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.
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
Look at the numbers. Which condition most affects the rate? Why?
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
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.
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.
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.
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.
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.
A critical distinction that examiners test every year
🌿 Leafy shoot
│
───────┴─────── ← airtight seal (rubber bung)
│ │
│ water-filled│ ← no air pockets in vessel
│ vessel │
│ │
└──────────┬──┘
│ capillary tube
│
│——│——│——│——│ ← calibrated scale (mm)
·
air bubble ← position recorded every minute
│
┌──────┴────┐
│ reservoir │ ← can reset bubble by opening tap
└────────────┘
Variables in a valid potometer experiment:
| Variable Type | Example | Why It Matters |
|---|---|---|
| Independent variable (IV) | Wind speed (fan on/off), temperature (incubator), humidity (mist chamber), light intensity (lamp distance) | The factor being tested — change only this one thing |
| Dependent variable (DV) | Distance bubble moves per unit time (mm/min) — converted to volume if tube diameter known | What is being measured as an indicator of transpiration rate |
| Controlled variables | Same species, same leaf area, same shoot length, same time of day, same starting bubble position, same time period | Ensuring changes in DV are caused only by the IV, not confounding variables |
| Reliability improvement | Multiple trials (at least 3) per condition — average results to reduce random error | Identifies outliers; increases confidence in the mean; the student's data above uses three trials per condition |
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.
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).
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.
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.
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.
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.
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.
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.
| Condition | Rate (mm/min) | Mechanism Explanation |
|---|---|---|
| Baseline (still air, 22°C, 60% humidity, dim light) | 1.3 | Moderate gradient, stomata partially open (dim light), no wind — transpiration occurs but at a baseline rate |
| Bright light added | 2.0 (+54%) | Light triggers guard cell K⁺ accumulation → osmotic water entry → stomata widen → more pathway for water vapour → rate increases. Temperature and humidity unchanged — gradient unchanged, only aperture changed. |
| Fan (wind) added | 2.4 (+85%) | Wind removes the humid boundary layer outside stomata → drier bulk air at stomatal pore → steeper water potential gradient → faster diffusion. Larger effect than light alone because it acts on the gradient, not just aperture. |
| High temperature + low humidity | 3.2 (+146%) | Both variables act simultaneously on the gradient: higher temperature → more evaporation within leaf → leaf air spaces more humid. Lower outside humidity → atmosphere drier. Both changes steepen the gradient dramatically — additive effect producing the largest rate increase. |
| High humidity (90%) | 0.4 (−69%) | Outside air nearly as humid as leaf interior → gradient almost eliminated → diffusion nearly stopped. Strongest effect in the dataset because it acts directly on the gradient. Confirms humidity is the limiting factor when at its extreme. |
Activities
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 |
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.
Assessment
Select the best answer — feedback shown immediately
1. A potometer is set up in a laboratory. The air bubble moves 12 mm in 5 minutes under normal conditions. When a fan is turned on (same temperature, same humidity), the bubble moves 20 mm in 5 minutes. Which explanation best accounts for this increase?
2. A plant in a tropical rainforest experiences 95% relative humidity throughout the day. Which of the following best predicts its transpiration rate compared to an identical plant in a dry savanna at 30% relative humidity?
3. A student uses a potometer to investigate the effect of temperature on transpiration rate. Their results show a positive relationship between temperature and bubble movement rate. Which of the following is a valid limitation of this investigation?
4. Which of the following xerophyte adaptations reduces transpiration by increasing the boundary layer effect?
5. In a potometer experiment, CO₂ is injected into the sealed chamber around the shoot. The air bubble moves significantly more slowly. Which of the following correctly explains this observation?
6. Explain why a plant wilts faster on a hot, dry, windy day compared to a cool, humid, still day. Refer to the effect of each factor on transpiration rate in your answer. 4 MARKS
7. A student claims "a potometer directly measures how much water a plant transpires." Evaluate this claim, identifying one inaccuracy and explaining what the potometer actually measures. 3 MARKS
8. Explain how sunken stomata reduce transpiration in a xerophyte. In your answer, refer to the water potential gradient and boundary layer. 3 MARKS
1. C — Wind's mechanism is boundary layer removal. In still air, water vapour transpired through stomata partially saturates the thin air layer immediately outside — reducing the effective gradient. Wind constantly sweeps this away, maintaining drier air at the stomatal pore. This is analogous to how a fan dries laundry faster — it removes the humid air immediately adjacent to the wet surface.
