What is Transpiration in Plants? Process and Types

What is Transpiration?

Transpiration is the process in plants where water is released into the atmosphere in form of water vapor or moisture. The water inside the plant evaporates into the atmosphere from aerial parts of the plant such as the stems, flowers, and small pores on the leaves.

Transpiration in plants is a very essential process because, without it, excess water will get accumulated in the plant cells, which will lead to the lysis of the cells. This process literally accounts for more than 10% of the earth’s moisture and is known to be a part of the water cycle.

Water is very essential for the metabolism and growth of plants and so the plant roots take up water from the soil for use, while the rest of the water (about 97-99.5%) is lost into the atmosphere by guttation and transpiration. The leaves of plants have pores called stomata which are more numerous on the undersides of the foliages of most plants.

These stomata make transpiration in plants possible as they are bordered by guard cells and stomatal accessory cells that open and close the pore. Hence, transpiration occurs through the stomata in leaves. A stoma and its surrounding cells are referred to as stomatal complex. The plant transpiration process takes place through the stomatal apertures and the opening of the stomata allows the diffusion of carbon dioxide gas from the air for photosynthesis.

Transpiration in plants

Plant transpiration is literally an invisible process. As we humans release water vapor when we breathe that is how plants transpire. The surest way to visualize the concept of plant transpiration is to put a plastic bag around some plant leaves. You would notice that the transpired water vapor will condense inside the bag. A leaf can transpire more water than its own weight during a growing season. For example, each day an acre of corn can give off about 3,000-4,000 gallons (11,400-15,100 liters) of water. Also per year, a large oak tree can transpire 40,000 gallons (151,000 liters) of water.

Plants put down their roots into the soil to absorb water and nutrients into their leaves and stems and some of this water is returned back to the atmosphere through transpiration. Transpiration rates can vary depending on the weather conditions. These conditions include land slope, humidity, temperature, availability of sunlight, the intensity of sunlight, precipitation, wind, soil type, and soil saturation. Transpiration during dry periods can contribute to moisture loss in the upper soil zone. This can in turn affect vegetation and food crop fields.

Transpiration pull

This is the pull created in the xylem when water evaporates through the leaves of the plant. Transpiration pull aids the upward movement of water into the xylem vessels. This process can be observed in higher plants and trees. It occurs in trees and higher plants because their stems are surrounded by bundles of fine tubes made from woody material (xylem).

Mechanism

The transpiration pull can also be called a suction force as this force is used to draw water from the roots of the plant to the leaves in an upward direction. Plants use the water received by the leaves for photosynthesis and the excess water is then released to the atmosphere through the stomata of the leaf. The mechanism of transpiration pull in plants is based on water moving up from the tip of the plant root to the aerial parts of the plant.

This upward movement of water and minerals from the root to the upper part of the plant body is referred to as the ascent of sap. Water molecules combine together in the xylem to form a column. Then, the pressure formed by transpiration pull applies a force on these combined water molecules. This force helps the molecules to move into the mesophyll in an upward direction. Therefore, a negative hydrostatic pressure is generated in the mesophyll cells during transpiration which draws water from the roots of the plants to the veins of the leaves.

Role

In plants, the transpiration pull is a result of the evaporation and excretion of water from the cell’s surface in the leaves. This pull protects the plant from embolism and helps in the appropriate flow of water within the plant.

Water molecules evaporate from the stomata during plant transpiration and in the process, water concentration is reduced in mesophyll cells. This results in lowering the cells sap of mesophyll compared to that of the xylem vessels. Then, negative pressure in xylem vessels is generated to pull the water from the soil which eventually leads to the upward pull of water from the root to the mesophyll cells.

Transpiration rate

The rate of transpiration in plants has a correlation with the absorption rate of water by the root of the plant. A plant may wilt if there is a scarcity of water or the plant roots get damaged. Once there is an increase in the transpiration rate, the absorption rate of water by the root increases as well. However, there are factors that affect the transpiration rate in plants as well as the water uptake by the plant.

Factors affecting transpiration in plants

• Cellular factors
• Environmental factors

Different factors affect the transpiration rate in plants and these factors are grouped into two:

Cellular factors

• The orientation of leaf
• Water status of the plant
• Structural Peculiarities of the leaf: A leaf that has a larger surface area will have more transpiration rate than the leaf with a smaller surface area.
• Total number and distribution of stomata in a leaf: The lower side of leaves in dicots have more stomata while in monocots, both sides of the leaves have an equal number of stomata.

