Hormonal Control of Growth

Lab 8: Hormonal Control of Growth

Objectives

By the end of this lab, you'll understand:

• How plants control branching through apical dominance (auxin)

• How plants respond to gravity (gravitropism)

• How plants grow toward light (phototropism)

• How mechanical stress changes plant form (thigmomorphogenesis)

• How plant hormones coordinate these responses

• The ecological significance of plant hormone responses

Background

Unlike animals, plants can't move—so they adjust their growth instead! Plant hormones coordinate these growth responses to environmental signals.

Key Plant Hormones:

1. Auxin (IAA):

• Promotes cell elongation

• Controls apical dominance (tip suppresses side branches)

• Redistributes in response to light and gravity

• Made in shoot tips and young leaves

2. Cytokinins:

• Promote cell division

• Stimulate lateral bud growth

• Work opposite to auxin (break apical dominance)

• Made in root tips

3. Ethylene:

• Stress hormone (drought, wounding, mechanical stress)

• Promotes leaf drop (abscission)

• Slows stem elongation, thickens stems

• Gas that moves through air between plants!

4. Gibberellins:

• Promote stem elongation

• Break seed dormancy

• Promote flowering

5. Abscisic Acid (ABA):

• "Stress hormone"

• Closes stomata during drought

• Promotes seed dormancy

Materials

Required:

• Measuring tape

• String

• Scissors

• Protractor (or angle-measuring app)

• Camera

• Cardboard box

• Compass (or compass app)

• Field notebook

• Permanent markers for labeling

Plants needed:

• 12 similar plants for apical dominance (cilantro, basil, or dill)

• 6 plants for gravitropism (same species, flexible stems)

• 6 plants for phototropism (in pots that can be moved)

• 15 plants for thigmomorphogenesis (3 groups of 5)

This lab runs for 2 WEEKS with regular check-ins.

Pre-Lab: Growing Your Experimental Plants

2 weeks before Day 1 of experiments:

Step 1: Plant seeds

1. Fill pots with moistened potting soil

2. Plant 2-3 seeds per pot

3. Cover lightly (1/4 inch depth)

4. Water gently

5. Label with date and species

Step 2: Germination

• Cilantro: 7-10 days

• Basil: 5-7 days

• Dill: 7-14 days

• Keep soil moist but not waterlogged

• Place in bright light (window or grow light)

Step 3: Thin seedlings

• Once seedlings have 2-4 true leaves, thin to 1 plant per pot

• Choose healthiest, most uniform plants

Step 4: Select experimental plants

• Choose plants of similar size and health

• Label clearly before experiments begin

Part 1: Apical Dominance Experiment

Goal: Demonstrate that the shoot tip suppresses side branch growth through hormone signals (auxin).

Apical dominance = The terminal bud (apex) at the top of the plant suppresses lateral buds from growing.

The mechanism:

1. Apex produces auxin hormone

2. Auxin moves down the stem (polar transport)

3. High auxin levels suppress lateral bud development

4. Remove apex → auxin production stops → lateral buds released → bushier plant!

Real-world applications:

• Pruning: Gardeners pinch plant tips to make them bushier

• Topiary: Repeated tip removal creates dense, shaped plants

• Agriculture: Understanding branching patterns improves crop yields

Step 1: Select Your Study Plants

Requirements:

• 12 plants of the same species

• Similar age (planted same day)

• Similar size (within 20% of each other in height)

• Healthy (no yellowing, good growth rate)

Recommended age for experiment:

• Cilantro: 14-21 days old, 8-15 cm tall

• Basil: 14-21 days old, 10-18 cm tall

• Dill: 14-21 days old, 12-20 cm tall

Step 2: Divide Into Treatment Groups

Create three groups of 4 plants each:

Group 1: CONTROL (No manipulation)

• Plants: C1, C2, C3, C4

• Treatment: None—don't touch these

• Purpose: Show normal growth pattern

• Label each plant with permanent marker on pot

Group 2: APEX REMOVED (Complete removal)

• Plants: R1, R2, R3, R4

• Treatment: Cut off terminal bud completely

• Purpose: Completely eliminate auxin source

• Prediction: Most lateral branching

Group 3: APEX PINCHED (Partial damage)

• Plants: P1, P2, P3, P4

• Treatment: Damage terminal bud but leave attached

• Purpose: Partially disrupt auxin production

• Prediction: Intermediate branching

Why three groups? The gradient (control → pinched → removed) shows dose-dependent response of auxin on branching.

