Photosynthetic Adaptations in Natural Light Gradients

Lab 5: Photosynthetic Adaptations in Natural Light Gradients

Objectives

By the end of this laboratory exercise, you will understand:

• How light availability varies in different habitats throughout the day

• How plant leaves and structures differ between sunny and shady environments

• Why these differences help plants survive and photosynthesize effectively

• How desert plants balance photosynthesis with water conservation

• The relationship between leaf characteristics and environmental adaptation

Background

The Challenge: Plants need light for photosynthesis, but too much light can damage cells and cause water loss. Plants in different environments solve this problem in different ways.

Key Concepts:

• Sun plants (heliophytes): Adapted to high light—thick leaves, protective coatings, multiple leaf layers

• Shade plants (sciophytes): Adapted to low light—thin leaves, more chlorophyll, horizontal orientation

Materials

Required:

• Smartphone with:

  • Light meter app

  • Camera

  • Compass app (built into most phones)

• Measuring tape (at least 10 feet)

• Thermometer (digital preferred for fast readings)

• Notebook and pencils

• Water bottle (stay hydrated!)

Study organisms:

• 30 plants total:

  • 10 from deep shade

  • 10 from partial shade

  • 10 from full sun

Part 1: Map the Light Environment

Goal: Understand how dramatically light conditions vary across short distances and throughout the day.

Step 1: Set Up Your Three Study Stations

Choose locations that form a gradient from darkest to brightest.

Station 1: Deep Shade

Where to find:

• Forest interior, under dense tree canopy

• North side of buildings (never gets direct sun)

• Dense shrub thickets

• Should feel noticeably darker and cooler

What to expect:

• Light levels: 500-5,000 lux

• Temperature: 5-15°F cooler than full sun

• Higher humidity (less evaporation)

Station 2: Partial Shade

Where to find:

• Forest edge

• Gap in canopy

• Under scattered trees or large shrubs

• East-facing walls (sun in morning only)

• Gets some direct sunlight but also shade during the day

What to expect:

• Light levels: 5,000-25,000 lux

• Temperature: Intermediate

• Moderate humidity

Station 3: Full Sun

Where to find:

• Open field, meadow, or clearing

• South-facing slopes

• No overhead obstacles blocking sunlight

• Parking lot edges (with plants!)

What to expect:

• Light levels: 30,000-100,000+ lux

• Temperature: Hottest location

• Lowest humidity (high evaporation)

Important tip: Stations should be close enough that you can visit all three during each time period (within 15-20 minutes of each other). This ensures your measurements are comparable.

Step 2: Take Measurements at Each Station

Why 5 replicates? Light can vary a lot even within a small area due to leaf movement, clouds, etc. Taking 5 readings and averaging gives you more reliable data.

At each of your three stations, take these measurements:

1. Light Intensity

What is lux? A unit measuring how much light hits a surface. Full sunlight = 50,000-100,000 lux. Indoor lighting = 300-500 lux.

How to measure:

1. Open your light meter app

2. Hold phone at plant height (roughly 1-3 feet above ground)

3. Point sensor upward (toward sky)

4. Record reading in lux or foot-candles

5. Take 5 readings from slightly different spots (spread out over ~10 feet)

6. Calculate average: (reading 1 + reading 2 + reading 3 + reading 4 + reading 5) ÷ 5

Recording format:

• Reading 1: _____ lux

• Reading 2: _____ lux

• Reading 3: _____ lux

• Reading 4: _____ lux

• Reading 5: _____ lux

• Average: _____ lux

Troubleshooting:

• If readings vary wildly (>50%), you may be in patchy shade—move to more uniform area

• Avoid shadows from your body

• Wait for consistent reading (5-10 seconds)

2. Air Temperature

Why at plant height? Temperature near the ground can be very different from head height, especially in deserts where ground heats up intensely.

How to measure:

1. Hold thermometer at plant height (where leaves are)

2. Keep thermometer in shade (not in direct sun—sun heats the thermometer itself!)

3. Wait 2 minutes for accurate reading (thermometers need time to equilibrate)

4. Record in °F or °C

Recording:

• Air temperature: _____ °F (_____ °C)

Arizona note: In summer, ground-level temperatures in full sun can exceed 140°F! This is why timing matters.

