Directional growth movements
b) Phototropism
c) Light
b) Geotropism
c) Water
c) Thigmotropism
b) Chemicals
b) Metabolism
c) Auxin
c) Auxin
c) ABA
c) ABA
b) Aging
c) Cytokinin
c) Gibberellin
b) Gibberellin
b) Phototropism
b) Positive geotropism
b) Negative geotropism
a) Unidirectional light
b) Gravity
b) Finding water
c) Climbing plants
b) Auxin
c) Auxin
b) Cytokinin
c) Ethylene
c) Cytokinin
c) ABA
a) All tropisms
b) Irreversible
c) Coleoptiles
b) IAA
b) Fungi
a) Coconut milk
b) Stress hormone
c) Gaseous hormone
b) Gibberellin
c) Auxin
c) Gibberellin
c) Growing tips
b) Shoots
b) Shoots
b) Roots
b) Shoot apex
b) Unidirectional
c) Basipetal
c) Auxin
b) Gibberellin
c) Auxin
c) Auxin
b) Gibberellin
b) Unequal growth on both sides
b) Shaded side
b) Unequal distribution of auxin
b) Roots
c) Grape vine
b) Slower than animals
b) Very low concentrations
b) Delayed
b) Same process
c) Can be both
b) Auxin
d) All of the above
b) 2,4-D
c) Both cell number and size
b) Adenine
b) Carotenoids
c) ABA
c) Cell maturation
c) Ethylene
c) Ethylene
b) Differential growth
c) De Candolle
b) Coleoptiles
c) Phototropin
d) Both b and c
b) Gravity
d) Statocytes
b) Only roots
a) Mechanical stimulus
b) Root
b) Non-directional
d) All of the above
d) All cellular compartments
c) Both primary and secondary messengers
d) All of the above
d) Independent of nervous system

Section B: One Mark Questions

Growth regulators.
Auxin.
Cytokinin.
Abscisic Acid.
Ethylene.
A directional growth movement of a plant in response to a directional stimulus.
Growth in response to light.
Geotropism.
Hydrotropism.
Growth in response to touch.
Auxin.
Abscisic Acid (ABA).
Ethylene.
Gibberellin.
Growth in response to chemicals.
Indole-3-acetic acid (IAA).
Gibberellin.
Abscisic Acid (ABA).
Cytokinin.
Auxin.
Senescence.
Gibberellin.
Gibberellin.
Ethylene and Abscisic Acid (ABA).
Abscisic Acid (ABA).
The inhibition of lateral bud growth by the apical bud.
Auxin.
The development of fruit without fertilization.
Synthetic auxins like 2,4-D.
Phototropin.
Starch-containing plastids that act as gravity sensors.
In specialized cells called statocytes, located in the root cap.
Roots.
Shoots.
Auxin.
The rapid elongation of a flower stalk from a rosette-forming plant.
Ethylene.
The induction of a plant's flowering process by exposure to the prolonged cold of winter.
2,4-Dichlorophenoxyacetic acid (2,4-D).
Cytokinin.
Carotenoids.
Cytokinin.
Abscisic Acid (ABA).
Shoot apex.
Unidirectionally, from the apex downwards (basipetal).
Basipetal (downward).
Auxin.
Cytokinin.
Irreversible growth movements.
Augustin Pyramus de Candolle.
Root cap.
The photoreceptor protein phototropin.
Epidermal cells.
Mechanical stimulus (touch).
Ethylene.
Very low concentrations.
No, they are delayed.
Yes.
Synergistic (acting together) and antagonistic (acting in opposition).
Auxin.
As herbicides, for rooting cuttings, and inducing parthenocarpy.
Abscisic Acid (ABA).
Gibberellin.
Cytokinin and Auxin.
Unequal growth on the two sides of the stem.
On the shaded side.
Unequal distribution of auxin.
Roots.
In climbing plants like grape vines.
Slower.
Cell membrane or cytoplasm.
In various cellular compartments, including the cell membrane and cytoplasm.
Primary and secondary messengers.
Changes in gene expression, enzyme activation, and metabolism.
Independent of the nervous system.
Auxin.
It delays senescence (aging).
Gibberellin.
Abscisic Acid (ABA).
Ethylene.
To promote stem elongation, germination, and flowering.
Cytokinin.
Auxin.
Ethylene.
Abscisic Acid (ABA).
Auxin.
Unequal distribution of auxin caused by light.
Auxin.
Auxin.
Cytokinin.
Unequal distribution of auxin due to gravity.
Gibberellin.
Ethylene.
Cytokinin.
Auxin.
Gibberellin.
Auxin.
Gibberellin.
Auxin.
Abscisic Acid (ABA) and Ethylene.

Section C: Two Marks Questions

Plants need chemical coordination to respond to environmental stimuli like light, gravity, and water. Since they lack a nervous system, plant hormones (growth regulators) coordinate growth and development, ensuring survival and adaptation.
Plant hormones are produced in various tissues and act locally or are transported, regulating growth and development. Animal hormones are typically produced in specialized glands and transported through the bloodstream to target specific organs.
Auxins promote cell elongation, which is crucial for stem and root growth. They are also vital for root formation, fruit development, and maintaining apical dominance.
Gibberellins promote stem elongation, making plants grow taller. They also break seed dormancy, leading to germination, and can induce flowering in some plants.
Cytokinins are essential for promoting cell division, which is fundamental for plant growth. They also delay senescence (aging) of leaves and promote lateral bud growth.
ABA is called a stress hormone because it helps plants cope with adverse conditions like drought. It promotes stomatal closure to reduce water loss and induces dormancy in buds and seeds.
Ethylene is a gaseous hormone that triggers the ripening process in fruits, causing them to soften and change color. It also promotes senescence, leading to the aging and shedding of leaves and flowers.
A tropism is a directional growth movement in response to a directional stimulus. It is important for plants to orient themselves optimally for resources, such as shoots growing towards light for photosynthesis.
Positive phototropism is the growth of a plant part towards a light source, as seen in shoots. Negative phototropism is the growth away from light, which is exhibited by roots.
Geotropism ensures that roots grow downwards into the soil (positive geotropism) to anchor the plant and absorb water and nutrients. It also ensures that shoots grow upwards (negative geotropism) towards light.
Hydrotropism is the growth of roots towards a water source. This is crucial for plant survival, especially in dry environments, as it allows the plant to efficiently find and absorb water.
Thigmotropism allows climbing plants to find and coil around a support structure. This response to touch enables them to grow upwards, reaching for better light exposure without needing a thick, self-supporting stem.
In shoots, light causes auxin to migrate from the illuminated side to the shaded side. The higher concentration of auxin on the shaded side promotes more cell elongation there, causing the shoot to bend towards the light.
In a horizontally placed root, gravity causes auxin to accumulate on the lower side. In roots, high concentrations of auxin inhibit cell elongation, so the upper side grows faster, causing the root to bend downwards.
The apical bud produces auxin, which is transported down the stem and inhibits the growth of lateral buds. This phenomenon, called apical dominance, results in the plant growing taller with limited branching.
Cytokinins delay the breakdown of chlorophyll and proteins in leaves. By promoting the synthesis of new proteins and mobilizing nutrients to the leaf, they effectively slow down the aging process.
Auxin promotes apical dominance, inhibiting lateral buds, while cytokinin promotes their growth. This antagonistic relationship helps regulate the overall shape and architecture of the plant.
During water stress, ABA levels increase, causing the stomata (pores on the leaf surface) to close. This closure reduces water loss through transpiration, helping the plant conserve water.
Ethylene promotes the synthesis of enzymes that digest the cell walls at the base of the leaf petiole, in a region called the abscission zone. This weakening leads to the shedding of the leaf.
Gibberellins break seed dormancy by stimulating the production of enzymes, like amylase, in the seed. These enzymes break down stored food reserves, providing energy for the embryo to grow and germinate.
