Plants use transpiration to regulate their internal water balance and to cool themselves. When transpiration rates are high, plants release more water vapour into the atmosphere, which helps to cool the plant body.
Water is necessary for plants, but only a tiny amount of water taken up by the roots is used for growth and metabolism! Most of the water they take up, usually up to 99%, is transpired. It may sometimes be detrimental to the plant to lose this water via transpiration, but it is a necessary process.
Transpiration also is an integral part of the hydrological cycle and plays a role in global climate change. Let's take a look at what factors affect transpiration in plants, including internal factors and external factors such as humidity and temperature.
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.
Transpiration is liquid water becoming water vapour on the plant's surface and dissipating into the atmosphere.
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.
Stomata are tiny pores, typically bordered by two guard cells, which open and close to regulate gas exchange.
When the stomata are open, water vapour escapes from the leaf, cooling the plant.
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.
Cuticular refers to the thin membrane (cuticle) which covers plant leaves.
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.
Some animals, like humans, also use transpiration to cool themselves down by sweating.
Some plants, such as the pineapple plant, do the opposite of what we expect: they keep their stomata open during the night and close them during the day!
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.
Internal factors | Description | Details |
Stomatal Density | The number and size of stomata. | The number of stomata present on a leaf surface affects the amount of water vapour that can be lost. Leaves with more stomata will lose more water vapour than those with fewer stomata. |
Leaf Area | Surface area to volume ratio; the more surface area exposed to the air, the more water vapour will be lost through transpiration. | Plants with more surface area exposed to the air generally have higher transpiration rates than those with less exposed surface area. Similarly, plants with thinner leaves will lose water more rapidly than those with thicker leaves. |
Cuticular Thickness | The thicker the layer of wax on the surface of a leaf, the less water vapour will be lost through transpiration. | When tissue is dry, it will absorb water from the surrounding air more readily, leading to increased transpiration. |
Water Potential Gradient | The pressure exerted by water loss through transpiration helps to pull up more water from the roots. | The water potential gradient is the difference in water potential between the inside of a cell and the outside environment—a higher water potential gradient results in a higher rate of transpiration. |
Table 1: Description of internal factors that affect transpiration rate in plants.
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.
For example, if the air around the leaves is arid, the plant may close its stomata to prevent further water loss. This can happen even if the light intensity is high. Similarly, if the temperature is very high, the plant may close its stomata to reduce water loss, even if the light intensity is low.
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.
In plant physiology, transpiration pull is the force created by water loss through transpiration which pulls water upwards through a plant. This force is created by the evaporation of water from the leaves, which creates a pressure gradient from the leaves to the roots. The amount of water a plant can pull up is determined by the strength of the transpiration pull and the resistance to flow within the plant.
A pressure gradient is the direction and rate of pressure increase in a particular location.
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.
External factors that increase humidity or reduce windspeed will generally decrease the transpiration rate.
Evapotranspiration from plants produces significant quantities of water vapour. This vapour leads to cloud formation, controls surface temperatures, and increases precipitation. It is an integral part of the hydrological cycle and influences weather patterns.
Fig. 6. Evapotranspiration is an important part of the water cycle.
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.
The greater the wind speed, the greater the evaporation rate, so the more significant the water loss from the leaves.
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.
The optimum temperature for plants in the temperate climate lies between 20-30 degree Celsius.
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.
Plant transpiration is an essential process in the water cycle, and it can be easily studied with two simple experiments.
A. For this experiment, you will need:
- clear glass jar/ clear bag
- plant with leaves
- clear tape
- thermometer
- First, place the plant in the jar, so the leaves are exposed to the air.
- Then, use the tape to seal the jar shut. Leave the jar in a sunny spot for 24 hours, but check the thermometer to record the mean temperature regularly.
- After 24h, check the plant to see how much water has evaporated from the leaves, by analysing or even weighing the droplets after removing the plant from the jar.
By repeating this experiment with different plants, you can learn more about how plant transpiration works and what factors affect the transpiration rate.
B. It is also possible to calculate the leaf mass. For this experiment, you'd need to weigh a leaf before and after it has been placed where humidity is controlled and left for a few hours.
The difference in the two weights is then used to calculate the rate of transpiration. We express the result in percentages (%) - the percentage decrease in leaf mass. By the way, by "weight", we mean mass!
Don’t forget that the number displayed by weighing scales is the mass of an object. Scale input otherwise comes from the weight, where weight is the pressure exercised on them, multiplied by the gravitation acceleration (9.8 m/s2).
Factors Affecting Transpiration - Key takeaways
Cuticular and lenticular transpiration are the two main ways plants lose moisture.
Internal factors affecting transpiration rates include cuticle thickness, hairiness, leaf shape and size, stomata size and density, etc.
External factors affecting transpiration rates include temperature, light, wind, etc.
Some plants may behave differently to thrive in certain areas, such as by transpiring at night instead of during the day.
Two practical experiments to observe transpiration are measuring a cut leaf's mass before and after being left in a controlled environment or quantifying the water droplets transpired by a plant sealed in a jar or bag.
References
- Fig. 1. Tomato leaf stomate 1-color (https://commons.wikimedia.org/wiki/File:Tomato_leaf_stomate_1-color.jpg) by Photohound, Public domain
- Fig. 3. Gean lenticels (https://commons.wikimedia.org/wiki/File:Gean_lenticels.JPG) by Rosser 1954 Roger Griffith, Public domain
- Fig. 4. Dew on Sage Leaves 2 (https://www.flickr.com/photos/leo_leibovici/1758285761/) by Leo Leibovici (https://www.flickr.com/photos/leo_leibovici/) under a CC BY 2.0 License (https://creativecommons.org/licenses/by/2.0/)
- Fig. 5. Salvia officinalis close up (https://commons.wikimedia.org/wiki/File:Salvia_officinalis_close_up.jpg) by Smartse (https://en.wikipedia.org/wiki/User:Smartse) under a CC BY-SA 3.0 License (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
- Fig. 7. World map indicating tropics and subtropics (https://commons.wikimedia.org/wiki/File:World_map_indicating_tropics_and_subtropics.png), by KVDP (https://commons.wikimedia.org/wiki/User:Genetics4good), Licensed by CC-BY-SA-3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
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