3.6.2 Transpiration

Key Steps in Transpiration:

1. Water is absorbed from the soil into the roots via osmosis.
2. It moves through the xylem by the cohesion-tension mechanism.
3. Water evaporates from the surfaces of mesophyll cells in the leaves into air spaces.
4. It diffuses out of the leaf through stomata as water vapour.

The Stomata:

• Stomata are small pores in the leaf surface that facilitate gas exchange.
• Carbon dioxide enters for photosynthesis, while water vapour and oxygen exit.
• The size and opening of stomata are controlled by guard cells to regulate water loss.

Factors Affecting Transpiration Rate:

1. Light:

• Stomata open in the light for photosynthesis, increasing transpiration.

2. Temperature:

• Higher temperatures increase evaporation and diffusion rates.

3. Humidity:

• Lower humidity increases the water vapour gradient, enhancing transpiration.

4. Air Movement:

• Wind removes water vapour from around the leaf, maintaining a steep gradient.

5. Number, Size, and Position of Stomata:

• More stomata or those on upper surfaces increase water loss.

6. Waxy Cuticle:

• Reduces evaporation by preventing water loss through the leaf surface.

7. Water Availability:

• Limited water reduces transpiration as the stomata close to conserve water.

Investigating Transpiration: Potometer:

• A potometer measures the rate of water uptake, which is an indirect estimate of transpiration rate.
• How it works:
  1. A shoot is cut underwater and attached to the potometer.
  2. Air bubbles or a meniscus in the capillary tube track water uptake.
  3. The distance moved by the bubble/meniscus over time is recorded.
• Variables to Control:
  • Light intensity, temperature, humidity, and air movement during the experiment.

Xerophytes: Plants Adapted to Dry Conditions

Xerophytes are plants adapted to minimise water loss and survive in arid environments. Their adaptations include:

8. Smaller Leaves:

• Reduced surface area decreases water loss through evaporation.

9. Thick Waxy Cuticle:

• Acts as a waterproof barrier, reducing water loss.

10. Densely Packed Mesophyll:

• Reduces internal evaporation of water.

11. Stomatal Modifications:

• Sunken stomata and hairs trap moist air, reducing the water vapour gradient.
• Stomata close in response to water stress.

12. Rolled Leaves:

• Trap moist air within the rolled structure, reducing the water potential gradient.

13. Extensive Root Systems:

• Increase water uptake from deep or widespread soil areas.

Key Terms:

• Transpiration: The loss of water vapour from plant leaves through stomata.
• Potometer: Apparatus to measure the rate of water uptake by a plant.
• Xerophytes: Plants adapted to dry conditions with features to conserve water.