Parthenocarpy is the development of fruit without prior fertilization. It can be induced by applying auxins to the flowers, which stimulates the ovary to develop into a seedless fruit.
Plant hormones can interact synergistically, where their combined effect is greater than the sum of their individual effects, or antagonistically, where they have opposing effects. This complex interplay finely regulates plant growth.
Synthetic auxins are widely used in agriculture. For example, they are used to promote rooting in cuttings, prevent premature fruit drop, and as herbicides to kill broadleaf weeds.
In plant tissue culture, the ratio of auxin to cytokinin in the growth medium is critical. A high auxin-to-cytokinin ratio typically promotes root formation, while a high cytokinin-to-auxin ratio promotes shoot formation.
Plant hormones bind to specific receptor proteins in the cell. This binding triggers a signal transduction pathway, often involving second messengers, which ultimately leads to changes in gene expression and cellular activity.
The transport of hormones allows for communication and coordination between different parts of the plant. For example, polar auxin transport from the shoot apex to the roots is crucial for establishing the plant's overall polarity and development.
Auxin, particularly when applied to cuttings, stimulates the formation of adventitious roots from the stem. This is a common practice in horticulture for vegetative propagation.
Genetic dwarfism in some plants is caused by a deficiency in gibberellin production. Applying gibberellins to these plants can overcome the genetic block and cause them to grow to a normal height.
Cytokinins delay the process of senescence by preventing the breakdown of chlorophyll and other cellular components. This "anti-aging" effect keeps leaves green and photosynthetically active for longer.
ABA triggers the closure of stomata by causing guard cells to lose turgor pressure. It promotes the efflux of potassium ions from the guard cells, leading to water loss and stomatal closure.
The triple response, caused by ethylene, is seen in seedlings growing in the dark. It consists of a shortened and thickened hypocotyl, an exaggerated apical hook, and reduced root elongation, which helps the seedling navigate around obstacles.
Tropisms are directional growth movements determined by the direction of the stimulus. Nastic movements are non-directional responses where the direction of movement is independent of the stimulus direction, such as the opening and closing of flowers.
The direction of light is the primary determinant for the direction of phototropic bending. The shoot will always bend towards the source of unilateral light, as auxin accumulates on the shaded side.
Gravity provides the primary cue for the vertical orientation of a plant. It ensures that roots grow down into the soil (positive geotropism) and shoots grow up towards the sky (negative geotropism).
Statoliths are dense, starch-filled amyloplasts found in specialized cells (statocytes) in the root cap. They settle at the bottom of the cells in response to gravity, triggering the geotropic response.
Plants perceive touch through mechanoreceptors in their cell membranes. This stimulus triggers a signal cascade, often involving changes in ion fluxes and hormone levels, leading to a growth response like coiling.
Chemotropism, the growth towards or away from a chemical stimulus, is significant for processes like the growth of the pollen tube towards the ovule during fertilization, guided by chemical signals.
Environmental factors like light intensity, temperature, and water availability can significantly influence the synthesis, transport, and sensitivity of plant hormones, thereby modifying the plant's response.
Phototropins are blue-light photoreceptor proteins located in the tip of the coleoptile. When they absorb blue light, they initiate the signal transduction cascade that leads to the redistribution of auxin.
The concentration of a hormone is crucial, as different concentrations can elicit different responses. For example, low concentrations of auxin promote root growth, while high concentrations can be inhibitory.
Plant hormone actions are generally slow, taking hours or days to manifest as visible growth changes. This is because their effects rely on processes like cell division, elongation, and differentiation.
Hormones often regulate gene expression by activating or repressing transcription factors. This leads to the synthesis of new proteins that carry out the specific cellular response.
Second messengers, such as calcium ions (Ca2+) and cyclic AMP (cAMP), are small molecules that relay the hormonal signal from the receptor at the cell surface to downstream targets within the cell, amplifying the signal.
Signal transduction is the process by which a cell converts an external signal (like a hormone binding to a receptor) into a specific cellular response. It involves a cascade of molecular events that amplify and transmit the signal.
Chemical coordination is a primitive and fundamental system in plants, having evolved as a way for these sessile organisms to adapt and respond to their environment long before the evolution of nervous systems in animals.
Plant hormones act as chemical messengers, coordinating the activities of different cells and tissues. They ensure that growth and development occur in a balanced and integrated manner, responding to both internal and external cues.
Hormones play critical roles in reproduction. For instance, gibberellins and auxins are involved in flower initiation and fruit development, while ethylene can influence sex expression in some species.
Dormancy is a period of arrested growth that allows plants to survive unfavorable conditions. It is primarily regulated by the hormone ABA, which induces dormancy, while gibberellins are responsible for breaking dormancy.
The transition to flowering is controlled by a complex interaction of hormones, particularly gibberellins and the hypothetical hormone "florigen". Environmental cues like day length (photoperiod) trigger hormonal changes that initiate flowering.
Plants use hormonal changes to respond to seasonal shifts. For example, decreasing day length and temperature in autumn can trigger an increase in ABA, leading to leaf senescence and dormancy in preparation for winter.
Auxin, produced by developing seeds, promotes the growth of the fruit tissue. Applying auxin to unfertilized flowers can lead to the development of seedless fruits (parthenocarpy).
Gibberellin promotes stem elongation by stimulating both cell division and cell elongation in the internodal regions of the stem, leading to a taller plant.
Cytokinins promote the growth of lateral (axillary) buds by counteracting the inhibitory effect of auxin produced by the apical bud. This leads to a bushier plant growth habit.
During drought, ABA synthesis increases dramatically. It travels to the leaves and causes stomata to close, reducing water loss, and can also promote root growth to explore for more water.
Ethylene stimulates the production of enzymes like polygalacturonase and cellulase, which break down cell walls and pectin, causing the fruit to soften. It also triggers the conversion of starches to sugars, making the fruit sweeter.
In coleoptiles, blue light is perceived at the tip, causing auxin to move to the shaded side. This auxin travels down the coleoptile, and the higher concentration on the shaded side causes those cells to elongate more, resulting in a bend towards the light.
Roots perceive gravity in the root cap, where dense statoliths settle in the direction of gravity. This triggers a signal that leads to the redistribution of auxin, inhibiting elongation on the lower side and causing the root to bend downwards.
The root cap is the primary site of gravity perception in roots. It contains the statocytes with their gravity-sensing statoliths and is essential for the positive geotropic response.
Roots exhibit hydrotropism by growing towards areas of higher water potential in the soil. The exact mechanism is still being researched but is thought to involve the root cap sensing the moisture gradient and signaling changes in growth direction.
Thigmotropism involves a mechanosensitive response where touch triggers a signal transduction pathway. This often leads to differential growth, causing the plant part (like a tendril) to coil around the object it has touched.
Cambial activity, which leads to secondary growth (thickening) in stems and roots, is regulated by hormones. Auxin and gibberellins promote the division of cells in the vascular cambium.
Auxin plays a key role in vascular differentiation, directing the formation of xylem and phloem tissues. Canals of auxin flow are thought to determine the pattern of vascular strands in the plant.
During seed germination, gibberellins diffuse from the embryo to the aleurone layer of the endosperm. There, they stimulate the synthesis and secretion of alpha-amylase, an enzyme that breaks down starch into sugars for the growing embryo.
Cytokinins are involved in the development and maintenance of chloroplasts, the site of photosynthesis. They promote the synthesis of chlorophyll and chloroplast proteins, thus delaying leaf senescence.
ABA is important for seed maturation and the induction of dormancy. It promotes the accumulation of storage proteins and lipids and prevents premature germination (vivipary) on the parent plant.
In some plants, like cucumbers and melons, ethylene application can promote the formation of female flowers, while gibberellins may promote male flower formation. This allows for the manipulation of the sex ratio in commercial cultivation.
