Transport in Plants
3. Factors Affecting Transpiration
Learning outcomes
- I can identify environmental factors that affect transpiration.
- I can explain how temperature influences transpiration rates.
- I can describe the effects of humidity on water loss.
- I can explain how wind speed affects transpiration.
- I can predict how changing environmental conditions influence plant water loss.
What Is Transpiration?
Transpiration is the loss of water vapour from the leaves and other aerial parts of a plant.
Most transpiration occurs through tiny openings in leaves called:
stomata
Water moves from the roots, through the plant, and eventually into the atmosphere.
A simplified pathway is:
soil → roots → xylem → leaves → stomata → atmosphere
Transpiration is therefore part of the continuous movement of water through a plant.
The Transpiration Stream
Water absorbed by the roots moves upward through tubes called:
xylem vessels
This movement of water through the plant is called the:
transpiration stream
The transpiration stream helps transport:
- water
- dissolved mineral ions
from the roots to the leaves and other parts of the plant.
How Does Water Leave a Leaf?
Water arrives in the leaf through the:
xylem
It moves into cells in the leaf.
Some water evaporates from moist cell surfaces into the air spaces inside the leaf.
Water vapour then diffuses through the stomata into the surrounding atmosphere.
The sequence is:
xylem → mesophyll cells → evaporation → leaf air spaces → stomata → atmosphere
Evaporation and Diffusion
Two important processes are involved in transpiration.
Evaporation
Liquid water changes into:
water vapour
Diffusion
Water vapour moves from an area of:
higher water-vapour concentration
to an area of:
lower water-vapour concentration
The air spaces inside a leaf are usually humid.
If the surrounding air is drier, water vapour diffuses out through the stomata.
Stomata
Stomata are microscopic pores in the epidermis of leaves.
A single pore is called a:
stoma
Each stoma is surrounded by:
guard cells
Guard cells can change shape and influence whether the stomatal pore is:
- open
- partly open
- closed
Stomata allow gases to move between the leaf and atmosphere.
Why Do Plants Have Stomata?
Plants need carbon dioxide for:
photosynthesis
Carbon dioxide enters through stomata.
However, when stomata are open, water vapour can also escape.
This creates an important biological trade-off:
open stomata → CO₂ enters more easily, but water can also be lost
Plants therefore have mechanisms that help regulate stomatal opening.
What Is Transpiration Rate?
Transpiration rate describes how quickly a plant loses water through transpiration.
For example, we could measure:
water lost per hour
or use another measurement that estimates the rate of water loss.
Transpiration rate is affected by environmental conditions.
Major factors include:
- temperature
- humidity
- wind speed
- light intensity
Temperature
In general:
higher temperature → higher transpiration rate
Why?
At higher temperatures, water molecules have greater average kinetic energy.
This generally increases the rate of:
evaporation
from moist surfaces inside the leaf.
More water vapour becomes available in the leaf air spaces and can diffuse out through open stomata.
Temperature and Particle Movement
At a higher temperature:
water molecules have greater average kinetic energy
↓
evaporation generally increases
↓
more water vapour enters leaf air spaces
↓
water vapour diffuses through stomata
↓
transpiration rate generally increases
This connects transpiration to the particle model.
Temperature Example
Imagine two identical plants.
Plant A is kept at:
20°C
Plant B is kept at:
30°C
All other conditions are kept approximately the same.
We would generally predict:
Plant B has the higher transpiration rate.
This is because the higher temperature promotes evaporation.
Extremely High Temperatures
The relationship is not always as simple as:
hotter = unlimited increase in transpiration
Under hot, dry conditions, a plant may begin losing water faster than its roots can replace it.
The plant may respond by closing some stomata.
Therefore, under severe water stress:
stomatal closure can reduce transpiration
This is an example of how biological systems can modify simple physical relationships.
Humidity
Humidity describes the amount of water vapour in the air.
