Simply put, transpiration is the natural evaporative cooling system by which plants take up water and nutrients through their roots, transport it throughout the plants’ tissues and then release the new moisture through the leaves and into the atmosphere.
We had known about transpiration since the early 1700s when English physiologist and botanist Stephen Hales created a way to measure the amount of water vapor plants emit.
He described the phenomenon as plants “imbibing” water and then “perspiring.”
He realized that the process of transpiration created a continuous flow of nutrients and water from the roots upward into the plants and that the liquid was then released through the leaves.
What Hales may not have realized at the time is that this process is far more a force than a flow.
Plants take water from the soil and carry it up (sometimes to great heights) against the flow of gravity to be released into the atmosphere through the plant’s leaves.
This is done through a pumping action made possible by water molecules’ strong hydrogen bonding properties and straw-like xylem water channels within the plant.
Evaporation creates a negative pressure (suction) to lift water from the earth and circulate it through plants and into their leaves to be taken into the atmosphere, as shown in this fascinating video.
Xylem & Transpiration
How Does Transpiration Work?
Plants release water through the leaves’ stomal openings, covering only about one or two percent of the leaf surface.
These openings also allow carbon dioxide to enter the leaf and let oxygen out as the plant’s photosynthesis.
For this reason, many consider transpiration a side effect of photosynthesis, which is necessary for the plant to survive and thrive.
The stomata regulates the evaporative process to some extent by using a pair of guard cells on each side of the minute openings.
As water flows upward into the guard cells, they become swollen and arch open. At this point, the water is released, and the guard cells can close again.
In addition to being sensitive to the amount of water being transported, the guard cells are also susceptible to the other factors that affect transpiration.
Such as the amount of light the plant is receiving, relative humidity, air movement and temperature, and the amount of carbon dioxide contained in the leaf.
If there is not enough carbon dioxide, the stomata open up to take in more for use in photosynthesis. When this happens, loss of water is inevitable.
This is one of the reasons why it is best to water plants early in the morning on hot summer days. They will tend to lose less water and have more available as the day heats up.
Different Types Of Plants Transpire At Different Rates & Amounts
Throughout the growing season, the leaves of plants transpire a great deal of water into the atmosphere. A mature oak tree typically occurs about forty-thousand gallons of water annually.
A mature maple tree may release as much as sixty gallons of water hourly in the summer heat. An acre of corn typically transpires up to four-thousand gallons of water daily.
Plants adapted to arid settings (xeric plants) often have small leaves with few stomata. They may also have slightly furry or silvery leaves and may produce essential oils.
All of these are adaptations that help them transpire less and preserve moisture.
Succulent plants and cacti tend not to transpire much because they have evolved to conserve water in their native arid homes.
These plants open their stomata in the evening to reduce transpiration rate as they intake carbon dioxide. They store this CO2 in their leaves for photosynthesis when the sun comes out.
Of course, in these settings, there is not much moisture in the soil for the plants to uptake.
Plants in areas where their roots have access to the water table (eg, next to a body of water) uptake more moisture and transpire more.
Environmental Conditions Affect Transpiration
Several things affect the number of water plants that transpire. For example, temperature plays a big part.
In warmer weather, plants tend to happen more because the plant cells that release the moisture are more open during warmer weather.
In very hot, dry weather, bubbles can form in a plant’s tissues that will block water flow.
Windier weather also stimulates transpiration because dry, moving air causes evaporation. When there is a breeze, moisture that tends to collect near or on the surface of plants’ leaves evaporates.
The relative humidity also plays a significant role in transpiration. If the air is more humid, plants transpire less because the air is already saturated with moisture.
Dry soil will also cause reduced transpiration because the plants need less water to uptake, circulate and release into the atmosphere.
If a plant cannot get enough moisture for transpiration, it will overheat and become wilted. This is a survival strategy intended to prevent water loss.
Of course, if this state of affairs goes on too long, the plant will dry out and die.
Transpiration & Photosynthesis Are Intertwined
About 98% of a plant’s energy is invested in transpiration, which may provide the plant with the energy it needs to conduct photosynthesis.
The moisture is drawn into the leaves and warmed by the sun so that it becomes vapor and evaporates.
This action may also help the plant stay cool in direct sunlight through the evaporation of water.
Too much transpiration is harmful to plants. If a plant loses more water than it takes in, its growth will be stunted in the short term.
If the lack of water continues, the plant will naturally dehydrate and die.
Transpiration & Life On Earth Are Intertwined
All plants participate in transpiration. It is the plant equivalent of breathing, an essential component of the maintenance of all life on earth.
It is a step in the water cycle by which moisture is taken from the ground and delivered back into the atmosphere to become clouds and produce precipitation.
Nearly 100% of the moisture taken in by plants is eventually released into the air as water vapor.
Transpiration contributes to humidity, making life on earth more comfortable for everyone and everything.
Areas that have large numbers of trees also have large amounts of precipitation.
Hence the abundant snowfall in the northern United States and the ample rain in the aptly named rain forests.
This is why responsible forest management is essential—precise cutting results in reduced transpiration and precipitation.