Plant Responses to Environmental Stimuli

Lab 9: Plant Responses to Environmental Stimuli

Objectives

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

• How plants move their leaves in daily rhythms (circadian movements)

• How plants respond to touch and vibration (rapid movements)

• How plants adjust leaf positions between day and night (sleep movements)

• How plants respond to environmental stresses (heat, drought, excess light)

• The speed and mechanisms of different plant responses

• The difference between internal clocks and external triggers

Background

Plants seem stationary, but they're constantly moving—just slowly! Without a nervous system or muscles, plants use changes in cell water pressure (turgor), hormones, and internal biological clocks to respond to their environment.

Key Concepts:

Plant movements fall into two categories:

1. Growth movements (slow, irreversible):

• Phototropism (bending toward light)

• Gravitropism (bending upward/downward)

• Covered in Lab 8

2. Turgor movements (rapid, reversible):

• Circadian rhythms (daily leaf movements)

• Nyctinasty (sleep movements)

• Stress responses (wilting, leaf folding)

• Covered in this lab!

The mechanism of turgor movements:

1. Special motor cells at base of leaf (pulvinus)

2. Water moves into or out of these cells

3. Cells swell (high turgor) → leaf rises

4. Cells shrink (low turgor) → leaf drops

5. Process is reversible and can repeat daily

Materials

Required:

• Smartphone with:

  • Camera

  • Timer

  • Flashlight

  • Time-lapse app

  • Protractor measuring app

• Measuring tape or ruler

• String

• Field notebook and pencils

Plants needed:

• 5-6 plant species showing leaf movements

• At least 1 plant each for circadian, nyctinasty, and stress experiments

• Can overlap (same plant used for multiple parts)

Part 1: Circadian Leaf Movements

Goal: Demonstrate that plants have internal biological clocks that control leaf movements on a ~24-hour cycle, even without light/dark cues.

Circadian rhythms = Internal biological "clocks" running on approximately 24-hour cycles

Key features:

• Persist in constant conditions (continuous light or dark)

• Free-running period ~24 hours (not exactly 24.0)

• Reset daily by environmental cues (light/dark transitions)

• Found in all organisms (plants, animals, fungi, bacteria!)

In plants:

• Leaf movements (up/down throughout day)

• Flower opening/closing

• Photosynthesis efficiency

• Gene expression patterns

Why have circadian rhythms?

• Anticipate daily changes (dawn, dusk)

• Coordinate physiology with environment

• Optimize processes for time of day

Real-world example: Prayer plants raise their leaves at night and lower them during the day. This happens even if you keep lights on constantly—proves it's an internal clock!

Step 1: Select Your Study Plants

Find 5-6 individual plants from different species that show leaf movements.

Excellent for Circadian Movements:

Fast-growing annuals (grow from seed):

• Beans (Phaseolus vulgaris) - Trifoliate leaves, dramatic movements

• Clover (Trifolium) - Trefoil leaves fold at night

• Alfalfa (Medicago sativa) - Compound leaves, clear rhythms

Garden plants:

• Wood sorrel (Oxalis) - Shamrock-like leaves, rapid movements

• Sensitive plant (Mimosa pudica) - If you can find it (touch-sensitive too!)

Arizona natives (if accessible):

• Mesquite (Prosopis) - Bipinnate leaves fold at night

• Catclaw acacia - Similar to mesquite

Houseplants (if working indoors):

• Prayer plant (Maranta) - Needs humidity, challenging in AZ

• Calathea - Similar care needs

What to Look For:

Leaf characteristics:

• Leaves on petioles (leaf stems) that can pivot

• Compound leaves (multiple leaflets) - show clearest movements

• Leaves that respond to day/night already

• Healthy, well-watered plants (stressed plants don't move well)

Best leaf types for observation:

• Trifoliate (3 leaflets, like beans)

• Pinnate (leaflets along central axis, like mesquite)

• Simple with flexible petiole (like prayer plants)

Step 2: Set Up Your Observation System

Create Reference Points

You need a fixed point to measure leaf movement from.

