Transpiration is faster in bright light, higher temperature, more wind and lower humidity, and slower in the opposite conditions. Each factor works by changing either how open the stomata are or how steep the concentration gradient is for water vapour.
This lesson builds on the water pathway from roots to leaves and leads into reading potometer data. It belongs to the plant transport module.
Why does each factor change the rate?
Do not memorise four results. Connect each to the two ideas underneath.
- Light. Light makes the guard cells open the stomata, mainly because the leaf is photosynthesising. Open stomata give water vapour a way out, so transpiration rises.
- Temperature. Warmth gives water molecules more kinetic energy, so they evaporate and diffuse faster.
- Wind. Moving air removes the damp layer beside the leaf, so the gradient stays steep.
- Humidity. Humid air is already full of vapour, so the gradient is shallower and diffusion is slower.
A table of invented data
The numbers below are invented for teaching. A leafy shoot was weighed for one hour in four different conditions, and the mass of water it lost was recorded.
| Condition | Temperature | Air movement | Humidity | Water lost in 1 hour (g) |
|---|---|---|---|---|
| A | cool | still | humid | 1.2 |
| B | warm | still | humid | 2.4 |
| C | warm | windy | humid | 3.6 |
| D | warm | windy | dry | 6.0 |
Worked example
Using the table, describe and explain the change between B and D, and give the water loss in D as a multiple of A.
Step 1, spot what changed: from B to D, the air movement changed from still to windy, and the humidity changed from humid to dry. The temperature stayed warm.
Step 2, describe with numbers: water loss increased from 2.4 g to 6.0 g, which is an increase of 6.0 − 2.4 = 3.6 g.
Step 3, explain: wind carries away water vapour, and drier air holds less, so the concentration gradient out of the leaf is steeper. Water vapour diffuses out faster through the stomata.
Step 4, the multiple: 6.0 ÷ 1.2 = 5. Water loss in D is five times that in A.
Check: 1.2 × 5 = 6.0, so the multiple is right.
The mistake to watch for
The most common error is reversing the humidity argument.
Mistaken answer: In humid air more water evaporates because the air has more water in it, so transpiration is faster.
This confuses “more water in the air” with “a bigger gradient”. The gradient gets smaller, not bigger.
The correction is to compare the two sides of the stomata. Humid air outside means the outside concentration is closer to the inside one, so the gradient is shallower and diffusion is slower. Always say “gradient” in a humidity answer.
Check yourself
Try these without looking back, then open each answer.
1. Which gives faster transpiration: (i) warm, windy, dry, bright air or (ii) cool, still, humid, dim air? Explain using two factors.
Show answer
(i) is faster. Wind removes water vapour from beside the leaf and dry air holds little vapour, so the concentration gradient is steep. Bright light also opens the stomata and warmth speeds evaporation.
2. A shoot loses 6.0 g of water per hour under condition D. How much does it lose in 90 minutes, if the rate stays the same?
Show answer
90 minutes is 1.5 hours. 6.0 × 1.5 = 9.0 g. Check: 6.0 in the first hour plus 3.0 in the half hour gives 9.0 g.
3. Give one reason why transpiration is lower at night even when the temperature stays warm.
Show answer
There is no light, so the stomata are mostly closed. Closed stomata leave little route for water vapour to diffuse out of the leaf.
Where this leads next
Now that you can predict the direction of change, interpret a potometer-style dataset to see how these effects are measured, and what the measurement cannot tell you. The plant transport practice set mixes this with the other four skills.
Some students end up writing the same sentence for all four factors. A teacher in online one-to-one Biology tuition can ask “why” after each answer until the reasoning is your own.