TRANSPIRATION

Transpiration is the loss of water vapour from different parts of the plant shoot. These parts include the stomata of leaves, the cuticle of both leaves and young stems, and the lenticels on tree trunks. Transpiration is described according to the venue through which the water vapour is lost. Hence water loss through the stomata is called stomatal transpiration. Water loss through the cuticle and lenticels are called cuticular and lenticular transpiration, respectively.

The greatest amount of transpiration occurs through the stomata. The water that is lost through the leaves is originally absorbed by the root hairs from the soil into the root cortex. The water is passed into the root xylem from where it is translocated to the stem xylem, to the veins and mesophylls of the leaf and finally to the stomatal surfaces from where the water evaporates into the atmosphere. This unbroken chain of water existing between the soil and the leaves constitutes the transpiration stream.

 

Experiment to demonstrate transpiration in a young shoot

Aim: To show that a potted plant loses water vapour from its leaves.

 

Method: A rubber sheet is used to cover the pot and part of the stem of the potted plant. This prevents water evaporating from the soil and the stem. The potted plant is placed on a vaselined glass plate. The potted plant is then covered with a dried bell jar ‘A’ and placed on a sunny place. The control experiment B is set in the same manner but with the leaves of the potted plant removed.

 

Result: After two hours, drops of colourless liquid collect on the inside of the bell jar A. No such drops of colourless liquid collect on the inside of the bell jar B.

 

Conclusion: Since the pot and soil are covered, the presence of water in the experiment shows that plant loses water through the leaves. Since the bell jar was not in contact with leaves, the loss of water is by vapour.

 

Precaution: It is important to invert the bell-jar over a flat vaselined glass-sheet on which the potted plant is placed. This is to prevent the leakage of air into and out of the bell-jar as this is likely to lead to loss of vapour from the bell-jar into the atmosphere. Air contains some amount of water vapour which can, also leak into the apparatus if unprevented.

 

Forces of transpiration

The role of the root hairs in the absorption of water and mineral salts has earlier been discussed. The continuous absorption of water from the soil into the plant is believed to be under the influence of two main forces. These are the root pressure which is known to be most active at night or the suction pressure set up on the leaves during the day in the presence of light.

 

Suction pressure on the leaves

The root pressure is most active under conditions of high humidity and low temperature and cannot account for the rise of water to the top of very tall trees. It has also been established that there is an unbroken column of water existing between the water in the soil and the water in the leaves (the transpiration stream).

As water evaporates through the stomata, the cells of the spongy tissue become more concentrated in sap than the cells of the inner tissues. A suction pressure is set up which draws up water from the cells of the inner tissues into the intercellular spaces. Water absorption is by osmosis.After losing water to the outer cells, the inner cells become concentrated and draw up water from the veins of the leaves. Water drawn from the veins is replaced by water in the xylem vessels of the stem and branches. The whole process sets up a tension which is transmitted to the root system, causing more water to be absorbed from the soil.

The xylem vessels are capillary tubes and the water moving up through them moves with a capillary force which ensures that the water column remains unbroken. It has been accepted that the actual force which causes the rise of water from the soil to the top of very tall trees is generated on the leaves as a result of the continuous evaporation of water through the stomata during the day in the presence of light. This force is called the “transpiration pull”.

 

Experiment to demonstrate the pathway of water to the leaves

Aim: To show the path through which water passes up the stem to the leaves of a flowering plant.

 

Method: A young sunflower plant is carefully dug to avoid damage to the roots. The soil is rinsed from roots with water. The roots are placed in a beaker of water coloured with eosin or red ink. The experiment is left for about 3 – 48 hours. When red colour appears in the veins of the leaves, a thin section of the stem is cut above the solution level, and examined under the microscope or hand lens.

 

Results: The colour was found to concentrate inside xylem. The red colour was not found in the phloem or other parts of the stem.

 

Conclusion: Water passes up through the xylem of the stem only.

 

Measurements of transpiration

There are various ways by which the rate of transpiration can be measured.

• By the weighing method.
• By the use of a potometer.
• By the use of cobalt chloride paper.

Which-ever method is used, the rate of transpiration depends on a number of factors.

