Transpiration is influenced by several environmental factors that affect the rate at which water is lost from the leaves. The main factors include temperature, humidity, wind speed, and light intensity.

1. Temperature (Higher temperature = Increased transpiration rate)

• At higher temperatures, water molecules gain more kinetic energy.

• This increases the rate of evaporation from the mesophyll cell surfaces into the air spaces in the leaf.

• Water vapour moves from the leaf air spaces to the external air through the stomata.

• Higher temperatures increase the rate of diffusion of water vapour. 

2. Humidity (Higher humidity = Decreased transpiration rate)

• Humidity refers to the amount of water vapour in the air.

• In high humidity, the air surrounding the leaf is already moist, reducing the water potential gradient between the inside and outside of the leaf.

• As a result, less water vapour diffuses out of the stomata, decreasing transpiration

3. Wind Speed (Air Movement) (Higher wind speed = Increased transpiration rate)

• Around the leaf surface, there is a thin layer of still, moist air.

• If there is little or no wind, this layer remains, reducing the water potential gradient and slowing transpiration.

• When wind speed increases, moving air removes the still, moist layer of air and replaces it with drier air.

• This maintains a steep water potential gradient, allowing water vapour to diffuse out of the leaf more rapidly.

4. Light Intensity (Higher light intensity = Increased transpiration rate)

• Stomata open in the presence of light to allow carbon dioxide (CO₂) into the leaf for photosynthesis.

• When stomata are open, water vapour can escape, increasing transpiration.

• Increased light intensity also raises leaf temperature, which increases evaporation rates and diffusion of water vapour.

A potometer is a device used to measure the rate of water uptake by a plant.

Method

1. Assemble the potometer underwater to prevent air bubbles forming in the xylem (air bubbles would break the continuous column of water in the xylem vessels.

2. Cut a leafy shoot underwater at a slant to prevent air from entering and blocking the xylem.

3. Insert the shoot into the rubber tubing of the potometer.

4. Seal all joints with petroleum jelly to prevent leaks and make sure the apparatus is airtight.

5. Introduce one bubble of air into the capillary tube.

6. Measure the distance the bubble moves over a set time to calculate the rate of water uptake.

7. Use units such as ml per minute.

8. Repeat 3 times and calculate a mean, keeping all other variables constant.

(A reservoir or a syringe filled with water can be used to move the air bubble back to the original, starting position so the experiment can be repeated)

Key Assumptions

• Water uptake is directly proportional to transpiration rate. This is not exactly true as some of the water that enters the shoot will be used by the plant for photosynthesis.

Factors Affecting Potometer Readings

We can alter the environmental conditions when using a potometer to investigate the effects on transpiration.

• Temperature → Higher temperatures increase evaporation, moving the bubble faster.

• Humidity → Higher humidity slows the rate of transpiration.

• Wind Speed → Higher wind speed removes water vapour, increasing transpiration.

• Light Intensity → More light opens stomata, increasing transpiration rate.