3. Relative Humidity

What is relative humidity? The percentage of water vapor in the air compared to the maximum it could hold at that temperature. High humidity = muggy. Low humidity = dry (use a weather app).

4. Canopy Openness

What does this measure? How much sky is visible vs blocked by vegetation. This affects not just light, but also temperature and moisture.

How to estimate:

1. Stand in the center of your station

2. Look straight up

3. Estimate what percentage of the sky is visible (not blocked by leaves, branches, rocks, buildings)

Scale:

• 0% = Completely covered (like under a dense tree)

• 25% = Mostly covered, small gaps

• 50% = Half sky visible, half blocked

• 75% = Mostly open, scattered shade

• 100% = Completely open (no overhead obstacles)

Recording:

• Canopy openness: _____ %

Tip: Take a photo looking straight up—this helps you estimate more accurately later.

5. Distance to Nearest Shade Source

Why does this matter? Shows how isolated or protected the site is. Plants far from shade sources experience different wind patterns and light quality.

How to measure:

1. Stand in the center of your station

2. Look around for the nearest tree, building, large shrub, or tall plant that creates shade

3. Measure the distance with tape measure or estimate by pacing

4. Record in meters or feet

Recording:

• Distance to nearest shade source: _____ m (_____ ft)

For deep shade: This might be 0 m (you're under the shade source) For full sun: Might be 10-50+ meters away from nearest shade

Step 3: Repeat at Three Times of Day

Why different times? Light intensity and angle change dramatically throughout the day. Some plants get morning sun but afternoon shade, or vice versa.

Visit all three stations during:

Time 1: Early Morning (8-9 AM)

• Sun is low in the east

• Temperatures relatively cool

• Long shadows

• Arizona timing: In summer, consider 6-7 AM to avoid heat

Time 2: Midday (12-1 PM)

• Sun directly overhead (highest intensity)

• Shortest shadows

• Hottest temperatures

• Peak light levels

Time 3: Late Afternoon (4-5 PM)

• Sun low in the west

• Temperatures starting to drop

• Long shadows again

• Arizona note: Still very hot in summer!

Step 4: Organize Your Data

Create a master data table like this:

After collecting data, look for patterns:

• Which station had the highest light levels? Lowest?

• How much did light change throughout the day at each station?

• Which station had the most stable conditions?

• Which had the biggest temperature difference?

Example interpretation: "Full sun station light increased from 45,000 lux at 8 AM to 98,000 lux at midday—more than doubled! Deep shade stayed relatively constant at 2,000-3,500 lux all day."

Part 2: Measure Plant Characteristics

Goal: Discover how plants adapt their physical traits to match their light environment. You'll find that sun plants and shade plants look dramatically different!

The hypothesis: Plants in high light will have smaller, thicker leaves with more protection. Plants in low light will have larger, thinner leaves to capture more of the limited light.

Step 1: Select Your Study Plants

At each station, choose 10 individual plants to measure (30 plants total).

Selection Tips:

Do:

• Include multiple species if available (more diversity = more interesting patterns)

• Choose healthy, mature plants (not tiny seedlings or damaged plants)

• Space out selections across the station area (don't just pick plants near each other)

• Look for plants actually adapted to that light level (not recently transplanted)

Don't:

• Pick diseased or damaged plants

• Choose plants that are clearly stressed (wilting, yellowing)

• Measure the same species at all stations (variety is good!)

Labeling system:

• S1-P1 = Station 1, Plant 1

• S1-P2 = Station 1, Plant 2

• S2-P1 = Station 2, Plant 1

• Continue through S3-P10

Take a photo of each labeled plant for reference later.

Step 2: Measure Leaf Traits

Why leaves? Leaves are the photosynthesis factories. Their size, thickness, color, and angle all affect how efficiently they capture light.

For each plant, examine 3 sun-exposed leaves (leaves getting the most light on that plant).

A. Leaf Area

What it tells us: Larger leaves capture more light (good in shade). Smaller leaves reduce water loss and overheating (good in sun).