Step 3: Apply Treatments (Day 0)

Timing: Do all treatments on same day, at same time

For CONTROL plants:

1. Label clearly (C1, C2, C3, C4)

2. Take initial measurements (see Step 4)

3. Don't touch the terminal bud

4. Photograph

For APEX REMOVED plants:

Procedure:

1. Use clean, sharp scissors or pruning shears

   • Wipe blades with rubbing alcohol (prevents disease transmission)

2. Identify the terminal bud:

   • The very tip of the main stem

   • Usually has tiny unopened leaves

3. Cut cleanly 1-2 cm below the terminal bud

   • Remove the apical meristem completely

   • Include 1-2 of the smallest leaves

   • Make a straight, clean cut (not jagged)

4. Label plant (R1, R2, R3, R4)

5. Note exact time of treatment

What you're removing:

• Terminal/apical bud (growing point)

• Apical meristem (actively dividing cells)

• Source of auxin production

For APEX PINCHED plants:

Procedure:

1. Use fingernails or forceps

2. Locate terminal bud

3. Squeeze or crush the growing tip

   • Apply firm pressure to damage tissues

   • Tip should look bruised or broken

   • But leave it attached to plant

4. The bud should be damaged but not removed

5. Label plant (P1, P2, P3, P4)

6. Note exact time of treatment

What you're doing:

• Damaging cells that produce auxin

• Reducing (but not eliminating) auxin production

• Testing whether partial removal has partial effect

Step 4: Measurement Schedule

Measure ALL 12 plants on Days: 0, 3, 6, 9, 12, 14

A. Main Stem Height

What to measure: Distance from soil surface to highest point on main stem

Procedure:

1. Place ruler vertically next to stem

2. Note soil surface level (0 cm)

3. Find highest point on main stem

   • For removed/pinched: may be where you cut

   • For control: terminal bud tip

4. Record to nearest mm

Recording: Height: _____ cm

Expected pattern:

• Control: Steady height increase

• Removed: Height increase slows or stops

• Pinched: Intermediate

B. Number of Lateral Branches

What to count: All visible side branches emerging from main stem

Counting rules:

• Only count branches >1 cm long (standardizes measurement)

• Count from base to top of plant

• Don't count tiny buds that haven't elongated

• Each node (leaf attachment point) can produce 1 lateral branch

Recording: # lateral branches: _____

Expected pattern:

• Control: Few lateral branches (0-2)

• Removed: Many lateral branches (4-8+)

• Pinched: Moderate (2-5)

C. Length of 3 Longest Lateral Branches

Why measure length? Shows not just number but vigor of lateral growth

Procedure:

1. Identify all lateral branches >1 cm

2. Find the 3 longest ones

3. For each, measure from:

   • Start: Where branch attaches to main stem

   • End: Tip of branch (including any leaves)

4. Record all three lengths

Recording:

• Longest branch: _____ cm

• 2nd longest: _____ cm

• 3rd longest: _____ cm

• Average: _____ cm

Expected pattern:

• Control: Short laterals (1-3 cm)

• Removed: Long laterals (5-15 cm)

• Pinched: Intermediate (3-8 cm)

D. Position of Most Vigorous Growth

Why track this? Shows where auxin levels are lowest (where branching is strongest)

Procedure:

1. Identify the lateral branch growing fastest

   • Look for longest, thickest, greenest branch

   • Compare growth between measurement days

2. Measure distance from soil to where this branch attaches

3. Record position: Lower third / Middle third / Upper third

Recording:

• Most vigorous branch position: _____ cm from soil

• Location: Lower  /  Middle  /  Upper

Expected pattern:

• Lower branches often grow most (lowest auxin concentration)

• But upper branches may have more light

E. Total Number of Leaves

Why count leaves? Shows total photosynthetic capacity and resource allocation

Procedure:

1. Count all leaves on main stem

2. Count all leaves on lateral branches

3. Add together

4. Only count fully expanded leaves (>1 cm long)

Recording: Total # leaves: _____

Expected pattern:

• Control: Moderate leaf number

• Removed: High leaf number (many on lateral branches)

• Pinched: Intermediate

F. Stem Diameter

Why measure diameter? Shows resource allocation (wider stem = more vascular tissue)

Procedure:

Method 1: String method (no calipers)

1. Wrap string around main stem 5 cm above soil

2. Mark where string overlaps

3. Measure string length = circumference (C)

4. Calculate diameter: D = C ÷ 3.14

5. Record in mm

Method 2: Caliper method (more precise)

1. Use digital calipers

2. Measure across widest point of stem 5 cm above soil

3. Record directly in mm

Recording: Stem diameter: _____ mm

Expected pattern:

• Control: Steady slow increase

• Removed: May thicken (supporting lateral branches)

• Pinched: Intermediate

Step 5: Photography Protocol

Take photos on measurement days (0, 3, 6, 9, 12, 14):

Individual plant photos:

• Straight-on view (front)

• Include ruler for scale

• Label visible in photo

• Consistent distance from plant

Group comparison photos:

• Line up all 4 plants of each treatment

• Same angle, lighting, background

• Labels visible

• Take photo of: all controls, all removed, all pinched

Close-up photos:

• Terminal bud area (show cut/pinch site)

• Lateral branch development

• Most vigorous branches

Step 6: Data Table

Create spreadsheet or table for all measurements:

Part 2: Gravitropism Study

Goal: Demonstrate that plants sense gravity and reorient growth to grow upward (negative gravitropism in shoots).

Gravitropism (geotropism) = Directional growth response to gravity

The mechanism:

1. Plant tipped sideways → gravity pulls statoliths (dense organelles) to lower side of cells

2. Statoliths trigger auxin redistribution

3. Auxin moves to lower side of horizontal stem

4. Lower side cells elongate faster → stem curves upward

5. Process continues until stem is vertical again

Types:

• Negative gravitropism: Shoots grow up (away from gravity)

• Positive gravitropism: Roots grow down (toward gravity)

Step 1: Select Plants

Requirements:

• 6 plants of same species

• Young, flexible stems (won't snap when tipped)

• Similar size

• Healthy growth

Best species:

• Sunflower (very responsive, large stem)

• Bean (classic demo plant)

• Basil or dill (work well)

Plant age: 10-14 days old, stems 8-15 cm tall

Step 2: Create Treatment Groups

Group 1: TIPPED (3 plants)

• Plants T1, T2, T3

• Will be placed horizontally

• Show gravitropic response

Group 2: CONTROL (3 plants)

• Plants GC1, GC2, GC3 (GC = gravitropism control)

• Stay vertical

• Show normal growth without gravitational stimulus

Step 3: Set Up Experiment (Day 0)

For TIPPED plants:

Procedure:

1. Carefully tip entire pot 90°:

   • Slowly lean pot onto its side

   • Stem should be horizontal

   • Don't bend or damage stem!

   • Secure pot so it doesn't roll (use books, blocks, tape)

2. Mark the "curve point":

   • Tie small piece of colored string or yarn around stem

   • Position: 5 cm below the growing tip

   • Tie loosely (don't restrict growth)

   • This marks where you'll measure curvature

3. Orient plant:

   • Growing tip should point in consistent direction for all 3 plants

   • Note compass direction if possible

4. Label clearly: T1, T2, T3

5. Take "before" photo:

   • Side view showing horizontal stem

   • String marker visible

   • Include protractor or angle reference

For CONTROL plants:

1. Keep pots vertical (normal position)

2. Tie colored string 5 cm below tip (for consistency)

3. Label: GC1, GC2, GC3

4. Take photo

Step 4: Measurement Schedule

Measure every 12-24 hours (more frequent = better data)

Best times:

• Morning (8 AM) and evening (8 PM), OR

• Every 24 hours at same time

Continue for 7-10 days or until stems are vertical again

Measurements to Record:

A. Angle of Stem from Vertical

What to measure: Angle between stem and true vertical at the string marker

Procedure:

1. Use protractor or angle-measuring app

2. Place against stem at string marker

3. Measure angle from vertical

Angle scale:

• 0° = Perfectly vertical (fully recovered)

