Transpiration — Factors and Measurement
A single maize plant loses around 200 litres of water by transpiration over a growing season. What controls how fast it loses water — and how do we measure it? Start with the data and figure it out.
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Before you begin this lesson, take a moment to think about what you already know about this topic. Jot down your ideas — you will revisit them at the end.
Know
- Define transpiration and explain its role in the cohesion-tension mechanism
- Explain how temperature, humidity, wind, and light affect transpiration rate
- Describe how a potometer works and what it actually measures
- Analyse potometer data to identify which factor most affects transpiration
- Explain structural adaptations that reduce or increase water loss in plants
Understand
- Investigate transpiration and the factors that affect it
- Analyse data from investigations using potometers
- Connect: L09 (stomata/guard cells), L16 (xylem, cohesion-tension, water pathway)
- Working Scientifically: identifying variables, analysing data, evaluating method
Can Do
- Explain the water potential gradient driving transpiration
- Predict the effect of each environmental factor on transpiration rate with reasoning
- Describe a valid potometer experiment identifying IV, DV, and controlled variables
- Identify a limitation of the potometer method and suggest an improvement
- Link xerophyte adaptations to specific reductions in transpiration rate
Content from this lesson that appears directly in HSC Biology exams
Analysing potometer data (graph or table) to identify factor effects on transpiration rate — tested as a 3–4 mark working scientifically question in most HSC papers. Must identify trend, explain mechanism, evaluate the method, and calculate rate from data.
Explaining why each factor (temperature, humidity, wind, light) affects transpiration rate. Tested as 2–3 mark mechanism questions — must state the factor, explain the mechanism (water potential gradient, stomatal aperture, or diffusion gradient), and predict the effect direction.
Explaining how structural adaptations of xerophytes (plants in dry environments) reduce transpiration. Tested as 2–3 mark application questions — must name the adaptation, describe the structure, and explain how it reduces water loss.
Designing a valid potometer experiment (IV, DV, controlled variables) and evaluating its limitations (measures water uptake, not transpiration directly). Tested as 2–3 mark Working Scientifically questions in Section II.
Start with the Data
Misconceptions to Fix
Wrong: Wind always increases transpiration rate.
Right: Moderate wind removes the boundary layer and increases transpiration. However, very high wind or hot, dry wind can cause stomata to close as a protective response, actually reducing transpiration. The relationship between wind speed and transpiration is not linear — stomatal regulation overrides physical effects at extreme conditions.
The Data — Four Conditions, One Plant
Look at the numbers. Which condition most affects the rate? Why?
What drives a plant to lose water faster?
A student uses a potometer to measure water uptake in a leafy shoot under four different conditions. In each trial, all other variables are held constant (same shoot, same time period, same starting conditions). The shoot is moved into each condition for 10 minutes and the distance the air bubble travels in the capillary tube is measured.
| Condition | Trial 1 (mm) | Trial 2 (mm) | Trial 3 (mm) | Mean (mm) | Rate (mm/min) |
|---|---|---|---|---|---|
| Still air, 22°C, 60% humidity, dim light | 12 | 14 | 13 | 13.0 | 1.3 |
| Still air, 22°C, 60% humidity, bright light | 19 | 21 | 20 | 20.0 | 2.0 |
| Fan (wind), 22°C, 60% humidity, dim light | 24 | 23 | 25 | 24.0 | 2.4 |
| Still air, 35°C, 30% humidity, dim light | 31 | 34 | 32 | 32.3 | 3.2 |
| Still air, 22°C, 90% humidity, dim light | 4 | 5 | 4 | 4.3 | 0.4 |
Your hypotheses — before reading Card 2: Rank the conditions fastest to slowest. For each, suggest why that condition affects water loss rate.
The Mechanism
Four Factors That Affect Transpiration Rate
Each factor acts on one of two things: the water potential gradient or stomatal aperture
Transpiration rate is determined by how steep the water potential gradient is between the leaf interior and the outside air, and by how open the stomata are. Every environmental factor acts through one of these two mechanisms — once you understand which, you can predict the direction of effect for any factor without memorising.
The five factors that affect transpiration rate — each acts on either the water potential gradient or stomatal aperture
Temperature
High temperature → faster transpiration
Mechanism 1 — Kinetic energy: Higher temperature increases the kinetic energy of water molecules — evaporation from mesophyll cell walls into leaf air spaces is faster, increasing the water vapour concentration in the leaf air spaces.
Mechanism 2 — Atmospheric holding capacity: Warm air can hold more water vapour than cool air. This means at high temperature, the atmosphere is further from saturation — the water potential difference between the humid leaf air space and the dry outside air is larger, increasing the diffusion gradient driving water vapour out through stomata.
Mechanism 3 — Stomatal response: High temperature also causes guard cells to open stomata wider (heat stress response), increasing stomatal conductance.
Humidity
High humidity → slower transpiration
Mechanism — Gradient reduction: Transpiration is driven by the difference in water potential (water vapour concentration) between the leaf air spaces (nearly saturated, ~99% relative humidity) and the outside air.
When outside air is already highly humid (high relative humidity), the water potential difference between leaf interior and atmosphere is small — the gradient driving diffusion of water vapour through stomata is reduced. At 100% relative humidity, no net transpiration occurs because there is no gradient.
The potometer data confirms this dramatically: the 90% humidity condition (0.4 mm/min) is 8× slower than the high temperature/low humidity condition (3.2 mm/min) — the single largest effect in the dataset.
Wind
Increased air movement → faster transpiration
Mechanism — Removal of boundary layer: In still air, a thin layer of humid air (the boundary layer) accumulates immediately outside the stomata, partially saturated with water vapour from previous transpiration. This boundary layer reduces the effective water potential gradient — water vapour must diffuse through this humid layer before reaching drier bulk air.
Wind constantly removes the boundary layer, replacing humid air around stomata with drier bulk air. This maintains a steep water potential gradient at the stomatal pore, increasing transpiration rate. This is why clothes dry faster on windy days — the same physics.
Note: very strong wind can cause stomatal closure as a water-conservation response, which would reduce transpiration.
Light Intensity
Bright light → faster transpiration (indirectly)
Mechanism — Stomatal opening: Light does not directly accelerate water evaporation. Instead, it acts through guard cell biology. In bright light, guard cells photosynthesise, producing ATP and accumulating K⁺ ions by active transport — this lowers guard cell water potential, causing osmotic water entry and inflation, which opens stomatal pores (the mechanism from L09).
Wider stomatal aperture = more pathway for water vapour to diffuse out. In darkness, stomata close (most species) and transpiration drops to near zero — only cuticular transpiration remains through the waxy cuticle.
The potometer data shows light is a moderate factor (1.3 → 2.0 mm/min) — it changes stomatal aperture, but the gradient itself (determined by temperature and humidity) determines maximum possible rate.
Figure: Four factors affecting transpiration rate. Coloured left strip identifies the factor. Temperature and humidity act on the water potential gradient; light acts on stomatal aperture; wind maintains the gradient by removing the boundary layer.
The Potometer — How It Works and What It Actually Measures
A critical distinction that examiners test every year
The distance the bubble moves per unit time = rate of water uptake (mm/min or cm³/min if tube diameter known).
Water uptake = transpiration + water used in photosynthesis and other metabolic processes + any water retained in growing cells
In practice, for most experimental conditions, the difference is small — over 95% of water taken up by a shoot is transpired. But this limitation must be stated when evaluating the method in an HSC response. The correct language is "the potometer measures water uptake as an indicator of transpiration rate" — not "the potometer measures transpiration."
A second limitation: the cut shoot is not a whole plant. The root system is absent, so root pressure and root resistance to water uptake cannot be studied. Results may not represent intact plant transpiration exactly.
Variables in a valid potometer experiment:
Xerophyte Adaptations — Reducing Transpiration in Dry Environments
Every adaptation acts on one of the four factors — or physically blocks the pathway
Xerophytes are plants adapted to environments with limited water availability — deserts, rocky slopes, coastal dunes. Their structural adaptations are not random; each one acts on a specific transpiration factor or directly reduces the water loss pathway. Understanding the mechanism is what separates Band 6 from Band 3 responses.
🌵 Thick, Waxy Cuticle
A thick layer of waxy cutin deposited on the leaf epidermis. Cutin is highly hydrophobic — essentially waterproof. Reduces cuticular transpiration (water diffusing through the cuticle rather than stomata).
🌿 Sunken Stomata
Stomata positioned in crypts or pits below the leaf surface rather than flush with the epidermis. Still air accumulates in the crypt, creating a high-humidity boundary layer immediately outside the stomatal pore.
🍃 Reduced Leaf Surface Area / Modified Leaves
Leaves reduced to spines (cacti), needles (pines), or small scale-like structures. Fewer and smaller leaves mean fewer stomata and less total surface area for evaporation. Some have leaves that roll up in dry conditions, trapping humid air.
🌾 Hairy Leaves (Trichomes)
Dense covering of fine epidermal hairs (trichomes) on the leaf surface. Hairs trap a still, humid boundary layer around stomata, reducing the water potential gradient between the stomatal pore and bulk air. Hairs also reflect light, reducing leaf temperature.
💧 Succulent Water Storage
Large vacuoles and parenchyma cells in leaves or stems that store water. Not a transpiration reduction adaptation directly — but provides a water reservoir that sustains the plant through dry periods when transpiration cannot be met by soil water uptake.
🌑 CAM Photosynthesis — Nocturnal Stomata
Crassulacean Acid Metabolism (CAM) plants open stomata only at night to fix CO₂ into organic acids. During the hot, dry day, stomata remain closed — CO₂ stored overnight is released internally for daytime photosynthesis. Eliminates daytime transpiration entirely.
Returning to the Data — What the Numbers Now Tell You
Reread the potometer results through the lens of the mechanisms you now understand
Return to the data table in Card 1. With the mechanisms from Cards 2 and 3, you can now explain every result — not just describe the trend. This is the difference between Band 4 and Band 6.
Limitation 1: The potometer measures water uptake, not transpiration directly — a small proportion of water taken up is used in photosynthesis and growth. This causes slight overestimation of transpiration.
Limitation 2: Only one variable changed at a time, but real environments change multiple factors simultaneously — results may not reflect field conditions.
Improvement: Use intact plants rather than cut shoots to include root resistance and root pressure. Use a humidity sensor in the leaf chamber to measure actual water vapour output (gravimetric method) for direct transpiration measurement.
Validity concern: The same shoot was used across conditions — but the order of conditions could affect results if the shoot deteriorates over time (e.g. cavitation in the cut end). Ideally, a fresh shoot should be used for each condition.
Copy into your books
▼Four Factors — Effect and Mechanism
- Temperature ↑ → transpiration ↑ (kinetic energy ↑, gradient ↑, stomata open wider).
- Humidity ↑ → transpiration ↓ (gradient ↓ — atmosphere closer to saturation).
- Wind ↑ → transpiration ↑ (removes boundary layer, restores gradient).
- Light ↑ → transpiration ↑ (stomata open wider via guard cell K⁺ accumulation).
Potometer — Key Points
- Measures water uptake rate — not transpiration directly (overestimates slightly).
- DV: distance air bubble moves per unit time.
- IV: one environmental factor at a time.
- Control: all other variables kept constant.
Xerophyte Adaptations
- Thick waxy cuticle → blocks cuticular transpiration.
- Sunken stomata → boundary layer in crypt → gradient ↓.
- Hairy leaves → trap boundary layer + reflect light (temp ↓).
- Reduced leaf area → fewer stomata, less evaporative surface.
- CAM photosynthesis → stomata closed during hot day.
The Two Mechanisms Transpiration Acts Through
- Water potential gradient (leaf interior vs atmosphere) — affected by temperature and humidity.
- Stomatal aperture — affected by light and CO₂ concentration.
- Wind affects gradient maintenance (boundary layer removal).
Activities
Potometer Data Analysis — Calculating and Explaining
The following additional data was collected from the same potometer experiment. The capillary tube has an internal diameter of 1.0 mm.
| Condition | Mean bubble distance (mm/10 min) | Rate (mm/min) | Volume rate (mm³/min) |
|---|---|---|---|
| Still air, 22°C, low humidity, no wind | 18 | ||
| Same conditions + fan (high wind) | 29 | ||
| Same conditions + fan + high humidity | 11 | ||
| Same conditions + fan + CO₂ injected | 6 |
- Calculate the rate in mm/min and volume rate in mm³/min for each condition. (Volume of a cylinder = π × r² × length. Use π = 3.14, r = 0.5 mm)
- Explain why adding a fan significantly increased water uptake rate, referring to boundary layer effects and water potential gradients.
- In the final condition, CO₂ was injected into the air around the plant. This is a much lower rate than even the baseline. Explain this result using your knowledge of guard cells and stomatal control.
- Identify one controlled variable that was not mentioned that the student should have controlled in this experiment. Explain why this variable could affect results if not controlled.
Xerophyte Design Challenge
A biologist discovers a new plant species living in a hot, dry coastal sand dune environment. The plant has the following structural features: very thick waxy cuticle, leaves reduced to small triangular scales lying flat against the stem, stomata only on the underside of the scales in deep pits, a dense coating of silver hairs on all surfaces, and roots extending 4 metres deep.
- For each of the five features described, explain how it reduces water loss or assists water acquisition, naming the specific mechanism.
- The plant's stomata are open at night and closed during the day. Identify the photosynthetic pathway this suggests and explain how this pattern minimises water loss.
- Despite all these adaptations, a student argues "this plant must still transpire." Explain why transpiration cannot be completely eliminated in any living plant, and why this is not necessarily a problem for the plant.