The relative balance between different hormones is often more important than the absolute concentration of any single hormone. For example, the auxin-to-cytokinin ratio determines whether roots or shoots are formed in tissue culture.
External stimuli, such as light, gravity, or touch, are perceived by receptors in the plant. This perception triggers a change in the synthesis or transport of hormones, which then mediate the appropriate growth response.
Hormonal pathways often involve feedback mechanisms. For example, high levels of a hormone can inhibit its own synthesis (negative feedback), which helps to maintain hormonal balance within the plant.
Plant hormones are central to stress tolerance. ABA helps with drought stress, while other hormones like salicylic acid and jasmonic acid are involved in the response to pathogens and herbivores.
Hormones help regulate the distribution of resources (sugars produced during photosynthesis) from "source" tissues (like mature leaves) to "sink" tissues (like growing fruits, seeds, or roots).
Plant hormones can influence photosynthesis by affecting stomatal opening (regulated by ABA), chloroplast development (regulated by cytokinins), and the overall growth and health of the leaves.
Senescence (aging) and abscission (shedding of organs) are active, genetically programmed processes. They are promoted by hormones like ethylene and ABA, and delayed by auxins and cytokinins.
The timing of hormonal signals is critical for proper development. For example, the sequential action of different hormones is required to coordinate fruit development, maturation, and ripening.
Plants integrate a wide array of environmental signals (light, temperature, water, etc.) through their hormonal system. This allows them to produce a coordinated response that optimizes growth and survival in their specific environment.
Hormones direct the flow of nutrients and energy to different parts of the plant based on developmental stage and environmental conditions. This ensures that resources are allocated to the most important sinks, such as developing seeds.
Plants respond to attack by pathogens or herbivores by producing defense-related hormones like salicylic acid and jasmonic acid. These hormones trigger the production of defensive compounds and proteins.
Gradients in hormone concentration are fundamental to pattern formation and development in plants. For example, an auxin gradient is crucial for establishing the apical-basal axis and for positioning new organs like leaves and flowers.
The formation of new organs (organogenesis) is controlled by the precise interplay of plant hormones. For instance, peaks of auxin concentration are thought to specify the locations where new leaves or flowers will form.
The transition from vegetative growth (producing leaves and stems) to reproductive growth (producing flowers) is a major developmental switch regulated by a complex interplay of hormones and environmental cues like photoperiod.
The directional, polar transport of auxin is unique among plant hormones and is fundamental to many aspects of development, including embryogenesis, organ formation, apical dominance, and tropisms.
The quality of light (i.e., the wavelength) can affect hormonal responses. For example, the ratio of red to far-red light, perceived by phytochromes, influences gibberellin synthesis and thus affects seed germination and stem elongation.
Temperature changes can affect the rates of hormone synthesis, transport, and degradation, as well as the sensitivity of tissues to the hormone. For example, cold temperatures are required for the vernalization response, which involves hormonal changes.
Plant hormones are involved in regulating circadian rhythms, the internal biological clock that controls daily processes. For example, the levels of some hormones oscillate on a 24-hour cycle, helping to time activities like stomatal opening and growth.
Hormone receptors are proteins that specifically bind to a hormone, initiating a cellular response. The presence and abundance of these receptors in a cell determine its sensitivity and ability to respond to that hormone.
Hormones, particularly ABA, play a central role in regulating a plant's water relations. ABA controls water loss through transpiration by regulating stomatal aperture.
The root and shoot systems of a plant are in constant communication, and hormones are the key signals. For example, cytokinins produced in the roots travel to the shoot to influence its growth, while auxin from the shoot influences root development.
Mechanical stress, such as wind or physical impedance, can lead to changes in hormone production, particularly an increase in ethylene. This results in thigmomorphogenesis, characterized by shorter, thicker stems that are more resistant to the stress.
Biotic stress, such as attack by insects or pathogens, triggers a complex hormonal response. Hormones like jasmonic acid and salicylic acid act as signals to activate the plant's defense mechanisms.
The initiation, growth, and final shape of leaves are regulated by a complex network of hormones. Auxin plays a key role in initiating leaf primordia, while other hormones influence their expansion and differentiation.
Hormones are involved in the development of fruits and seeds in ways that aid dispersal. For example, ethylene-induced ripening can make fruits more attractive to animals, which then disperse the seeds.
The synthesis, degradation, and conjugation of hormones are tightly regulated processes that control the active amount of a hormone in a tissue. This hormone metabolism is crucial for maintaining hormonal homeostasis.
Plant hormones can influence the uptake of mineral nutrients from the soil. For example, auxin can stimulate the activity of proton pumps in the root cell membrane, which facilitates ion uptake.
The overall architecture of a plant—its height, branching pattern, and root system structure—is determined by the complex interplay of plant hormones, primarily the balance between auxin, cytokinin, and gibberellin.
The timing of hormone application can be critical for achieving the desired effect in agriculture. For example, applying auxin to prevent fruit drop is most effective during a specific window of fruit development.
Plant growth is a result of coordinated cell division and cell expansion. Cytokinins primarily promote cell division, while auxins and gibberellins primarily promote cell expansion.
Programmed cell death (PCD) is an essential process in plant development, involved in things like the formation of xylem vessels and the removal of tissues during senescence. Hormones like ethylene and salicylic acid are known to be involved in regulating PCD.
Hormone crosstalk refers to the complex interactions between different hormone signaling pathways. A plant's final response to a stimulus is often the result of the integration of signals from multiple hormone pathways.
Environmental stresses can significantly modify a plant's hormonal balance. For example, drought increases ABA levels, while flooding can increase ethylene levels, each triggering a different set of adaptive responses.
Future research in plant hormones holds great promise for agriculture. A deeper understanding of hormone action could lead to the development of new growth regulators to improve crop yield, enhance stress tolerance, and reduce the need for fertilizers and pesticides.

Section D: Three Marks Broad Questions

Auxins:
- Structure: The primary natural auxin is Indole-3-acetic acid (IAA), derived from the amino acid tryptophan. Synthetic auxins include IBA, NAA, and 2,4-D.
- Functions: Auxins are master regulators of plant growth. Their primary function is to promote cell elongation in stems and coleoptiles. They are synthesized in the shoot apex and young leaves and transported in a polar (unidirectional, basipetal) manner. Key roles include:
  - Apical Dominance: Maintains the dominance of the central stem over lateral branches.
  - Root Formation: Stimulates the formation of adventitious and lateral roots, crucial for vegetative propagation.
  - Tropisms: Mediates phototropism and geotropism by causing differential growth.
  - Fruit Development: Promotes fruit growth and prevents premature fruit drop.
- Applications: Synthetic auxins are vital in horticulture and agriculture. IBA and NAA are used as rooting hormones for cuttings. 2,4-D is used as a selective herbicide against broadleaf weeds. Auxins are also used to induce parthenocarpy (seedless fruit development).
Gibberellins (GAs):
- Roles: Gibberellins are a large family of hormones (over 100 types) that regulate various developmental processes. Their most prominent roles are:
  - Stem Elongation: GAs cause dramatic stem and leaf elongation by promoting both cell division and cell elongation. This is known as the "bolting" effect in rosette plants.
  - Seed Germination: They break seed dormancy by stimulating the synthesis of hydrolytic enzymes (like α-amylase) that digest stored food reserves in the endosperm.
  - Flowering and Fruit Growth: They can induce flowering, particularly in long-day plants, and increase the size of fruits like grapes.
- Mechanism and Significance: GAs work by promoting the degradation of DELLA proteins, which are growth repressors. By removing these repressors, GAs allow growth-promoting genes to be expressed. This is significant in agriculture for increasing fruit size, promoting germination of malting barley, and overcoming genetic dwarfism in crops.