Method 1: Photo analysis (most accurate)

1. Place leaf on white paper with ruler for scale

2. Take photo from directly above

3. Use ImageJ software (free) to calculate area:

   • Tutorial: https://www.youtube.com/watch?v=kXFShlKSXbM

   • ImageJ traces leaf outline and calculates area

4. Record in cm²

5. Measure 3 leaves and calculate average

Method 2: Grid method (no computer needed)

1. Trace leaf outline on graph paper (1 cm squares)

2. Count full squares inside leaf

3. Estimate partial squares (2 halves = 1 full)

4. Total squares = area in cm²

Recording:

• Leaf 1 area: _____ cm²

• Leaf 2 area: _____ cm²

• Leaf 3 area: _____ cm²

• Average leaf area: _____ cm²

Expected pattern:

• Shade plants: 20-100+ cm² (large)

• Sun plants: 1-20 cm² (small)

• Desert sun plants: Often <5 cm² (very small!)

B. Leaf Thickness

What it tells us: Thick leaves have more protective layers and water storage (good in sun). Thin leaves allow more light penetration (good in shade).

Method 1: Feel test (quick)

1. Gently pinch leaf between thumb and forefinger

2. Compare thickness by feel across all your plants

3. Rate as: Very thin / Thin / Medium / Thick / Very thick

Method 2: Measurement (more precise)

1. Stack 10 similar leaves flat

2. Measure total thickness with calipers or ruler

3. Divide by 10 to get average single-leaf thickness

4. Record in mm

Recording:

• Leaf thickness rating: _____

• OR exact measurement: _____ mm

Expected pattern:

• Shade leaves: Thin (0.1-0.3 mm), almost papery

• Sun leaves: Thick (0.5-2+ mm), leathery or succulent

Arizona adaptation: Desert sun plants often have extremely thick leaves (succulence) for water storage—think agave, aloe.

C. Leaf Color Darkness

What it tells us: Darker green = more chlorophyll. Shade plants need extra chlorophyll to capture limited light. Sun plants don't need as much and may have protective pigments.

How to rate:

Use this scale consistently for ALL leaves:

• 1 = Very light green, yellowish-green

• 2 = Light green

• 3 = Medium green (typical grass color)

• 4 = Dark green

• 5 = Very dark green, almost blue-green

Tips for consistency:

• Compare all leaves to the same reference (like grass)

• Take photos on white paper in consistent lighting

• Rate all leaves in one session if possible

Recording:

• Leaf color darkness: _____ (1-5 scale)

Expected pattern:

• Shade plants: 4-5 (very dark green)

• Sun plants: 2-3 (lighter green or even gray-green in desert)

D. Chlorophyll Content (Photo Method)

What it tells us: A more objective measure of leaf greenness than visual rating.

How to measure using ImageJ:

1. Place leaf flat on white paper

2. Take photo in consistent lighting (cloudy day is ideal—no shadows)

3. Open photo in ImageJ

4. Tutorial for measuring color intensity: https://www.youtube.com/watch?v=lih0YEXJXMc

5. ImageJ will give you a "green channel intensity" value (0-255)

6. Higher values = more chlorophyll

Recording:

• Green intensity value: _____ (0-255 scale)

Expected pattern:

• Shade plants: 100-200 (high chlorophyll)

• Sun plants: 50-120 (lower chlorophyll, may have protective waxes or hairs that reduce greenness)

Alternative without ImageJ:

• Just use your visual color rating from section C

E. Leaf Angle

What it tells us: Horizontal leaves (0°) maximize light capture. Vertical or angled leaves (45-90°) reduce midday sun exposure and overheating.

