Transpiration, Practice Problems and FAQs

What happens to the extra water in your body? It’s lost from the body along with the urine, right? Did you know that only 1% of the water absorbed by the plants from the soil is used for photosynthesis? So what happens to all the extra water that the plants absorb throughout the day? It is also lost from the plant body as water vapour with the help of a process known as transpiration. 

You will be amazed to know that transpiration is one of the major factors that is responsible for the upward movement of water from the roots to different parts of the plant, against the force of gravity. But do the plants have special organs to get rid of this extra water from their body? Well, maximum transpiration in the plants occurs through special pores on the leaf surface, known as the stomata. Come, let us learn more about it.

List of contents:

• Transpiration
• Types of transpiration
• Transpiration and ascent of sap
• Transpiration and physical properties of water
• Measurement of rate of transpiration
• Factors affecting transpiration
• Significance of transpiration
• Practice problems
• FAQs

Transpiration

The loss of water in the form of vapour from the aerial parts of the plant is known as transpiration. Transpiration plays a major role in the transportation of water in plants. It mainly supports long distance transportation. 

Types of transpiration

Depending upon the structure involved in transpiration, it is mainly of four types: 

• Stomatal transpiration
• Cuticular transpiration
• Lenticular transpiration
• Bark transpiration

Stomatal transpiration

Stomata are tiny pores on the epidermis of the leaves of a plant that allow exchanges of gases during respiration and photosynthesis and are also responsible for loss of water vapour during transpiration. Transpiration occurring through the stomata is known as stomatal transpiration. Stomatal transpiration accounts for about 50–97% of the total transpiration. 

The number of stomata differs in dicot and monocot leaves. In dicot leaf (dorsiventral leaf) the lower surface has more stomata and in monocot leaf (isobilateral leaf), both surfaces have an equal number of stomata.

Each stoma consists of a complex called stomatal apparatus. It consists of the following:

• Stomatal aperture - It is the pore present in between the guard cells. Movement of gases takes place through the pore.

• Guard cells - Cells surrounding the stomatal aperture or pore are called guard cells. The guard cells possess chloroplasts and are capable of photosynthesising. The outer wall of a guard cell is thin whereas the inner wall is thick. Guard cells are bean shaped in dicots and dumb-bell shaped in monocots. 

• Subsidiary cells - Subsidiary cells are specialised epidermal cells seen around guard cells. When subsidiary cells lie above the guard cells, it is called sunken stomata.

Opening and closing of stomata

Stomata open during the daytime and closes at night. The change in the turgidity of guard cells regulates the closing and opening of the stomata. When solute concentration in the guard cells is high, the water from neighbouring cells enters the guard cells by endosmosis and the guard cells become turgid and expand. Due to the presence of a thick and rigid inner wall and a thin outer wall, expansion causes the guard cells to bulge towards the outer side and results in opening of the stomatal aperture. This mostly happens during the day when the guard cells produce sugars by photosynthesis.

When the solute concentration in the guard cells becomes less, the water from the guard cells leaks out by exosmosis. This causes the guard cells to become flaccid and collapse which is why the stomatal aperture closes.

Role of radial orientation of cellulose microfibrils in guard cells

The cellulose microfibrils in the guard cells are arranged radially for better functioning of the stomata.

The shape taken by the guard cells is dependent on cellulose microfibrils. They fan out radially from the pore. They are somewhat similar to radial tyres. The cellulose microfibrils are rigid and they do not stretch when water has entered the cell. The thick cell walls surrounding the stomatal opening prevent that side of the guard cell from expanding. Therefore, when pressure in the cell increases due to water entry, the guard cell does not widen, but rather the outer edge stretches disproportionately more than the inner edge. The pores between the two guard cells are formed by this unequal stretching.

Cuticular transpiration

In cuticular transpiration the loss of water takes place from the waxy layer (cuticle) coating surface of the epidermal cells of leaves and herbaceous stems. It accounts for 3–10% of the total transpiration.

Lenticular transpiration

Lenticular transpiration occurs through aerating pores called lenticels. Lenticels are mainly present in the cork. Lenticular transpiration accounts for 0.1% of the total transpiration.

Bark transpiration 

The transpiration that takes place through the bark of woody stems is called bark transpiration. It accounts for around 1% of the total transpiration.

Physical properties of water

Ascent of sap occurs due to transpiration. There are some physical properties of water like cohesion, adhesion and surface tension that play a major role in the process of ascent of sap.