Cytokinins:
- Role: Cytokinins are adenine derivatives primarily involved in promoting cell division (cytokinesis). They are synthesized mainly in the roots and transported upwards. Their key functions include:
  - Cell Division and Morphogenesis: They are essential for growth and are a key component of plant tissue culture media.
  - Delaying Senescence: They delay the aging of leaves by preventing chlorophyll and protein degradation and promoting nutrient mobilization.
  - Lateral Bud Growth: They promote the growth of lateral buds, counteracting the effect of apical dominance caused by auxin.
- Interaction and Applications: The ratio of cytokinin to auxin is crucial for morphogenesis. A high cytokinin/auxin ratio promotes shoot formation, while a low ratio promotes root formation in tissue culture. Commercially, they are used to prolong the shelf life of green vegetables and cut flowers due to their anti-aging properties.
Abscisic Acid (ABA):
- Role as Stress Hormone: ABA is the primary "stress hormone" in plants, helping them adapt to adverse environmental conditions. It is synthesized in response to stresses like drought, salinity, and cold.
- Functions and Mechanisms:
  - Drought Tolerance: In response to water deficit, ABA levels rise, causing stomata to close rapidly. It does this by triggering an efflux of K+ ions from guard cells, reducing their turgor and closing the stomatal pore, thus conserving water.
  - Seed Dormancy: ABA induces and maintains dormancy in seeds, preventing them from germinating during unfavorable conditions (e.g., in winter). It inhibits the synthesis of enzymes required for germination.
  - Growth Inhibition: ABA generally acts as a growth inhibitor, counteracting the growth-promoting effects of auxins and gibberellins.
Ethylene:
- Functions: Ethylene is a unique gaseous hormone that diffuses through the plant's tissues and plays diverse roles, particularly in senescence and stress responses.
  - Fruit Ripening: It initiates and accelerates the ripening of climacteric fruits (like bananas, tomatoes). It stimulates the breakdown of cell walls (softening), conversion of starch to sugar, and production of volatile compounds associated with aroma.
  - Senescence and Abscission: Ethylene promotes the aging (senescence) of leaves and flowers and the shedding (abscission) of leaves, fruits, and flowers by inducing the formation of an abscission layer.
  - Stress Response: It is produced in response to various stresses, including wounding, flooding, and mechanical stress. It causes the "triple response" in seedlings, helping them navigate obstacles underground.
Comparison of Plant Hormones:
- Auxin: Promotes cell elongation, apical dominance, rooting. Transport is polar.
- Gibberellin: Promotes stem elongation, germination, flowering. Transported in xylem and phloem.
- Cytokinin: Promotes cell division, lateral bud growth, delays senescence. Transported from roots to shoots.
- ABA: Inhibits growth, promotes dormancy, closes stomata (stress hormone). Transported in xylem and phloem.
- Ethylene: Gaseous hormone, promotes ripening, senescence, abscission. Diffuses locally.
- Interactions: Their effects are coordinated. Auxin/cytokinin ratio controls root/shoot formation. Gibberellin and ABA have antagonistic effects on seed germination. This balance allows plants to finely tune their growth to environmental conditions.
Phototropism:
- Concept: Phototropism is the directional growth of a plant in response to a light stimulus. Shoots are positively phototropic (bend towards light), while roots are negatively phototropic. This response maximizes light capture for photosynthesis.
- Molecular Mechanism: The response is initiated by blue-light photoreceptors called phototropins located at the plant tip. When unilateral blue light strikes the tip, it causes the hormone auxin to be transported laterally to the shaded side. The higher concentration of auxin on the shaded side stimulates more rapid cell elongation there compared to the illuminated side. This differential growth results in the stem bending towards the light source.
- Adaptive Significance: This is a crucial adaptation for plants, especially seedlings growing in dense vegetation, as it allows them to actively seek out light, which is essential for their survival and growth.
Geotropism (Gravitropism):
- Concept: Geotropism is the growth of a plant in response to gravity. Shoots are negatively geotropic (grow upwards, against gravity), and roots are positively geotropic (grow downwards, with gravity).
- Perception and Mechanism: Gravity is perceived in the root cap (for roots) and in endodermal cells of the shoot. Specialized cells called statocytes contain dense, starch-filled organelles called statoliths (or amyloplasts). These statoliths settle at the bottom of the statocytes in the direction of gravity. This settling triggers a signal that leads to the redistribution of auxin. In roots, auxin accumulates on the lower side, inhibiting cell elongation, causing the root to bend down. In shoots, auxin also accumulates on the lower side but stimulates elongation, causing the shoot to bend up.
- Significance: This response ensures that roots grow into the soil to anchor the plant and find water/nutrients, and shoots grow towards the sky for light.
Hydrotropism:
- Phenomenon: Hydrotropism is the directional growth of roots towards a moisture gradient. It is a distinct response from geotropism and can override it in dry conditions.
- Mechanism: The perception of the water potential gradient is thought to occur in the root cap. While the exact mechanism is still under investigation, it is known to involve the hormone ABA. The root cap senses differences in water availability and signals the elongation zone to alter its growth direction, bending the root towards the wetter region of the soil.
- Ecological Significance: This is a vital survival mechanism, particularly for plants in arid or semi-arid environments. It allows the root system to efficiently forage for water, which is often patchily distributed in the soil, thereby maximizing water uptake and ensuring the plant's survival.
Thigmotropism:
- Phenomenon: Thigmotropism is the directional growth of a plant in response to a touch or mechanical stimulus. It is most famously observed in climbing plants and their tendrils.
- Mechanosensitive Response: When a tendril touches a solid object, the epidermal cells perceive the stimulus. This triggers a signal transduction pathway, likely involving ion fluxes and hormones like auxin and ethylene. The response is a rapid differential growth: cells on the side opposite the touch stimulus elongate faster than the cells on the touched side. This causes the tendril to coil around the support.
- Adaptive Advantage: This adaptation provides support for plants with weak stems, allowing them to climb upwards towards better light conditions without investing heavily in structural tissue. It is a highly efficient strategy for competing for light in crowded plant communities.
Chemical vs. Nervous Coordination:
- Chemical Coordination (Plants): Relies on hormones transported through vascular tissues or by diffusion. It is slow, long-lasting, and affects broad regions of the plant. It's well-suited for a sessile organism coordinating growth and development over time.
- Nervous Coordination (Animals): Uses electrical impulses transmitted along nerves. It is extremely fast, allowing for rapid responses to immediate stimuli. It is highly specific, targeting particular muscles or glands.
- Comparison: The primary limitation of chemical coordination is its speed, making rapid responses impossible. However, its advantage is its ability to regulate slow, sustained processes like growth. Nervous coordination's advantage is speed, but it is metabolically expensive to maintain.
Hormone Interactions:
- Concept: Plant responses are rarely controlled by a single hormone. Instead, the final outcome depends on the interaction and balance between multiple hormones. These interactions can be synergistic or antagonistic.
- Synergistic Effect: Two hormones work together to produce a greater response than either could alone. Example: Auxin and gibberellin both promote stem elongation, and their combined effect is additive.
- Antagonistic Effect: Two hormones have opposing effects. Example: ABA promotes seed dormancy, while gibberellin breaks it. The ratio of ABA to gibberellin determines whether a seed will germinate or remain dormant. Another key example is the auxin/cytokinin ratio controlling apical dominance and lateral branching.
Hormonal Regulation of Plant Life Cycle:
- Germination: ABA maintains dormancy. A shift in the ABA/gibberellin ratio, favoring gibberellin, breaks dormancy and initiates germination.
- Vegetative Growth: The auxin/cytokinin balance controls plant architecture (apical dominance vs. branching). Gibberellins promote stem elongation.
- Flowering: The transition to reproductive growth is triggered by environmental cues (like photoperiod) that lead to the production of "florigen" (a protein) and changes in gibberellin levels.
