Transpiration is the loss of water vapour from the leaves and stems of plants. Water is absorbed by the roots, travels up the stem in the xylem vessels in the vascular bundles to the leaves, and is lost by evaporation through stomata.

Most plants grow in areas where the amount of water in the air, the humidity, is less than in the leaves. During the day when the stomata are open, water vapour leaves the air spaces in the spongy mesophyll through stomata in the lower epidermis and the stem. The evaporating water is drawn from the vascular bundles in the leaf and stem. The vascular bundles are continuous with those in the leaf and stem. The vascular bundles are continuous with those in the xylem from the roots so a column of water is formed connecting the roots, stem, leaves and air spaces. This is known as the transpiration stream.

Transpiration also carries minerals through the plant, and serves to cool leaves in warm conditions.

The movements of water through a plant water moves from the soil to the air (form where there is more water to where there is less water)

Transpiration in plants

Water evaporates from the parts of a plant that are exposed to the atmosphere – for example, the whole shoot system of a terrestrial plant and the upper leaf surfaces of a floating aquatic plant.

The greatest loss of water takes place through the stomata (singular stoma), minute pores on the leaf surface. There are usually more stomata on the lower surface of leaves than on the upper surface. The lower surface is less exposed to the warming effects of Sun’s radiation, which would speed up the evaporation rate.

In the leaves, water molecules leave the xylem vessels and move from cell to cell. They move through the spongy mesophyll layer by osmosis along a concentration gradient. Water then evaporates into spaces behind the stomata and diffuses through the stomata into the surrounding air.

Transpiration is the evaporation of water at the surfaces of the mesophyll cells, followed by loss of water vapour from plant leaves, through the stomata.

Water in the leaf cells forms a thin layer on their surface. The water evaporates into the air spaces in the spongy mesophyll. This creates a high concentration of water molecules. They diffuse out of the leaf into the surrounding air, through the stomata, by diffusion.

Mechanism of water movement through a plant

Water molecules are attracted to each other (cohesion) à water vapour evaporating from a leaf crates a kind of suction, pressure of water at the top of the vessels is lower than that of the bottom à water moves up the stem in the xylem, more water is drawn into the leaf from the xylem. This creates a transpiration stream, pulling water up from the root.

Cohesion-tension theory

The movement of water in the xylem can be explained by the cohesion-tension theory.

  • Loss of water vapour from the stomata in the leaves results in ‘tension’ or negative pressure in the xylem vessels.
  • Water vapour re-enters the air spaces in the leaf from the xylem vessels.
  • Continuous columns of water are drawn up the xylem due to cohesion between water molecules in the xylem and forces of adhesion between the water molecules and the xylem vessel walls. Cohesion is due to hydrogen bonding between water molecules and adhesion is caused by the hydrogen bonds between water molecules and molecules in the walls of the xylem vessels.
  • The tension in the xylem is strong due to loss of water and there would be a tendency for xylem vessels to collapse inwards. The thickening provided by lignin prevents this happening.
  • Water is drawn in from the cortex in the roots to replace water that is lost in transpiration.
  • The tension caused by transpiration also causes water to be drawn into the roots from the soil.

Transpiration is largely controlled by the pairs of guard cells that surround the stomata. Guard cells have unevenly shaped cell walls with more cellulose on the side adjacent to the stoma. The inner part of the cell wall is less elastic, so that when guard cells take up water and become turgid, they take on a sausage-like shape and an opening – the stoma – is formed between them. When the guard cells lose water, the cell walls relax and the stoma closes.

The opening and closing of stomata is controlled by the concentration of potassium ions. In darkness, these ions move out of the guard cells into surrounding cells. In light conditions, potassium ions are actively pumped into the vacuoles of guard cells. This creates an increased solute concentration so that water enters by osmosis, making the cells turgid and opening the stomata. A plant hormone called abscisic acid, produced in the roots during times of drought, affects potassium ion movement in guard cells. When abscisic acid is present, potassium ions leak out and water follows by osmosis. This means that the guard cells lose turgor and stomata closes, thus conserving water.

Xylem vessels are not alive and have no plasma membrane, so water can easily move in and out of them

Water movement through a plant begins with the diffusion of water vapour out of the leaf and evaporation from the leaf surface (spongy mesophyll). 98% of the water taken up by a plant is lost to the atmosphere by transpiration.

The rate of transpiration can be affected by several factors:

Several abiotic environmental factors (notably light, temperature, humidity and wind speed) influence the rate of transpiration in plants.

  • Light affects transpiration directly by controlling the opening and closing of stomata. As light intensity increases, stomata opens, speeding up the rate of transpiration. In darkness, stomata closes, thus restricting transpiration.
  • Temperature affects transpiration because heat energy is needed for the evaporation of water. As the temperature rises, the rate of transpiration also rises as water evaporates from the air spaces in the spongy mesophyll and diffuses out of the stomata.
  • An increase in atmospheric humidity reduces the rate of transpiration. Air in the mesophyll air spaces tends to be saturated with water vapour so if atmospheric air becomes more humid, the concentration gradient between the air space and the atmosphere is reduced and transpiration is slowed down.
  • An increase in wind speed increases the rate of transpiration because it blows away air just outside the stomata, which is saturated with water vapour. Reduced humidity near the stomata enables water vapour to diffuse more readily from the spongy mesophyll, where the air is very humid, to the air just outside the leaf, which has lower humidity.

Factors affecting transpiration rate

Factor Explanation
↑ temperature↑ the kinetic (movement) energy of water molecules → they diffuse faster
↑ air movement
(wind…..)
Removes water molecules as they pass out of the leaf → maintaining a steep concentration gradient for diffusion
↓ humidity↓ the concentration of water molecules outside the leaf → steeper concentration gradient for diffusion
↑ light intensityStomata open to allow exchange for photosynthesis → water vapour can diffuse out of the leaf

How wilting occurs

Young plant stems and leaves rely on their cells being turgid to keep them rigid. If the amount of water lost from the leaves of a plant is > than the amount taken into the roots à the plant will have a water shortage as cells become flaccid (soft) and will no longer press against each other à stems and leaves lose their rigidity, and wilt.

Adaptations of the leaf, stem and root to different environments

Plants which live in extreme environments have adaptations to control their transpiration rate. Most modifications are adaptations to very dry (arid) environments.

Water plants have no problems of water shortage. They do not need adaptations to conserve water as desert plants.

Plants modified to cope with a lack of water are called xerophytes. Living in deserts where water is scarce and evaporation is rapid, or in windy habitats where evaporation can also be rapid, they have to cut down water loss.

  • Very long roots to search for water deep down in sand dunes.
  • Leaves that roll up in dry weather to increase humidity around stomata, reducing transpiration.
  • Sunken stomata to create high humidity and reduce transpiration.
  • Fine hairs around stomata, reducing air movement so humidity builds up and transpiration is reduced.

Xerophytes

Marram grass is well adapted to survive in the dry conditions found in sand dunes. Its leaves are rolled into tube-like shapes, which are protected on the outside by a thick waxy cuticle. Stomata of marram grass are protected deep inside pits which themselves are rolled up inside the leaf. A lining of hairs on the inner side of the leaf keeps humid air trapped inside the rolled up leaves. Hairs prevent water loss by diffusion. When the humidity of the air rises, marram grass is able to unroll its leaves using specialised hinge cells. When the leaf is unrolled, leaf hairs help conserve a supply of water by trapping moist air. This air remains inside the leaf when it rolls up again.

The Crassulaceae, a group of succulent plants have evolved a mechanism allowing them to keep their stomata closed during the heat of the day, to reduce water loss. At night, the stomata opens and carbon dioxide diffuses into the leaf. It is fixed temporarily in cells, and then is released for photosynthesis during the day when the stomata are closed. This is called crassulacean acid metabolism (CAM).

Prickly pear cactus (Opuntia)

  • Leaves reduced to spines – this reduces the surface area for transpiration and also acts as a defence against herbivores.
  • Reduces number of stomata.
  • Stomata closed during the day – when conditions for transpiration are most favourable.
  • Fleshy stem – to store water.

Pine tree (Pinus)

  • Leaves needles-shaped to reduce surface area for transpiration and to resist wind damage.
  • Sunken stomata to create high humidity and reduce transpiration.
  • Thick waxy cuticle on the epidermis to prevent evaporation from leaf surface.

Water plants may have stomata on the tops of their leaves

Water hyacinth (Ecichhornia csassipies)

  • Roots do not attach to the bed of the river or pond where they grow, but just float freely in the water.
  • The stems and leaf stalks have hollow spaces in them, filled with air à help to float on the top of the water where they can get plenty of light for photosynthesis.
  • Leaves and stomata are on both surfaces, not just on the underside as in most plant à allow to absorb CO2 from the air, for photosynthesis.
  • The cuticle on the upper and lower surfaces of the leaves is much thinner than in plants that don’t live in water, there is no need to prevent water loss from the leaves.

Importance of transpiration

A tree, on a hot day, may draw up hundred of litres of water from the soil. Most of this water evaporates from the leaves; only a tiny fraction is retained for photosynthesis and to maintain the turgor of the cells. The advantage to the plant of this excessive evaporation is not clear. A rapid water flow may be needed to obtain sufficient mineral salts, which are in very dilute solution in the soil. Evaporation may also help to cool the leaf when it is exposed to intense sunlight.

Against the first possibility, it has to be pointed out that, in some cases, an increased transpiration rate does not increase the uptake of minerals.

The second possibility, the cooling effect, might be very important. A leaf exposed to direct sunlight will absorb heat and its temperature may rise to a level that could kill the cytoplasm. Water evaporating from a leaf absorbs its latent heat and cools the leaf down. This is probably one value of transpiration. However, there are plants whose stomata close at around midday, greatly reducing transpiration. How do these plants avoid overheating?

Many biologist regard transpiration as an inevitable consequence of photosynthesis. In order to photosynthesise, a leaf has to take in carbon dioxide from the air. The pathway that allows carbon dioxide in will also let water vapour out whether the plant needs to lose water or not. In all probability, plants have to maintain a careful balance between the optimum intake of carbon dioxide and a damaging loss of water. Plants achieve this balance in different ways.

Practical work

To demonstrate water loss by a plant

The apparatus shown in Figure is called a weight potometer. A well-watered potted plant is prepared by surrounding the pot with a plastic bag, sealed around the stem of the plant with an elastic band or string. The plant is then placed on a top-pan balance and its mass is recorded. After a measured time period e.g. 24 hours, the plant is re-weighed and the difference in mass calculated. Knowing the time which has elapsed, the rate of mass loss per hour can be calculated. The process can be repeated, exposing the plant to different environmental conditions, such as higher temperature, wind speed, humidity or light intensity.

A weight potometer

Reults

The plant loses mass over the measured time period. Increases in temperature, wind speed and light intensity result in larger rates of loss of mass. An increase in humidity would be expected to reduce the rate of loss of mass.

Interpretation

As the roots and soil surrounding the plant have been sealed in a plastic bag, it can be assumed that any mass lost must be due to the evaporation of water vapour from the stem or leaves (transpiration). Increases in temperature, wind speed and light intensity all cause the rate of transpiration to get higher, so the rate of loss of mass from the plant increases. An increase in humidity reduces transpiration, so the rate of loss of mass slows down.

Rates of water uptake in different conditions

The apparatus shown in Figure is called a potometer. It is designed to measure the rate of uptake of water in a cut shoot.

  • Fill the syringe with water and attach it to the side arm of the 3-way tap.
  • Turn the tap downwards (i) and press the syringe until water comes out of the rubber tubing a the top.
  • Collect a leafy shoot and push its stem into the rubber tubing as far as possible. Set up the apparatus in a part of the laboratory that is not receiving direct sunlight.
  • Turn the tap up (ii) and press the syringe until water comes out of the bottom of the capillary tube. Turn the tap horizontally.
  • As the shoot transpires, it will draw water from the capillary tube and the level can be seen to rise. Record the distance moved by the water column in 30 seconds or a minute.
  • Turn the tap up and send the water column back to the bottom of the capillary. Turn the tap horizontally and make another measurement of the rate of uptake. In this way obtain the average of three readings.
  • The conditions can now be changed in one of the following ways:
    1. Move the apparatus into sunlight or under a fluorescent lamp.
    2. Blow air past the shoot with an electric fan or merely fan it with an exercise book.
    3. Cover the shoot with a plastic bag.
  • After each change of conditions, take three more readings of the rate of uptake and notice they represent an increase or a decrease in the rate of transpiration.
A potometer

Results

  1. An increase in light intensity should make the stomato open and allow more rapid transpiration.
  2. Moving air should increase the rate of evporation and, therefore, the rate of uptake.
  3. The plastic bag will cause a rise in humidity round the leaves and suppress transpiration.

Interpretation

Ideally you should change only one condition at a time. If you took the experiment outside, you would be changing the light intensity, the temperature and the environment. When the rate of uptake increased, you would not know which of thee three changes was mainly responsible.

To obtain reliable results, you should really keep taking readings until three of them are nearly the same. A change in conditions may take 10 or 15 minutes before it produces a new, steady rate of uptake. In practice, you may not have time to do this, but even your first three readings should indicate a trend towards increased or decreased uptake.

  • The plant stem can be attached directly to a length of capillary tubing with a short section of rubber tubing. This is best carried out in a bowl of water.
  • While still in the water, squeeze the rubber tubing to force out any air bubbles.
  • Remove the potometer from the water and rub a piece of filter paper against the end of the capillary tubing to introduce an air bubble. The capillary tubing does not need to have a scale: a ruler can be clamped next to the tubing.
  • Record the distance moved by the bubble over a measured period of time. Then place the end of the capillary tubing in a beaker of water and squeeze out the air bubble.
  • Introduce a new air bubble as previously described and take further readings.

Limitation of the potometer

Although we use the potometer to compare rates of transpiration, it is really the rates of uptake that we are observing. Not all the water taken up will be transpired; some will be used in photosynthesis; some may be absorbed by cells to increase their turgor. However, these quantities are very small compared with the volume of water transpired and they can be disregarded.

The rate of uptake of a cut shoot may not reflect the rate in the intact plant. If the root system were present, it might offer resistance to the flow of water or it could be helping the flow by means of its root pressure.

To find which surface of a leaf loses more water vapour

  • Cut four leaves of about the same size from a plant (do not use an evergreen plant). Protect the bench with newspaper and then treat each leaf as follows:
    • Smear a thin layer of Vaseline (petroleum jelly) on the lower surface.
    • Smear Vaseline on the upper surface.
    • Smear Vaseline on both surfaces.
    • Leave both surfaces free of Vaseline.
  • Place a little Vaseline on the cut end of the leaf stalk and then suspend the four leaves from a retort stand with cotton threads for several days.

Results

All the leaves will have shrivelled and curled up to some extent but the ones that lost most water will be the most shrivelled.

The results of evaporation from leaves subjected to different treatments

Interpretation

The Vaseline prevents evaporation. The untreated leaf and the leaf with its upper surface sealed show the greatest degree of shrivelling, so it is from the lower surface that leaves lose most water evaporation.

More accurate results may be obtained by weighing the leaves at the start and the end of the experiment. It is best to group the leaves from the whole class into the respective batches and weigh each batch. Ideally, the weight loss should be expressed as a percentage of the initial weight.

More rapid results can be obtained by sticking small squares of blue cobalt chloride paper to the upper and lower surface of the same leaf using transparent adhesive tape. Cobalt chloride paper changes from blue to pink as it takes up moisture. By comparing the time taken for each square to go pink, the relative rates of evaporation from each surface can be compared.

To find which surface of a leaf loses more water vapour

The results of either experiment can be correlated with the numbers of stomata on the upper and lower epidermis. This can be done by painting clear nail varnish of ‘Germoline New-skin’ over each surface and allowing it to dry. The varnish is then peeled off and examined under the microscope. The outlines of the guard cells can be seen and counted.

Water movement