Factors Affecting Transpiration
A single large tree can lose hundreds of litres of water on a hot, dry, windy day, and almost none on a cool, still, humid one. The plant has not changed; the weather has. Transpiration is the engine that pulls water up the xylem, but how fast that engine runs is set by four environmental factors acting on the leaf surface. This lesson explains how light, temperature, wind and humidity each speed up or slow down water loss.
Orient yourself
Predict, scan the goals and key terms, and clear the misconception this lesson exists to fix.
Before You Read
warm-upImagine drying two identical wet towels: one is hung in a hot, breezy, dry garage; the other in a cool, still, steamy bathroom.
Before reading: predict which towel dries faster, and list the conditions that speed up evaporation. Then predict whether the same conditions would speed up water loss from a leaf.
Learning Intentions
goalsKnow
- What transpiration is and where it happens on the leaf
- The four environmental factors: light, temperature, wind, humidity
- The direction each factor pushes the rate of transpiration
Understand
- Why light increases transpiration by opening the stomata
- Why wind and high temperature steepen the water-vapour gradient
- Why high humidity slows transpiration by flattening that gradient
Can Do
- Explain how each of the four factors affects transpiration rate
- Predict the effect of a change in conditions on water loss
- Justify each prediction using the water-vapour gradient or stomata
Scan these before reading
vocabMisconceptions To Fix
watch outOdd one out: Three of these statements about transpiration are correct. Which one is wrong?
Light and Temperature
Two factors that increase the rate by opening stomata and energising molecules
Core Content
Light and Temperature
+5 XPTwo factors that increase the rate by opening stomata and energising molecules
Transpiration is fastest in bright, warm conditions because light opens the doorways out of the leaf and heat gives the water molecules more energy to escape.
Light: in the light, guard cells take up water and open the stomata so that carbon dioxide can enter for photosynthesis. With more stomata open, more water vapour can diffuse out, so increasing light intensity increases the rate of transpiration, mainly because it opens the stomata. In the dark, most stomata close and transpiration falls sharply.
Temperature: heat gives water molecules more kinetic energy, so they evaporate from the moist leaf surfaces faster and the warm air can also hold more vapour. Increasing temperature increases the rate of transpiration because evaporation speeds up and the water-vapour gradient out of the leaf becomes steeper.
Pause, copy the highlighted effects of light and temperature into your book before moving on.
Light increases transpiration mainly because it causes the leaf's _____ to open.
Wind and Humidity
Two factors that act on the water-vapour gradient at the leaf surface
Wind and Humidity
+5 XPTwo factors that act on the water-vapour gradient at the leaf surface
We just saw light and temperature raise the rate. That raises a question: what about the air around the leaf, does moving it or moistening it matter? This card answers it → both change the steepness of the gradient that water vapour diffuses down.
Whether water vapour keeps diffusing out depends on how dry the air just outside the leaf is, wind keeps it dry, humidity makes it moist.
Wind: still air lets a layer of humid air (the boundary layer) build up against the leaf, flattening the gradient. Wind blows this layer away, replacing it with drier air. Increasing wind speed increases the rate of transpiration because it removes the humid boundary layer, keeping the water-vapour gradient steep. (Extreme wind can cause stomata to close, eventually reducing the rate.)
Humidity: the more water vapour already in the surrounding air, the smaller the difference between inside and outside the leaf. Increasing humidity decreases the rate of transpiration because it reduces the water-vapour gradient between the leaf and the air.
Text description
- Increase the rate: more light (opens stomata); higher temperature (faster evaporation); more wind (removes the humid boundary layer).
- Decrease the rate: higher humidity (flattens the water-vapour gradient).
- Darkness and high humidity both slow water loss.
Pause, copy the highlighted effects of wind and humidity, and the diagram's two columns, into your book.
Increasing the humidity of the surrounding air increases the rate of transpiration.
Wind increases transpiration by removing the humid boundary layer of air at the leaf surface.
In the light, open stomata allow more water vapour to diffuse out, raising transpiration.
Predict the Conditions
Do the activity in your book, then check what you have understood.
Activity 1
Predict the Conditions
Pattern B, Predict and justify
In your book, complete a four-row table headed Factor, Change, Effect on rate, Why. For each of light, temperature, wind and humidity, state what happens to the transpiration rate when the factor increases, and give a one-line reason. Then answer: a gardener wants to keep cut flowers fresh for longer, which two conditions should they create, and why?
Match each factor to the mechanism through which it acts:
- Light
- Temperature
- Wind
- Humidity
- Strips away the humid boundary layer, keeping the gradient steep
- Opens the stomata, so more vapour can leave
- Flattens the vapour gradient, so less vapour diffuses out
- Speeds evaporation and lets the air hold more vapour
Putting the Four Factors Together
One underlying idea explains all four
Putting the Four Factors Together
+5 XPOne underlying idea explains all four
We just saw wind and humidity act on the gradient. That raises a question: is there a single principle linking all four factors? This card answers it → every factor works by changing either how open the stomata are or how steep the water-vapour gradient is.
All four environmental factors change the transpiration rate through just two mechanisms: how wide the stomata are open, and how steep the water-vapour gradient is out of the leaf.
Light increases the rate by opening the stomata. Temperature increases it by speeding evaporation and steepening the vapour gradient. Wind increases it by removing the humid boundary layer (keeping the gradient steep). Humidity decreases it by flattening the vapour gradient. Hot, bright, windy, dry conditions give the fastest transpiration; cool, dark, still, humid conditions give the slowest.
This is why plants in hot, dry, windy habitats often have adaptations to slow water loss, sunken stomata, thick waxy cuticles and reduced leaves, which you will meet again in Year 11 Focus Area 3 when studying adaptations.
Add the highlighted summary of all four factors to your notes before the check below.
Which set of conditions would produce the HIGHEST rate of transpiration?
Drive It: Change One Factor at a Time
+XP for exploringLight, temperature, humidity, wind and the stomata each change the rate of water loss
Use the simulation. Keep every other factor the same and raise the humidity. What happens to the transpiration rate, and why?
Use the simulation. True or false: when the stomata close, transpiration stops completely.
Why farmers watch the weather to schedule irrigation
Where this biology is doing real work outside the classroom.
Why farmers watch the weather to schedule irrigation
Australian farmers and horticulturalists use exactly these four factors to decide when and how much to water. Agencies publish "evapotranspiration" forecasts that combine temperature, wind, humidity and sunshine into a single predicted water-loss figure for crops. On a hot, dry, windy day a wheat or cotton crop can transpire several millimetres of water, so irrigation is scheduled for cooler, calmer, more humid periods (often overnight) when transpiration is lowest, saving water and reducing the stress that causes crops to wilt and yield less.
Explain how light, temperature, wind and humidity each affect the rate of transpiration in plants, and show how all four effects reduce to two underlying mechanisms.
Explain means cause and effect, so each factor needs a direction and a mechanism. "Light increases transpiration" is half a mark; "light increases transpiration because it opens the stomata" is a whole one.
6 marks. Give each of the four factors its direction and reason, then name the two mechanisms and assign each factor to one. Watch the sign on humidity — it is the one that decreases the rate.
Increasing light intensity increases the rate of transpiration. In the light, guard cells take up water and open the stomata so that carbon dioxide can enter for photosynthesis; with more stomata open, more water vapour can diffuse out. In darkness most stomata close and transpiration falls sharply. Light therefore acts mainly by controlling how far the stomata are open, rather than on evaporation directly.
Increasing temperature increases the rate of transpiration. Heat gives water molecules more kinetic energy, so they evaporate from the moist internal leaf surfaces more rapidly, and warmer air can hold more water vapour, which steepens the water-vapour gradient from the inside of the leaf to the air outside.
Increasing wind speed increases the rate of transpiration. In still air a layer of humid air, the boundary layer, builds up against the leaf surface and flattens the water-vapour gradient. Wind blows this layer away and replaces it with drier air, keeping the gradient steep so vapour continues to diffuse out. In extreme wind the stomata may close, which eventually reduces the rate again.
Increasing humidity decreases the rate of transpiration. The more water vapour already present in the surrounding air, the smaller the difference in water-vapour concentration between the air spaces inside the leaf and the air outside it, so less vapour diffuses out. Humidity is the only one of the four factors whose increase slows transpiration.
All four factors work through just two mechanisms: how wide the stomata are open, and how steep the water-vapour gradient out of the leaf is. Light acts on stomatal aperture; temperature, wind and humidity all act on the steepness of the gradient, the first two by steepening it and humidity by flattening it. The fastest transpiration therefore occurs in hot, bright, windy, dry conditions and the slowest in cool, dark, still, humid ones.
Increasing light intensity increases the rate of transpiration, because in the light the guard cells take up water and open the stomata to admit carbon dioxide for photosynthesis, and more open stomata let more water vapour diffuse out; in darkness most stomata close and the rate falls sharply. Increasing temperature also increases the rate, because heat gives water molecules more kinetic energy so they evaporate faster from the moist internal leaf surfaces, and warmer air holds more vapour, which steepens the water-vapour gradient out of the leaf. Increasing wind speed increases the rate as well, because still air allows a humid boundary layer to build up against the leaf and flatten the gradient, while wind strips that layer away and replaces it with drier air, keeping the gradient steep — although extreme wind may close the stomata and eventually reduce the rate. Humidity is the exception: increasing humidity decreases the rate, because moist surrounding air reduces the difference in water-vapour concentration between the leaf's internal air spaces and the outside air, so less vapour diffuses out. All four therefore act through only two mechanisms, the width of the stomatal opening and the steepness of the water-vapour gradient: light acts on stomatal aperture, while temperature, wind and humidity act on the gradient, the first two steepening it and humidity flattening it. Transpiration is consequently fastest in hot, bright, windy and dry conditions, and slowest in cool, dark, still and humid ones.
Compare the effects of wind and humidity on the rate of transpiration, explaining why two factors that act on the same mechanism push the rate in opposite directions.
Compare calls for similarities and differences on the same criterion. The question hands you the criterion — both factors act on the water-vapour gradient — so the interesting part is why the same mechanism gives opposite results.
5 marks. Establish the shared mechanism first, then take each factor through it, then state the contrast and what it predicts in practice.
Water vapour leaves the leaf by diffusion through the stomata, and the rate of that diffusion depends on the water-vapour gradient: the difference in water-vapour concentration between the air spaces inside the leaf, which are close to saturated, and the air immediately outside it. Both wind and humidity affect transpiration by altering the steepness of this gradient, and neither acts on the stomata directly.
In still air, water vapour diffusing out of the leaf accumulates in a layer of humid air against the leaf surface, the boundary layer. Because that layer is moist, it reduces the difference between the inside of the leaf and the air just outside it, so diffusion slows. Wind removes this boundary layer and replaces it with drier air from further away, restoring a steep gradient. Increasing wind speed therefore increases the rate of transpiration.
Humidity is the amount of water vapour already present in the surrounding air. The more there is, the smaller the difference between the leaf's nearly saturated internal air spaces and the outside air, so less vapour diffuses out. Increasing humidity therefore decreases the rate of transpiration.
The two factors act in opposite directions because they change opposite ends of the same difference. Wind changes which air sits at the leaf surface, continually replacing moist air with drier air and so steepening the gradient; humidity changes how moist that air is in the first place, and so flattens it. They can therefore cancel each other out: a windy but very humid day may produce little more transpiration than a still, dry one, because wind can only expose the leaf to whatever air is available, and if that air is already near saturation there is no steeper gradient to restore. This is why a florist keeping cut flowers fresh should aim for still, humid air, and why crops lose most water on hot, dry, windy days.
Water vapour leaves the leaf by diffusion through the stomata at a rate set by the water-vapour gradient — the difference in water-vapour concentration between the leaf's nearly saturated internal air spaces and the air immediately outside — and both wind and humidity act on transpiration by altering the steepness of that gradient rather than on the stomata directly. In still air, vapour diffusing out accumulates as a humid boundary layer against the leaf surface, reducing the difference between inside and outside and slowing diffusion; wind strips that layer away and replaces it with drier air, restoring a steep gradient, so increasing wind speed increases the rate of transpiration. Humidity is the water vapour already present in the surrounding air, and the more there is the smaller the difference between the leaf's internal air spaces and the outside air, so less vapour diffuses out and increasing humidity decreases the rate. The two act in opposite directions because they change opposite ends of the same difference: wind changes which air sits at the leaf surface, continually replacing moist air with drier, while humidity determines how moist that air is to begin with. They can therefore cancel out, since a windy but very humid day may produce little more transpiration than a still, dry one — wind can only bring the leaf whatever air is available, and if that air is already near saturation there is no steeper gradient to restore. This is why cut flowers keep best in still, humid air, and why crops lose most water on hot, dry, windy days.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 3 questions · 13 marks
+5 XPUnderstandBand 5(5 marks) 1. Explain how light, temperature, wind and humidity each affect the rate of transpiration in plants.
1 mark each for a correct effect-with-reason for light, temperature and wind · 1 mark for humidity (decreases rate) with reason · 1 mark for correctly linking the effects to stomata opening and/or the water-vapour gradient
ApplyBand 4(3 marks) 2. A florist wants cut flowers to stay fresh for as long as possible. Recommend two changes to the conditions in the shop that would slow transpiration, and justify each using the mechanism by which that factor acts.
1 mark: one valid change named (cooler, dimmer, still air or more humid) · 1 mark: a second valid change named · 1 mark: each justified by the correct mechanism, stomatal opening or the water-vapour gradient
AnalyseBand 5–6(5 marks) 3. Explain how light, temperature, wind and humidity each affect the rate of transpiration, and analyse why all four can be reduced to just two underlying mechanisms.
1 mark: light increases the rate by opening stomata · 1 mark: temperature increases the rate by speeding evaporation and steepening the gradient · 1 mark: wind increases the rate by removing the humid boundary layer · 1 mark: humidity decreases the rate by flattening the gradient · 1 mark: identifies the two mechanisms as stomatal aperture and the steepness of the water-vapour gradient, and assigns each factor to one
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (5 marks): Light increases the rate of transpiration because in the light the guard cells open the stomata (to admit carbon dioxide for photosynthesis), and more open stomata allow more water vapour to diffuse out. Temperature increases the rate because heat gives water molecules more kinetic energy so evaporation from the moist leaf surfaces is faster, and warmer air holds more vapour, steepening the water-vapour gradient out of the leaf. Wind increases the rate because it blows away the humid boundary layer of still air at the leaf surface, replacing it with drier air and so keeping the water-vapour gradient steep. Humidity is the exception, increasing humidity decreases the rate, because moist surrounding air reduces the difference in water-vapour concentration between the inside of the leaf and the air outside, so less vapour diffuses out. In short, every factor acts either by changing how open the stomata are (light) or how steep the water-vapour gradient is (temperature, wind, humidity).
Q2 (3 marks): First, raise the humidity around the flowers, for example by misting them or keeping them in a closed cabinet. Humid air already holds a lot of water vapour, so the difference in water-vapour concentration between the moist leaf interior and the air outside is small, the gradient is flattened and less vapour diffuses out. Second, keep the shop cool and out of draughts. Lower temperature gives water molecules less kinetic energy so evaporation from the leaf surfaces is slower, and still air lets a humid boundary layer build up against each leaf, which flattens the gradient further instead of being blown away. A third acceptable answer is to keep the flowers out of bright light, because light causes the guard cells to open the stomata and open stomata are the route by which most water vapour escapes.
Q3 (5 marks): Light increases the rate: in the light the guard cells open the stomata to admit carbon dioxide for photosynthesis, and open stomata are the route out for water vapour. Temperature increases the rate: heat gives water molecules more kinetic energy so evaporation from the moist mesophyll surfaces is faster, and warmer air holds more vapour, both of which steepen the gradient out of the leaf. Wind increases the rate: still air lets a humid boundary layer build against the leaf surface, and moving air strips that layer away and replaces it with drier air, keeping the gradient steep. Humidity is the exception and decreases the rate: moist surrounding air narrows the difference between the leaf interior and the outside, so less vapour diffuses out. Analysing these together, all four reduce to two mechanisms: how far the stomata are open, which only light controls directly, and how steep the water-vapour gradient is, which temperature, wind and humidity all act on. That is why the fastest transpiration occurs in bright, hot, windy, dry conditions, every factor is pushing the same way, and the slowest in dark, cool, still, humid conditions.
Retrieve and reflect
Use the spaced-review session, then compare how your thinking has changed since your opening prediction.
Check what actually stuck
A short retrieval session mixing this lesson with earlier ones, so the ideas move into long-term memory instead of fading by the exam.
Lock it in
bossYou can now explain how light, temperature, wind and humidity each change the rate of transpiration. Next lesson puts this to the test in a practical, where you measure transpiration rate while changing one environmental factor.
↩ Back to focus-area hubHow did your thinking change?
At the start you compared drying towels. The towel in the hot, breezy, dry garage dries faster, and the same conditions, high temperature, wind and low humidity, speed up water loss from a leaf too. The one factor unique to the leaf is light, which opens the stomata; a towel has no stomata to open.
If you predicted that high humidity would increase transpiration, revisit the misconception box: transpiration is diffusion of water vapour out of the leaf, so it needs a steep gradient, humid air flattens that gradient and slows the rate.