- Fruit Development & Ripening: Auxin and gibberellins promote initial fruit growth. Ripening is then initiated by a burst of ethylene.
- Senescence & Abscission: As organs age, cytokinin levels decrease, while ABA and ethylene levels increase, promoting senescence and the eventual shedding of the organ.
Hormones in Agriculture & Horticulture:
- Rooting: Synthetic auxins (IBA, NAA) are widely used as powders or solutions to stimulate root formation in cuttings, enabling mass propagation of desirable plants.
- Weed Control: Synthetic auxins like 2,4-D are used as selective herbicides. At high concentrations, they disrupt normal growth processes in broadleaf weeds, causing them to die, while monocots like corn or wheat are largely unaffected.
- Fruit Production: Gibberellins are sprayed on grapes to increase fruit size and produce looser clusters. Auxins are used to prevent premature fruit drop in citrus and apples. Ethylene is used to synchronize ripening in fruits like tomatoes and bananas for easier harvesting and marketing.
Molecular Mechanisms of Hormone Action:
- Signal Perception: Hormones, the primary messengers, bind to highly specific receptor proteins. These receptors can be located on the cell membrane (e.g., for ethylene) or inside the cell, in the cytoplasm or nucleus (e.g., for auxin and ABA).
- Signal Transduction: The hormone-receptor binding triggers a cascade of intracellular events. This often involves second messengers like Ca2+ ions, which amplify the initial signal. It also frequently involves protein phosphorylation cascades, where a series of protein kinases activate each other, transmitting the signal towards the nucleus.
- Cellular Response: The ultimate target of the signal transduction pathway is often a change in gene expression. The signal activates or deactivates transcription factors, which then turn specific genes on or off. The resulting new proteins carry out the final response, such as cell wall loosening for growth or synthesis of defense compounds.
Environmental Factors and Hormone Responses:
- Light: Light is a major factor. Photoperiod (day length) influences flowering through the phytochrome system, which interacts with gibberellin pathways. Light quality (e.g., red/far-red ratio) affects seed germination and stem elongation. Unilateral light causes the auxin redistribution seen in phototropism.
- Temperature: Temperature affects the rates of all biochemical reactions, including hormone synthesis and breakdown. Cold temperatures are required for vernalization (flowering promotion), a process linked to hormonal changes. High temperatures can be a stress that increases ABA production.
- Water & Stress: Water deficit is a powerful stimulus for the synthesis of ABA, leading to stomatal closure and other drought-response adaptations. Other stresses like salinity, flooding (which increases ethylene), and pathogen attack all trigger specific hormonal signaling pathways (e.g., salicylic acid, jasmonic acid) to coordinate a defense response.
Evolution of Chemical Coordination:
- Ancient Origins: The basic hormonal signaling pathways are ancient and are found even in algae and mosses, indicating they evolved early in plant history. The core set of hormones (auxin, cytokinin, etc.) was likely established before the divergence of major land plant lineages.
- Increasing Complexity: As plants evolved and became more complex, moving from aquatic to terrestrial environments and developing complex body plans (roots, stems, leaves, flowers), their hormonal systems also became more intricate. The roles of the hormones expanded, and the crosstalk and interactions between them became more complex to coordinate the development of these new structures.
- Adaptive Significance: The evolution of this system was a key adaptation for a sessile lifestyle. It allowed plants to perceive and respond to their environment in a coordinated way, optimizing growth, reproduction, and survival without the need for movement.
Apical Dominance:
- Concept: Apical dominance is the phenomenon where the central, apical bud grows more strongly while the growth of the lateral (axillary) buds below it is inhibited. This results in a plant that grows tall and has a conical shape (like a pine tree).
- Hormonal Regulation: This is a classic example of hormonal interaction. The apical bud produces a high concentration of auxin, which is transported down the stem. This high level of auxin inhibits the outgrowth of the nearby lateral buds. Cytokinins, which are produced in the roots and transported upwards, promote the growth of lateral buds.
- Balance: The fate of a lateral bud depends on the ratio of auxin (coming from the apex) to cytokinin (coming from the roots). Near the apex, the high auxin/cytokinin ratio inhibits growth. Further down the stem, the auxin concentration is lower, and the cytokinin effect can dominate, allowing the lateral buds to grow out. If the apical bud is removed (e.g., by pruning), the auxin supply is cut off, and the lateral buds are released from inhibition and begin to grow, making the plant bushier.
Hormones in Reproductive Development:
- Flower Initiation: The switch from vegetative to reproductive growth is controlled by a complex interplay of environmental cues (especially photoperiod) and hormones. Gibberellins are known to promote flowering, particularly in long-day plants. The signal from the leaves that perceives the photoperiod is a protein called "florigen" (FT protein), which travels to the shoot apex and interacts with hormonal pathways to initiate flower primordia.
- Fruit Development: After fertilization, the developing seeds become a major source of auxin. This auxin promotes the growth of the ovary wall, leading to the development of the fruit. Without this hormonal signal, the fruit would typically abort.
- Seed Formation: ABA plays a crucial role during the late stages of seed development (embryogenesis). It promotes the accumulation of storage reserves (proteins, lipids) and induces dormancy, preventing the seed from germinating prematurely.
Hormonal Control of Stomatal Behavior:
- Primary Regulator (ABA): The hormone Abscisic Acid (ABA) is the primary regulator of stomatal closure. When a plant experiences water stress, ABA is synthesized in the roots and leaves.
- Mechanism of Closure: ABA binds to receptors on the guard cell membrane. This triggers a signal cascade that opens ion channels, causing an efflux of potassium (K+) and other ions from the guard cells. This loss of solutes causes water to leave the guard cells via osmosis. The guard cells become flaccid, lose turgor pressure, and the stomatal pore closes, significantly reducing water loss through transpiration.
- Other Factors: While ABA is the main closing signal, other hormones can have minor effects. For example, auxins can promote stomatal opening in some conditions. The overall response integrates various environmental signals like light, CO2 concentration, and humidity, largely mediated through the ABA signaling pathway.
Hormones in Root Development:
- Primary Root Growth: The growth of the primary root is controlled by a delicate balance of hormones in the root apical meristem. A specific concentration of auxin is required to maintain cell division in the meristem.
- Lateral Root Formation: Lateral roots initiate from the pericycle, an internal layer of cells in the root. This process is primarily triggered by auxin transported down from the shoot. Peaks of auxin accumulation in the pericycle signal the cells to divide and form a new lateral root primordium.
- Root Architecture: The overall root system architecture is shaped by the interplay of hormones. Cytokinin generally inhibits lateral root formation, acting antagonistically to auxin. Therefore, the auxin/cytokinin ratio helps determine the density of lateral roots. Other hormones like ethylene and ABA also modulate root growth in response to soil conditions.
Senescence and Hormonal Regulation:
- Concept: Senescence is the process of aging in plants, particularly in organs like leaves, flowers, and fruits. It is not simply a passive decay but an active, genetically programmed process that allows the plant to recover and reallocate nutrients from the dying organ to other parts of the plant (like developing seeds).
- Hormonal Control: The process is regulated by a shift in hormonal balance. Cytokinins are powerful anti-senescence hormones; they delay the breakdown of chlorophyll and proteins. As a leaf ages, its cytokinin levels decline. In contrast, the levels of ethylene and ABA increase. These hormones promote senescence by activating genes that encode for degradative enzymes (like proteases and nucleases), leading to the breakdown of cellular components and the eventual death of the organ.
Seed Germination and Dormancy:
- Dormancy: Seed dormancy is a state of arrested growth that ensures germination only occurs under favorable environmental conditions. This state is induced and maintained primarily by the hormone Abscisic Acid (ABA). ABA inhibits cell growth and the expression of genes required for germination.
