Stomatal Transpiration: Plants, like other living organisms, require an excretory system to expel excess water from their bodies. The removal of surplus water from the plant body is known as Stomatal transpiration in plants. The evaporation of water from the surface of the leaves is the most common cause. The roots use some of the soil’s water, while the remainder evaporates into the atmosphere. In other terms, the stomatal transpiration mechanism is the process through which water evaporates from plant leaves and other parts into the atmosphere.

Water molecules in plant tissues are removed from the aerial regions of the plants during the transpiration process. Only a small portion of the water received by plants is used for growth and development. The remainder is expelled. Read the article for more details.

What is Stomatal Transpiration in Plants?

The loss of water in the vapour form from the exposed part of a plant is called transpiration.
Water loss is very high due to transpiration. Like in the case of a sunflower plant, it is (2,{text{L}}) per day. It varies from plant to plant. Almost (95% ) of the water is lost due to transpiration, while only (5% ) is used for its own growth and development.

Stomatal transpiration occurs through the stomatal apertures and is a required “cost” of opening the stomata to allow carbon dioxide gas to diffuse from the air for photosynthesis.

Stomatal Transpiration Definition

Transpiration occurring through stomata is called stomatal transpiration. Stomatal transpiration constitutes about (50 – 97% ) of the total transpiration.

Stomata are meant for gaseous exchange. Also, maximum transpiration occurs through it.

Structure of Stomata

1. The stomata are mostly found in the lower epidermis of the leaves.

Fig: Leaf Showing Stomata with Cuticle

2. These are minute, microscopic pores present in the epidermal surface of the leaves, young stems and in certain fruits.
3. It consists of kidney-shaped guard cells which surround the stomatal pores.
4. These guard cells are much smaller than the other epidermal cells and therefore are affected by a small change in turgor pressure.
5. Guard cells have thick walls towards the pore while thin walls are opposite to it.
6. Each guard cell consists of cytoplasm, nucleus and chloroplasts, which perform photosynthesis.
7. The cellulose microfibrils in the guard cell walls are oriented radially rather than laterally.

 Fig: Cellulose Microfibrils in Stomata Wall

8. The guard cell walls are with special elastic properties.
9. The adjoining cell walls of two guard cells around a pore are and not attached to each other. These special properties help them to stretch laterally during stomatal opening.
10. The main function of stomata is to let in $\mathrm{C}{\mathrm{O}}_2$ during photosynthesis. Transpiration occurs along with photosynthesis, i.e., water is lost from the stomatal pore when $\mathrm{C}{\mathrm{O}}_2$ gets in during photosynthesis, which occurs during daytime.
11. Due to this, it can be said that transpiration is the price that plants pay for photosynthesis.

Stomatal Transpiration Mechanism

Stomatal transpiration takes place through the following steps:

1. The water from the xylem of the leaf moves to the intercellular space above the stomata through osmotic diffusion.
2. Opening and closing of stomata.
3. Movement of water from the intercellular space to the external environment through stomata.

Theories about Mechanism of Stomatal Movement

A. Sugar Concentration Theory

This is the old theory, and according to it, during the daytime, guard cells perform photosynthesis and produce sugar (glucose) in it. Due to this, there is a decrease in water potential in the guard cells, which let the water enter it from nearby epidermal cells. This results in turgidity of the cells, which bulge outward due to their thinner outer walls—resulting in the opening of the pore.
During the nighttime, when no photosynthesis occurs, there is no sugar (glucose) formation in them, and guard cells lose their water content due to exosmosis, resulting in no turgidity of guard cells which become flaccid, resulting in the closure of stomata.

The objection to sugar concentration theory

1. Sugar formation is less in guard cells.
2. It is not confirmed that guard cells have photosynthetic activity as their chloroplasts do not have much strength for photosynthesis.
3. In some plants, stomatal pores remain open during nighttime also.

B. Starch $\rightleftharpoons$ Sugar Hypothesis

1. This theory was formulated in $1923$ by J.D Sayre and modified by Steward in $1964.$
2. Photosynthesis occurs in light by using $\mathrm{C}{\mathrm{O}}_2$ present in the intercellular spaces. Due to this ${\mathrm{H}}^{+}$concentration of cell sap decreases, resulting in increased $\mathrm{p}\mathrm{H}$ of the guard cell.
3. High $\mathrm{p}\mathrm{H}$ favours the formation of glucose-$1$- phosphate by the activity of the enzyme phosphorylase. This glucose-$1$-phosphate is changed to glucose-$6$-phosphate and finally to glucose and phosphate.
4. This glucose and phosphate increase the concentration of cell sap.
5. This concentrated cell sap decreases the water potential of the guard cells. Due to which there is the movement of water into the guard cells from the surrounding cells, resulting in turgidity of the guard cells.
6. This allows the stomatal pore to open.
7. At dark, there is no photosynthesis, resulting in a decrease of $\mathrm{p}\mathrm{H}.$
8. At low $\mathrm{p}\mathrm{H}$ glucose is converted back into starch. This results in the high water potential of the guard cell.
9. Guard cells lose water to the surrounding cells causing the guard cells to become flaccid, and the pore closes.

