Plant Growth and Development

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Plant Growth and Development — Key Facts for Sri Lanka A/L Examination

Plant Growth Regulators (Hormones):

Hormone	Primary Function	Example Effect
Auxin	Cell elongation	Phototropism (bend toward light)
Gibberellin	Stem elongation	Increased stem growth
Cytokinin	Cell division	Delay leaf senescence
Ethylene	Fruit ripening	Climacteric fruit ripening
Abscisic acid	Stress response	Seed dormancy, stomatal closure

Growth Patterns:

• Primary growth: Length increase (apical meristems)
• Secondary growth: Width increase (lateral meristems - vascular cambium, cork cambium)
• Indeterminate growth: Plants grow continuously

Growth Measurement:

Parameter	Method
Cell size	Microscope with micrometer
Fresh weight	Balance
Dry weight	Oven drying (constant weight)
Length	Ruler or graph paper

⚡ A/L Exam Tip: Remember the “A” in Auxin stands for “Apical dominance” - auxin inhibits lateral bud growth, causing trees to grow tall rather than bushy!

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Plant Growth and Development — Detailed Study Guide

Auxin Functions

Discovery (Darwin’s experiments, 1880):

• Coleoptile tip receives light
• Tip produces signal transmitted to growing region
• Bending occurs below the tip

Properties of Auxin (IAA - Indoleacetic acid):

Property	Description
Synthesis	Tip regions (shoots, root tips)
Transport	Polar (base to tip in shoots, tip to base in roots)
Storage	Inactive forms bound to proteins
Destruction	Light destroys auxin (photodegradation)

Effects of Auxin:

1. Phototropism: Light destroys auxin, causing accumulation on shaded side → differential growth → bending toward light
2. Gravitropism: Gravity causes auxin redistribution → differential growth
3. Apical dominance: Auxin from terminal bud inhibits lateral bud growth
4. Root initiation: Stimulates adventitious root formation (used in cuttings)

⚡ A/L PYQ: “How does auxin cause phototropism in a plant shoot?” Answer: Light causes auxin to move from light side to shaded side, causing cells on shaded side to elongate more, resulting in bending toward light.

Gibberellins

Discovery:

• “Foolish seedling disease” in rice (Gibberella fujikuroi fungus)
• Caused excessive stem elongation
• Isolated gibberellic acid (GA₃)

Functions:

Function	Mechanism
Stem elongation	Stimulates cell division AND elongation
Seed germination	Breaks dormancy, stimulates amylase
Fruit growth	Stimulates cell enlargement
Bolting	Flowering stalk elongation

Agricultural Uses:

• Increase grape berry size
• Break dormancy of potato tubers
• Accelerate malting in brewing

Cytokinins

Discovery:

• Called “cytokinin” because stimulates cytokinesis (cell division)
• First cytokinin discovered: Kinetin (from DNA)
• Natural cytokinin: Zeatin (from corn kernels)

Functions:

Function	Description
Cell division	Essential for mitosis
Shoot promotion	Stimulates shoot formation
Delay senescence	Prevents chlorophyll breakdown
Nutrient mobilization	Attracts nutrients to treated area

In Tissue Culture:

• Cytokinin + Auxin ratio determines organogenesis
• High cytokinin/low auxin → shoots
• Low cytokinin/high auxin → roots
• Balanced → callus formation

Ethylene

Properties:

• Only gaseous plant hormone
• Synthesized in all tissues, especially ripening fruits
• Simple to apply (released from ethephon)

Functions:

Function	Application
Fruit ripening	Bananas, tomatoes
Abscission	Leaf and fruit drop
Senescence	Aging processes
Stress response	Flooding survival (aerial roots)

Triple Response in Seedlings:

1. Inhibition of stem elongation
2. Horizontal growth (diageotropism)
3. Exaggerated curvature (epinasty)

Abscisic Acid (ABA)

Discovery:

• Found in young leaves, developing seeds
• Called “abscisic” because it promotes abscission (though it’s not the primary cause)

Functions:

Function	Mechanism
Seed dormancy	Inhibits germination
Stomatal closure	Promotes K⁺ efflux from guard cells
Stress response	Drought, salinity, cold tolerance
Inhibitor of growth	Counteracts growth-promoting hormones

⚡ A/L Important: ABA is the “stress hormone” - when plants experience drought, ABA levels rise, causing stomata to close and reduce water loss!