Procedure:

1. Choose 2-3 healthy, mature leaves per plant

   • Not brand new (still growing)

   • Not old (may be dying)

   • Leaves with clear view for camera

2. Tie small piece of colored string to main stem just BELOW each leaf

   • This is your "zero point"

   • All measurements relative to this point

3. Label each leaf with small piece of tape

   • Label: "Plant 1 - Leaf A", "Plant 1 - Leaf B", etc.

   • Write small, don't block leaf

Set Up Camera Position (If Using Time-Lapse)

Procedure:

1. Position camera on tripod or stable surface

   • Sturdy (won't get bumped for 24-48 hours)

   • Pointed at plant from consistent angle

2. Frame plant so tracked leaves are clearly visible

   • Fill frame with plant

   • Leaves should be in focus

3. Test lighting

   • Adequate for daytime photos (check)

   • Adequate for nighttime photos (add dim LED lamp if needed)

   • Don't use bright light at night (will interfere with rhythms!)
     
     
     
     

4. Lock focus on plant

   • Autofocus may drift over 24 hours

   • Manual focus if possible

5. Test one photo

   • Take test shot

   • Check framing, focus, exposure

   • Adjust as needed

Time-lapse app settings:

• Interval: 1 photo every 10-15 minutes

• Duration: 24-48 hours minimum

• Frame rate: 10-15 fps when compiling video

• Storage: Make sure you have space (several hundred photos!)

Step 3: Measurement Schedule

Measure every 2 hours for at least 24 hours (48 hours better!)

At Each Time Point, Measure:

A. Leaf Angle from Horizontal

What to measure: Angle of main leaf axis (petiole + blade) relative to horizontal

Procedure:

1. View leaf from side (perpendicular to leaf blade)

2. Align protractor with horizontal (0°)

3. Measure angle to leaf petiole

Angle conventions:

• 0° = Leaf perfectly horizontal (parallel to ground)

• Positive angles (+) = Leaf raised above horizontal

  • Example: +45° = leaf pointing 45° upward

• Negative angles (-) = Leaf drooping below horizontal

  • Example: -30° = leaf hanging 30° downward

Recording: Leaf angle: _____ °

Expected pattern (typical):

• Daytime: Leaves horizontal to angled down (0° to -30°)

• Nighttime: Leaves raised up (+30° to +60°)

• Pattern repeats every 24 hours

B. Leaflet Angle (If Compound Leaves)

What to measure: Angle between individual leaflets on compound leaves

Procedure:

1. For leaves with multiple leaflets (beans, clover, mesquite)

2. Measure angle between opposite leaflets

3. Like measuring how "open" or "closed" the leaf is

Angle scale:

• 180° = Leaflets fully spread apart (maximally open)

• 90° = Leaflets perpendicular to main axis

• 0° = Leaflets pressed together (closed/folded)

Recording: Leaflet angle: _____ °

Expected pattern:

• Day: Leaflets spread (120-180°)

• Night: Leaflets folded (30-90°)

C. Position of Leaf Tip

What to measure: Vertical distance from reference string to leaf tip

Direct measure of how much leaf has moved up or down

Procedure:

1. Hold ruler vertically next to plant

2. Measure from reference string (on stem) to tip of leaf blade

3. Record in cm

Recording: Tip distance: _____ cm

Expected pattern:

• Day: Lower distance (leaf hanging down)

• Night: Higher distance (leaf lifted up)

• Range typically 5-15 cm depending on plant size

D. Environmental Conditions

Record these to look for correlations:

Light intensity:

• Bright sun (direct sunlight)

• Moderate (bright indirect or partly cloudy)

• Dim (heavy clouds, early/late day)

• Dark (nighttime)

Temperature:

• If you have thermometer: _____ °F

• If not, estimate: Hot / Warm / Moderate / Cool / Cold

Weather:

• Clear  /  Partly cloudy  /  Overcast  /  Windy  /  Rain

E. Take Photos

Photography protocol:

