Define Transpiration and its significance?
- The loss of water in the form of vapor from the living tissues of aerial parts of plant such as leaf, stem, leaves etc. is termed as transpiration.
- The plants uptake abundant quantity of water from the soil through their root hairs. Some portion of the water is utilized in the metabolic activities of the plant whereas rest of them are evaporated from the stem and the leaves.
- Transpiration takes place through stomata, lenticels or cuticle.
- Transpiration is a metabolic process regulated by protoplasm and may be decreased or increased where needed by the nature.
- It differs from evaporation in fact that transpiration being a physiological process while evaporation is a physical process.
- The rate of transpiration is measured by potometer.
Types of transpiration in plants:
- On the basis of site of transpiration, there are three types of transpiration. They are:
- Stomatal transpiration: It occurs through the stomata situated on the leaves and sometimes on the green stems. It is the most important one. Almost 90-97% of the total transpiration occurs through the stomata.
- Lenticular transpiration: It occurs through the lenticels found on the stem. The stomata remain closed during night and the plant transpire through lenticels and cuticle.
- Cuticular transpiration: It takes place through the cuticle found on the surface of the stem and leaves.
Factors affecting transpiration:
External factors:
- Light:
- Transpiration is indirectly affected by light.
- The rate of transpiration is always greater in light than in darkness.
- In presence of light, the stomata remain open and hence transpiration takes place whereas in night time, the stomata are closed and transpiration is checked.
- Only lenticular and cuticular transpiration occurs during night.
- The efficient transpiration is induced by blue light followed by red light.
- In presence of green light, UV rays, and infrared light, stomata never opens.
- Atmospheric temperature:
- The rate of transpiration is directly proportional to atmospheric temperature.
- When the temperature is high, transpiration from the leaf surface occurs rapidly.
- Atmospheric moisture:
- Dry and low humidity enhances the rate of transpiration.
- The rate of transpiration decreases in saturated atmosphere.
- Wind velocity:
- The rate of transpiration increases in windy condition, however, when the atmosphere is calm, transpiration rate decreases because of moist air in proximity of transpiring plants.
- Solar radiation:
- Solar radiation is deeply related to transpiration.
- Due to solar radiation, the temperature of the leaves rises, thus increasing the rate of transpiration.
- Soil environment:
- It indirectly affects transpiration.
- If the absorption of water from soil is abundant, the transpiration occurs to a larger extent and vice-versa.
Internal factors:
- The root system:
- If the roots are deeply penetrated to the soil in moist layers, there is abundant absorption of water and at same time, transpiration increases too.
- However, if the roots are limited to the upper layers of the soil, there is less absorption of water and less transpiration.
- The stem:
- The rate of transpiration depends on diameter of xylem vessels present in the stem.
- If the xylem vessels are wider, there is increase in transpiration and vice-versa.
- Leaf structure:
- Several adaptations in plants are present in order to check transpiration.
- In case of xerophytes the area of leaves is decreased because of many divisions of leaves.
- Sometimes, the leaves are completely absent and the stems are flat, angular or rounded. These are called phylloclades. Ex- Ruscus, Opuntia.
- Transpiration is less from the surface of phylloclades.
- The leaves of some plants bear special depressions where stomata are situated and are surrounded by hairs, this aids in checking transpiration. Ex. Nerium, Banksia, etc.
Significance of transpiration:
- As transpiration helps in the movement of xylem sap, it increases the absorption of mineral nutrients by the roots from the soil.
- It causes cooling effect on leaf and plant surface.
- It produces suction pressure for absorption, ascent of sap, mineral translocation and distribution if minerals.
- Transpiration decreases heating of leaves by solar radiations.
- It maintains turgidity as well as aids in hydrological cycle.
disadvantages of transpiration:
- The energy used during absorption is wasted.
- Unwanted loss of water.
- Excess transpiration causes wilting that is harmful for plants.
- It increases acidity, alkalinity or aridity of soil.
Mechanism of stomatal transpiration:
Structure of stomata:
- The stomata (stoma, singular) are microscopic apertures commonly found on the epidermis of leaves, green fruits and herbaceous stems.
- Stomata are never present in roots.
- It is biconvex elliptical in structure.
