Factors Affecting Transpiration

What is Transpiration?

Transpiration is the movement of water through a plant and the evaporation from aerial parts (as opposed to terrestrial, e.g. roots), such as from leaves, stems, and flowers.

Plants lose water mainly through the stomata of their leaves. This is called cuticular transpiration. Stomata are like tiny gaps. Only one opening will be called a stoma.

Fig. 1: Tomato leaf stoma, viewed through an electron microscope.

Fig. 2: Diagram of a plant leaf, also showing the cuticle. The cuticle can reduce transpiration rates.

Plants also transpire through lenticels, which are small gaps in their bark. Lenticular transpiration is affected by the same factors as stomatal transpiration (air humidity, soil moisture, etc.) but is usually less significant in terms of water loss.

Fig. 3. Lenticels in the trunk of a tree.

Factors Affecting Transpiration in Plants

The main driving force for transpiration is the evaporation of water from the surfaces of cells in the plant body. This process occurs when water vapour diffuses out of cells and into the atmosphere. This is also called cuticular transpiration.

Most transpiration occurs when the stomata open. However, the stomata open primarily to allow the plant to exchange gases for photosynthesis. Two gases exchanged through a plant's stomata are oxygen (O₂) and carbon dioxide (CO₂).

Several factors can affect the rate of transpiration, including temperature, humidity, air movement, and light intensity.

• Warmer temperatures generally lead to higher transpiration rates, while higher humidity levels decrease the rate of water loss.

• Additionally, windy conditions and bright sunlight also increase the rate of transpiration.

Together, these factors play an essential role in determining the overall rate of water loss for a given plant.

One reason plants transpire is that they need to cool down. But plants can also control their rate of transpiration. When night falls, plants minimize transpiration by closing their stomata.

Internal Factors Affecting Transpiration

Several internal factors contribute to the rate of transpiration, including leaf area and the number of stomata, as it is summarised in table 1.

Fig. 4: Sage plant covered in dew presents adaptations to dry mediterranean conditions, such as the thickening of its waxy cuticle, hairiness, and even reduced stomatal density.

Fig. 5: Hairs on a leaf surface reflect sunlight away from the cuticle and reduce air movement, leading to lower transpiration rates.

External Factors Affecting Transpiration

There are several external factors which can affect transpiration, as mentioned in the sections above, such as:

• Light intensity - higher light intensity levels will generally result in greater transpiration rates. This is because plants open their stomata in response to light, allowing water vapour to escape from the leaves. However, there are some exceptions to this rule.

• Temperature - as temperature increases, so does the rate of transpiration. The lower the temperatures, the lower the transpiration rate.

  Guard cells tend to close the stomata in windy conditions to prevent that.

• Wind - generates more water loss because they lead to higher evaporation rates.

  Guard cells tend to close the stomata in windy conditions to prevent that.

• Humidity - low humidity means the plant needs to retain more water and typically develops leaves with a lesser surface area, thicker cuticles, etc.

The Effects of Humidity on Transpiration

When the air is humid, there is less of a difference between the atmospheric pressure of water vapour and the pressure inside the leaf cells. As a result, water molecules are less likely to diffuse out of the leaf cells, and transpiration rates decrease. There is already a lot of water in the atmosphere in this scenario. However, plants can adapt to high humidity conditions by opening their stomata, or pores, to allow more water vapour to escape. This process will enable plants to continue transpiring rapidly, even in humid conditions.

The Effect of Wind on Transpiration

The faster the wind speed, the greater the transpiration rate (if all other conditions are constant). This is because the movement of air across the surface of leaves causes evaporation of water from the mesophyll cells, and this loss of water must be replaced by water moving up the xylem vessels from the roots.

To maintain a constant water balance, plants must increase their transpiration rate.

The Effect of Temperature on the Rate of Transpiration: How to Measure This in an Experiment

The rate of transpiration increases with an increase in temperature. The rise in temperature causes an increase in the evaporation of water from the surface of leaves. This results in an increased demand for water by the plants, leading to an increased transpiration rate.

Fig. 7: The temperate climate zones are located north and south of the subtropics.

At higher temperatures, the stomata of the leaves close to prevent further water loss, reducing the transpiration rate. However, at very low temperatures, the plants enter a state of dormancy and transpiration comes to a standstill. Therefore, temperature is a major external factor affecting plant transpiration.