Environmental factors

• Light
• Humidity
• Temperature
• Air
• Wind speed or velocity
• Water

Light

Once the light intensity is increased, the stomata open wider to admit more carbon dioxide into the leaf for photosynthesis. Therefore, the presence of light is directly proportional to the transpiration rate. This is because stomata close in the dark and open during the day when there is sunlight.

Humidity

The relative humidity is one of the factors affecting transpiration. If the leaf is surrounded by moist air, the diffusion of water vapor out of the leaf slows down. This means that the rate of transpiration is inversely proportional to relative humidity. Therefore, the more the relative humidity, the less the transpiration rate.

Temperature

Once there is an increase in temperature, evaporation and diffusion are faster. They tend to be faster at higher temperatures. The fact is a high temperature would lower the relative humidity and even in darkness open the stomata. Due to this, the rate at which plants transpire increases.

Air

The transpiration rate would be low if the air is still. This is because water vapor gathers around the transpiring organs, thus, reducing the diffusion pressure deficit of the air. So, once the air is moving, the transpiration rate increases as the saturated air around the leaves are removed.

Therefore, when moving air removes water vapor from the leaves, the rate of water vapor diffusion from the leaf is increased. The amount of water vapor in the air at a particular temperature and time is expressed as the percentage of water vapor needed for saturation at the same temperature.

Wind speed or velocity

There is an increase in the diffusion rate of water vapor from leaves because moving air tends to remove water vapor from the leaves.

Water

The availability of water is one of the factors affecting transpiration in plants. Once there is a decrease in the water absorption, the closure of stomata and wilting takes place. This reduces the rate of transpiration. Therefore, the rate of transpiration is directly proportional to the absorption of water from the soil by the plant’s root.

Transpiration process

The process of transpiration involves the removal of water molecules from the aerial parts of the plants. Plants utilize only a small amount of water for growth and development and end up eliminating the rest through the process of transpiration. The stomatal opening plays a major role in the transpiration process. They admit carbon dioxide to the leaf interior and during photosynthesis, allow oxygen to escape.  Transpiration occurs through the leaves and encourages a continuous upward flow of dissolved nutrients and water from the roots. Studies show that 99% of this water taken up by the root is released into the air as water vapor.

Plants like other extant organisms need an excretory system to enable them to discharge excess water from their body. This is why transpiration takes place in plants in order to eliminate the excess water from their body. It involves water evaporating from the surface of their leaves. However, in as much as transpiration helps eliminate excess water, a plant can be affected adversely if transpiration is excessive. The growth of a plant can be retarded if the water loss exceeds the water intake and due to dehydration, the plant can even die.

Furthermore, transpiration changes the osmotic pressure of the plant cells, cools the plants, and allows the mass flow of nutrients and minerals from the roots to shoots.  The rate of water flow from the soil to the roots is affected by two main factors. These factors include the magnitude of the pressure gradient through the soil and the hydraulic conductivity of the soil. They both influence the rate of bulk flow of water that moves from the roots to the stomatal pores in the leaves through the xylem.

Capillary action drives the mass flow of water from the roots to the leaves in parts. However, this mass flow of water is driven primarily by the water potential differences. Water vapor will travel down the gradient and move from the leaf airspace to the atmosphere if the water potential in the ambient air is lower than the one in the leaf airspace of the stomatal pore. This movement lowers the water potential in the leaf airspaces and causes water to evaporate from the mesophyll cell walls.

The evaporation then increases the tension on the water menisci in the cells. It also decreases the radius of the cell wall and the tension exerted on the water in the cells. However, due to the cohesive properties of water, the tension moves through the leaf cells to the xylem of the leaf and stem. As water is pulled up the xylem from the root, a momentary negative pressure is created. On the surface of the leaf, evaporation takes place as the properties of adhesion and cohesion work sequentially to pull water molecules from the root. The water molecules are passed through xylem tissue and then through the stomata, it goes out of the plant.