Let’s discuss more about them.

Cohesion

The attractive force that exists between the molecules of the same substance is known as cohesion. For example, rain falls in droplets rather than a fine mist. Droplets are formed, because of the strong cohesion which pulls the water molecules tightly together. This force tends to unite the molecules of a liquid.

Adhesion

The attractive force between the molecules of different substances is known as adhesion. Example: The adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass.

Surface tension

The water molecules are more attracted to each other in the liquid phase than in the gaseous phase. This creates a tendency of the liquid surfaces at rest to shrink into a minimum surface area. This is called surface tension.

Capillary action

It is the tendency of a liquid to rise up against gravity when confined within a narrow tube. It occurs due to cohesion, adhesion, and surface tension. The xylem serves as the provider for this capillary action. It is probably the longest part of the pathway that the water takes on its way to the leaves of a plant. Capillarity is aided by the small diameter of tracheids and vessels (tracheary elements).

Tensile strength is another important property attained by the water due to the physical properties like cohesion, adhesion and surface tension.

Transpiration and ascent of sap

Transpiration causes the ascent of sap (water and dissolved minerals) through the xylem vessels. 

Air enters intercellular spaces between the mesophyll cells of leaves through the stomata. On coming in contact with the moist walls of the mesophyll cells, it gets saturated with water vapour. As this moist air moves out of the intercellular spaces, water vapour is lost.

The loss of water vapour creates a negative diffusion pressure deficit or pressure potential in the mesophyll cells which increases their ability to accept more water molecules by diffusion from the adjoining cells that has more water and higher diffusion pressure deficit. This creates a suction force or transpiration pull that extends to the xylem vessels and water molecules keep moving upwards as a continuous column due to their cohesive properties. Since, there is a continuous water column in the xylem vessels, the transpiration pull exists from the leaves, to the petioles, stem and finally to the roots, leading to upwards movement of water and dissolved minerals.

The theory of transpiration pull was given by Dixon and Jolly in 1894. Because the water molecules stick together so well due to hydrogen bonding, water can be pulled up trees 130 metres tall (that’s over 300 feet).

Measurement of rate of transpiration

Transpiration can be measured by an instrument known as Ganong’s potometer. A cut plant stem is sealed into the potometer using a rubber bung. An air bubble is introduced to the capillary tube. The distance the bubble travels shows how much water the stem has taken up. The rate of movement of the bubble through the tube indicates the rate of transpiration.

Factors affecting transpiration

Transpiration is affected by both internal and external factors. Some of the external factors are, light, temperature, carbon dioxide concentration, wind and humidity. Let's discuss more about the factors affecting transpiration.

External factors

Light

In general, as the intensity of light increases, the rate of transpiration also increases until all stomata are open.

Temperature

Increase in temperature will cause an increase in the rate of transpiration. However, at higher temperatures, stomata normally close, thereby decreasing transpiration.

Carbon dioxide concentration

An increase beyond the normal level (0.03 %) can cause stomatal closure. Lower concentration usually induces stomatal opening.

Humidity

High humidity causes a decrease in the rate of transpiration.

Wind

Strong winds can promote the rate of transpiration. If wind is not blowing, water vapours accumulate above the transpiring leaves that decreases the rate of transpiration.

Availability of Clean, Fresh Water

The rate of transpiration is inversely related to the rate at which the roots absorb water from the soil. The closing of stomata and wilting are caused by a reduction in water absorption, which lowers the rate of transpiration. 

Internal factors

Higher leaf area results in higher transpiration. 

Rolling and twisting of leaves causes a decrease in transpiration.

Thicker the cuticle, the lower the transpiration. 

Sunken stomata, as in desert plants, also decrease the rate of transpiration.

The direction in which a leaf is facing, the plant's water status, peculiarities in leaf morphology, leaf stomata count and dispersion also affect the rate at which water is lost from the plant body via transpiration.

Significance of transpiration

• As a result of transpiration pull, water and minerals are transported upwards from the roots to various regions of the plants' bodies.
• The water balance inside the plant is maintained because water is continuously eliminated from the plant body.
• It keeps osmosis going and the cells intact.
• The leaves are kept wet by hydrophilic salts that build up on the surface of them.
• It keeps the cells turgid and aids in cell division.
• The correct development of the plants is assisted by optimal transpiration.
• The evaporation of water from a tree's leaves is responsible for its cooling effect.