1. Same angle every time (critical!)

2. Include string reference point

3. Include ruler or scale

4. Time-stamp if camera allows

5. Consistent lighting (use flash/lamp at night if needed)

6. Focus on tracked leaves

Organize photos:

• Label with plant ID, leaf ID, date, time

• Example: "Plant1_LeafA_Day1_0800.jpg"

Step 4: Time-Lapse Compilation (If Using)

After 24-48 hours of collection:

1. Stop time-lapse

2. Export photos from app

3. Compile into video:

   • Most time-lapse apps do this automatically

   • Or use video editing software

   • Frame rate: 10-15 fps (makes movements smooth)

4. Add annotations:

   • Time stamp overlay

   • Indicate day vs. night

5. Watch for patterns:

   • Do leaves move in cycles?

   • Are cycles ~24 hours?

   • Do movements continue in constant conditions?

Step 5: Data Table

Create comprehensive table for all measurements:

Repeat for all leaves on all plants

Analysis for Part 1:

After collecting data, create graphs:

Graph 1: Leaf Angle Over Time

Type: Line graph

Axes:

• X-axis: Time (hours, 0-48)

• Y-axis: Leaf angle (degrees, -60° to +60°)

Lines:

• One line per leaf tracked

• Mark day/night periods (shaded regions)

What to look for:

• Cyclic pattern: Angle goes up and down rhythmically

• Period: Time from peak to peak (~24 hours?)

• Amplitude: How much does angle change? (range of movement)

• Phase: When does peak occur? (what time of day?)

Graph 2: All Plants Compared

Show whether different species have similar rhythms

Axes: Same as Graph 1 Lines: One per plant species (average of leaves)

Questions:

• Do all species show 24-hour rhythms?

• Do some move more than others?

• Are peaks at same time or different?

Part 2: Nyctinasty - Sleep Movements

Goal: Document day-night leaf movements that are directly triggered by light/dark transitions (different from circadian rhythms which persist in constant conditions).

Nyctinasty = Leaf movements in response to day/night cycles

Key differences from circadian rhythms:

Control

Biological clock genes

Phytochrome (light receptor) + clock

In reality: Most "sleep movements" involve BOTH circadian clock AND light sensing working together!

The mechanism:

1. Phytochrome (light receptor protein) detects red/far-red light

2. At dusk: Low red light → phytochrome signals "night"

3. Signal triggers:

   • K+ ions move out of motor cells (pulvinus)

   • Water follows ions (osmosis)

   • Motor cells shrink → leaf moves

4. At dawn: High red light → signal reverses → leaf moves back

Why do plants have sleep movements?

• Reduce heat loss at night (leaves vertical reduce radiation to sky)

• Reduce water loss (smaller surface area exposed)

• Avoid frost damage (in cooler climates)

• Reduce herbivory (harder for insects to land on vertical leaves)

Real-world example: Clover leaflets fold down at night like closing an umbrella. In morning, they reopen when sunlight hits them.

Step 1: Select Study Species

Find 3 different plant species that show clear day-night movements:

Good Choices for Arizona:

Easy to grow from seed:

• Clover (Trifolium) - Trefoil leaves fold downward at night

• Beans/Peas - Trifoliate leaves, dramatic movements

• Wood sorrel (Oxalis) - Shamrock leaves fold completely

Available as plants:

• Albizia (silk tree/mimosa tree) - If you can find one

• Mesquite - Native, bipinnate leaves fold at night

• Sensitive plant (Mimosa pudica) - Also responds to touch!

Houseplants (need humidity):

• Prayer plant (Maranta leuconeura) - Leaves raise at night

• Calathea - Similar to prayer plant

• Difficult in Arizona unless indoors with humidifier

Select criteria:

• Clear visible movement between day and night

• Healthy plants (well-watered, not stressed)

• Accessible for repeated measurements

• At least 1 plant per species (3+ better for replication)

Step 2: Observation Schedule

Four key time points per day for 2 days minimum (3 days better):

Daily schedule:

1. Dawn (6-7 AM) - Leaves just "waking up," starting to open

2. Noon (12-1 PM) - Leaves fully open, maximum spread

3. Dusk (6-7 PM) - Leaves starting to close

4. Midnight (12 AM) - Leaves fully closed, night position

Ensures pattern repeats (confirms it's not random)

Step 3: At Each Time Point

For each of your 3 species:

1. Photograph from Same Position

Consistency is critical!