The decrease in hydrostatic (water) pressure in the upper parts of the plants overcomes the force of gravity in taller plants and trees. This is due to the diffusion of water out of the stomata and into the atmosphere. At the roots, water is absorbed by osmosis and any dissolved mineral nutrients move with it through the xylem. Therefore, how leaves pull water through the xylem is explained by the cohesion-tension theory. Water molecules exhibit cohesion and this is why a water molecule pulls an adjacent water molecule as it evaporates from the leaf surface. This creates a continuous flow of water through the plant.

Leaf transpiration

The majority of the transpiration process takes place in the leaf. Leaf stomates are the main sites of leaf transpiration and they consist of two guard cells. These guard cells form a small pore on the surfaces of leaves. The stomata, therefore, consist of a pair of guard cells with an aperture in between the cells.

It is these guard cells that regulate the opening and closing of the stomata in response to several environmental stimuli. They can also regulate the leaf transpiration rate to reduce water loss. The stomata during the day remain open and close at night. The reason for the stomata opening and closing is a result of the turgidity of the guard cells.

The guard cells have an interior wall that presents towards the aperture. This interior wall is flexible and dense giving out a crescent shape presentation while the exterior wall bulges out. When the turgidity of the guard cells increases, the stomata open and when the turgidity of the guard cells decreases, it closes.

The turgidity of the guard cells decreases as a result of water loss and as the stomata close, the interior wall forms a crescent shape returning back to its original shape. Moreso, the radial orientation of the microfibrils in the guard cells also plays an essential role in the opening of the stomata. This radial orientation makes it easier for the stomata to open.

Factors like darkness and internal water deficit tend to close stomata and reduce leaf transpiration. While, factors like optimum temperature, illumination, and ample water supply open stomata and increase transpiration. Under high-temperature conditions or high concentrations of carbon dioxide gas, many plants close their stomata to reduce evaporation. The lower side of leaves in dicots have more stomata while in monocots, both sides of the leaves have an equal number of stomata.

Adaptation

There are a number of adaptations that plants adapt to help reduce water loss during the process of transpiration. Usually, plants that inhabit areas with low humidity possess leaves with less surface area so that evaporation is limited. Whereas, plants inhabiting humid areas like those in low light conditions such as understory vegetation may possess large leaves. This is because the need for enough sunlight is heightened and there is a low risk of detrimental water loss.

During drought periods, there are several desert plants that possess minute leaves that are deciduous. These leaves during the dry season nearly eliminate water loss. Plants like the cacti even lack leaves. Leaf adaptations like waxy cuticles, sunken stomates, trichomes (leaf hairs), and others help to reduce evaporation and transpiration rate in the plant. This is done by protecting the surface of the leaf from air currents that increase evaporation or by keeping the surface of the leaf cool.

Furthermore, some plants in order to minimize transpiration have evolved alternative photosynthetic pathways e.g crassulacean acid metabolism (CAM). These plants as well as many succulents, at night, open their stomata to take in carbon dioxide and close them when the conditions are usually dry and hot during the day.

Plants can regulate the rate of evaporation and transpiration by controlling the size of their stomatal apertures. Transpiration rate is also influenced by the evaporative demand of the atmosphere that surrounds the leaf such as humidity, wind, boundary layer conductance, and incident sunlight. Stomatal opening and transpiration rate can be influenced also by above-ground factors, soil moisture, and temperature.

Transpiration in water cycle

One of the processes involved in the water cycle is transpiration. The water cycle involves the continuous circulation of water within the earth and the atmosphere. There are many processes involved in the water cycle and the most important include transpiration, evaporation, precipitation, condensation, and runoff. Within the cycle, the total amount of water remains essentially constants. However, the distribution of water among the various processes changes continually.

Evaporation is one of the major processes in the water cycle that involves the movement of water from the surface of the earth to the atmosphere. Through the process of evaporation, water in its liquid state is changed to a gaseous or vapor state. This evaporation process occurs when some of the molecules in the water mass have gotten sufficient kinetic energy to exit themselves from the water surface.

The primary factors that affect evaporation include wind speed, humidity, temperature, and solar radiation. However, measuring evaporation directly is quite difficult and only possible at point locations. The ocean is the main source of water vapor, though evaporation also takes place in soils, ice and snow. When there is evaporation from snow and ice which is the direct conversion of solid to vapor, it is called sublimation.