- Germination: The breaking of dormancy and the initiation of germination are triggered by environmental cues (like water, temperature, light) that lead to a decrease in ABA levels and an increase in Gibberellin (GA) levels.
- Role of Gibberellin: GA is the key hormone promoting germination. It diffuses from the embryo to the aleurone layer (a layer of cells surrounding the endosperm). Here, it stimulates the synthesis and secretion of hydrolytic enzymes, most notably α-amylase. This enzyme breaks down the stored starch in the endosperm into sugars, which provide the energy and building blocks for the growing embryo to emerge from the seed coat.
Hormones in Stress Responses:
- Abiotic Stress: These are environmental stresses like drought, salinity, and extreme temperatures. The primary hormone mediating the response to these is ABA. It triggers adaptive responses like stomatal closure (drought), synthesis of protective proteins, and changes in root growth to help the plant survive. Ethylene is also produced under stresses like flooding and mechanical damage.
- Biotic Stress: This involves attacks from pathogens (fungi, bacteria) or herbivores (insects). The plant's immune response is largely coordinated by two key defense hormones:
  - Salicylic Acid (SA): Generally associated with defense against biotrophic pathogens (which feed on living tissue). It triggers a response called Systemic Acquired Resistance (SAR), which provides long-lasting, broad-spectrum resistance throughout the plant.
  - Jasmonic Acid (JA): Typically involved in defense against necrotrophic pathogens (which kill tissue and feed on it) and insect herbivores. It induces the production of proteinase inhibitors and other toxic compounds that deter feeding.
Photoperiodism and Hormonal Basis:
- Concept: Photoperiodism is the physiological response of a plant to the length of day and night. It is the primary mechanism that allows plants to sense the time of year and coordinate seasonal activities, most importantly, flowering. Plants are classified as short-day plants (flower when nights are long), long-day plants (flower when nights are short), or day-neutral plants.
- Perception and Signal: The photoperiod is perceived by photoreceptor proteins called phytochromes in the leaves. In response to the correct photoperiod, the leaves produce a mobile flowering signal.
- Hormonal Basis: This mobile signal is a protein called florigen (the product of the FLOWERING LOCUS T, or FT gene). Florigen is a hormone-like molecule that travels from the leaves through the phloem to the shoot apical meristem. At the meristem, it interacts with other transcription factors and hormonal pathways (particularly involving gibberellins, which are often required for the flowering signal to be effective) to switch the meristem from producing leaves to producing flowers.
Cambial Activity and Secondary Growth:
- Concept: Secondary growth is the process that increases the girth (thickness) of stems and roots in woody plants. It results from the activity of the vascular cambium, a lateral meristem that produces secondary xylem (wood) to the inside and secondary phloem to the outside.
- Hormonal Control: The activity of the vascular cambium is under strong hormonal control.
  - Auxin: Transported down from the shoot apex and young leaves, auxin is a primary promoter of cell division in the cambium. It also plays a role in the differentiation of the new cells into xylem vessels.
  - Gibberellins: Also promote cell division in the cambium and are particularly important for fiber elongation and normal wood development.
  - Cytokinins: Transported up from the roots, they also stimulate cambial division. The balance between these hormones, influenced by seasonal cues, regulates the rate of wood formation, leading to the formation of annual growth rings.
Fruit Development and Ripening:
- Fruit Set and Growth: Following successful pollination and fertilization, the developing seeds begin to produce hormones, primarily auxin and gibberellins. These hormones signal the ovary wall to grow and develop into the fruit (the pericarp). This phase is characterized by rapid cell division and expansion. If fertilization fails, the hormone signal is absent, and the flower is typically aborted.
- Ripening: Ripening is a distinct process that marks the transition from a mature but unripe fruit to a ripe one. In climacteric fruits (e.g., apples, bananas, tomatoes), this process is triggered by a massive burst in the synthesis of ethylene.
- Role of Ethylene: Ethylene initiates a cascade of changes: it stimulates the activity of enzymes that break down chlorophyll (loss of green color), soften the fruit (cellulases, polygalacturonases), convert starches to sugars (sweetening), and produce the volatile organic compounds that give the fruit its characteristic aroma.
Leaf Development and Morphology:
- Initiation: The formation of a new leaf begins at the shoot apical meristem. The precise location where a new leaf primordium will form is determined by a localized peak in the concentration of auxin. This auxin peak acts as a trigger, signaling the cells to start dividing and differentiating to form the leaf.
- Expansion and Shape: As the leaf primordium grows, its final shape and size are orchestrated by a complex interplay of hormones. Gibberellins and auxins promote cell expansion, contributing to the overall size of the leaf blade. Cytokinins promote cell division, ensuring the leaf has the correct number of cells. The balance and distribution of these hormones across the developing leaf blade are critical for establishing its specific morphology (e.g., simple vs. compound, smooth vs. serrated margins).
Hormones in Tissue Culture:
- Concept: Plant tissue culture (or micropropagation) is a technique used to grow and propagate plants in a sterile, in vitro environment on a nutrient medium. It relies on the totipotency of plant cells (the ability of a single cell to regenerate into a whole plant).
- Hormonal Manipulation: The key to controlling development in tissue culture is the manipulation of plant hormones in the growth medium, specifically the ratio of auxin to cytokinin.
  - Callus Formation: An explant (a small piece of plant tissue) is placed on a medium with a balanced level of auxin and cytokinin. This induces the cells to de-differentiate and divide, forming an undifferentiated mass of cells called a callus.
  - Shoot Regeneration: To induce the callus to form shoots, it is transferred to a medium with a high cytokinin-to-auxin ratio.
  - Root Regeneration: To induce rooting from the regenerated shoots, they are transferred to a medium with a high auxin-to-cytokinin ratio. This allows for the creation of a complete plantlet, which can then be transferred to soil.
Hormone Transport:
- Significance: The transport of hormones from their site of synthesis to their site of action is crucial for coordinating development across the entire plant. It allows different parts of the plant to "communicate" with each other.
- Polar Auxin Transport: The most famous example is the polar transport of auxin. Auxin is transported directionally, primarily from the apical shoot tip downwards towards the roots (basipetal transport). This is not due to gravity but is an active, energy-dependent process involving specific influx (AUX/LAX) and efflux (PIN) carrier proteins on the cell membranes. This directional flow is fundamental for establishing the plant's apical-basal axis, apical dominance, and tropisms.
- Other Hormones: Other hormones are transported through the plant's vascular system. Cytokinins are mainly synthesized in the roots and transported upwards via the xylem. ABA and gibberellins can move in both the xylem and phloem, allowing for long-distance signaling throughout the plant. Ethylene, being a gas, primarily diffuses through air spaces between cells to act locally.
Thigmomorphogenesis:
- Concept: Thigmomorphogenesis refers to the changes in a plant's growth pattern in response to chronic mechanical stimulation, such as wind or repeated touching. It is a general developmental response, distinct from the directional growth of thigmotropism.
- Response: Plants exposed to mechanical stress typically exhibit a "sturdy" phenotype: they are shorter, have thicker stems, and may have smaller leaves. This is an adaptive response that makes the plant more resistant to mechanical damage.
- Hormonal Basis: The primary hormone mediating this response is ethylene. Mechanical stress induces a rapid increase in ethylene production. Ethylene inhibits cell elongation (leading to shorter stems) and promotes radial cell expansion (leading to thicker stems). Other hormones like auxin and gibberellins are also involved, but ethylene plays the central role in this adaptive response to mechanical challenges.
Source-Sink Relationships:
- Concept: In a plant, "sources" are tissues that produce an excess of photosynthates (sugars), primarily mature leaves. "Sinks" are tissues that require energy and carbon for their growth, such as roots, developing fruits, seeds, and young leaves. The allocation of sugars from sources to sinks is a tightly regulated process.
- Hormonal Coordination: Plant hormones play a key role in regulating this allocation. They can influence both "source strength" (the rate of photosynthesis and export) and "sink strength" (the ability of a tissue to attract and import sugars).