Procedure:

1. Mark camera position (tape on floor)

2. Use same angle, distance, height every time

3. Include scale reference (ruler)

4. Lighting:

   • Daytime: Natural light okay

   • Nighttime: Use flash or lamp

   • Be consistent across all plants

Photo checklist:

• Plant clearly visible

• Tracked leaves in frame

• Scale reference included

• Time/date noted

• Proper focus

2. Measure Leaf Angle

For 3 representative leaves per plant:

Procedure:

1. Use protractor or angle app

2. Measure angle from horizontal

3. Record for each leaf

4. Calculate average

Recording:

• Leaf 1 angle: _____ °

• Leaf 2 angle: _____ °

• Leaf 3 angle: _____ °

• Average: _____ °

3. Describe Leaf Position

Use consistent terminology:

Day position options:

• Open - Leaflets spread wide

• Horizontal - Leaves flat, perpendicular to stem

• Spread - Leaves angled outward

• Relaxed - Natural daytime position

• Other: _____ (describe)

Night position options:

• Closed - Leaflets pressed together

• Folded - Leaflets folded downward/upward

• Raised - Leaves lifted upward (prayer position)

• Lowered - Leaves drooped downward

• Other: _____ (describe)

Add details:

• Angle of fold: _____ °

• Tightness of closure: Loose / Moderate / Tight

• Symmetry: All leaves similar / Some different

4. Note Synchronization

Are all individuals coordinated?

• High synchronization → strong environmental control

• Low synchronization → individual variation, maybe stress

Questions to answer:

• Are all leaves on ONE plant in same position? Yes  /  No  /  Mostly

• Are all plants of SAME species synchronized? Yes  /  No  /  Mostly

• Are DIFFERENT species synchronized? Yes  /  No  /  Varies

Step 4: Create Movement Composite

After collecting all photos, create visual summary:

Photo Grid

1. Arrange photos in 4-panel layout

2. Labels: Dawn | Noon | Dusk | Midnight

3. Same plant/leaf shown at 4 times

4. Clearly shows movement range 

Step 5: Test in Constant Darkness

Question: Does movement still occur without light cues?

This distinguishes nyctinasty from circadian rhythm!

Procedure:

1. Select one plant of each species

2. Place in completely dark environment:

   • Dark closet or box

   • No light leaks

   • Stable temperature

3. Leave for 24-48 hours

4. Check at 4 time points using brief flashlight/headlamp:

   • 6 AM, 12 PM, 6 PM, 12 AM

   • Use red light if possible (doesn't reset clock)

   • Quick measurement (<1 min)

5. Record leaf positions

Predictions:

If pure nyctinasty (light-triggered):

• Leaves should stay in ONE position (no movement)

• Position depends on when placed in dark

• No 24-hour rhythm

If circadian rhythm involved:

• Leaves should continue moving up/down

• Movement follows ~24-hour cycle

• May gradually drift from real time

Most plants: Show BOTH components (rhythm continues but gets weaker)

Step 6: Data Table

Analysis for Part 2:

Questions to answer from your data:

1. Movement amplitude:

• Day angle: _____ ° (average across all measurements)

• Night angle: _____ ° (average)

• Total movement range: _____ ° (difference)

2. Timing:

• What time do leaves start closing? _____ (should correlate with dusk)

• What time do leaves start opening? _____ (should correlate with dawn)

• Lag time after light change: _____ minutes

3. Species comparison:

• Which species showed largest movement? _____

• Which was fastest to respond? _____

• Which was most synchronized? _____

4. Dark experiment results (if done):

• Did leaves move in constant dark? Yes  /  No  /  Partially

• Conclusion: Pure nyctinasty  /  Pure circadian  /  Both components

Part 3: Environmental Stress Responses

Goal: Document how plants respond to environmental stresses (heat, drought, excess light) through rapid protective movements and physiological changes.

