Plant Transpiration Basics

What is Plant Transpiration?

Plant Transpiration is a term that describes what occurs in all living trees, grass and plants. Transpiration in plants is the upward internal movement of water that begins in the roots and ends when water is released into the air, via the stomata, as water vapor.

(The term “Translocation” refers the downward movement through the phloem of sugars produced in the leaf by photosynthesis.) Transpiration in plants occurs in all living species with few exceptions, so whether you own an orchard or are growing a lawn, transpiration is an essential active process.

In fact, almost 100 percent of water taken in by plants is eventually released into the atmosphere. Water traveling through channels within the plant is used to transport essential nutrients, ensure proper plant function, and necessary for survival.

Large trees can absorb many gallons of water each day. The water entering through the roots slowly moves toward the leaves where it is released through the stomata. The stomata are tiny openings on the leaf surface through which gas exchanges take place. Here is what happens.

In trees, for example, water enters the plants though millions of tiny root hairs. One of the function of roots is to filter out particles and bubbles that could hinder water movement. Any bubbles that escape is later captured in other places in the plant. After a time, the water starts to move upward through channels in the xylem.

How Does Water Move Upward in Plants?

You may be wondering how water moves upward against gravity. For effective plant transpiration to occur, water molecules bond together in a process called cohesion, known as the “Cohesion-Theory”.

This cohesion, sometimes referred to as surface tension, is what allows some insects, small rodents such as shrews, and certain lizards to actually walk across the surface of water. Inside the plant, water molecules bond to each other forming a continuous strand within the xylem.

Molecules at the leaf’s surface escape through the open stomata. The cohesion of water molecules actually pulls the remaining water upward as it takes the place of the evaporated water molecules.

Why is this Large Intake and Water Loss Necessary?

It is all wrapped up in photosynthesis. The absorption of carbon dioxide is necessary for photosynthesis and plants absorb a tremendous amount of CO₂. Plant photosynthesis produces all of the food the plant uses to live on.

However, carbon dioxide cannot enter a cell in a gas form. It must be diffused first and water is necessary for diffusion. Plant transpiration and photosynthesis are closely tied together and one cannot exist for long without the other.

The Stomata’s Role in Plant Transpiration

Large plants will have literally thousands of stomata. The stomata (singular is Stoma) are tiny openings on the leaf’s surface that opens and closes and is responsible for the release and absorption of gases.

Plant transpiration may be controlled by several factors, but nothing happens unless the stomata are open. The major factors controlling stomata are air temperature, humidity, availability of soil water, and light intensity.

Guard cells around the stomata are responsible for its opening and closing. During the day carbon dioxide is absorbed through the stomata, while oxygen and water vapor is released.

Carbon dioxide is necessary for carbon fixation, which is an essential element of photosynthesis. In the evening the opposite occurs where oxygen is absorbed and carbon dioxide is released.

Gas exchange cannot take place when the stoma is closed, therefore no transpiration in plants occurs in winter on deciduous plants after the leaves have fallen.

Drought Conditions and the Stomata

During drought when water is scarce, the stomata will close to prevent water from escaping and causing the tree to weaken or die. We can apply chemicals to close the stomata to prevent the tree from being damaged. See information below on antitranspirants.

Note: Dormant season can be pruning season for a lot of trees. However, some trees should never be pruned during the dormant season. Trees, such as maples, elms, etc. have positive pressure inside the tree when no leaves are present and when plant transpiration is not occurring. If a branch is cut in the winter or if no leaves are on the tree, the tree will often bleed sap for weeks.

The term “bleeding” describes how water and plant juices ooze from a wound or cut on a tree and may continue for days. Sometimes it develops a bad smell. It's called "wetwood", when this happens because bacteria enter the wound creating not only dark patches on the bark, but the bacteria is also responsible for the bad odor.

Many gallons of fluids can ooze from wounds of larger trees, even pooling on the ground, creating an unsightly mess. Once the bud breaks in spring and leaves are forming, transpiration in plants resume again. Plant transpiration equalizes the pressure and no bleeding will occur when the tree is pruned.

Using Antitranspirants to Slow Transpiration

Antitranspirants are chemicals that are sprayed on plants to protect them from drying out too quickly. They come in two types: metabolic antitranspirants and film-forming antitranspirants.

Metabolic types work by influencing the closing of stomata. Film-forming types work by coating the leaf surface and blocking the stomata opening. Both types are designed to prevent water loss through plant transpiration, but the film-forming types generally have a greater longevity.

Antitranspirants are used on Christmas tree farms, on cut flowers, and are sprayed on trees during drought conditions and are used on newly transplanted plants.

Common Disagreements about Antitranspirant Research

While antitranspirants are widely used in many parts of the green industry, no all groups agree with the popular research, especially concerning live plants. For example, many manufacturers of antitranspirants state that gas exchange is not hindered, yet is very effective in slowing water vapor loss.

However, other researchers say that blocking of the stomata does prohibit essential gas exchange. Since plants need to absorb carbon dioxide for photosynthesis to take place, blocking or preventing the opening of the stomata only harms the plant. The concern is not really a problem for cut plants and flowers.

For best results on living plants, use antitranspirants as directed and only when needed. Plant transpiration is a necessary function of living plants, but preventing water loss under certain conditions may outweigh the concerns.

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