 

1) The weighing method

This is about the most reliable method of measuring the rate of transpiration. It involves the use of an intact living potted plant. The plant is well watered in the morning. The body of the pot together with the soil surface is covered with an aluminum foil to prevent the evaporation of water from the soil. This is an important precaution. The initial weight of the potted plant is taken with a sensitive balance at about 8.00am.

 

The pot is kept in an open place where the plant can receive maximum sunlight. Weighing is repeated at one hour interval until about 7.00pm.

 

It may be necessary to record the atmospheric temperature at each time of weighing and relate it to the amount of water loss.

 

The graph in figure 7.18. indicates that:

1. The rate of transpiration increases as the atmospheric temperature increases.
2. The rate of transpiration is highest in the afternoon of a sunny day between 2 and 4pm.
3. Transpiration begins at a lower rate in the mornings becoming faster as the temperature increases and after reaching a peak in the afternoon it becomes lower in the evenings and is likely to cease at night after 8pm.

 

2) The potometer method

A potometer is an instrument for measuring the rate of transpiration of a plant shoot at a very short interval and under different environmental conditions. It can be used in the laboratory where the condition is humid. It can be placed under fan to determine the effect of wind, or used in an open ground where there is bright sunlight.

The setting up of a potometer is done with some degree of precautions.

The shoot to be used is cut under water to prevent the entry of air which will likely block up the xylem vessels in the stem and disturb the movement of water through them.

The whole apparatus is carefully filled with water. There should be no air space in it.

When the shoot is quickly introduced through the rubber cork, it is immediately greased with vaseline to keep the apparatus air-tight.

With all these precautions taken, transpiration occurs and as water is being lost an equivalent amount of water is absorbed into the shoot.

The rate of water absorption into the shoot can be measured as the water meniscus moves through the graduated capillary tube. The average distance covered per minute can be measured with a stop clock. If the diameter of the capillary tube is known, the volume of water absorbed per minute or hour over a given distance can be calculated. The meniscus can be driven back to the zero mark by opening the reservoir tap for repeat recordings.

As a matter of fact, the potometer measures the rate of water absorption rather than the rate of transpiration. It works on the assumption that the rate of transpiration is very nearly equal to the rate of water absorption.

 

3) The cobalt chloride paper method

This method is used to estimate the relative rate of transpiration of different plants or different leaves on the same plant. It can also be used to estimate the relative rate of transpiration through the upper and lower epidermis of the same leaf.

Cobalt chloride paper is blue when dry. The colour changes to pink when it is wet. Cobalt chloride paper can therefore be used to measure the rate of transpiration based on time it takes the dry blue paper to change to pink when brought in contact with a transpiring leaf. This method does not however give a very accurate result, as two individuals may disagree on the exact time the pink colour is obtained. To minimise the errors in judgement, the cobalt chloride paper is sand-wiched between standard blue and pink papers.

 

The cobalt chloride paper when dry has the same colour with the standard blue paper. They are fastened to the surface of a transpiring leaf with a cellotape and the time taken to change to exact colour of the standard pink paper is noted.

The change in colour is brought about by the condensation of the water which would have been lost into the atmosphere either through the stomata or through the leaf cuticle as vapour. The water vapour on condensing, wets the cobalt chloride paper.

 

Factors affecting the rate of transpiration

There are three groups of factors which affect the rate of transpiration. These are:

• Plant factors.
• Atmospheric factors.
• Soil factors.

 

A) PLANT FACTORS

1) Leaf area: Experiments on transpiration have shown that the larger the leaf area, the greater the amount of water loss. This is because larger leaf areas occupy greater number of stomata through which water is lost. Using two plants of the same species, the one with greater number of leaves of approximately the same size will have more water transpired, than the other with reduced number of leaves. In determining the rate of transpiration of any plant, it is important to note the total leaf area. The rate of transpiration is therefore the amount of water loss per unit area of leaf per unit time. For example, the rate of transpiration of maize can be expressed as X gramme of water per cm’ per hour.

 

2) Leaf structure: The leaves of certain plants are specially adapted to live under dry habitats. These plants are known as xerophytes. Generally they posses thick cuticles, thick cells walls, compact mesophylls and sunken (or hidden) stomata. Sometimes the leaves possess a covering of dead epidermal hairs. Plants like the whistling pines have their leaves reduced to very tiny needle-like structure while in cactus there are no leaves at all. These are various adaptations of the plants to maintain a low rate of transpiration as excessive loss of water will lead to the death of the plant.