2. A — Transpiration rate depends on the water potential gradient between leaf air spaces (near-saturated, ~99% humidity) and the outside atmosphere. At 95% humidity, the gradient is 99 − 95 = ~4 percentage points. At 30% humidity, the gradient is 99 − 30 = 69 percentage points — approximately 17 times larger. Transpiration rate reflects this proportional difference.
3. D — The key limitation of a potometer is that it measures water uptake, not transpiration directly. At higher temperatures, photosynthesis rate also increases — meaning more water is incorporated into carbohydrates and other metabolic processes. This additional non-transpiratory water use would cause the potometer to slightly overestimate transpiration's specific contribution to water uptake. This is a valid methodological limitation specific to temperature experiments.
4. B — Sunken stomata in crypts create a still-air micro-environment within the pit. Transpired water vapour accumulates there rather than being dispersed, forming a humid boundary layer immediately outside the pore. This reduces the effective water potential gradient — the driving gradient is between the pore and the humid crypt air, not between the pore and dry bulk air. The waxy cuticle blocks the cuticular pathway (not boundary layer); CAM changes timing (not boundary layer); succulents store water (not boundary layer).
5. A — Elevated CO₂ signals guard cells to close stomata. CO₂ enters guard cells and is converted to bicarbonate, which triggers signalling pathways leading to K⁺ efflux from guard cells. Losing K⁺ raises guard cell water potential, water exits by osmosis, turgor falls, and the stomatal pore narrows or closes. This is the reverse of light-induced stomatal opening and is an adaptive response to high CO₂ — when CO₂ is abundant, there is less need for open stomata to capture more.
On a hot, dry, windy day, all three environmental factors combine to dramatically increase transpiration rate relative to a cool, humid, still day.
High temperature increases the kinetic energy of water molecules in the leaf mesophyll, accelerating evaporation into leaf air spaces and increasing the water vapour concentration inside the leaf. Additionally, warm air has greater capacity to hold water vapour, so even at the same absolute humidity, warm air is further from saturation — the atmosphere can absorb much more water vapour, creating a larger gradient driving diffusion through stomata.
Low humidity means the atmosphere has very low water vapour concentration — the water potential difference between the near-saturated leaf interior (~99% humidity) and the dry outside air (~20–30% humidity) is very large, driving rapid diffusion of water vapour through open stomata.
Wind removes the humid boundary layer that would otherwise accumulate just outside the stomata in still conditions. By constantly replacing this humid air with dry bulk air, wind maintains the maximum possible gradient at the stomatal pore throughout the day.
Together, these factors push transpiration rate far above the rate of water uptake from soil via roots, causing a progressive water deficit in leaf and stem cells. As water leaves cells faster than it is replaced, turgor pressure falls, the cells become flaccid, and the plant wilts.
The claim is inaccurate. A potometer measures the rate of water uptake by the cut shoot — not transpiration directly.
Water uptake includes all water absorbed by the shoot, which is then used for: (1) transpiration — evaporation through stomata and cuticle (the dominant pathway, ~95%+ of uptake), (2) photosynthesis — water split in the light-dependent reactions, (3) cell expansion and growth — water retained in vacuoles of growing cells.
Therefore, the potometer slightly overestimates transpiration rate, particularly at conditions that stimulate photosynthesis (bright light, high temperature) where non-transpiratory water use increases. A more accurate statement is that the potometer measures water uptake as an indirect indicator of transpiration rate — valid because transpiration accounts for the vast majority of water uptake in a leafy shoot under normal conditions.
In a xerophyte with sunken stomata, the pores are positioned in recessed pits or crypts below the level of the leaf surface. Water vapour transpired through the stomata accumulates within the crypt rather than being swept away into bulk air. This creates a humid still-air boundary layer immediately outside the stomatal pore, with water vapour concentration significantly higher than the dry bulk atmosphere outside the leaf.
The water potential gradient driving diffusion of water vapour is determined by the difference in water vapour concentration between the leaf air spaces (near-saturated, ~99%) and the air immediately outside the stomatal pore. With sunken stomata, the relevant comparison is between the leaf interior and the humid crypt air — not between the leaf interior and the dry bulk atmosphere. This smaller gradient reduces the driving force for diffusion, slowing transpiration rate.
In effect, the crypt creates a microenvironment that mimics the effect of high humidity outside the leaf, reducing the apparent water potential gradient even when the bulk atmosphere is dry. This adaptation is particularly effective in calm conditions; strong wind may partially disrupt the crypt boundary layer.
Tick when you've finished all activities and checked your answers.