Environmental factors change water uptake
Definition
Transpiration
The loss of water vapour from the surfaces of a plant, mainly through the stomata of the leaves.
- Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through leaf stomata.
- As transpiration removes water from leaves, water is pulled through the xylem and replaced by water absorbed from the soil.
- A factor that increases transpiration therefore usually increases the rate of water uptake by the roots.
Key Idea
Cause and effect: environmental factor, change to stomata or diffusion, change in transpiration, change in water uptake.
Light intensity opens stomata
- Increasing light intensity usually increases water uptake because stomata open to admit carbon dioxide for photosynthesis.
- Wider stomatal pores allow more water vapour to diffuse out of the leaf.
- This increases transpiration and pulls water through the xylem more quickly.
- In darkness, most stomata close, reducing both water loss and water uptake.
Air movement maintains a steep gradient
- Moving air removes the moist layer of air beside the leaf surface.
- The water vapour concentration outside the stomata falls, making the concentration gradient from the leaf to the air steeper.
- Water vapour diffuses out faster, so transpiration and water uptake increase.
- In still air, moist air remains near the leaf and the smaller gradient reduces diffusion.
Temperature increases particle movement
- At a higher temperature, water molecules have more kinetic energy.
- Water evaporates more rapidly from mesophyll cell surfaces and water vapour diffuses through stomata more quickly.
- Transpiration and water uptake therefore increase, provided the plant has enough water and the stomata remain open.
Common Mistake
Do not stop at stating that heat increases transpiration; explain the increase in kinetic energy, evaporation and diffusion.
Practical
Measuring the effect of an environmental factor
- Aim: to determine how light intensity, air movement or temperature affects the rate of water uptake by a leafy shoot.
- Apparatus: bubble potometer, freshly cut leafy shoot, beaker of water, sharp blade, petroleum jelly, ruler, stop clock, lamp, fan or controlled-temperature surroundings, and eye protection.
- Method:
- Fill the apparatus completely with water.
- Cut the shoot under water at an angle and fit it without letting air enter the xylem.
- Seal every joint with petroleum jelly.
- Introduce one air bubble into the capillary tube and record its starting position.
- Apply one chosen condition and allow the shoot to adjust.
- Measure the distance moved by the bubble in a fixed time.
- Reset the bubble and repeat at least three times.
- Change only the selected factor and repeat across a suitable range.
- Variables: change one environmental factor and measure bubble movement per unit time; control the shoot species, leaf area, measurement time and all other environmental factors.
- Results: brighter light, greater air movement or higher temperature usually makes the bubble move faster because water uptake increases.
- Maths: calculate rate=distance movedtime\text{rate}=\frac{\text{distance moved}}{\text{time}}rate=timedistance moved, then compare the mean rates for the different conditions.
- Watch out: air leaks make the bubble move without water entering the shoot, while a different leaf area changes the surface available for transpiration.
- Safety: cut the shoot on a tile away from your fingers, wear eye protection and keep water away from electrical lamps and fans.
Explaining experimental results
- Use repeat readings to identify anomalies and calculate a mean, which makes the comparison more reliable.
- Describe the pattern in the data before explaining it with stomatal opening, kinetic energy or the water vapour gradient.
Exam technique
- Explanation chain: increased air movement removes moist air, steepens the water vapour gradient, increases diffusion and raises water uptake.
- Fair-test mark: name a specific controlled variable and explain why changing it would affect transpiration.
Self review
- How does increased light intensity affect water uptake?
- Why does moving air increase transpiration?
- How does temperature affect evaporation and diffusion?
- Why must leaf area be controlled in a potometer investigation?
