transpiration Practically most of the water absorbed by plants is ultimately lost into the atmosphere. Only a very small fraction of water (generally less than 5%) is utilized in plant development and metabolic processes. The loss of water in the form of vapours from the living tissues of aerial parts of the plant is termed as transpiration.
Basically it is an evaporation phenomenon but it differs from the general process of evaporation. Evaporation is referred to the loss of water vapours from a free surface, whereas in case of transpiration, the water passes through the epidermis with its cuticle or through the stomata
The main differences between transpiration and evaporation are listed below:
|1. It is a physiological process and occurs in plants.||1. It is a physical process and occurs on any free surface.|
|.2. Any liquid can evaporate. The living epidermis and stomata are not involved.||.2. Any liquid can evaporate. The livingepidermis and stomata are not involved.|
|3. Living cells are in- volved.||. 3. It can occur from bothliving and non-living surfaces.|
|.4. Various forces (such as vapour pressure, diffusion pressure, osmotic pressure, etc) are involved..4. Various forces (such as vapour pressure, diffusion pressure, osmotic pressure, etc) are involved.||4. Not much forces are in- volved|
|2. The water moves through the epidermis with its cuticle or through the stomata.||.5. It causes dryness of the free surface.|
Expt To demonstrate the phenomenon of transpiration by the bell-jar method.
A small potted plant, glass plate, bell jar, oil cloth, grease, thread.
Take a thoroughly watered potted plant. Cover its open area of soil with the help of oil cloth to avoid evaporation. Place the pot on a glass plate and cover it with a bell jar. Make the apparatus air tight by smearing grease in the space between glass plate and bell jar. Leave the apparatus for some time and observe.
Observation and Conclusions.
Small drops of water start appearing on the inner side of bell jar due to condensation of water vapour transpired from the plant. The experiment demonstrates the phenomenon of transpiration
The Magnitude of Transpiration
As far as the magnitude of transpiration is concerned, Mayer (1956) had reported that some of herbaceous plants, under favourable conditions, transpire the entire volume of water which a plant has and it is replaced within a single day. A corn plant may transpire upto 54 gallons of water in one growing season (which equals to about 100 times of its own weight). Similarly from a deciduous forest during one complete year the water loss can be equal to the 30% of rainfall of the area.
Actually, the amount of water absorbed and lost by various species differs considerably. In a crop, as a matter of fact, from 90 to 500 kg. of water is lost for the production of 1 kg of dry matter. This means that of all the water absorbed by plants, approximately 95 per cent is lost by transpiration, and 5 per cent or less is used in the plant. If it were not for the loss of water by transpiration, a single rain or irrigation would have been provided the enough water for the growth of an entire crop. Types of Transpiration
There are three main sources through which water loss from a given plant can be affected:-
(1) Stomatal transpiration.
It is the loss of water which occurs through specialized apertures which are present in leaves called stomata. In herbaceous plants the stomata can also occur in the epidermis of green stem. It accounts for 80-90% of the total water loss from the plants
(2) Cuticular transpiration.
The cuticle provides a relatively impermeable covering. If it is thin and green, up to 20% of the total transpiration may take place through it, but as its thickness is increased (e.g., in xerophytes and xeromorphs) the extent of water vapour loss is significantly reduced.
(3) Lenticular transpiration.
Water is lost through the lenticels of fruits and woody stems, although to a much less extent than through stomata. Such a transpiration is termed lenticular transpiration.
Mechanism of Water Loss
The leaves absorb visible (light) and invisible (infrared) radiation from their surroundings during the day time. At night, they absorb heat from the surrounding air. Increase in the temperature due to radiant energy during the day time and due to heat energy during the night, cause vapourisation of water from the cells of transpiring plant organs (leaves). Vaporisation or evaporation of water is a powerful cooling process.
It reduces the temperature of plant organs about 2-5°C below that of atmosphere. During vaporisation, the turgid cells of parenchyma (chlorenchyma) lose water into the intercellular spaces. As a result, the intercellular spaces get saturated with water vapours. The atmospheric air, on the other hand, is dry.
The waxy the cuticle on leaf surface restricts loss of water vapours. However, the water vapours diffuse out from the intercellular spaces into the dry atmosphere mainly through the openings between the guard cells of stomata. A minor amount of water is lost directly from the open surface of leaf through cuticle. The loss of water vapours continues till the stomatal apertures are open. Therefore, transpiration is regulated by opening and closing of stomata.
To compare the rate of transpiration from the two surfaces of the leaf by cobalt chloride paper method.
A potted plant having dorsiventral leaves, filter paper, cobalt chloride solution (5%), glass slides, thread, and desiccator.
Dip a filter paper in slightly acidic 5% cobalt chloride solution. Squeeze the excess solution and dry it in an oven or desiccator. Cut small pieces of filter paper and store them in a desiccator. Now select a healthy dorsiventral leaf of the potted plant.
Clean its upper and lower surfaces with dry cotton. Place the dry cobalt chloride strips on both upper and lower surfaces of the leaf and immediately cover them with dry glass slides. Press them together and tie their sides by means of threads. Observe the change in colour of strips
Observation and Conclusions.
Dry cobalt chloride strips are blue in colour. The strip in touch with lower epidermis (lower surface) turns pink within a few minutes whereas the strip on the upper surface takes à longer time to turn pink. This indicates that loss of water from lower sur- face is comparatively much higher than the upper surface.
Dry cobalt chloride strip is blue in colour. It turns pink as soon as it gets hydrated (CoCl2.2H2O or CoCl2.4H2O). The strip of the lower sur- face gets hydrated faster because the number of stomata per unit area are more in the lower surface,
The stomata are tiny pores present in the epidermal surface of leaves, young stems and in certain fruits (e.g., Banana, Citrus, Cucumber, etc.). They are also found in almost all the young aerial parts of the plant body.
A typical stoma is microscopic and usually consists of two kidney-shaped guard cells surrounding a pore. The plural of stoma is stomata. The guard cells are usually much smaller in size as compared to other epidermal cells. They are, therefore, rapidly affected even by a small change in turgor. A stomatal pore is generally elliptical in surface view. The dimension of stomatal pore varies from species to species, but on an average, it measures about 20 μm long and about 10-20 μm wide when fully open.
In some species the two guard cells are sur- rounded by accessory or subsidiary cells which dif- fer in shape and size from other epidermal cells. Guard cells generally have thick walls towards pore and thin walls on opposite side. Moreover, the cellulose micelles in guard cell walls are oriented radially rather than laterally. The guard cell walls have special elastic properties. The adjoining cell walls of two guard cells around pore are free and not attached with each other. These properties help them to stretch laterally during stomatal opening.
Physiology of stomata.
The guard cells differ from other epidermal cells in mainly two respects-
(i) they contain chloroplasts and perform the phenomenon of photosynthesis, and
(ii) they have special type of wall thickenings.
The photosynthesis in guard cells differs from normal photosynthesis of mesophyll chloroplasts. The guard cell chloroplasts possess pigments of both photosystems I and II and thus, they have ability to perform photophosphorylation. But they lack ribulose bisphosphate carboxylase (RUBISCO) and NADP+-linked triose phosphate dehydrogenase enzymes. Thus the photosynthetic carbon reduction cycle is absent.
The guard cells are heterotrophic. However, starch is synthesized in guard cell chloroplasts by sugars transported from adjacent mesophyll cells. The guard cells are characterised by accumulation of starch during the night (in dark) and their degradation during the day (in light). The mesophyll cells, on the other hand, accumulate starch during the day and decrease during the night. This property helps in the opening and closing of stomata. Figure 1:19 shows the structure of open and closed stomata.
Factors affecting stomatal movement
The stomatal movement, i.e., opening and closing of stomata is affected by a number of external and internal factors.
The important among them are-
1. Carbon dioxide concentration,
4. Availability of water
5. pH of guard cell-sap
6. Hormones, etc.
1. Carbon dioxide concentration:
Low con centration of CO2 (ie., low partial pressure of CO2) in the intercellular spaces of leaves causes stomata to open whereas its higher concentration inside the leaf causes the closure of stomata. The opening has also been observed if CO2 free air is blown across the leaves even in darkness and its closure has been observed by merely breathing near the leaves. However, the opening and closing of stomata is dictated by the shortage or excess of the internal carbon dioxide rather than the atmospheric carbon dioxide.
The importance of internal CO₂ can be shown by keeping the plants in CO2-free environment in dark. The stomata remain closed. That means, the external CO2 is absent but the CO₂ released by the process of respiration is sufficient to cause stomatal closure. Similar situation in light causes stomata to open. This happens because the internal CO2, released by respiration, is utilized by photosynthesis so that the stomata open.
A turgid leaf in the dark, with closed stomata, shows stomatal opening shortly after ex- posure to light. On returning back to darkness, closing mechanism starts. That means stomata open in the presence of light and close in darkness. The opening of stomata in light generally requires about an hour and closing is gradual throughout the after- noon. However, some succulent plants (eg, Bryophyllum, Pineapple, Agava, etc.),
exhibiting Crassulacean Acid Metabolism (CAM), act in an opposite manner, i.e., they open their stomata at night and close during the day. The unique be- haviour of stomata in CAM plants is a kind of adaptation to conserve moisture.noi The maximum light intensity required for opening of stomata is about 1/1000 to 1/30 of full sunlight.
There are few plants which may be induced to open their stomata by bright moon light. How- ever, the amount and intensity of light required to open stomata varies from plant to plant.
The stomata remain closed at 0°C or lower than 0°C even under continuous light. Rise in tempera- ture upto 30°C causes stomatal opening. High temperatures above 30°C again cause stomatal closing-a phenomenon termed as mid day closure. Closure due to high temperature may be due to high CO2 concentration in the intercellular spaces caused by in- creased rate of respiration
4. Availability of Water:
Under conditions of less water availability and high transpiration rate, the plants undergo water deficit (increased water stress or decreased water potential). Water stress results in the closure of stomata to conserve water by leaves which otherwise may get lost by transpiration.
5. Responses of pH
: Rise in the pH of guard cells results in the opening of stomata and decreasein the pH causes its closure.
6. Growth hormones :
Abscisic acid (ABA), an inhibitor hormone, causes stomatal closure. The cytokinins are required for the opening of stomata.
7. Other factors:
The stomata remain open in humid environment. Their opening also depends on the availability of oxygen and K ions. The stomata get closed by mechanical shock and in dry environment.
Mechanism of Stomatal movement (Opening and closing of stomata);
A variety of external and internal stimuli affect the opening and closing of stomata by altering the size of stomatal pores. The important among them are light and dark, CO2 concentration, water supply, pH of the cell sap, etc. (Fig. 1.20). In most of the cases when water supply is adequate, the stomata tend to open during day time in response to light and close at night.
The opening and closing of stomata operates as a result of turgor changes in the guard cells. When the guard cells become turgid, their thin walls get extended and thick walls become slightly concave so that the stomatal aper- ture opens. On the other hand, the guard cells be- come flaccid when they lose water. Their thick walls revert back to the original position resulting in the closure of the stomatal pore.
Investigations on the submicroscopic anatomy of guard cell walls suggest that the special orientation of cellulose microfibrils and micelles are mainly responsible for the opening and closing of stomatal pores (Krikorian et. al., 1973). These studies have shown that cellulose microfibrils and micelles are arranged around the circumference of the elongated guard cells. This arrangement is called radial micellation.
Such guard cells, when take up water and expand, cannot increase much in diameter be- cause the microfibrils do not stretch much in elongation. On the other hand, they increase in length. Since the two guard cells remain attached to each other at both ends, they bend outward on swelling and result opening of stomatal pore.
Factors Affecting the Rate of Transpiration
(A) External Factors
1. Atmospheric humidity.
The rate of transpiration depends upon the difference in the vapour pressure of the internal atmosphere of leaf and external atmosphere of air. If the outer air is humid it will reduce the diffusion of water vapour from inter-cellular spaces of leaf to the outer atmosphere. In other words the rate of transpiration decreases with the increase in relative humidity of the atmosphere. The rate of transpiration is enhanced by lowering the atmospheri
The rate of transpiration in- creases with the increase in atmospheric temperature. Rise in temperature increases the rate of water evaporation from cell surface, opens the stomata and decreases the relative humidity of atmosphere. All these conditions favour transpiration.
Light indiretly affects the rate of transpiration by (1) opening the stomata and (2) increasing the temperature. If all other conditions viz., water supply and leaf temperature, etc. are adequate, light induces stomatal opening and darkness stomatal closure. Thus, the rate of transpiration increases in light and decreases in dark.
4. Wind velocity.
A transpiring surface of leaf continuously adds water vapours to the atmospheric air. Once the immediate area becomes saturated, it reduces the rate of transpiration. Wind velocity removes the air of that area, which is replaced by fresh air and results in an increase in the rate of transpiration.High wind velocity some times decreases the of transpiration because it causes hinderance in diffusion and results in stomatal closure by lowering the temperature of transpiring surface
.5. Atmospheric pressure.
Lowering of atmospheric pressure reduces the density of environment which permits more rapid diffusion of water vapours. For example, plants growing at hills show higher rate of transpiration, but this effect is neutralised by low temperature prevailing at the hills, with the result transpiration is normal in the plants growing on the hills.
6. Available soil water.
If the available water in the soil is not sufficient the rate of transpiration is decreased. Under internal water deficiency the stomata are partially or completely closed.
(B) Internal Factors
(1) Leaf area.
By reducing the size of leaves, plants decrease their rate of transpiration (xerophytic character). Larger leaves lose more water than smaller leaves.
(2) Leaf structure.
The following anatomical features reduce the rate of transpiration-
(i) Thick cuticle on the surface of plant parts.
(ii) Wax, resin and suberin coating on the surface of plant parts.
(iii) Compact mesophyll cells in the leaves.
(iv) Reduction in number of stomata.
(v) Sunken stomata.
(3) Age of plants.
The rate of transpiration is slow at the seedling stage, maximum at maturity and gradually decreases near the senescence.
It has already been mentioned earlier that almost all the water absorbed by the plants is practically lost by the process of transpiration. To avoid the water loss people are trying to find out substances that can reduce transpiration without hindering the other processes. These substances are termed as antitranspirants. An antitranspirant is, therefore, a substance applied to the plant for the purpose of reducing transpiration without causing significant effect on other plant processes, such as photosynthesis and growth.
Several substances viz., colourless plastics, silicone oils, low viscosity waxes have been used but failed to give promising results. These substances make a colourless transparent film over the surface of leaf which is permeable to O2 and CO2 but not to water vapours. The fungicide-phenylmercuric acetate when sprayed at 10-4 M concentration resulted a partial closure of stomata for about 2 weeks, but this substance has certain toxic effects on fruits and vegetables. Similarly, ‘aspirin’ and abscisic acid (ABA) also induce the closure of stomata when applied to the leaves.
These substances have comparatively little toxic effects, but they are costly and cannot be frequently used. Carbon dioxide (CO2) is yet another effective antitranspirant when used at a concentration slightly higher than the natural 0.03% in the atmosphere. It cannot be effectively used because higher concentration of CO₂ causes complete closure of stomata and affect the process of photosynthesis. However, people are trying to evolve a better antitranspirant which will certainly solve the problem. read more