Transpiration in the water cycle is also a crucial process that involves the evaporation of water from plants through the minute pores or stomata in the plant’s leaves. Water is very crucial to plants but they take up only a small amount. The small amount of water that the roots of the plants take up is used for metabolism and growth while the remaining 97-99.5% of water is lost by guttation and transpiration.

Capillary action may drive the mass flow of liquid from the root of the plant to the leaves. However, the water vapor travels down the gradient and move from the leaf airspace to the atmosphere once the water potential in the ambient air is lower than the water potential in the leaf airspace. This means that the water potential difference is the primary driving force for the mass flow of water.

The amount of water plant losses can also depend on the plant size and the amount of water it absorbs from the soil with its root. Transpiration accounts for most of the water loss by plants through their leaves and young stems. It serves to evaporatively cool plants because the evaporating water carries heat energy away due to the larger latent heat of vaporization of 2260 kJ per liter.

Hence, in the water cycle, the transpiration and evaporation of water from all water bodies, soils, vegetation, ice, snow, and other surfaces are summed together and called evapotranspiration or total evaporation.

Evapotranspiration

Evapotranspiration is the sum-up of water loss through the evaporation and transpiration of water from a surface area to the atmosphere. Water is lost from the soil through evaporation from the surface of the soil and by transpiration from the leaves of plants that grow on the soil. Therefore evapotranspiration is a very crucial part of the water cycle.

Evaporation accounts for the movement of water from sources such as canopy interception, soil, and water bodies to the air. Transpiration, on the other hand, accounts for the movement of water within a crop and the subsequent exit of the water from the crop as water vapor. In vascular plants, water exits the plants through the stomata in the leaves whereas, in nonvascular plants, it exits through the phyllids. This means that evapotranspiration (ET) accounts for the majority of water that is lost from the soil while a plant grows. A plant is said to be an evapotranspirator when it contributes to evapotranspiration.

Estimating the rates of evapotranspiration is therefore important when planning irrigation schemes. However, there are factors that affect the rate of evapotranspiration which include the amount of solar radiation, temperature, soil factors, wind, and atmospheric vapor pressure. Soil factors like the available soil moisture, the depth of the water table, and the density of vegetation really have a great influence on evapotranspiration. Moreso, factors like plant morphology, plant cover, crop geometry, and root depth of the plant also influence it.

Since evapotranspiration is characterized as the total losses of water from vegetation, it is difficult to determine evaporation separately from transpiration in the cropped areas. Hence, the evaporation process of water from the soil surface and the transpiration process from plants are combined in the single term evapotranspiration.

Types of transpiration

• Stomatal transpiration
• Lenticular transpiration
• Cuticular transpiration

Transpiration in plants are of three types:

Stomatal Transpiration

The evaporation of water from the stomata of plants is stomatal transpiration. The majority of the water transpired by plants is done through this type of transpiration. When the stomata of the plants open, the water near the leaves’ surface changes into vapor and evaporates.

Lenticular Transpiration

The evaporation of water from the lenticels of plants is lenticular transpiration. The bark of branches and twigs of plants have minute openings called lenticels. These lenticels, however, are not present in all plants, and only a minimal amount of water is lost through them.

Cuticular Transpiration

The evaporation of water from the cuticle of plants is called cuticular transpiration. There is a waxy covering on the surface of the plant leaves called the cuticle. The stomata close during dry conditions and as a result, more water is lost through the cuticles. Through cuticular transpiration, about 5-10% of the total transpiration of water is lost. This type of transpiration depends on the following:

• The thickness of the cuticle on the surface of the leaves
• Absence or presence of wax coating on the leaves’ surface

Generally, xerophytic plants in order to check cuticular transpiration have a very thick cuticle and wax coating on their leaves and stem.

Why is transpiration important for plants?

• One of the importance of transpiration is the conduction of minerals and water to different parts of the plants.
• There is a balance of water maintained in the plant as water is continuously transpired from the body of the plant.
• Another importance of transpiration is that it maintains osmosis and keeps the plant cells rigid.
• The transpiration pull force that is created as a result of transpiration helps in the upward movement of water in transpiring plants.
• The cooling effect that a tree has is a result of the evaporation of water from its leaves.
• In transpiring plants, certain hydrophilic salts gather on the surface of the leaves that helps to keep the leaves moist.
• Transpiration is important in plants because it helps in their growth.