• 45° = Halfway between horizontal and vertical

• 90° = Horizontal (starting position)

Recording: Angle from vertical: _____ °

Expected pattern:

• Starts at 90° (horizontal)

• Decreases over time: 90° → 70° → 50° → 30° → 10° → 0°

• Response fastest in first 24-48 hours

B. Distance to New Vertical Growth

What to measure: How much of the stem has curved back to vertical

Procedure:

1. Find the string marker (your reference point)

2. Follow stem from marker toward tip

3. Find where stem becomes vertical (within 10° of vertical)

4. Measure distance from marker to this point

Recording: Distance to vertical: _____ cm

Expected pattern:

• Day 0: 0 cm (nothing vertical yet)

• Day 2: 2-4 cm (curvature developing)

• Day 4: 5-8 cm (significant reorientation)

• Day 7-10: Full stem length (fully recovered)

C. Track Recovery Time

What to record: How long until stem is essentially vertical

Criteria for "recovered":

• Angle ≤10° from vertical

• No further straightening occurring

Recording:

• Recovery time: _____ hours (or days)

• Angle at recovery: _____ °

Expected time:

• Fast-responding plants: 2-4 days

• Typical: 5-7 days

• Slow: 7-10 days

D. Stem Growth Rate During Recovery

Why measure this? Shows whether reorientation slows overall growth

Procedure:

1. Measure total stem length (soil to tip)

2. Calculate growth rate: (Day 7 length - Day 0 length) ÷ 7 days

3. Compare tipped vs. control plants

Recording:

• Total stem length: _____ cm

• Growth rate: _____ cm/day

Expected pattern:

• Control: Steady growth (~1-2 cm/day)

• Tipped: May grow slower during reorientation (energy goes to curvature)

Step 5: Draw Curvature Profiles

At each measurement time, sketch the stem shape:

What to include:

• Outline of stem from soil to tip

• String marker location (mark clearly)

• Angle measurement

• Day/hour label

Draw on same page for comparison (shows progression over time)

Step 6: Data Table

Part 3: Phototropism Documentation

Goal: Demonstrate that plants grow toward light through differential cell elongation controlled by auxin redistribution.

Phototropism = Directional growth response to light

The mechanism:

1. Light hits one side of stem

2. Blue light receptors (phototropins) detect light direction

3. Auxin redistributes to shaded side of stem

4. Shaded side cells elongate faster → stem bends toward light

5. Process continues until plant faces light source

Real-world example: Houseplants lean toward windows!

Step 1: Create Directional Light Situations

Goal: Expose plants to light coming from ONE direction only

Box method

1. Get cardboard box large enough for plants

2. Cut opening on ONE side only (10×10 cm)

3. Paint inside white (reflects light evenly)

4. Place plants inside, opening faces window or lamp

5. Light enters only through opening

Arizona consideration: Strong sun through windows can overheat plants. Use sheer curtain or do experiment in spring/fall.

Step 2: Initial Documentation

For each of your 6 plants:

A. Document Light Direction

Record:

• Compass direction of light source

  • Example: "Light from South", "Light from East window"

• Time of day light is strongest

  • Example: "Morning sun only (6-10 AM)", "Afternoon (2-6 PM)", "All day"

• Type of light

  • Natural (window) or artificial (lamp)

Why this matters: Sun moves! Morning east-facing vs. afternoon west-facing produces different responses.

Recording:

• Light direction: _____ (N/S/E/W)

• Light timing: _____

• Light type:  Natural  /  Artificial

B. Measure Initial Orientation

What to measure: Is plant already leaning toward or away from light?

Procedure:

1. View plant from side (perpendicular to light direction)

2. Determine angle of stem relative to light

3. Use this scale:

   • 0° = Stem perpendicular to light (90° from light)

   • 45° = Stem angled toward light

   • 90° = Stem pointing directly at light

Recording: Initial orientation: _____ ° toward light

C. Measure Stem Curvature

What to assess: Is stem straight or already curved?

Procedure:

1. View stem from side

2. Look for curves or bends

3. If curved, measure maximum angle of curve

Recording:

• Stem shape:  Straight  /  Slightly curved  /  Strongly curved

• Curve angle (if present): _____ °

D. Identify and Mark Internodes

What are internodes? Sections of stem BETWEEN leaves (nodes)

Why mark them? Different-aged internodes respond differently—young internodes bend most!

Procedure:

1. Count nodes (leaf attachment points) from bottom up

2. Number internodes:

   • Internode 1: Between ground and 1st leaf

   • Internode 2: Between 1st and 2nd leaf

   • Internode 3: Between 2nd and 3rd leaf

   • Continue to top

3. Mark with small dots or tie colored thread

4. Note which are straight vs. curved

Recording:

• Total number of internodes: _____

• Which internodes are curved: _____

E. Measure Leaf Position

For plants with distinct leaves (not relevant for grasses):

What to observe: Have leaves turned to face light?

Procedure:

1. Note orientation of largest leaves

2. Are leaf blades perpendicular to light? (maximum capture)

3. Measure angle of leaf blade from horizontal

Recording:

• Leaf orientation:  Facing light  /  Random  /  Away from light

• Leaf angle: _____ ° from horizontal

Step 3: Daily Measurements

Measure at the SAME TIME each day (light angle changes through day!)

Recommended: Morning (8-9 AM) or evening (5-6 PM)

Duration: 10-14 days

Daily Measurements to Record:

1. Stem/leaf angle toward light:

• Has angle increased (bending more toward light)?

• Measure in degrees

2. Stem curvature at each internode:

• Which internodes show most bending?

• Measure angle of curve at each marked internode

• Typically: younger (upper) internodes curve more

3. Leaf movement distance:

• If tracking specific leaves, measure how far they've turned

• Use compass bearing or angle measurement

4. Calculate daily response rate:

• Response rate = Change in angle ÷ Days

• Example: Went from 10° to 60° in 5 days = 50° ÷ 5 = 10°/day

5. Take photos:

• Same angle, same time of day

• Shows progression clearly

Step 4: Data Table

Part 4: Thigmomorphogenesis Observation

Goal: Demonstrate that mechanical stimulation (touch, wind, flexing) causes plants to grow shorter and stockier.

Thigmomorphogenesis = Growth changes in response to mechanical stimulation

The mechanism:

1. Touch/wind causes mechanical stress on stems

2. Stress triggers ethylene and jasmonic acid production

3. These hormones:

   • Slow stem elongation

   • Increase radial growth (thickening)

   • Shorten internodes

   • Strengthen cell walls

4. Result: Shorter, thicker, sturdier plant

Real-world examples:

• Trees on windy mountaintops: Short, gnarled, dense canopy

• Same species in sheltered valley: Tall, thin, spreading canopy

• Greenhouse vs. outdoor plants: Greenhouse plants are "leggy" (tall/thin) without wind

Step 1: Create Plants in Three Exposure Categories

Plant 15 seeds total: 5 for each category

Grow for 10-14 days before beginning treatments, then continue treatments for 2 full weeks

Category 1: SHELTERED (5 plants)

• Location: Indoors, still air, protected

• Treatment: NO touching, NO movement

• Purpose: Baseline—shows growth without mechanical stress

• Label: S1, S2, S3, S4, S5

Setup:

• Place in room with no air circulation

• No fans, no open windows

• Minimal human traffic

• Water carefully (don't bump plants)

Category 2: EXPOSED (5 plants)

• Location: Outdoors or in front of fan

• Treatment: Constant wind/air movement

• Purpose: Natural mechanical stimulation

• Label: E1, E2, E3, E4, E5

Setup - Option A: Outdoors (best)

• Place outside in open area

• Exposed to natural wind

• Arizona: Wind common in spring afternoons!

• Protect from extreme heat/sun if needed

Setup - Option B: Indoor with fan

• Place small fan 1-2 feet from plants

• Set to low-medium speed

• Run fan 8-12 hours per day

• Simulate gentle breeze (not hurricane!)

