What is it called when water evaporates through the stomata of leaves?

Dave Campbell explains that evaporation occurs when water changes from a liquid state to a gaseous state. It can happen anywhere there is water – in the soil, lakes, oceans and plants. When it occurs in plants, water is lost through microscopic pores in the plant’s leaves (stomata). This process is called transpiration. Transpiration differs from evaporation not only because it occurs in plants, but also because the plants have some control over how much water they lose. Plants can actively open and close their stomata, limiting how much water the plant will lose.

Points of interest: Think about what affects the rate of transpiration.

Transcript

DR DAVE CAMPBELL
Evaporation is defined as the physical process by which water changes state from a liquid to a gas, so wherever there is liquid water – whether it’s in the oceans, or in a river, or a lake, or whether it's in the soil, or within plants – that liquid water can be turned to water vapour, and those water vapour molecules can move into the atmosphere.

Transpiration is just a subset of the evaporation process, and it’s the water that is inside plants, and then when that evaporates and moves into the atmosphere, that process is called transpiration. And it’s very much part of the living processes of the plant. It’s all occurring within a cycle – and I guess when it comes to plants, we talk about a daily cycle – and that cycle is driven by the sun. Obviously at night time, there is no solar radiation, so there is a lot less energy for driving evaporation or transpiration. And during the middle part of the day, around noon, we've got the maximum amount of solar radiation, so there is more energy, so you tend to get highest evaporation rates during the middle part of the day. Plants are a little bit different. They are active, actively involved – the microscopic pores on the surface of plant leaves, called stomata, are very active in restricting or regulating water loss, so that is another factor that comes in.

Acknowledgements:
Dr David Campbell, Waikato University

Evaporating lake image by Michael Shake, licensed through 123RF Ltd
Geothermal steam still image by Kavram, licensed through 123RF Ltd
Steam on road, Dance Floor Lizard
Atmosphere image courtesy of Aerospaceweb.org

Why do plants transpire?

Evaporative cooling: As water evaporates or converts from a liquid to a gas at the leaf cell and atmosphere interface, energy is released. This exothermic process uses energy to break the strong hydrogen bonds between liquid water molecules; the energy used to do so is taken from the leaf and given to the water molecules that have converted to highly energetic gas molecules. These gas molecules and their associated energy are released into the atmosphere, cooling the plant.

Accessing nutrients from the soil: The water that enters the root contains dissolved nutrients vital to plant growth. It is thought that transpiration enhances nutrient uptake into plants.

Carbon dioxide entry: When a plant is transpiring, its stomata are open, allowing gas exchange between the atmosphere and the leaf. Open stomata allow water vapor to leave the leaf but also allow carbon dioxide (CO2) to enter. Carbon dioxide is needed for photosynthesis to operate. Unfortunately, much more water leaves the leaf than CO2 enters for three reasons:

  1. H2O molecules are smaller than CO2 molecules and so they move to their destination faster.  
  2. CO2 is only about 0.036% of the atmosphere (and rising!) so the gradient for its entry into the plant is much smaller than the gradient for H2O moving from a hydrated leaf into a dry atmosphere.  
  3. CO2 has a much longer distance to travel to reach its destination in the chloroplast from the atmosphere compared to H2O which only has to move from the leaf cell surface to the atmosphere.

This disproportionate exchange of CO2 and H2O leads to a paradox. The larger the stomatal opening, the easier it is for carbon dioxide to enter the leaf to drive photosynthesis; however, this large opening will also allow the leaf to lose large quantities of water and face the risk of dehydration or water-deficit stress. Plants that are able to keep their stomata slightly open, will lose fewer water molecules for every CO2 molecule that enters and thus will have greater water use efficiency (water lost/CO2 gained). Plants with greater water use efficiencies are better able to withstand periods when water in the soil is low.

Water uptake: Although only less than 5% of the water taken up by roots remains in the plant, that water is vital for plant structure and function. The water is important for driving biochemical processes, but also it creates turgor so that the plant can stand without bones.

What is the evaporation of water from the stomata called?

Transpiration is the evaporation of water from plants occurring at the leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis. From: Key Concepts in Environmental Chemistry, 2012.

When water evaporates from plants What is it called?

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. The remaining 97–99.5% is lost by transpiration and guttation.

Why does water evaporate from stomata?

Because there is a higher concentration of water vapor inside the leaf than outside, the water vapor moves out of the leaf through the stomata by diffusion. This process is known as evaporation.