How to measure:

Using a protractor or smartphone angle app:

• 0° = Leaf horizontal (parallel to ground)

• 45° = Leaf at 45° angle

• 90° = Leaf vertical (perpendicular to ground, pointing straight up)

Quick estimation method:

• Horizontal: flat like a table

• Slightly angled: 30-45°

• Steeply angled: 60-80°

• Vertical: 90° (standing straight up)

Recording:

• Leaf angle: _____ degrees

Measure 3 leaves and average:

• Leaf 1: _____ °

• Leaf 2: _____ °

• Leaf 3: _____ °

• Average: _____ °

Expected pattern:

• Shade plants: 0-30° (horizontal to capture maximum light)

• Sun plants: 45-90° (angled or vertical to avoid excess midday sun)

Arizona example: Desert agave leaves often angle upward at 60-80° to shed intense sun and funnel rain to roots.

F. Leaf Layers

What it tells us: Multiple overlapping leaf layers help shade plants capture light at different levels. Sun plants may have fewer layers to avoid self-shading.

How to count:

1. Look at plant from the side

2. Count distinct horizontal layers of leaves

3. Example: A single-stemmed plant with all leaves at the same height = 1 layer

4. Example: A branching plant with leaves at ground level, mid-height, and top = 3 layers

Recording:

• Number of leaf layers: _____

Expected pattern:

• Shade understory plants: 1-2 layers (low light can't support many layers)

• Sun plants: 3-5+ layers (abundant light allows complex structure)

Step 3: Measure Whole Plant Traits

Now step back and look at the entire plant architecture. How is the whole organism adapted to its environment?

A. Plant Height

What it tells us: Tall plants may reach for light above competitors. Short plants may be conserving resources or avoiding wind/herbivores.

How to measure:

1. Measure from ground level to tallest point (tip of highest leaf or flower)

2. Use measuring tape held vertically

3. Record in cm or inches

Recording:

• Plant height: _____ cm (_____ inches)

Expected pattern:

• Shade plants: Often tall and spindly (reaching for light)

• Sun plants: More variable (desert plants often low-growing to avoid wind and heat)

B. Number of Leaves

What it tells us: More leaves = more photosynthetic surface area, but also more water loss and resource investment.

How to count:

For small plants (<50 leaves):

• Count directly

For large plants (50-100 leaves):

• Count one representative branch

• Count total branches

• Multiply: (leaves per branch) × (number of branches)

For very large plants (100+ leaves):

• Count leaves in one 30 cm × 30 cm section

• Estimate total plant coverage area

• Extrapolate total

Recording:

• Number of leaves: _____ (or estimated _______)

C. Total Estimated Leaf Area

What it tells us: The total photosynthetic factory size. Some plants achieve large total area with many small leaves, others with few large leaves.

How to calculate:

• Total leaf area = (average single leaf area) × (number of leaves)

• Example: 25 cm² average × 40 leaves = 1,000 cm² total

Recording:

• Total estimated leaf area: _____ cm²

Expected pattern:

• Shade plants: High total area (need to capture limited light)

• Sun plants: Variable (desert plants often low total area to conserve water)

D. Branching Pattern

What it tells us: Open branching allows light penetration. Compact branching creates self-shading but protects inner leaves.

How to classify:

Compact:

• Branches close together

• Dense, bushy appearance

• Little space between branches

• Inner leaves well-shaded

Open:

• Branches spread far apart

• Sparse, airy appearance

• Lots of space between branches

• Most leaves exposed to light

Recording:

• Branching pattern:  Compact / Open / Intermediate

Expected pattern:

• Shade plants: Often open (don't want self-shading)

• Sun plants: Often compact (outer leaves protect inner leaves)

E. Stem Color

What it tells us: Green stems photosynthesize too! Important for desert plants that drop leaves during drought.

How to classify:

Green:

• Actively photosynthesizing

• Contains chlorophyll

• Common in young growth

Brown/Woody:

• Not photosynthesizing

• Protective bark layer

• Mature, older stems

Recording:

• Stem color:  Green / Brown-woody / Mix of both

Arizona adaptation: Many desert plants have green stems (palo verde = "green stick," ocotillo) to photosynthesize when leafless!

Step 4: Create Your Data Table

Organize all measurements for your 30 plants:

Part 3: Estimate Photosynthetic Activity

Goal: Use a simple experiment to estimate how actively plants from different environments are photosynthesizing and transpiring.