Category 3: MANUALLY STIMULATED (5 plants)

• Location: Indoors, sheltered (like Category 1)

• Treatment: Brush or gently flex stems 2× daily

• Purpose: Controlled mechanical stimulation

• Label: M1, M2, M3, M4, M5

Treatment protocol (VERY IMPORTANT—be consistent):

Twice daily (morning and evening):

1. Gently brush palm of hand back and forth across plant tops

2. 10 strokes each time

3. Light pressure—like petting a cat

4. Just enough to bend stems slightly

5. Stems should flex but not break

6. Do this EVERY day for 14 days

Or alternatively:

1. Gently flex each main stem

2. Bend 10-15° to each side

3. Hold 2 seconds, release

4. Repeat 10 times

5. Twice daily

Record treatment times in notebook (ensures consistency)

Step 2: Measurement Schedule

Measure all 15 plants on Days: 0, 3, 6, 9, 12, 14

Consistency critical:

• Same time of day

• Same measurement tools

• Same person (if possible)

• Measure before treating (don't stimulate right before measuring)

Measurements to Record:

A. Stem Diameter at Base

What to measure: Thickness of main stem at soil level

Procedure:

1. Wrap string around stem at soil surface

2. Mark where string overlaps

3. Measure string length = circumference

4. Calculate diameter: C ÷ 3.14 = diameter

5. Record in mm

Recording: Base diameter: _____ mm

Expected pattern:

• Sheltered: Thin (2-4 mm)

• Exposed: Thicker (4-7 mm)

• Manual: Thicker (3-6 mm)

B. Stem Diameter at Mid-Height

What to measure: Thickness at halfway point of stem

Why measure here too? Shows if thickening is uniform or concentrated at base

Procedure:

1. Measure total plant height

2. Divide by 2 to find midpoint

3. Wrap string at midpoint

4. Calculate diameter (C ÷ 3.14)

Recording: Mid-height diameter: _____ mm

C. Stem Height

What to measure: Total height from soil to tip

Procedure:

1. Place ruler vertically next to plant

2. Measure soil to highest point

3. Record in cm

Recording: Height: _____ cm

Expected pattern:

• Sheltered: Tallest (15-25 cm)

• Exposed: Shortest (8-15 cm)

• Manual: Intermediate (10-18 cm)

D. Internode Length

What to measure: Distance between consecutive leaves (shows compactness)

Procedure:

1. Select 3 internodes in middle of plant (not top or bottom)

2. Measure each internode length

3. Calculate average

Recording:

• Internode 1: _____ cm

• Internode 2: _____ cm

• Internode 3: _____ cm

• Average internode length: _____ cm

Expected pattern:

• Sheltered: Long internodes (2-4 cm) → stretched appearance

• Exposed: Short internodes (0.5-1.5 cm) → compact appearance

• Manual: Intermediate (1-2.5 cm)

E. Calculate Sturdiness Index

What it measures: Relative sturdiness (thickness vs. height)

Formula: Sturdiness Index = Base Diameter (mm) ÷ Height (cm)

Example:

• Plant 1: 5 mm diameter, 50 cm tall → 5/50 = 0.10

• Plant 2: 8 mm diameter, 25 cm tall → 8/25 = 0.32 (much sturdier!)

Higher number = Sturdier, stockier plant

Recording: Sturdiness Index: _____

Expected pattern:

• Sheltered: 0.10-0.20 (thin and tall)

• Exposed: 0.30-0.50 (thick and short)

• Manual: 0.20-0.35 (intermediate)

Step 3: Additional Observations

F. Stem Flexibility

Test on Day 14 only (don't want to stress plants during growth):

Procedure:

1. Gently bend main stem to one side

2. Measure maximum angle before feeling resistance

3. More flexible = larger angle before resistance

Recording: Flexibility: _____ ° (angle of bend)

Expected pattern:

• Sheltered: Very flexible (bends 30-45°)

• Exposed: Stiff (bends only 10-20°)

• Manual: Intermediate (20-30°)

Why? Mechanical stress strengthens cell walls (lignification, cellulose deposition)

G. Color and Vigor

Subjective but important observations:

Recording:

• Stem color:  Light green  /  Medium green  /  Dark green  /  Purple-tinged

• Leaf size:  Large  /  Medium  /  Small

• Overall vigor:  Robust  /  Normal  /  Stressed

Stressed plants (exposed/manual) may show:

• Darker green color (denser chloroplasts)

• Smaller but thicker leaves

• Purple/red tints (anthocyanin stress pigments)

Step 4: Data Table

Comprehensive measurements for all 15 plants: