Notes on Transpiration:- 1. Meaning of Transpiration 2. Factors Affecting Transpiration 3. Factors Controlling 4. Significance.
Meaning of Transpiration:
If a bell jar is placed on a pot, a film of moisture appears on its dry inner surface indicating loss of water as vapours which condense on the cool inner surface of the bell jar. The loss of water from the aerial parts of the plant in the form of water vapours is called transpiration. Leaves are the principal organs of transpiration and most of the transpiration takes place through their stomata.
This is termed as stomatal transpiration. A small amount of water vapours is lost from the leaves and herbaceous stems by direct evaporation from the epidermal cells through the very thin cuticle. This is called cuticular transpiration.
The amount of water lost through stomatal transpiration is very high. The loss of water vapours also takes place through lenticels of fruits and woody stems. This is called lenticular transpiration. In woody stems, transpiration takes place through bark or cork.
Factors Affecting Transpiration:
Whenever the rate of transpiration exceeds the rate of absorption, a water deficit is created in the plants and results in the incipient wilting of leaves. The development of internal water deficit in a plant causes Ψ gradient between the guard cells and the mesophyll and epidermal cells surrounding the cells.
This gradient favours outward movement of water from the guard cells and the reduced turgor causes the complete or partial closure of the stomata. Yemm and Willis (1954) found that conditions causing an internal water deficit also cause an increase in starch content of the guard cells.
It is also noteworthy that under the same circumstances (wilting) the starch of the mesophyll cells is rapidly hydrolysed. This increases their Ψs thus enabling them to draw water from the guard cells. The higher concentration of sugar in mesophyll cells may also depress the utilization of CO2 by photosynthesis and CO2 concentration may increase due to continued respiration.
The increased CO2 concentration results in increased acidity and the latter favours conversion of sugar into starch in guard cells. The osmotic concentration of the guard cells falls. Water leaves the guard cells and enters the leaf parenchyma.
The guard cells change their shape, become flaccid and the stomata close. Thus the basic mechanism for closure of stomata by water deficiency is the same as that for closure by darkness and both are controlled by increased acidity resulting from an increase in CO2 concentration.
Stomatal mechanism is sensitive to carbon dioxide concentration. Stomata close in the presence of high concentrations of CO2.
This gas is essential for the opening of stomata. Its deficiency quickens stomatal closure.
ABA causes stomatal closure whereas cytokinins are essential for the intake of K+. Moreover, ABA works only in the presence of CO2.
Generally, stomata open when exposed to illumination and close in the dark. In majority of the cases, blue light absorbed by a special pigment system, affects opening.
Similarly the phytochrome system may also lead to opening of the stomata. Early workers assumed that guard cells when exposed to light and warmth increased their output of osmotically active substances through photosynthesis. The resulting increase in osmotic pressure and turgor resulted in the opening of stomata.
The objection to this assumption is that photosynthesis occurs in the guard cells only at a reduced rate and could not account for the amount of osmotically active substances required to cause opening.
The theory also cannot explain the fact that stomata open in carbon dioxide-free air in the dark as well as in the light when there is no photosynthesis. Light may provide ATP for K+ transport into guard cells.
When all other factors are equal, stomatal opening increases with rise in temperature up to 25°- 30°C and decreases at still higher temperatures.
In most species stomata fail to open at or near 0°C. In some species high temperatures of about 40°C cause stomatal opening instead of closing even in the dark.
With the deficiency of any of minerals such as, nitrogen, phosphorus and potassium, the stomatal movements become sluggish. Different ions affect the stomatal aperture differently.
Stomata close when the leaf is exposed to high wind velocities. This may be due to the loss of water by the guard cells during transpiration or the effect may be indirect through leaf temperature.
Various types of shocks, such as rough handling of leaves may result in stomatal closure. It is found that stomata may close in light when nearby tissue is wounded.
Factors Controlling Transpiration:
The rate of transpiration is influenced by plant factors such as the root-shoot ratio, leaf area and structure, environmental factors and availability of soil water.
Parker (1949) found that the rate of transpiration increased with an increase in root-shoot ratio, provided other conditions were favourable for transpiration.
Sorghum with more extensive root system than that of corn, transpired at a higher rate per unit of leaf surface than corn. The sorghum root system provides more water to the shoot than the corn root system.
Greater the leaf area more will be the magnitude of water loss but there is no proportionality between the leaf area and the water loss. Although the greater amount of water was lost by the larger corn plant, the amount of water lost per unit area was greater in the smaller plant.
The removal of leaves (decreasing leaf area) from a plant increases the rate of transpiration per unit leaf area of the plant but the total water loss is greater in the un-pruned ones. This is because the root system of pruned plants is providing a greater amount of water to a smaller number of leaves, thus increasing the transpiration efficiency.
Plants growing in dry areas show a number of structural modifications, especially in leaves, such as thick cuticle, thick cell well, well-developed palisade, sunken stomata, a covering of hairs, etc. These features reduce water loss. Under dry conditions the stomata are closed and cuticular transpiration is the main source of water loss.
In xerophytic species, well supplied with water, stomatal transpiration exceeds that of mesophytes. This is due to higher number of stomata per unit area, more extensive venation, greater internal evaporating surfaces exposed to the internal atmosphere in xerophytic leaves as compared to mesophytic leaves.
In addition leaf size and shape, spacing, distribution and structure of stomata, water content of leaves greatly affect the rate of transpiration. The rate of transpiration was greatly enhanced in the diseased plants as compared to the healthy ones.
Environmental or external factors:
Light affects directly by heating the leaf and thus raising its temperature and the latter augments the saturation pressure deficit and indirectly favours the opening of the stomata.
The rise in temperature accelerates evaporation from the outer surface of the cell walls of mesophyll cells into the intercellular spaces of the leaf and the open stomata help diffusion. Thus, the rate of transpiration increases in light. In the dark, stomata are closed and transpiration ceases.
Saturation pressure deficit or vapour pressure deficit:
Transpiration is composed of two processes i.e. evaporation from the cell walls of the mesophyll cells into the inter-cellular spaces of the leaf and diffusion of water vapour from these spaces into the leaf and diffusion depends upon the difference between the vapour pressure inside and outside the leaf.
Greater is this difference (VPD), greater is the rate of transpiration. At a given temperature the vapour pressure of a space varies directly with the relative density or concentration of water vapour (Boyle’s Law).
A rise in temperature of the leaf or both leaf and air, within a certain physiological range, will increase the rate of transpiration. This is due to the effect of temperature on stomatal movements and VP gradients. Stomata close at temperature approaching 0°C and increase in aperture with increase in temperature up to about 30°C.
In addition to its effect on the opening of the stomata, an increase in temperature steepens the VP gradient between the internal atmosphere of the leaf and the surrounding atmosphere. A rise in leaf temperature to 30°C results in an increased evaporation from cell walls of mesophyll cell so that the air spaces remain at 100% RH.
The VP is thus increased to 31.85 mm Hg at this temperature. This means greater VPD at higher temperature than at the lower temperature. This results in an increase in the rate of transpiration.
Wind increases evaporation and consequently transpiration by removing the moist air from the surface of the leaf and replacing it by drier air, thereby increasing the saturation pressure deficit.
When plants are suddenly exposed to wind, there is sharp increase in the rate of transpiration followed by a gradual falling of this increase. This is because of cooling effect which wind produces on the evaporating surfaces lowering their vapour pressure gradient. In addition, winds of high velocity may cause stomatal closure.
Availability of soil water:
If the supply of water to the leaves is not adequate the rate of transpiration decreases. If this condition is prolonged, a water deficit will result and the plant will appear wilted.
The supply of water can be inadequate due to the following reasons:
(i) Water in the soil about the roots reaches wilting co-efficient,
(ii) Root system is not adequate to supply the top,
(iii) A low soil temperature, and
(iv) A high concentration of soil solution.
The rate of evaporation is inversely proportional to the pressure of other gases. A reduction of atmospheric pressure from 1 to 1/2 atm. would double the rate of evaporation.
On top of mountains the rate of transpiration is increased because of reduced atmospheric pressure.
Significance of Transpiration:
Several beneficial and harmful effects are ascribed to transpiration.
There are three possible advantages of transpiration to the plants and these are discussed below:
I. Transport of minerals:
Usually, high transpiration rates cause high rates of mineral absorption. However, deep understanding of diffusion and differentially permeable membranes point towards the independent absorption of minerals.
It is generally held that minerals absorbed by the plant from the soil usually move up through the plant via transpiration stream.
The quantum of minerals reaching the leaves is dependent upon the rate of absorption of minerals by the roots rather than the rate of transpiration.
Rates of transpiration do not seem to affect the availability of minerals in the leaf. On the contrary when the mineral in the soil are in abundance then the rate of transpiration is vital for their translocation.
II. Lowering of leaf temperature:
Transpiration of water from a surface of the leaf lowers the temperature of that organ since the loss of water molecules of a relatively high kinetic energy; the molecules having highest kinetic energy are the first to evaporate.
Rough estimates show around that transpiration removes about 600 calories per gram of evaporated water.
There are other ways e.g., radiation and convection by which heat is removed. If transpiration stops, there would be an enhanced loss of heat by radiation and convection because of the increased leaf temperature.
III. Optimum turgidity:
It is generally argued that there is an optimum level of water potential within the plant. Many and varied functions are slowed down or are rendered inefficient both above or below this level.
In the absence of transpiration, plants tend to become over turgid and will cease to grow. Similarly when the water potential becomes highly negative growth also stops.
IV. Bringing water to the top of a plant:
It is also suggested that importance of transpiration is embodied in its essentiality to pull water to the top of the trees.
The conclusive fact about transpiration is that it is harmful. It is loss of one of the essential components of life and one of the substrates of most important process for the plant life- photosynthesis. It may be added that the existence of transpiration in plants may not be taken to mean that it is useful to the plant body.
For photosynthesis wet cell walls and open stomata are essential and these involve transpiration. This is accomplished through diffusion. The danger is when the amount of diffused water exceeds the amount entering the roots, the wilting may be caused and this results in lowering the yield or even the death of a plant.
It is well accepted that wilting may have several deleterious effects which include stomatal closure, dehydration of enzymes, lowering of membrane permeability, and reduction of photosynthesis. In brief, transpiration appears to be a necessary evil. Leaves are primarily meant to perform the process of photosynthesis and transpiration is just incidental.
Several practices are employed to reduce or at least minimize water losses, and these include removal of weeds adjacent to crop plants. Greenhouses are whitewashed to reduce light intensity, temperature and thus transpiration. Periodically the atmosphere is moistened by spraying water to saturate the greenhouse and to reduce transpiration. This reduces the periodicity of watering the crop plants.
In vegetative propagating plants, transpiration is reduced since there is low or no water absorption because of lack of roots. The cuttings are denudated of leaves or covered with bell jars of plastic covers. In such vegetative propagated portions, transpiration is reduced to avoid wilting of organs or tissues since flaccid cells do not grow.
These are chemical or physical agents which reduce the rate of transpiration by closing the stomatal apertures. Antitranspirants are used in agricultural practises to control loss of water through the stomata under water stress conditions. Hence antitranspirants are chemicals which on application to crop plants reduce transpiration.
There are two types of antitranspirants:
(i) Film forming chemicals:
These are basically inert compounds form a film over the leaf and cover the stomata and thus reduce the cuticular transpiration. Some of the commonly used emulsions are dimethyl silicone or polyethylene or plastic films, hexadecanol, kaolin etc.
Sometimes wax emulsions are also sprayed as film coating compounds. However, most of these emulsions or film coatings reduce CO2 and O2 exchange and are not preferred ideal compounds for use in agriculture. The compounds used should permit exchanges of gases but preclude water vapours.
(ii) Stomatal inhibitors:
Large numbers of synthetic compounds are available which reduce stomatal-opening by hindering stomatal mechanism. Several herbicides, fungicides and growth regulators act as inhibitors of stomatal opening. One of the most widely used antitranspirants compounds is acetylsalicylic acid which is used in low concentrations and also endogenously reported in some plants.
Phenylmercuric acetate-a fungicide is also used to check stomatal opening.
Conceivably the ideal antitranspirants should have the following features:
i. Less expensive,
ii. Should be non-toxic to various life forms,
iii. Should be specific in function; should close the stomata but should have low mobility within the plant,
iv. The closure effect on stomata should be reversible, and
v. The effect of the chemical should persist for some-time.
Within the plants some natural antitranspirants are synthesized and these include ABA and salicylic acid.
Within the plant ABA regulates the water economy of plant under water stress conditions antitranspirants. Some studies have demonstrated triazoles as antitranspirants. CO2 is also shown to act as an antitranspirants, high concentrations of CO2 partially close stomata and increase the rate of photosynthesis.