- The two kidney-shaped special epidermal cells termed as guard cells surrounds each stoma.
- The guard cells are filled with thin layer of cytoplasm and central large vacuole.
- The cell wall of guard cells surrounding the stomatal pores is thicker and inelastic because of the formation of secondary layer of cellulose, while rest cell wall is thin and elastic.
- The epidermal cells that surrounds the guard cells of the stoma are termed as accessory or subsidiary cells.
- The guard cells are always living and consists of small chloroplasts unlike other epidermal cells.
- In case of dicotyledonous leaves, the stomata are found scattered whereas in case of monocotyledonous leaves, the stomata are arranged in parallel rows.
- The stomata may be found on both the surface of the leaf, but their number is always greater on the lower surface.
Shape, size and number of stomata:
- The shape of guard cells in case of dicots is reniform or kidney shaped whereas in case of monocots, it is dumb-bell shaped.
- The size of stoma varies from species to species and measures 3- 12 μ.
- The number of stomata can vary from thousands to lacs per square centimetre on the surface of the leaf.
Mechanisms of opening and closing of stomata:
- In normal condition, the stomata remain closed in the absence of light.
- In the day time or in the presence of light, stomata are always open.
- Under each stoma, a respiratory cavity is present.
- The mechanism of the closing and opening of the stomata relies upon the presence of sugar and starch in the guard cells.
- Photosynthetic guard cells:
- According to Von Mohl and Schwendener, sugar is produced by the chloroplasts of the guard cells through photosynthesis.
- The sugar is soluble and hence increases the concentration of sap of guard cells.
- The increase of osmotic pressure of guard cells leads to endosmosis of water from neighbouring cells into guard cells and they become turgid.
- This results in opening of stomata.
- Starch-sugar inter conversion hypothesis:
- This hypothesis states that the opening and closing of stomata is controlled by phosphorylase enzyme.
- During daytime, the starch converts into glucose (sugar) by the activity of phosphorylase enzyme.
- The increasing concentration of sugar in the guard cells causes endosmosis from neighboring cells.
- Hence, the guard cells become turgid and stomata opens
- The sugar present in guard cell converts into the starch in the absence of light or during night.
- The starch is insoluble, and hence the cell sap of the guard cell remains of much lower concentration in comparison to neighboring cells.
- Exosmosis from the guard cells takes place by making them flaccid and the stomata is closed.
- The starch-sugar inter-conversion depends upon the acidity (pH) and alkalinity of the cell sap of guard cells.
- During night, photosynthesis is absent thus the carbon dioxide gets accumulated in the guard cells.
- This converts the cell sap in to weak acidic starch.
- The carbon dioxide is utilized in the process of photosynthesis during daytime and the cell sap becomes alkaline and the starch converts in to sugar.
- Concentrations of CO2 hypothesis:
- This hypothesis for opening and closing of stomata was proposed by Bonner and Galston.
- It relies upon the concentration of the carbon dioxide (CO2) present in the stomatal chamber.
- It is independent of the presence or absence of light.
- Normally 0.03% of carbon dioxide is present in the atmosphere.
- When the density of the CO2 in the sub stomatal chamber also becomes 0.03%, then the guard cells become flaccid and the stomata becomes closed.
- As the density of CO2 decreases gradually, the stoma starts to open and it opens gradually lengthwise until the density of CO2 becomes 0.01%.
- Now the stomata are completely open and they are not open further beyond this density.
- The photosynthesis occurs in day time and much of the carbon dioxide is being utilized in the process, the density becomes lesser than 0.03% and the stomata stays open during day time.
- During night or in the darkness, photosynthesis is absent, the density of carbon dioxide remains 0.03%.
- The guard cell remains flaccid and the stomata remains closed.
- Active potassium (K+) theory:
- This theory is also termed as hormonal regulation theory or malate switch theory or potassium malate theory.
- This theory was proposed by Levitt in 1974.
- The role of potassium (K+) in stomatal opening is now most accepted world-wide.
- In 1967, Fujino, for the first time observed that opening of stomata takes place due to the influx of K+ ions concentration.
- The osmotic concentration of guard cells is increased by the influx of K+ and causes stomatal opening.
- The uptake of potassium K+ controls the gradient in the water potential.
- This in turn triggers endosmosis into the guard cells increasing the turgor pressure.
- ATP aids in entry of K+ ions into the guard cells.
- Levitt (1974) observed that proton (H+) uptake by the guard cell’s chloroplasts occurs with the help of ATP.
- This leads to rise of pH in guard cells.
- Increase in pH converts starch into organic acid, such as malic acid.
- Malic acid again dissociates to form H+ and malate anion.
- The absorption of potassium K+ ions is balanced by one of the following:
- Uptake of Cl-
- Transport of H+ ions from organic acids, such as malic acid
- By negative charges of organic acids when they lose H+ ions
- The accumulation of large concentration of K+ ions in guard cells is ionically balanced by the uptake of negatively charged ions, i.e., chloride and malate.
- The hydrolysis of starch causes the accumulation of high amount of malate in guard cells of open stomata.
- A passive or highly catalyzed excretion of K+ and Cl from the guard cells to the epidermal tissue results in stomatal closure in general and subsidiary cells in particular.
- It is considered that subsidiary cells have an active re-absorption mechanism of K+.
Factors Affecting Stomatal Movement:
- The most likely factors that affect opening and closing of stomata include:
- i) Light:
- It has intense controlling affect on stomatal movements.
- Generally, stomata open in light and close in darkness.
- Exception, the stomata of plants showing CAM (Crassulacean Acid Metabolism) such as pineapple agave, aloe, opens during night and closes in day time. Even moonlight is enough for the opening of stomata.
- The concentration of light needed to achieve optimum stomatal opening differs from species to species. For instance, some plants, such as tobacco need low light intensities, while others may need full sunlight.
- However, light intensity required for stomatal opening is very low than the intensity required for photosynthesis.
- The duration during which stomata remain open in daylight and close at night changes from species to species of plants.
- Different wavelengths of light affect stomatal movement on different ways.
- ii) Temperature:
- Generally, the rise in temperature causes the stomatal opening unless water is a limiting factor.
- Even under continuous light at 0oC, stomata remain closed in some plant species.
- For example, in Camellia (tea Plant), stomata remain closed at very low temperature (below 0o C) even in strong light.
- There is decline in stomatal opening at temperatures higher than 30oC in some species.
- iii) Water availability:
- In the condition, where water availability is less and the rate of transpiration is high, plants goes through water stress.
- Water stress is also termed as water deficit or moisture deficit.
- Such plants start to show signs of wilting and are referred as water-stressed plants.
- Under such conditions, most of the mesophytes, close their stomata tightly and completely to protect them from the damage that may result due to extreme water shortage.
- The stomata reopen only when water potential of these plants is restored.
- This type of control of stomatal movement by water is called hydro-passive control.
- In the guard cells of several water stressed plants, accumulation of phytohormone abscisic acid (ABA) is now well established.
- The closing of stomata of such plants is caused by ABA.
- When water potential of water-stressed plant is restored, the stomata reopen and ABA gradually disappears from the guard cells.
- This type of control of stomata by water, communicated through ABA, is termed as hydro-active control.
- ABA, when applied externally to leaves of normal plants also includes closure of stomata.
- iv) Carbon-dioxide (CO2) concentration:
- CO2 concentration has noticeable effect on opening and closing of stomata.
- Opening of stomata is favored by reduced CO2 concentrations while an increase in CO2 concentration causes stomatal closing.
- This occurs even in presence of the light.
- In specific species of plants, stomata also close merely by breathing near leaves.
- Stomata, that are forced to close by raised CO2 concentration, do not reopen easily simply by draining the leaf with free and dark CO2 air.
- However, such stomata soon open during subsequent light exposure.
- This occurs because, during light exposure, CO2 stored within the leaf is absorbed in photosynthesis.
- This suggests that the internal leaf CO2 concentration is responsible for stomatal opening rather than ambient carbon dioxide.
- The cuticle over the guard cells and epidermal cells, however, is rather impermeable to CO2 and ensures that stomata respond to the CO2 present in the leaf rather than the external atmosphere.
- Some endogenous factors also affect stomatal movement such as K+, Cl+ and H- ions and organic acids.