 

3) Root system: The major function of the root is for the absorption of water and mineral salts. The degree of water absorption depends largely on the extent of the roots, the number of root hairs, the osmotic value of the cell sap and the permeability of the cell membranes. There is a very close relationship between the rate of water absorption and transpiration. Under all other conditions which favour transpiration, the rate of transpiration increases as the rate of water absorption becomes increased. But whereas both events occur during the day, transpiration stops at night when the stomata are closed, water absorption continues at night, although at a much reduced rate.

 

B) Atmosphere factors

1) Sunlight: Sunlight is about the most important factor which influences transpiration. Sunlight is responsible for the opening of stomata. This is why transpiration occurs during the day when the stomata are open and ceases at night when the stomata become closed.

 

2) Humidity: Generally the higher the relative humidity of the atmosphere the lower the rate of transpiration. This is because at high relative humidity, the atmosphere is getting nearer the point at which it becomes saturated with water vapour. Such a condition does not favour the evaporation of water. At high relative humidity, the layer of air immediately above the leaf surface is easily saturated with water vapour which evaporates at a very slow rate.

 

3) Temperature: An increase in temperature increase the rate of transpiration, provided that the increase does not disturb the functioning of the plant. Temperature influences the opening of stomata. Furthermore, an increase in temperature increases the kinetic energy which changes the liquid water to vapour, thus increasing the rate of transpiration. The effect of temperature changes in transpiration is shown in figure 7.18.

 

4) Wind: Wind has two effects on transpiration. A gentle wind increases the rate of transpiration. Wind helps to sweep away the water vapour over the surface of the leaf thereby preventing the surface from being saturated. The water becomes replaced as fast as it evaporates. However, when the wind is too strong and persistent, like in the harmattan, it not only acts against evaporation of water, it also increases dust particles on the leaves which are likely to block up the stomata and reduce the rate of transpiration.

 

C) Soil factors

1) Availability of soil water: For transpiration to be effective, water must be available in the soil in adequate amount. The soil water must remain at a lower osmotic value than the osmotic values of the cell sap of the plant root hairs.

 

2) Temperature: The rate of water absorption increases with an increase in the temperature of the soil. Generally at lower temperatures, water becomes more viscous and the cell membranes become less permeable to the water molecules, thus reducing the rate of water absorption. For transpiration to progress, the rate of absorption of soil water must at least be equal to the rate of transpiration. If the rate of transpiration by far exceeds the rate of water absorption, the plant will wilt. This is the case when the soil is in short supply of absorbable water or when the osmotic value of soil water is higher than that of the cell sap of the root hairs.

 

Importance of transpiration

1) Cooling effect: Transpiration involves the evaporation of water which is a cooling process. It is believed that this cooling effect is likely to prevent the plant from overheating on a very hot day.

 

2) Translocation of mineral salts: Water absorption by the plant roots is a passive process which does not involve the use of metabolic energy. Salt absorption is an active process which involves the use of energy and is independent of water absorption. However, once the salt reaches the xylem vessels in the root, it is translocated to the other parts of the plant under the influence of transpiration pull.

 

3) Good growth of the plant: Most plants do not grow well when they are maintained under conditions of high humidity conditions which do not favour transpiration. Often there is a great reduction in the size of the plant. In some cases the buds fail to grow and no flower is produced. It can be argued that under conditions of high humidity, the rates of water absorption and transpiration are greatly reduced and the plant lacks the required amount of water to carry out the vital metabolic processes. Transpiration is a process which ensures the continuous availability of water in the plant body.

 

GUTTATION

This is a loss of water in a liquid form through the margins of leaves of plants growing in a warm and moist soil in a humid atmosphere. Guttation is usually experienced at the early hours of the morning and is common among grasses. Such plants possess specialized ducts at the tip of the veins in the leaves. These specialized ducts are known as hydathodes.

A condition in which a plant is growing in a warm and moist soil with a humid atmosphere favours the absorption of water with low rate of transpiration. The rate of water absorption soon exceeds the rate of transpiration and since the excess water must be removed, it is removed not as water vapour through the stomata, but as liquid through the hydathodes.

Guttation water may not be pure water. It contains some dissolved organic and inoraganic substances. Palm wine which is tapped from palm trees, is brought to the top of the tree under the influence of guttation initiated by root pressure.