C. ${\mathrm{K}}^{+}$ion Concentration Theory

1. This theory was proposed by Levitt in $1974$ and elaborated by Raschke and Bowling in $1975$ and $1976,$ respectively.
2. This theory appears to be the most satisfactory mechanism.
3. During the daytime, starch present in the guard cells gets converted into malic acid in the presence of light.
4. This malic acid dissociates into malate anion and ${\mathrm{H}}^{+}$
5. ${\mathrm{H}}^{+}$ is transported to the nearby epidermal cells, and ${\mathrm{K}}^{+}$ are taken into guard cells. (shown in the figure)
6. There occurs an exchange of ${\mathrm{H}}^{+}$ and ${\mathrm{K}}^{+}$ ions which requires ATP.
7. Some chlorine, $\mathrm{C}{\mathrm{l}}^{–}$ is also taken in to neutralize a small percentage of ${\mathrm{K}}^{+}.$ (shown in the figure)
8. Increased ${\mathrm{K}}^{+}$ and malate ion in guard cells make them hypertonic. Due to which more water from adjacent cells is drawn into the cell, causing the cell to swell or become turgid. This makes the formation of a stomatal pore.
9. The reverse happens at night. The ${\mathrm{K}}^{+}$ ion leaks out, thus reducing the turgor of guard cells resulting in flaccid guard cells and stomatal pore closes.

Fig: Stomata Showing Ion Exchange

Factors Affecting Stomatal Transpiration

1. Atmospheric Humidity– If outer air is humid, the rate of transpiration will decrease.
2. Temperature– With the increase in temperature, transpiration increases.
3. Light– Transpiration increases in light and decreases in the dark.
4. Wind Velocity– It increases the rate of transpiration
5. Atmospheric Pressure– Lowering atmospheric pressure increases the rate of transpiration.
6. Soil Water– The decrease of soil water decreases the rate of transpiration.

Summary

From the above discussion, we came to know about stomatal transpiration and its mechanism. Stomatal transpiration refers to the transpiration occurring through stomata. Stomata are the microscopic structures present mostly in leaves having kidney-shaped guard cells, which open and close. In guard cells, cellulose microfibrils are arranged radially. The opening and closing of stomata are due to the change in turgor in the guard cells. When guard cells swell due to water influx, it opens while the reverse happens, i.e., it closes when guard cells become flaccid. Different scientists gave different theories for stomatal opening and closing. But ${\mathrm{K}}^{+}$ ion concentration theory is the most satisfactory mechanism proposed by Levitt in $1974.$

Frequently Asked Questions (FAQs) on Stomatal Transpiration

Q.1. How does stomatal transpiration occur?
Ans: Stomata have guard cells that surround a pore. When water enters the guard cell, it becomes turgid. The wall of the guard cell, which is thin, bulges outwards and with it pulls the thick wall side of the guard cell towards itself, resulting in the opening of the guard cell by making a crescent shape pore. When guard cells lose water, they become flaccid, resulting in the closing of stomata.

Q.2. What are the three types of transpiration?
Ans: Three types of transpiration are:-
a. Stomatal transpiration
b. Cuticular transpiration
c. Lenticular transpiration

Q.3. How much is the stomatal transpiration percentage?
Ans: Stomatal transpiration constitutes about $50–97\mathrm{\%}$ of the total transpiration.

Q.4. How do stomata work in photosynthesis?
Ans: The main function of stomata is to let in $\mathrm{C}{\mathrm{O}}_2$ during photosynthesis through the stomatal pore. This $\mathrm{C}{\mathrm{O}}_2$ is utilized by cells for the formation of glucose, the main product of photosynthesis. Transpiration occurs along with photosynthesis, i.e., water is lost from the stomatal pore when $\mathrm{C}{\mathrm{O}}_2$ gets in during photosynthesis, which occurs during daytime.

Q.5. What is an example of transpiration?
Ans: Stomatal transpiration is an example of transpiration.

PRACTICE QUESTIONS RELATED TO STOMATAL TRANSPIRATION

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