Humidity strongly affects the concentration gradient for water vapour between:
inside the leaf
and:
the surrounding air
Low Humidity
Low humidity means the surrounding air is relatively dry.
Inside the leaf:
high water-vapour concentration
Outside the leaf:
lower water-vapour concentration
This creates a relatively steep concentration gradient.
Therefore:
low humidity → steeper water-vapour gradient → faster diffusion → higher transpiration
High Humidity
When humidity is high, the surrounding air already contains substantial water vapour.
The difference between the leaf and surrounding air becomes smaller.
Therefore:
high humidity → smaller water-vapour gradient → slower diffusion → lower transpiration
Comparing Humidity
Imagine two identical plants.
Plant A:
30% relative humidity
Plant B:
90% relative humidity
If other conditions are the same, Plant A would generally lose water more rapidly.
Why?
Because the surrounding air is much drier.
Humidity and Diffusion
Remember:
diffusion occurs down a concentration gradient
A larger difference in water-vapour concentration generally produces faster net diffusion.
Therefore:
dry air
creates a larger gradient.
humid air
creates a smaller gradient.
This is why humidity has such a strong influence on transpiration.
Wind Speed
Wind also affects transpiration.
In general:
higher wind speed → higher transpiration rate
To understand why, we need to consider the air immediately surrounding the leaf.
The Boundary Layer
As water vapour leaves a leaf, the air immediately next to the leaf becomes more humid.
This creates a thin region of relatively still, humid air called the:
boundary layer
The boundary layer reduces the water-vapour concentration gradient between the leaf and the surrounding air.
This can slow further water loss.
What Does Wind Do?
Moving air removes some of the humid air surrounding the leaf.
Therefore:
wind removes humid boundary layer
↓
drier air replaces it
↓
water-vapour concentration gradient increases
↓
diffusion increases
↓
transpiration increases
Still Air
In still conditions, humid air can accumulate around the leaf.
Therefore:
still air → thicker humid boundary layer → smaller gradient → slower transpiration
Windy Air
In windy conditions, the humid boundary layer is continually disturbed or removed.
Therefore:
windy air → thinner boundary layer → steeper gradient → faster transpiration
Wind Speed Example
Plant A is placed in still air.
Plant B is placed in front of a gently operating fan.
Everything else is kept constant.
We would generally predict:
Plant B loses water faster.
The fan removes humid air from around the leaves.
Very Strong Wind
Extremely strong wind can sometimes produce more complicated responses.
For example, strong wind may:
- cause stomata to close
- damage leaves
- increase water stress
Therefore, the simple relationship:
more wind → more transpiration
is most useful when comparing moderate environmental changes while other factors remain controlled.
Light Intensity
Although the main targets emphasize temperature, humidity, and wind, another important environmental factor is:
light intensity
In many plants:
greater light intensity → greater stomatal opening → increased transpiration
Stomata commonly open in light because carbon dioxide is required for photosynthesis.
Open stomata also allow more water vapour to escape.
Light and Transpiration
The general relationship is:
increased light
↓
stomata tend to open
↓
greater pathway for water vapour diffusion
↓
transpiration generally increases
At night, stomata of many plants partially or mostly close.
This generally reduces transpiration.
There are exceptions among plants with different photosynthetic adaptations.
The Four Major Factors
A useful summary is:
| Environmental Factor | Change | General Effect on Transpiration |
|---|---|---|
| Temperature | Increases | Increases |
| Humidity | Increases | Decreases |
| Wind speed | Increases | Increases |
| Light intensity | Increases | Usually increases |
These relationships describe general trends, assuming other conditions remain reasonably similar.
A Simple Memory Pattern
Conditions that tend to increase transpiration are:
Hot + Dry + Windy + Bright
Conditions that tend to decrease transpiration are:
Cool + Humid + Still + Dark
This is a useful starting point for predictions.
Predicting Transpiration
Consider a plant on a:
hot, dry, windy afternoon
Temperature:
high
Humidity:
low
Wind:
high
All three factors tend to:
increase transpiration
Therefore, we predict:
a high rate of water loss
Example 2: Cool and Humid
A plant is placed in:
cool, humid, still air
Temperature:
low → decreases evaporation
Humidity:
high → decreases the water-vapour gradient
Wind:
low → allows a humid boundary layer to remain
Prediction:
low transpiration rate
Example 3: Hot but Humid
Suppose conditions are:
high temperature + high humidity
Temperature tends to:
increase transpiration
Humidity tends to:
decrease transpiration
The factors are working in opposite directions.
Without more information, we cannot always determine exactly how large the overall change will be.
This is an important scientific idea:
multiple variables can influence the same process simultaneously.
Example 4: Cool but Windy
Conditions:
low temperature + high wind speed
Lower temperature tends to:
decrease transpiration
Wind tends to:
increase transpiration
Again, the factors act in opposite directions.
We should avoid assuming that one factor always determines the final result.
Environmental Factors Interact
In nature, temperature, humidity, wind, and light do not normally change independently.
For example, a sunny afternoon may involve:
- increased light
- increased temperature
- decreased relative humidity
- increased wind
Several factors may therefore increase transpiration simultaneously.
This can produce rapid water loss.
Why Does Transpiration Matter?
Transpiration is not simply wasted water.
It contributes to several important processes.
Water transport
Transpiration helps maintain movement of water through the xylem.
Mineral transport
Mineral ions dissolved in water are transported from roots toward other plant tissues.
Cooling
Evaporation of water from leaves can remove thermal energy.
Water supply
Water transported to leaves is needed for processes including photosynthesis.
Transpiration and Cooling
Evaporation requires energy.
When water evaporates from leaf surfaces, thermal energy is transferred.
This can help cool the leaf.
This is similar to the cooling effect of:
sweating in humans
Both involve evaporative cooling.
The Problem of Excessive Water Loss
Plants require water for:
- photosynthesis
- transport
- maintaining cell turgor
- biochemical reactions
- growth
If transpiration becomes too rapid:
water loss > water absorption
The plant may become water stressed.
Turgor Pressure
Plant cells contain large vacuoles.
When cells contain sufficient water, internal pressure helps keep tissues firm.
This is called:
turgor pressure
If too much water is lost:
cells lose water
↓
turgor decreases
↓
leaves and stems may wilt
Wilting
Wilting occurs when plant tissues lose enough turgor that leaves and stems become less rigid.
Wilting can reduce:
- leaf exposure to sunlight
- stomatal opening
- photosynthesis
Temporary wilting may be reversible if water becomes available.
Prolonged severe water shortage can damage tissues.
Plants Can Reduce Water Loss
Plants have several mechanisms and adaptations that reduce excessive transpiration.
These can include:
- closing stomata
- waxy cuticles
- reduced leaf area
- hairs on leaves
- sunken stomata
- rolled leaves
These features are particularly important in plants living in dry environments.
The Waxy Cuticle
Leaves are covered by a protective layer called the:
cuticle
The cuticle contains waxy substances that reduce water loss directly through the epidermis.
Plants adapted to dry environments often have particularly thick waxy cuticles.
Sunken Stomata
Some plants have stomata located in small depressions.
These are called:
sunken stomata
Water vapour can accumulate inside the depression.
This creates a more humid microenvironment.
Therefore:
sunken stomata → more humid trapped air → smaller concentration gradient → reduced transpiration
Leaf Hairs
Leaf hairs can trap relatively still air near the leaf surface.
This helps maintain a:
humid boundary layer
Therefore:
leaf hairs → reduced air movement → reduced water-vapour gradient → reduced transpiration
This is similar to the effect of still air.
Reduced Leaf Area
Some plants living in dry environments have very small leaves.
Others have leaves modified into:
spines
Smaller leaf area means:
less surface area for water loss
This helps reduce transpiration.
Desert Plant Adaptations
Plants adapted to dry conditions are called:
xerophytes
Possible adaptations include:
- thick waxy cuticles
- reduced leaves
- sunken stomata
- leaf hairs
- water-storage tissues
- extensive root systems
These adaptations help plants survive where water is limited.
Measuring Transpiration
Scientists can investigate transpiration using an instrument called a:
potometer
A potometer measures the rate at which a plant shoot takes up water.
Water uptake is used as an estimate of transpiration rate.
It is not exactly the same as directly measuring water loss because some absorbed water is used by the plant.
Bubble Potometer
A common potometer contains:
- leafy shoot
- water-filled tubing
- capillary tube
- air bubble
- scale
As the shoot takes up water, the air bubble moves along the capillary tube.
Faster bubble movement indicates:
faster water uptake
which usually indicates:
higher transpiration rate
Calculating Water Uptake
Suppose an air bubble moves:
24 mm in 6 minutes
Average movement rate:
24 ÷ 6 = 4 mm/min
If another condition produces:
8 mm/min
the water uptake is faster under the second condition.
This can be used to compare environmental factors.
Investigating Temperature
To investigate temperature:
Independent variable:
temperature
Dependent variable:
rate of water uptake
Control variables might include:
- plant species
- leaf area
- light intensity
- wind speed
- humidity
- duration of experiment
Only one factor should intentionally change.
Investigating Wind Speed
A fan can be used to investigate the effect of air movement.
For example:
Trial 1:
fan off
Trial 2:
fan at low speed
Trial 3:
fan at higher speed
Prediction:
As wind speed increases, water uptake will generally increase because humid air is removed from around the leaves.
Investigating Humidity
Humidity can be changed by altering the moisture in the air surrounding the plant.
Prediction:
higher humidity → lower transpiration rate
because the water-vapour concentration gradient between the leaf and atmosphere becomes smaller.
Investigating Light Intensity
A lamp can be used to investigate light intensity.
However, there is an experimental problem.
A lamp can also:
heat the plant
Therefore, changing light intensity may accidentally change:
temperature
A good investigation must control this variable.
Designing a Fair Test
When investigating one factor:
change one variable
measure one outcome
keep other important factors constant
For example, when investigating wind speed:
Change:
wind speed
Measure:
water uptake
Keep constant:
- temperature
- humidity
- light intensity
- plant species
- approximate leaf area
This improves the validity of the investigation.
Reliability
A single measurement may be affected by random variation.
Scientists can improve reliability by:
- repeating measurements
- conducting multiple trials
- calculating a mean
- identifying unusual results
For example:
Trial 1 = 4.2 mm/min
Trial 2 = 4.4 mm/min
Trial 3 = 4.3 mm/min
Mean:
(4.2 + 4.4 + 4.3) ÷ 3
= 4.3 mm/min
Graphing Transpiration Data
Experimental results can be represented using graphs.
For example:
x-axis: temperature
y-axis: transpiration rate
A graph allows us to:
- identify trends
- compare conditions
- detect anomalies
- make predictions
Predicting a Temperature Graph
Within a normal physiological range, we would generally expect:
temperature increases → transpiration rate increases
The graph would therefore show an overall:
positive relationship
However, at extreme temperatures, stomatal closure or plant stress may alter the pattern.
Predicting a Humidity Graph
As humidity increases:
transpiration rate generally decreases
Therefore, we expect an overall:
negative relationship
High humidity reduces the water-vapour concentration gradient.
Predicting a Wind-Speed Graph
As moderate wind speed increases:
transpiration rate generally increases
The graph should show an overall positive relationship.
Wind removes humid air from the boundary layer surrounding the leaf.
Applying the Ideas: Greenhouses
Greenhouse growers can control environmental conditions such as:
- temperature
- humidity
- ventilation
- light
- irrigation
If a greenhouse becomes:
very hot + dry
plants may lose water rapidly.
Increasing humidity or reducing excessive temperature can reduce water loss.
Understanding transpiration therefore has practical importance in agriculture.
Applying the Ideas: Irrigation
Farmers need to consider environmental conditions when deciding how much water crops require.
Hot, dry, windy conditions can increase water loss.
Therefore, crops may require more water during periods of high transpiration.
This connects plant biology to:
- agriculture
- weather
- water management
- food production
Applying the Ideas: Indoor Plants
Imagine placing a houseplant beside:
a heater
The surrounding air may become:
- warmer
- drier
Both conditions can increase transpiration.
Therefore, the plant may require more frequent watering than the same plant in cooler, more humid conditions.
Applying the Ideas: Windy Conditions
A plant on an exposed balcony experiences strong air movement.
Wind removes the humid boundary layer around the leaves.
Therefore:
wind → increased concentration gradient → increased transpiration
The plant may lose water more rapidly than a similar plant in a sheltered location.
Connecting Transpiration to Diffusion
Transpiration provides an excellent biological example of diffusion.
Inside the leaf:
high water-vapour concentration
Outside the leaf:
usually lower water-vapour concentration
Therefore:
water vapour diffuses out
The size of this gradient is influenced by:
humidity and wind
Connecting Transpiration to Evaporation
Temperature affects transpiration partly because it affects:
evaporation
Higher temperatures increase the average kinetic energy of water molecules.
This increases the likelihood that molecules escape from liquid water into the gas phase.
Therefore:
temperature → evaporation → transpiration
Connecting Transpiration to Osmosis
Water entering plant roots moves through tissues partly by:
osmosis
Water then enters the xylem and travels upward.
Therefore, plant water transport connects several processes:
osmosis → xylem transport → evaporation → diffusion
Connecting Transpiration to Photosynthesis
Plants need carbon dioxide for photosynthesis.
Carbon dioxide enters through:
stomata
But open stomata also allow:
water vapour to escape
Therefore, plants face a trade-off:
CO₂ uptake ↔ water conservation
This is one of the major challenges faced by land plants.
Common Misconception: Plants Lose Water Only When It Is Hot
Plants can transpire under many environmental conditions.
Temperature affects the rate of transpiration, but it is not the only factor.
Transpiration is also influenced by:
- humidity
- wind
- light
- stomatal opening
- leaf structure
Common Misconception: High Humidity Increases Transpiration
High humidity generally:
decreases transpiration
because the air surrounding the leaf already contains substantial water vapour.
This reduces the concentration gradient.
Remember:
Dry air → faster water loss
Humid air → slower water loss
Common Misconception: Wind Stops Water Loss
Wind generally increases transpiration because it removes the humid boundary layer around the leaf.
Therefore:
wind → drier air beside leaf → steeper gradient → increased water loss
Common Misconception: Transpiration Is Completely Harmful
Water loss can become harmful if excessive.
However, transpiration also contributes to:
- water movement
- mineral transport
- leaf cooling
Therefore, transpiration is a normal and important plant process.
Common Misconception: Potometers Directly Measure Transpiration
A potometer measures:
water uptake
It does not directly measure water leaving the leaf.
Because most water absorbed by a cut shoot is eventually lost through transpiration, water uptake provides a useful estimate.
But scientifically:
water uptake ≠ exactly the same as water loss
Challenge: Predict the Highest Transpiration
Four identical plants are placed under different conditions.
Plant A: cool, humid, still
Plant B: warm, humid, still
Plant C: warm, dry, windy
Plant D: cool, dry, still
Which would probably have the highest transpiration rate?
Plant C
because it combines:
- higher temperature
- low humidity
- high wind speed
All three factors generally increase transpiration.
Challenge: Predict the Lowest Transpiration
Which plant would probably have the lowest transpiration rate?
Plant A
because:
- cool temperature reduces evaporation
- high humidity reduces the concentration gradient
- still air maintains the humid boundary layer
Did You Know?
A mature tree can move a surprisingly large amount of water from the soil into the atmosphere through transpiration.
Across entire forests, transpiration contributes significantly to the movement of water into the atmosphere.
This means plant transpiration is not only important for individual plants.
It also contributes to the:
water cycle
Key Terms
- Transpiration: Loss of water vapour from the aerial parts of a plant, mainly through stomata.
- Transpiration rate: Rate at which water is lost through transpiration.
- Transpiration stream: Movement of water through a plant from roots through xylem toward the leaves.
- Xylem: Vascular tissue that transports water and mineral ions.
- Stoma: Microscopic pore in the epidermis of a leaf.
- Stomata: Plural of stoma.
- Guard cells: Specialized cells that control stomatal opening.
- Evaporation: Change of liquid water into water vapour.
- Diffusion: Net movement of particles from higher concentration toward lower concentration.
- Humidity: Amount of water vapour present in the air.
- Relative humidity: Amount of water vapour present relative to the maximum possible at that temperature.
- Concentration gradient: Difference in concentration between two regions.
- Boundary layer: Thin layer of relatively still air surrounding a surface such as a leaf.
- Turgor pressure: Pressure of cell contents against the cell wall resulting from water inside the cell.
- Wilting: Loss of rigidity in plant tissues due to reduced turgor.
- Cuticle: Waxy protective layer covering the epidermis of leaves and young stems.
- Xerophyte: Plant adapted to dry environments.
- Potometer: Apparatus used to estimate transpiration by measuring water uptake.
- Independent variable: Variable deliberately changed in an investigation.
- Dependent variable: Variable measured in response to the independent variable.
- Control variable: Variable kept constant to make an investigation fair.
Key Relationships
Temperature:
↑ temperature → ↑ evaporation → generally ↑ transpiration
Humidity:
↑ humidity → ↓ water-vapour concentration gradient → ↓ transpiration
Wind:
↑ wind → boundary layer removed → ↑ concentration gradient → generally ↑ transpiration
Light:
↑ light → stomata tend to open → generally ↑ transpiration
Dry conditions:
low humidity → steep gradient → rapid diffusion of water vapour
Water stress:
water loss > water uptake → reduced turgor → wilting
Plant water pathway:
soil → roots → xylem → leaves → evaporation → stomata → atmosphere
Key Takeaways
- Transpiration is the loss of water vapour from the aerial parts of a plant.
- Most transpiration occurs through stomata in leaves.
- Water moves from the soil through the roots and xylem to the leaves.
- Water evaporates from moist surfaces inside the leaf.
- Water vapour then diffuses through stomata into the atmosphere.
- Transpiration rate is affected by environmental conditions.
- Temperature, humidity, wind speed, and light intensity are important environmental factors.
- Higher temperatures generally increase evaporation and therefore increase transpiration.
- High humidity generally decreases transpiration.
- Low humidity creates a larger water-vapour concentration gradient and increases water loss.
- Wind generally increases transpiration by removing the humid boundary layer surrounding a leaf.
- Still air allows humid air to remain near the leaf and reduces transpiration.
- Increased light often increases transpiration because stomata tend to open.
- A useful general rule is hot + dry + windy + bright = higher transpiration.
- Cool + humid + still + dark = lower transpiration.
- Environmental factors can work together or oppose one another.
- Extreme environmental conditions can cause stomatal closure, making relationships more complex.
- Transpiration contributes to water and mineral transport through the plant.
- Evaporation during transpiration can help cool leaves.
- Excessive water loss can cause cells to lose turgor and plants to wilt.
- Plants can reduce water loss using stomatal closure and structural adaptations.
- Xerophytes may have thick cuticles, sunken stomata, leaf hairs, or reduced leaves.
- A potometer measures water uptake and can be used to estimate transpiration rate.
- Experiments investigating transpiration should change one environmental factor while controlling the others.
- Repeated measurements improve the reliability of experimental results.
- Transpiration connects several biological processes, including osmosis, xylem transport, evaporation, diffusion, gas exchange, and photosynthesis.
- The central idea is: environmental conditions change evaporation and diffusion, which changes how quickly a plant loses water.