- Examples: Cytokinins can increase sink strength, promoting the movement of sugars to the tissues where they are present. Auxin produced by developing fruits and seeds creates a strong sink, drawing resources to them. The hormonal balance helps the plant prioritize resource allocation to the most important sinks at any given developmental stage (e.g., favoring fruit development after flowering).
Flower Sex Determination:
- Concept: In monoecious plants (which have separate male and female flowers on the same plant, like cucumbers and melons), the sex of a developing flower can be influenced by hormones.
- Hormonal Control: The balance between gibberellins and ethylene is often the deciding factor.
  - Gibberellins tend to promote maleness. Applying gibberellins can increase the number of male flowers.
  - Ethylene promotes femaleness. Applying ethylene-releasing compounds (like Ethephon) is a common commercial practice to increase the number of female flowers, and therefore the number of fruits, in cucurbit crops.
- Significance: This hormonal control allows the plant to modulate its reproductive strategy based on environmental conditions and provides a tool for agriculturalists to increase crop yields.
Hormones and Circadian Rhythms:
- Concept: Circadian rhythms are internal biological clocks that allow organisms, including plants, to coordinate their physiological processes with the daily cycle of day and night. These rhythms persist even in constant light or darkness and have a period of approximately 24 hours.
- Hormonal Involvement: The plant's internal clock regulates the expression of a large number of genes, including those involved in hormone synthesis and signaling. Consequently, the levels and sensitivity to many hormones oscillate with a daily rhythm.
- Examples: The expression of genes for gibberellin and auxin synthesis often peaks at specific times of the day, contributing to rhythmic growth patterns. The sensitivity of stomata to ABA also varies throughout the day, which is part of the reason stomata typically open during the day and close at night. This integration of the circadian clock and hormonal signaling allows the plant to anticipate environmental changes and optimize its activities (like photosynthesis and growth) for the appropriate time of day.
Hormone Gradients:
- Concept: A hormone gradient is a progressive change in the concentration of a hormone across a tissue or organ. These gradients are fundamental to development (morphogenesis) because they provide positional information to cells, telling them where they are and what they should become.
- Role in Development: Instead of a simple on/off switch, a gradient allows for a more nuanced response. Cells can respond differently to high, medium, and low concentrations of the same hormone.
- Examples: The most well-studied example is the auxin gradient. The high point of an auxin gradient at the shoot apical meristem determines where a new leaf will form. The overall apical-basal auxin gradient is essential for establishing the polarity of the entire plant. Similarly, gradients of auxin and cytokinin are thought to pattern the development of the root meristem. These gradients are the foundation of pattern formation in plants.
Abscission Processes:
- Concept: Abscission is the controlled shedding of plant organs, such as leaves, flowers, or fruits. It is an active process that occurs at a specific, pre-determined layer of cells called the abscission zone, typically at the base of the organ.
- Hormonal Regulation: The process is controlled by an interaction between auxin and ethylene/ABA.
  - Prevention: As long as an organ (like a leaf) is healthy and active, it produces a steady stream of auxin. This auxin flowing through the abscission zone prevents it from becoming sensitive to ethylene and keeps the organ attached.
  - Induction: As the organ ages (senesces) or experiences stress, its auxin production declines. This drop in auxin makes the abscission zone sensitive to ethylene and ABA. These hormones then promote the synthesis of cell-wall-degrading enzymes (like cellulases and polygalacturonases) within the abscission zone. These enzymes digest the middle lamella that holds the cells together, weakening the connection and allowing the organ to be shed by wind or its own weight.
Pathogen Defense Responses:
- Concept: Plants have an innate immune system to defend against pathogens. This system relies on the ability to recognize pathogen-associated molecular patterns (PAMPs) and trigger a defense response, which is coordinated by hormones.
- Key Defense Hormones:
  - Salicylic Acid (SA): This is the primary hormone involved in defense against biotrophic pathogens (which feed on living cells). Upon infection, SA levels rise, triggering both local defense (e.g., programmed cell death to contain the pathogen) and a plant-wide response called Systemic Acquired Resistance (SAR). SAR "immunizes" the rest of the plant against subsequent attacks.
  - Jasmonic Acid (JA) and Ethylene (ET): These hormones are typically associated with defense against necrotrophic pathogens (which kill cells first) and insect herbivores. They work synergistically to induce the expression of aenes encoding defense proteins, such as proteinase inhibitors (which disrupt insect digestion) and enzymes that produce anti-fungal compounds.
- Crosstalk: There is often antagonistic crosstalk between the SA and JA pathways, meaning the plant has to prioritize one type of defense over the other.
Plant Architecture:
- Concept: Plant architecture refers to the 3D structure of a plant, including its height, branching pattern, and the arrangement of its leaves and roots. This structure is not random but is determined by the plant's genetic makeup and modulated by environmental factors.
- Hormonal Control: The overall form of a plant is largely dictated by the interplay of the main growth hormones.
  - Apical Dominance (Auxin/Cytokinin): The ratio of auxin (inhibiting branches) to cytokinin (promoting branches) is the primary determinant of whether a plant will be tall and unbranched or short and bushy.
  - Stem Elongation (Gibberellin): The height of a plant is strongly influenced by gibberellins, which promote internode elongation. Dwarf varieties of plants are often deficient in gibberellin.
  - Tropisms (Auxin): Phototropism and geotropism, mediated by auxin, orient the shoots and roots correctly in their environment, contributing significantly to the plant's final form.
Vernalization:
- Concept: Vernalization is the process by which some plants require exposure to a prolonged period of cold temperature to acquire the competence to flower. It's a mechanism to ensure that flowering occurs in the spring after winter has passed, not during a warm spell in autumn.
- Mechanism: The cold period is perceived by the shoot apical meristem. The molecular basis involves epigenetic changes, specifically the silencing of a potent flowering repressor gene called FLOWERING LOCUS C (FLC). The cold causes modifications to the chromatin around the FLC gene, shutting down its expression.
- Hormonal Role: While the primary mechanism is epigenetic, hormones are involved in the final steps. After the FLC repressor is silenced by the cold, the plant can then respond to flowering-promotive signals (like long days). Gibberellins are often involved in this downstream pathway; after vernalization, the plant becomes sensitive to gibberellins, which can then promote the expression of genes that lead to flowering.
Hormone Homeostasis:
- Concept: Hormone homeostasis refers to the maintenance of a stable, optimal concentration of the active form of a hormone within a specific tissue. Plant cells must be able to tightly regulate hormone levels because too much or too little can be detrimental.
- Mechanisms: Plants achieve homeostasis through several mechanisms:
  - Synthesis and Degradation: The rates of hormone biosynthesis and catabolism (breakdown) are tightly controlled by feedback loops. For example, high levels of a hormone can sometimes inhibit the enzymes that produce it (negative feedback).
  - Conjugation: Hormones can be temporarily inactivated by conjugating (attaching) them to other molecules like sugars or amino acids. These conjugated forms are inactive but can be stored and later re-activated by removing the conjugate, allowing for rapid changes in the active hormone pool.
  - Transport: The movement of hormones into and out of cells and tissues is another way to control local concentrations.
Coordination with Nutrition:
- Concept: Plant growth is dependent on the availability of mineral nutrients from the soil (e.g., nitrogen, phosphorus, potassium). Hormonal signals are crucial for integrating the plant's nutritional status with its growth and development programs.
- Hormonal Integration:
  - Nitrogen: The availability of nitrogen influences cytokinin synthesis in the roots. High nitrogen levels promote cytokinin production, which signals to the shoot that there are ample resources for growth.
  - Phosphate: Phosphate starvation can alter auxin transport and distribution, leading to changes in root architecture (e.g., more shallow lateral roots) to better explore the topsoil where phosphate is often concentrated.
  - Overall Growth: Hormones act as central processors, translating the information about nutrient availability into developmental decisions, such as whether to invest in further root growth to find nutrients or in shoot growth to increase photosynthesis.
Temporal Regulation of Hormone Action:
- Concept: The timing of hormonal signals and responses is as critical as their location. Developmental processes in plants follow a strict timeline, and hormones are the key regulators of this schedule.
- Examples:
  - Fruit Development: There is a clear temporal sequence of hormone action. First, auxin and gibberellins from the seed promote the growth phase of the fruit. Only after this phase is complete does a burst of ethylene trigger the ripening phase. Applying ethylene too early will not cause an unripe fruit to ripen properly.
  - Germination: ABA is dominant during seed maturation and dormancy. The switch to gibberellin dominance must occur at the right time, in response to favorable environmental cues, to trigger germination.
- Significance: This temporal control ensures that developmental events happen in the correct order and at the appropriate time in the plant's life cycle and in relation to the seasons.
Cell Fate Determination:
- Concept: Cell fate determination is the process by which a cell becomes committed to a specific developmental pathway, leading it to become a particular cell type (e.g., an epidermal cell, a xylem vessel, a root hair cell).
- Hormonal Role: Hormone gradients and local concentrations are key determinants of cell fate. A cell's position within a hormonal gradient provides it with the information it needs to adopt the correct fate.
- Examples:
  - Root Hairs: In the root epidermis, cells that are in contact with two underlying cortical cells are exposed to a different hormonal environment (low ethylene) than cells that touch only one cortical cell (high ethylene). This difference determines which cells will differentiate into root hairs.
  - Xylem/Phloem: The differentiation of procambial cells into either xylem or phloem is thought to be controlled by the local concentrations and ratios of auxin and cytokinin.
Response to Flooding:
- Concept: Flooding or waterlogging creates anaerobic (low oxygen) conditions in the soil, which is a major stress for most plants as roots require oxygen for respiration.
- Hormonal Adaptations: The primary hormonal signal in response to flooding is ethylene. The precursor to ethylene (ACC) is synthesized in the oxygen-deprived roots and transported to the shoot. In the shoot, where oxygen is available, ACC is converted to ethylene.
- Ethylene-Mediated Responses: Ethylene then triggers several adaptive responses:
  - Epinasty: The downward bending of leaves, which reduces water loss and metabolic activity.
  - Aerenchyma Formation: Ethylene can induce programmed cell death in the root cortex, creating air channels (aerenchyma) that allow oxygen to diffuse down from the shoot to the submerged roots.
  - Adventitious Root Growth: It can stimulate the growth of new roots from the stem, above the waterlogged soil, to aid in water and nutrient uptake.
Response to Salt Stress:
- Concept: High salt concentration in the soil is a major stress that causes both osmotic stress (making it hard for roots to take up water) and ion toxicity (from high levels of sodium and chloride).
- Hormonal Coordination: The plant's response is complex and involves the interaction of several hormones.
  - ABA: As with drought, salt stress leads to a rapid increase in ABA. ABA helps by triggering stomatal closure to conserve water and by activating genes that encode for proteins involved in ion transport and osmotic adjustment. It helps regulate the Na+/H+ antiporters that sequester excess sodium into the vacuole.
  - Other Hormones: The balance between other hormones is also shifted. For example, salt stress can reduce the levels of growth-promoting hormones like cytokinins and gibberellins, thus slowing down growth to conserve resources and focus on survival.
Hormonal Priming:
- Concept: Hormonal priming, or stress priming, is a phenomenon where a plant's prior exposure to a mild stress or a hormone treatment makes it more resistant and quicker to respond to a subsequent, more severe stress event. The plant enters a "primed" state.
- Mechanism: Priming doesn't necessarily involve a constant high level of defense. Instead, it often involves epigenetic changes and the accumulation of dormant signaling molecules. When the second stress hits, the primed plant can activate its defense gene expression more rapidly and robustly.
- Example: Treating a plant with a low dose of ABA or salicylic acid can prime it to be more tolerant to a future drought or pathogen attack. This concept is being explored in agriculture as a way to enhance crop resilience without the fitness cost of having defenses constantly activated.
Photosynthetic Efficiency:
- Concept: Photosynthetic efficiency is the rate at which a plant converts light energy into chemical energy (sugars). This process is influenced by many factors, including stomatal conductance, leaf health, and the allocation of resources.
- Hormonal Regulation:
  - Stomatal Control (ABA): The most direct link is through ABA, which controls stomatal aperture. By closing stomata during water stress, ABA reduces CO2 uptake and thus lowers the photosynthetic rate, but this is a crucial trade-off to prevent dehydration.
  - Leaf Health (Cytokinins): Cytokinins delay the senescence of leaves by preventing the breakdown of chlorophyll and photosynthetic proteins. A higher cytokinin level can keep leaves photosynthetically active for longer, increasing the plant's overall carbon gain.
  - Source-Sink Balance: Hormones regulate the transport of sugars away from the leaves (sources). Efficient transport prevents the buildup of sugars in the leaf, which can otherwise inhibit photosynthesis through a feedback mechanism.
Nastic Movements:
- Concept: Nastic movements are plant movements that are independent of the direction of the stimulus. This contrasts with tropisms, where the direction of movement is determined by the stimulus direction.
- Hormonal Basis: These movements are typically caused by rapid changes in turgor pressure in specialized cells in a structure called a pulvinus, and these turgor changes are often mediated by hormones.
- Examples:
  - Nyctinasty (Sleep Movements): The daily opening and closing of leaves or flower petals (e.g., in legumes). This is driven by the plant's internal circadian clock and involves rhythmic movements of ions and water in the pulvini, a process influenced by auxin and other hormones.
  - Thigmonasty: The rapid, non-directional response to touch, famously seen in the Venus flytrap and the Mimosa pudica (sensitive plant). The touch triggers an electrical signal that propagates through the leaf, causing a rapid loss of turgor in the pulvini and the folding of the leaflets. Hormones are thought to be involved in the subsequent recovery.
Coordinating Reproductive Timing:
- Concept: The timing of reproduction (i.e., flowering and seed set) is arguably the most critical decision in a plant's life. It must be timed to coincide with favorable environmental conditions and the presence of pollinators to maximize success.
- Hormonal Coordination: Plants integrate environmental cues (photoperiod, temperature) with their internal developmental state through hormonal signals to make this decision.
  - Flowering: As discussed under photoperiodism and vernalization, the decision to flower is the result of a complex signaling network involving the mobile signal florigen and its interaction with gibberellins at the shoot apex.
  - Fruit and Seed Set: After flowering, hormones like auxin (from pollen and developing seeds) and ethylene coordinate the processes of fertilization, fruit growth, and ripening, ensuring that seeds are mature and ready for dispersal when conditions are optimal.
Future Directions in Plant Hormone Research:
- Hormone Crosstalk: A major focus is on unraveling the complex signaling networks and understanding how the different hormone pathways "talk" to each other to produce a coordinated response. This involves systems biology approaches to model these complex interactions.
- Climate Resilience: With climate change, there is a huge interest in using our knowledge of stress hormones like ABA to engineer crops that are more tolerant to drought, heat, and salinity. This could involve developing new chemical primers or genetically modifying hormone signaling pathways.
- Synthetic Biology: Researchers are designing novel synthetic signaling pathways to control plant growth in predictable ways. This could lead to "smart plants" that can be programmed to have specific architectures, flower at a desired time, or have enhanced nutritional content.
- New Hormones: While the "classic five" hormones plus others like brassinosteroids, salicylic acid, and jasmonates are well-known, researchers continue to discover new signaling molecules (peptides, strigolactones) that play hormonal roles, opening up new avenues for understanding and manipulating plant growth.

Chemical Coordination in Plants

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