The connection: Photosynthesis and transpiration (water loss) both happen through stomata (tiny pores on leaves). When stomata open to take in CO₂ for photosynthesis, water escapes. More transpiration = more active photosynthesis (usually).

The Transpiration Bag Test

What we're measuring: Water vapor released by leaves accumulates as condensation inside sealed bags. More condensation = more transpiration = more active photosynthesis.

Step 1: Collect Leaves

What you need:

• 1 healthy leaf from a plant in each light environment

• Choose similar-sized leaves if possible

• Keep them fresh (place in plastic bag during transport)

• Label each leaf with its origin

Collection:

• Deep shade leaf: Species: _____

• Partial shade leaf: Species: _____

• Full sun leaf: Species: _____

Important: Cut leaves in early morning when fully hydrated for best results.

Step 2: Set Up the Experiment

We'll test each leaf type in BOTH sunny and shady conditions to see how they respond to different environments.

Materials needed:

• 6 resealable plastic bags (quart or gallon size)

• Paper towels

• Water

• Tape for labels

• Marker

For Sunny Conditions:

1. Take 3 plastic bags

2. Label each bag with:

   • Leaf source (deep shade plant / partial shade plant / full sun plant)

   • "SUNNY TREATMENT"

   • Date and time

3. Add a small piece of damp paper towel to each bag (NOT soaking wet—just moist)

   • Why? Maintains humidity so leaf doesn't dry out immediately

4. Place one leaf in each bag

5. Seal bag with minimal extra air inside

   • Remove most air by pressing gently before sealing

6. Place all bags in direct sunlight for 30 minutes

   • Lay flat or stand up against something

   • Make sure all bags get equal sun exposure

For Shady Conditions:

1. Repeat the same process with 3 more bags

2. Use fresh leaves from the same plants (or different individuals of same species)

3. Label bags with "SHADY TREATMENT"

4. Place these bags in deep shade instead of sun

5. Also wait 30 minutes

Setup summary:

• 6 bags total:

  • 3 in sun (one leaf from each environment)

  • 3 in shade (one leaf from each environment)

Step 3: Observe Results

After 30 minutes, examine each bag carefully WITHOUT opening it.

What to look for:

• Water droplets on inside of bag

• Fog/mist obscuring the view

• Leaf surface (is it glistening with moisture?)

Count and describe:

• Small droplets: How many? (estimate if too many to count)

• Large droplets: How many?

• Fog level: None / Light / Moderate / Heavy

• Where is water concentrated? (Top of bag / All over / Bottom)

Rate condensation level:

• None: Bag still clear, no visible moisture

• Light: Few small droplets, mostly clear

• Moderate: Many droplets, some fog, can still see leaf clearly

• Heavy: Bag very foggy, large droplets, hard to see leaf

Take photos of each bag before opening!

Step 4: Record Your Observations

Create a data table:

Additional observations to note:

• Did any leaves wilt or change color?

• Which combination produced the most condensation?

• Any unexpected results?

Step 5: Interpret Your Results

Expected patterns:

Question 1: Which leaf source produced the most transpiration?

• Prediction: Full sun plants have more stomata and higher photosynthetic rates → more transpiration

• BUT: Full sun plants also have adaptations to REDUCE water loss (thick cuticles, sunken stomata)

• Desert twist: Sun plants may show LESS transpiration due to water-conserving adaptations!

Question 2: How did bag location affect transpiration?

• Sunny bags: Higher temperature and more light stimulate photosynthesis → more transpiration

• Shady bags: Lower activity → less transpiration

• But: Shade-adapted leaves in sun may stress and close stomata (less transpiration than expected)

Question 3: What's the best match?

• Hypothesis: Each plant type transpires most actively in its native environment

• Full sun plant in sunny bag: High transpiration (comfortable environment)

• Shade plant in sunny bag: May be lower (stressed, stomata closing)

• Sun plant in shady bag: Lower (not enough light to justify water loss)

Your interpretation:

Based on your data:

1. Which plant-environment combination showed the highest transpiration? _____

2. Which showed the lowest? _____

3. Did your results support the hypothesis that plants work best in their native light environment? _____

4. Any surprises or exceptions? _____