Disadvantages of transpiration 

During transpiration, a large amount of energy is lost. As long as transpiration continues, plants continue absorbing a lot of water that isn't essential.

Practice Problems

Q 1. Which of the following factors would increase the rate of transpiration

1. High temperature
2. Low light intensity
3. Low relative humidity
4. High wind velocity
5. Sunken stomata

a. III, I, IV, V
b. I, II, III, IV, V
c. III, I, IV 
d. III, I, IV, II, V
Answer: With increase in temperature, the rate of evaporation will increase and hence rate of transpiration will also increase.Rate of transpiration increases with increase in light intensity as long as all the stomata open up. Low relative humidity promotes more evaporation of water and hence rate of transpiration increases.High wind velocity can promote the rate of transpiration. If wind is not blowing, water vapour accumulates above the transpiring leaves that decreases the rate of transpiration.Sunken stomata in leaves is an adaptation to reduce the rate of transpiration.

Hence the correct option is c.

Q 2. Describe the role of radially arranged microfibrils in the opening and closing of stomata.
Answer: The shape taken by the guard cells is dependent on cellulose microfibrils which fan out radially from the pore. The cellulose microfibrils are rigid and they do not stretch when water enters the cell. The thick cell walls surrounding the stomatal opening prevent that side of the guard cell from expanding. Therefore, when pressure in the cell increases due to water entry, the inner wall of the guard cell does not widen, but rather the outer edge stretches disproportionately more than the inner edge. The pores between the two guard cells are formed by this unequal stretching.

Q 3. Match the following:

a. A - 2, B - 1, C - 3
b. A - 3, B - 2, C - 1
c. A - 1, B - 3, C - 2
d. A - 2, B - 3, C - 1
Answer: Stomatal transpiration accounts for 50-97% of the total transpiration in a plant. Cuticular transpiration accounts for 3-10% and lenticular transpiration accounts for only 0.1 % of the total transpiration of the plant.

Hence the correct option is b.

Q 4. Ascent of sap through xylem due to transpiration pull depends upon?

a. Mutual attraction between water molecules
b. Attraction of water molecules to other polar molecules 
c. Surface tension
d. all of the above 
Answer: The mutual attraction between water molecules is called cohesion. The adhesion of water molecules is the attraction of water molecules to the surface of xylem elements. Surface tension: Water has high surface tension due to hydrogen bonding in water molecules and in the liquid phase, there is greater attraction between water molecules than in the gaseous phase. All the above said properties of water give high tensile strength and capillary action. Tensile strength gives the ability to resist the pulling force which is caused by transpiration. Capillary action allows water to move up through the thin tubes of tracheary elements.
Hence the correct option is d.

FAQs

Q 1. What causes stomata to close in cold weather?
Answer: Plants dry up as a result of limited water intake at low temperatures. Rapid stomatal closure after initially being exposed to cold stress for hours, minimises water loss in these settings. Cold-tolerant species have this mechanism, but cold-sensitive species do not.

Q 2. How did stomata evolve?
Answer: The evolution of stomata is poorly documented in the fossil record, but they were present in land plants by the middle of the Silurian epoch. Plants which include alga-like ancestors may have modified conceptacles (specialised cavities present in algae), allowing them to develop into stomata. Stomata must have evolved at the same time as the waxy cuticle, as the combination of these two features provided a significant advantage to early terrestrial plants.

Q 3. Which trees have the most transpiration?
Answer: The areca palm, also known as Chrysalidocarpus lutescens, has one of the greatest transpiration rates of any houseplant and is particularly good at supplying moisture to indoor air.

Q 4. What is the relationship between cavitation and transpiration?
Answer: Plants must continuously ingest water with their roots to sustain the pressure gradient required for them to remain healthy. They must be capable of meeting the demands of water lost through transpiration. When a plant is unable to bring in enough water to keep up with transpiration, a phenomenon called cavitation occurs. Cavitation occurs when a plant's xylem is unable to deliver enough water, and instead of being filled with water, the xylem becomes filled with water vapour. Within the plant's xylem, these water vapour particles clump together and form blockages. This makes it impossible for the plant to move water across its vascular system. There is no discernible pattern to where cavitation occurs in the xylem of the plant. Cavitation can cause a plant to reach its permanent wilting point and perish if it is not properly managed