When environmental conditions exceed optimal range, plants activate stress responses:

Types of stress:

• Heat stress - Temperature >95°F for most plants

• Water stress - Soil moisture insufficient for demand

• Light stress - Too much or too little light

• Cold stress - Temperature <50°F for warm-season plants

Common stress responses:

• Wilting (loss of turgor pressure)

• Leaf angle changes (reduce exposure)

• Leaf rolling/folding (reduce surface area)

• Stomatal closure (reduce water loss)

• Color changes (damage or protective pigments)

Hormonal control:

• Abscisic acid (ABA) - "Stress hormone"

  • Produced during drought, heat, salt stress

  • Closes stomata

  • Triggers protective gene expression
    
    
    

• Ethylene - Also increases during stress

  • Promotes leaf drop if severe

  • Slows growth

Experiment A: Heat Stress Response

Step 1: Find or Create Heat-Stressed Plants

Option 1: Find Naturally Stressed Plants

Where to look:

• Summer afternoon (2-4 PM), full sun

• South-facing slopes or walls

• Parking lots, disturbed areas

• Plants without irrigation

What to look for:

• Leaves drooping or wilted

• Leaves rolled or folded

• Dull color (not shiny)

• Plant looks "tired"

Best candidates:

• Non-native garden plants (not adapted to heat)

• Recently transplanted plants

• Shallow-rooted species

Option 2: Create Mild Heat Stress (Controlled)

Safe methods:

1. Move potted plant to full sun on hot day (85-95°F)

2. Use lamp near plant (12-18 inches away)

   • Monitor closely (check every 5 minutes)

   • Remove if leaves start browning

3. Place in enclosed space (car, greenhouse) for brief period

   • Monitor temperature constantly

   • Don't exceed 100°F

Step 2: Document Stress Progression

Measure at regular intervals to track how stress develops

Before Stress (Baseline - Time 0):

Record:

1. Leaf angle: _____ ° (should be normal, horizontal to slightly raised)

2. Leaf temperature:

   • Touch leaf gently (should feel cool)

   • Or use IR thermometer if available: _____ °F

3. Turgidity (firmness):

   • Gently try to bend leaf

   • Rating: 5 = Very stiff/rigid (healthy)

4. Visual appearance:

   • Color:  /  Bright green  /  Normal  /  Dull

   • Surface:  /  Shiny  /  Matte

   • Overall posture:  /  Upright  /  Relaxed  /  Drooping

5. Environmental conditions:

   • Air temperature: _____ °F

   • Humidity: _____ % (or Low/Moderate/High)

   • Light intensity:  /  Full sun  /  Partial  /  Shade

6. Take photos (front, side, close-up of leaves)

During Stress (Check Every 15-30 Minutes):

At each check, record:

1. Elapsed time: _____ minutes

2. Leaf angle changes:

   • Current angle: _____ °

   • Change from baseline: _____ ° (positive = raised, negative = drooped)

3. Leaf temperature:

   • Touch or measure: _____ °F

   • Compared to baseline: Cooler  /  Same  /  Warmer  /  Hot

4. Turgidity rating (1-5):

   • 5 = Very stiff, rigid (no stress)

   • 4 = Firm, slight give

   • 3 = Moderately soft, flexible

   • 2 = Soft, bends easily

   • 1 = Limp, completely wilted

5. Visual symptoms:

   • Leaves drooping

   • Leaves curling/rolling

   • Color dulling

   • Edges browning

   • Whole plant wilting

   • No change yet

6. Plant behavior notes:

   • Any protective responses?

   • All leaves affected equally?

   • Speed of wilting (fast/slow)?

7. Environmental data:

   • Air temperature: _____ °F

   • Has it changed?: _____

8. Photos (same angles as baseline)

Continue until:

• Clear wilting observed (~30-60 minutes typically)

• OR 2 hours passed

• OR browning/damage begins (STOP immediately!)