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Plant Growth and Development — Complete Notes for A/L Sri Lanka

Photoperiodism and Flowering

Photoperiodism:

• Response to day length (photoperiod)
• Plants measure night length, not day length

Plant Categories:

Category	Flowering Trigger	Examples
Long-day	Short night (<critical period)	Wheat, spinach
Short-day	Long night (>critical period)	Rice, soybean
Day-neutral	Not affected by photoperiod	Tomato, cucumber

Critical Concepts:

• Phytochrome: Light receptor protein
• Pfr form (far-red, active) → Pr form (red, inactive)
• Red light converts Pr → Pfr
• Far-red converts Pfr → Pr
• Darkness: Pfr slowly converts to Pr

Florigen Theory:

• Mobile flowering signal produced in leaves
• Transmitted to shoot apex
• Induces flowering
• Grafting experiments show signal can move between plants

Tropisms and Nastic Movements

Tropisms (directional growth responses):

Tropism	Stimulus	Direction
Phototropism	Light	Toward/away
Gravitropism	Gravity	Up/down
Thigmotropism	Touch	Coiling
Hydrotropism	Water	Toward
Chemotropism	Chemicals	Toward/away

Nastic Movements (non-directional):

Movement	Stimulus	Example
Nyctinasty	Light/dark cycle	Sleep movements (leaves)
Seismonasty	Touch/mechanical	Mimosa pudica (touch-me-not)
Thermonasty	Temperature	Tulip opening
Chemonasty	Chemicals	Sundew trapping

Mimosa pudica Response:

1. Touch stimulus received
2. Electrical signal transmitted
3. Potassium ions pumped out of bulliform cells
4. Water follows osmotically
5. Cell flaccidity → leaf drooping

Secondary Growth

Vascular Cambium:

• Lateral meristem
• Produces secondary xylem (wood) inward
• Produces secondary phloem outward
• Continuous ring formation

Cork Cambium (Phellogen):

• Produces cork (phellem) outward
• Produces phelloderm inward
• Cork contains suberin (waterproofing)

Wood Anatomy:

Feature	Description
Early wood	Large vessels, light color (spring)
Late wood	Small vessels, dark color (autumn)
Annual rings	One early + one late wood
Heartwood	Inner, non-functional, dark
Sapwood	Outer, functional, light

Dendrochronology:

• Study of tree rings
• Ring width indicates climate conditions
• Widely used in temperate regions
• Less applicable in tropical Sri Lanka

Plant Senescence and Abscission

Senescence:

• Aging and deterioration
• Breakdown of chlorophyll → yellowing
• Programmed cell death
• Recycling of nutrients to storage organs

Abscission:

• Shedding of plant parts
• Leaves, flowers, fruits
• Protective layer (abscission layer) forms
• Ethylene promotes, auxin inhibits

Types of Senescence:

Type	Description	Example
Whole plant	Entire plant dies	Annual plants
Shoot senescence	Shoots die, roots survive	Perennials
Organ senescence	Individual organs die	Deciduous trees

GCE A/L Sri Lanka Past Paper Tips

Common Questions:

1. “Describe the role of auxin in phototropism” (8 marks)
2. “Explain how gibberellins promote stem elongation” (6 marks)
3. “What is apical dominance? How does it relate to auxin?” (8 marks)
4. “Describe the effects of ethylene on plant development” (6 marks)
5. “Compare the roles of auxin and gibberellin in growth” (10 marks)

Diagram Questions:

1. Draw and label the response of a plant shoot to unilateral light (6 marks)
2. Draw LS of a root showing geotropic response (6 marks)
3. Draw transverse section of a one-year-old stem showing secondary growth (10 marks)

Practical Work:

• Effect of IAA on root/shoot growth (cutTINGS in solutions)
• Phototropism experiment ( unilaterally illuminated seedlings)
• Effect of ethylene on fruit ripening
• Observe thigmotropism in curling tendrils

⚡ A/L Strategy: For plant hormone questions, always relate the hormone function to its practical application. For example, auxin’s role in rooting powder for plant propagation!

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