Transpiration in Plants – SmartStudent Academy

Transpiration is a process that involves loss of water vapour through the stomata of plants. Transpiration is thought to be a ‘necessary cost or evil’ to allow the plant to absorb water from the soil. It is an inevitable process. Turgidity, or turgor pressure, refers to the water content of cells and how this lends structural support to the plant. When cells absorb water, the vacuoles fill up and the cytoplasm increases, pushing against the cell membranes, which in turn push against the rigid cell walls. This makes the cells rigid, or turgid. Transpiration is important in plants for three major reasons:

Cooling of the plant: the loss of water vapour from the plant cools down the plant when the weather is very hot.
The transpirational pull: when the plant loses water through transpiration from the leaves, water and mineral salts from the stem and roots moves, or is `pulled’, upwards into the leaves. Water and is therefore taken up from the soil by osmosis and finally exits the plants through the stomata.
Plant structure: young plants or plants without woody stems require water for structural support. Transpiration helps maintain the turgidity in plants.

Transpirational pull: results from the evaporation of water from the surfaces of the mesophyll layer in the leaf to the atmosphere, through the stomata. Evaporation of water from the leaves surface causes a negative pressure (suction force) in the xylem that pulls water from the roots and soil. This results in water being drawn up the xylem vessel.

Transpirational pull draws water from the roots to the leaves because of the effects of capillary action. The primary forces that create the capillary action are adhesion and cohesion. Adhesion is the attraction that occurs between water and the surface of the xylem, and cohesion is the attraction between water molecules.

We will revisit transpirational pull and capillarity later in the chapter when we examine how water is transported in the plant.

Capillary action occurs when the adhesion of water molecules to the walls of the vessel is stronger than the cohesive forces between the water molecules. Have you ever seen fluid in a drinking straw move higher than the level of the fluid in the glass? This happens due to capillary action. The narrower the straw, the greater the capillary action, and therefore, the higher the fluid will rise in the straw.

Cohesion refers to the intermolecular, attractive forces that hold molecules in solids and liquids together. Imagine a drop of water on a waxy surface like wax paper. Even if the drop slides and rolls around, the water molecules will stay together due to the cohesive forces. Adhesion is the ability of a substance to stick to an unlike substance. If you were to take the same piece of wax paper and turn it upside down, some water droplets would still adhere to the paper. This indicates that there must be an attraction between the water and the wax paper. However, in this case the water-water cohesive force is stronger than the adhesive force between the molecules of the wax paper and the water.

Factors affecting the rate of transpiration

There is close inter-relationship between transpiration and leaf structure . the rate at which transpiration occurs refers to the amount of water lost by plants over a given period of time . Plants regulate the rate of transpiration by opening and closing the stomata. There are, however, a number of external factors that affect the rate of transpiration, namely: temperature, light intensity, humidity, and wind.

Figure 2. The opening and closing of stomata. Different environmental conditions trigger both the opening and closing of stomata.

Temperature

Temperature affects the transpiration rate in two ways. Firstly, at warmer temperatures water molecules move faster, and the rate of evaporation from stomata is therefore much faster. Secondly, the water-holding capacity of warm air is greater than that of cold air. Assuming that cold air and warm air contain the same amount of water, the cold air may be saturated, and therefore have a shallow water concentration gradient, while the warm air may will be able to hold more water vapour, and will therefore have a steeper water concentration gradient.

Temperature vs transpiration rate.

Light intensity

At high light intensity, the rate of photosynthesis increases. As photosynthesis increases, the amount of stored glucose in the guard cells increases. This lowers the water potential of the leaf (i.e. the contents of the leaf are less dilute). As the water potential decreases, more water enters the guard cells making them more turgid. The turgor pressure of the guard cells leads to an opening up of stomata resulting in transpiration.

Transpiration vs light intensity.

Relative humidity

The amount of water vapour in the air is referred to as the humidity. Water always moves down a concentration gradient. Therefore when the humidity is high (lots of water vapour in the air) the water potential gradient between the inside of the leaf stomata and the atmosphere is shallow and the rate of transpiration will be low. However, if the atmosphere is dry, there will be a steep water concentration gradient between the humid inside of the stomata and the outside air and the rate of transpiration will therefore be fast.

Transpiration vs humidity.

Wind

When water is lost from the leaf it forms a thin layer outside the leaf. This reduces the water potential between the leaf and the atmosphere outside. When there is wind, this layer is blown away, thus maintaining the water potential gradient across the leaf.

Wind speed vs transpiration.

Structural adaptations of plants to reduce rate of transpiration

When the rate of transpiration is too high, it can have detrimental effects on the plant, as you will see in the next section on wilting and guttation. For this reason, plants have developed structural adaptations to minimise the amount of water loss.

Position of stomata: Stomata are found on both surfaces of the leaf but there are usually more on the ventral (lower) surface of the leaf. This means that less water vapour is lost because the ventral side of the leaf is in the shade and therefore does not get as hot.

Sunken stomata: some plants such as xerophytes have sunken stomata as a way of preventing water loss. Xerophytes (pronounced “zero-phytes”) are plants that are normally found in hot, dry areas such as deserts. The sunken stomata creates a small pocket of moist air. The high humidity in the air pocket reduces the water potential gradient between the leaf air spaces and the exterior, and therefore decreases the rate of transpiration.

Thickened cuticle: Some plants that occur in dry places have a thick cuticle that reduces transpiration.

Hairs on leaves: Hairs trap a small layer of water vapour that works in three ways to reduce transpiration:

Creates a pocket of moist air to reduce the water potential gradient.
Increases the sheen on leaves to make them more reflective.
The combination of the above effects result in a cooling effect that also decreases transpiration.

Reduction of leaf size: Small leaves have a smaller surface area for transpiration to occur.

Leaf spines: Some plants have spines instead of leaves. Spines usually have thicker cuticles and a very small surface area, which decreases transpiration.

Leaf arrangement: vertical leaf arrangement (like proteas) decrease the surface area exposed to the sun in the heat of the day, In rosette arrangements the upper leaves shield the lower leaves from the Sun.

Rolling of leaves: When leaves roll up, water vapour gets trapped in the tunnel made by the leaf, therefore reducing the water potential gradient, and therefore reducing the rate of transpiration.

Uptake of water and minerals in the roots

In the first section of this chapter, we looked at the structure of the dicotyledonous root and stem and compared the different cells in the specialised tissues of the plant root and stem. Now we will look at how these specialised cells help the plant to absorb water from the soil and transport it to the stem, where it can then be transported to the rest of the plant.

Movement of water through the dicotyledonous root

Water is found in the spaces between the soil particles. Water and mineral salts first enter through the cell wall and cell membrane of the root hair cell by osmosis. Root hair cells are outgrowths at the tips of plants’ roots. They function solely to take up water and mineral salts. Root hair cells do not perform photosynthesis, and do not contain chloroplasts as they are underground and not exposed to sunlight. These cells have large vacuoles which allow storage of water and mineral salts. Their small diameter (5-17 micrometres) and greater length (1500 micrometres) ensure they have a large surface area over which to absorb water and mineral salts. Water fills the vacuole of the root hair cell.

The following list summarises how the root hair is adapted to absorb water from the soil:

There are many, elongated root hairs to increase the total root surface area for water absorption.
They have thin walls to speed up the intake of water by osmosis.
They have large vacuoles to absorb water quickly and transport it to the next cells.
The vacuoles have salts, which speed up water absorption from soil water.
Root hairs do not have cuticles, as this would prevent water absorption.

Water can now move from the root hair cells and across the parenchyma cells of the cortex in two major ways. Some water passes through the cells by osmosis. Most water travels either in, or between the cell walls (of the parenchyma cells) by simple diffusion. The water must pass through the endodermis to enter the xylem. Once water is in the xylem of the root, it will pass up the xylem of the stem.

Step-by-step transport of water in plants, from the roots to the xylem

Transport of water and minerals to leaves

We have dealt with the transport of water in plants from the soil into the root xylem. Now we need to discuss how the water is transported against gravity from the roots to the leaves where it is needed for the process of photosynthesis.

Water travels to the leaves via the stem. Recall, that three processes are necessary for the transport of water in plants, namely; transpiration, capillarity and root pressure. All three of these processes are passive and do not require an input of energy.

Transpiration: constant water loss via transpiration from the leaves causes a negative water pressure in the leaves. The negative pressure in the leaves works like a ‘suction’ force, pulling the water up the stem.

Capillary Action: water moves up the stem in response to the ‘suction’ caused by transpiration because of two forces: adhesion and cohesion. Cohesion is the tendency for water molecules to stick together and adhesion is the tendency for water molecules to stick to other surfaces, such as the inside of the xylem vessels. Stem xylem is structurally adapted to take advantage of capillarity, because they are very long with a narrow diameter.

Root Pressure: water can also be moved up the stem via a ‘push’ force from the roots. Water is constantly being absorbed by the roots due to the negative water potential in the root cells. This movement of water into the roots can cause the water pressure inside the roots to become high, resulting in a force that ‘pushes’ water up the stem xylem.

Capillarity: refers to the ability of a liquid to flow through narrow spaces (capillary pressure).

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