After Relief (Shade or Water):

Provide relief:

• For heat stress: Move to shade or cool area

• For heat + water stress: Water thoroughly AND provide shade

Monitor recovery:

Measure every 15 minutes:

1. Time after relief: _____ minutes

2. Leaf angle:

   • Current: _____ °

   • Compared to wilted state: Same  /  Slightly raised  /  Mostly recovered  /  Fully recovered

3. Turgidity:

   • Rating (1-5): _____

   • Improvement: None  /  Slight  /  Moderate  /  Full

4. Visual appearance:

   • Color: Still dull  /  Brightening  /  Normal

   • Leaves: Still wilted  /  Partially lifted  /  Fully recovered

5. Time to first improvement: _____ minutes

   • When did you first notice recovery starting?

6. Time to full recovery: _____ minutes

   • When did plant look completely normal again?

7. Photos showing recovery progression

Expected recovery time:

• Mild stress: 15-30 minutes

• Moderate stress: 30-60 minutes

• Severe stress: 1-3 hours (or may not fully recover)

Experiment B: Water Stress Response

This demonstrates wilting due to water deficit (not heat)

Step 1: Find or Create Water-Stressed Plants

Option 1: Find Naturally Wilted Plants

Where to look:

• End of hot afternoon

• Areas without irrigation

• Sandy soil (dries fast)

• Plants in small pots (limited water)

What to look for:

• Obvious wilting

• Soil very dry (dust when touched)

• Plant not recovering in evening

Good candidates:

• Garden vegetables

• Ornamental flowers

• Non-native species

Option 2: Withhold Water (Requires Planning)

Procedure:

1. Start with well-watered plant

2. Stop watering

3. Check daily for wilting

4. When wilting begins, proceed with measurements

5. Don't let plant stay wilted >24 hours

Time to wilt:

• Small pots: 2-4 days

• Large pots: 4-7 days

• Depends on temperature, plant size

Step 2: Document Wilting Progression

Measure hourly for 3-4 hours (or until clearly wilted)

At each time point:

1. Leaf angle from horizontal:

• Measure 3 leaves

• Record each: _____ °, _____ °, _____ °

• Calculate average: _____ °

• Track progressive drooping

2. Turgidity rating (1-5):

Scale:

• 5 = Very stiff, rigid

  • Leaves hold position against gravity

  • No flexibility

  • Healthy, well-watered

• 4 = Firm

  • Slight give when touched

  • Still mostly rigid

  • Beginning of stress

• 3 = Moderately soft

  • Noticeably flexible

  • Some drooping

  • Moderate stress

• 2 = Soft

  • Bends easily

  • Obvious drooping

  • Severe stress

• 1 = Limp, no resistance

  • Completely wilted

  • Hangs straight down

  • Critical stress

Recording: Turgidity: _____ (1-5)

3. Visual appearance:

Note all symptoms:

Color changes:

• Bright green (normal)

• Dull green

• Gray-green

• Yellow-green

• Brown edges

Leaf modifications:

• Flat (normal)

• Edges rolling inward (reduces surface area)

• Folded along midrib

• Twisted

• Curled

Overall posture:

• Upright

• Slightly drooping

• Moderately wilted

• Severely wilted

• Collapsed

4. Soil moisture check:

Squeeze test:

1. Dig down 2-3 inches

2. Grab handful of soil

3. Squeeze firmly

4. Classify:

   • Saturated - Water drips out

   • Wet - Forms ball, feels very moist

   • Moist - Forms ball, slightly damp

   • Dry - Crumbles, dusty feel

   • Very dry - Powder, no cohesion

Recording: Soil condition: _____

5. Environmental data:

• Time: _____

• Air temperature: _____ °F

• Humidity: High  /  Moderate  /  Low

• Wind: Calm  /  Light breeze  /  Windy

• Sun exposure: Full sun  /  Partial  /  Shade

6. Take photos every hour showing progression

Step 3: Water and Document Recovery

After wilting is clearly established:

Water thoroughly:

1. Add water slowly until:

   • Soil is saturated

   • Water drains from bottom (if potted)

   • Top 4-6 inches completely moist

2. Don't over-water (can damage stressed plant)

3. Place in shade if in full sun

4. Note time watering completed: _____

Measure every 15 minutes during recovery:

Record:

1. Time after watering: _____ minutes

2. Leaf angle:

• Current average: _____ °

• Compared to wilted angle: +_____ ° improvement

3. Turgidity:

• Rating (1-5): _____

• Change: +_____ points from wilted state

4. Visual improvement:

• No change yet

• Slight improvement (leaves lifting slightly)

• Moderate improvement (clear recovery)

• Nearly recovered

• Fully recovered

5. Percent recovered:

• Estimate: _____%

• 0% = still wilted

• 50% = halfway back to normal

• 100% = fully turgid again

6. Photos showing recovery

Recovery milestones:

First signs of recovery:

• When?: _____ minutes after watering

• What changed?: _____

50% recovery:

• When?: _____ minutes

• Appearance: _____

Full recovery:

• When?: _____ minutes (or hours)

• Total time: _____ minutes

Expected recovery times:

Fast recoverers (30-60 min):

• Herbaceous plants (beans, lettuce)

• Well-established root systems

• Mild-moderate stress

Moderate recoverers (1-3 hours):

• Woody plants

• Severe stress

• Damaged roots

Slow recoverers (3-12+ hours):

• Very severe stress

• Root damage

• May show lasting leaf damage

Experiment C: Light Stress Response

Plants adapted to one light level stressed when moved to different level

Step 1: Find Plants in Wrong Light Level

Option 1: Shade Plants in Full Sun

Look for:

• Shade-adapted plants recently moved to sun

• Understory plants after tree removal

• Houseplants placed outdoors

• Forest plants in open areas

Symptoms:

• Leaf bleaching (white/yellow spots)

• Brown scorch marks

• Wilting despite adequate water

• Leaf curling

Arizona examples:

• Ferns in full sun (dramatic stress!)

• Shade perennials in desert sun

• Tropicals moved outside

Option 2: Sun Plants in Deep Shade

Look for:

• Sun-loving plants shaded by new structure

• Garden plants under trees

• Desert plants brought indoors

Symptoms:

• Elongated growth (etiolation)

• Pale green color

• Thin leaves

• Drooping (less common)

Step 2: Document Responses

Measure over several hours (or days for slower responses)

Time-series measurements:

Every 1-2 hours (for acute stress):

1. Leaf position changes:

• Angle from horizontal: _____ °

• Orientation relative to light:

  • Perpendicular (maximum exposure)

  • Parallel (reduced exposure)

  • Angled away from light

2. Leaf modifications:

• Flat (normal)

• Drooping (reduce exposure)

• Rolling (reduce surface area)

• Folding along midrib

• Twisting

3. Color changes:

• Normal green

• Paling/yellowing

• Bleaching (white spots)

• Browning (scorch)

• Purple/red tints (protective anthocyanins)

Daily measurements (for chronic stress):

1. Leaf damage symptoms:

Sun damage (high light):

• Bleaching - White spots where chlorophyll destroyed

• Necrosis - Brown dead tissue

• Scorching - Crispy brown edges/tips

• Chlorosis - General yellowing

Shade damage (low light):

• Etiolation - Elongated, spindly growth

• Pale color - Light green, chlorotic

• Thin leaves - Less dense tissue

• Leaf drop - Lower leaves falling

2. Growth responses:

• New leaf characteristics (size, color, thickness)

• Stem elongation rate

• Overall vigor

3. Protective responses:

• Development of waxy coating (sun protection)

• Increased hair density (sun protection)

• Leaf angle adjustments

• Production of protective pigments

4. Quantify damage:

Percent damage:

• Count total leaves: _____

• Count damaged leaves: _____

• Percent damaged: (damaged ÷ total) × 100 = _____%

Severity of damage:

• Mild (<25% of leaf area affected)

• Moderate (25-50% affected)

• Severe (>50% affected)

Step 4: Data Tables

Heat/Water Stress Progression:

Recovery Data: 

Light Stress Documentation: