TRANSPIRATION



 Transpiration, Types of Transpiration: &Stomatal Apparatus

Transpiration: 

The loss of water in the vapour form from the exposed parts of a plant is called transpiration. 98_99% of the water absorbed is lost in transpiration. Only 0.2% is used in photosynthesis while the remaining is retained in the plant during growth. Water loss from a leaf can be studied using cobalt chloride paper, which turns colours ( from blue to pink) on absorbing  water. 



The Magnitude of Transpiration: 

As far as the magnitude  of transpiration  is concerned, Mayer ( 1956) had reported that some of the herbaceous  plants, under favourable  conditions, transport the entire volume of water a plant has and it is replaced within a single day. This means that of all the water absorbed by plants, approximately 95 per cent is lost  by transpiration, and 5 per cent or loss 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: 

Most of the transpiration  occurs through foliar surface or surface of the leaves. It is known as foliar transpiration. Transpiration  from stems is called cauline transpiration. Depending upon the plant surface transpiration  is of the following  four types _ stomatal, cuticular, lenticular and bark. 

1. Stomatal Transpiration: It is the most important type as it constitutes about 50 _ 97% of the total transpiration. It occurs through the stomata which are found mostly on the leaves. A few occur on the young stems, flowers and fruits. The stomata expose the wet interior  of the plant to the atmosphere. The stomatal transpiration   till the stomata are kept open. 


2. Cultural Transpiration: It occurs through  the cuticle of epidermal cells of the leaves and other exposed parts of the plant. I  common land plants it is only 3__ 10%  of the total transpiration. In herbaceous  shade loving  plants where the cuticle is very thin, it may be upon 50% of the total. It continues throughout  day and night. 


3. Lenticular  or Lenticellate Transpiration: It is found only in the woody branches of  the trees where lenticels occur. It is only 0.1% of the total transpiration. It continuous  day and night, as lenticels  have no mechanism  of closure. The lenticels connect the atmospheric air with the cortical tissue of the stem through the intercellular  spaces present between  the complementary cells. It constitutes the major part of water loss by deciduous  trees during leafless stage. 


4. Bark Transpiration  : It occurs through corky covering  of the stems. It is very little but its measured rate is often more than lenticular transpiration  due to large area. It occurs continuously  during day and night. 



Mechanism of water Loss :

To form vapours, water inside the exposed  parts of the plant requires a source of heat energy. It is the radiant energy during the day and heat energy  from the transpiring  organ during the night. In both conditions the temperature  of the transpiring organs comes to lie 2_ 5°C below that of the atmosphere.  

 The atmosphere  is rarely  saturated  with water vapours. The dry air of the atmosphere  has a high DPD ( or low water potential). The intercellular  spaces of the transpiring  organ are almost  saturated with water vapours. When the stomata are open, the water vapours  are drawn from the substomatal cavities to the outside air due to high DPD of the air. This increases the DPD of the substomatal air which drawn more water vapours from the intercellular  spaces. The latter in turn get water vapours from the wet walls of mesophyll  cells. Mechanism  of lenticular transpiration  is similar to that of stomatal  transpiration.  

Cuticle is not  musch  permeable  to water. Its molecules  absorb water from the epidermal cells by imbibition. The imbibed water is slowly  lost to  the atmosphere. It.is reduced by  the thickness  of cuticle. Cuticle is strunked and thicker during the day but at night it expands and becomes loose. Therefore, culticular transpiration  can be more at night. Mechanism  of bark transpiration  is similar to that of cuticular transpiration.  



STOMATAL APPARATUS : 

Stomatal are tiny pore complexes found in the epidermis of leaves  and other  aerial parts. They are meant for the gaseous exchange and transpiration. Each stomate or stoma is  surrounded by two specialized  green epidermal cells called guard cells. 

Because  of their small size, they are rapidly  influenced by turgor changes. They are connected with the adjacent  epidermal cells through plasmodesmata. 

They contain chloroplasts with peripheral  reticulum  characteristic of chloroplasts showing  C4 photosynthesis. They  also possess vacuoles  and microbodies. They store starch with the exception of a few. The walls are  differentially  thickened and elastic. They have folds for expansion.

In most of the plants  the guard cells are kidney shaped. The concavo _ convex curvature of the two guard cells is variable  and causes the stomatal  pore to open and close. Their walls are thickened on the inner side. They are thinner and more elastic on the outer side. When the stomata are to open, these guard cells swell up on the outer side by the development  of a high turgor pressure.  The inner concave sides also bend out slightly  so as to create a pore in between  the two guard cells. The opening  of the stoma is also aided due to the orientation  of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally making it easier for the stoma to open. During closure movement, the reverse changes occur.

 In cereals the guard cells are dumb_ bell shaped in outline. Their expanded ends are thin_ walled while the middle  portions  are highly thick_ walled . In such cases the opening and closing of the Stomatal pore is caused by expansion and contraction of the thin_ walled ends of the guard cells. 



Stomatal Periodicity:

Stomata usually  open during the daytime and close during night. In  succulents or crassulacean plants, the stomata remain closed during the day and open during night, e.g., Opuntia. There  are three  categories  of stomata.  


(i) Barley or Cereal Type: They are dumb_ bell shaped. The stomata usually  open during the day for a few hours. 


(ii) Leucerne or Alfalfa Type: They  open during  day and close during night under mesophytic conditions. Under  xerophytic conditions,  the stomatal behavior becomes that of crassulacean  plants, e.g., Pea, Brassica,  and Radish. 

(iii) Potato Type: Under mesophytic conditions,  the stomata can remain open throughout  day and night but close down for different  periods and at various times under conditions of less water availability, e.g., Cucurbita. 

Mechanism  of Stomatal Movement: 

Stomata function as turgor _ operated values because their opening and closing movement is governed by turgor changes of the guard cells. The  guard cells swell up due to increased  turgor, movements: 


1. Hypothesis of Guard Cell Photosynthesis: Guard cells contain chloroplasts.  During  day the chloroplasts perform photosynthesis  and produce sugar. Sugar increases osmotic concentration of guard cells. It causes absorption  of water from nearby epidermal cells. The turgid cells bend outwardly  and create a pore in between. Photosynthetic  activity of guard fails to explain the stomatal  opening during the dark in succulents.  

2. Classical Starch Hydrolysis Theory :  The guard cells contain starch. At low carbon dioxide concentration ( in the morning  achieved through photosynthesis  by mesophyll and guard cells),  PH of guard cells rises. It stimulates enzyme phosphorylase  which converts starch into glucose. Glucose increases  osmotic concentration  of  guard cells. Because  of it, the guard cells absorb water from neighboring cells, swell up and create a pore in between  them. Evening closure of stomata is brought about by increased carbon dioxide  content ( due to stoppage  of photosynthesis) of leaf. It decreases pH  of guard cells and stimulates  starch synthesis.  As a result, osmotic, osmotic concentration of guard cells falls. They lose water to adjacent epidermal cells. With the loss  of turgidity,  the guard cells shrink and close the pore in between  them. 

Objections to starch Hydrolysis  Theory : 

(i) Starch _ sugar interconversion is too slow to account for the rapid stomatal  response. 

(ii) Change in CO2 concentration  in the leaf interior cannot cause such a wide variation of pH  ( as from 5 to  7 or vice versa).

(iii) Glucose is not detectable  in the guard cells at the time of   stomatal   opening 

(iv)  Some plants  like onions do not possess starch or other related polysaccharide that can be hydrolysed to glucose. 

(v) It fails to explain extra_ effectiveness  of blue _ light during stomatal opening.  


3. Malate or K+ ion Pump Hypothesis ( Modern Theory) : 

According to this theory, pH of the guard cell can rise due to active H+ uptake by guard cell chloroplasts or mitochondria  and CO2 assimilation by mesophyll  and guard cells.  A rise  in pH causes hydrolysis of starch to form organic acids, especially  phosphoenol pyruvate . Starch ➡️ Hexose Phosphate ➡️ Phosphoglyceric Acid ➡️ Phosphoenol Pyruvate.  


   Phosphoenol pyruvate ( PEP)  can also be found by pyruvic acid of respiratory  pathway. It combines with available  CO2 to produce malic acid with the help  of enzymes  PEP carboxylase and malate dehydrogenase. Malic acid dissociates into H+ and malate. H+ ions pass out of the guard  cells in exchange  for K+ ions. Some K+ ions ( accompanied  by Cl- ions) are also absorbed actively from adjacent epidermal cells with the help of cAMP, ATP  and cytokinins. K+ ions are balanced  by organic anions like malate and Cl+ ions. ATP is synthesized in cyclic photophosphorylation  as well as respiration  .K+ ions combine with malate. In the combined state they pass into the small vacuoles   and increase  the osmotic  concentration  of the guard cell. As a result guard cells absorb  water from the nearby epidermal cells through endosmosis , swell up and create a pore in between  them. 

Stomatal closure generally occurs towards  evening when  light begins to decrease. In the absence of light photophosphorylation, and hence ATP  synthesis  stops. Nonavailability of ATP  stops efflux of H+ ions and influx  of K+ ions. The H+ ions diffuse out of the guard cell cytoplasm. Any malate present in the cytoplasm combines with  H+ to form malic acid. Excess of malic acid inhibits its own biosynthesis.   High CO2 concentration  also has a similar effect. Undissociated malic acid promotes leakage of ions. As a result K+ ions dissociate from malate and pass out of the guard cells. Formation of abcissic acid ( as during drought or midday) also promotes reversal of H+  à  K+ pump  and increase availability  of H+ inside the guard cell cytoplasm  .Loss of K+ ions decreases  osmotic concentration  of guard cells as compared to adjacent  epidermal cells. This causes exosmosis and hence turgidity  of the guard cells decreases. It closes  the pore between  the guard cells. The  organic acids are metabolised to produce starch. 

READ AND DIGEST 

● The chloroplasts  of guard cells have peripheral reticulum similar to those of C4 plants. They have both photosyntems I and II, but carbon  assimilation  is absent due to absence  of RuBP Carboxylase and NADP linked triose phosphate  dehydrogenase. Guard cells are, therefore,  heterotrophic. They may import sugars from the adjacent  cells. 

● Stomatal index is defined as the percentage  number of stomata as compared to all the epidermal cells ( including  stomata) in a unit  area of leaf. 

                              

Here  I = stomatal index, S = number  of stomata per unit area, E = number of epidermal cells per unit area. 


● The stomatal index,  is independent  of the environmental  conditions, age of the plant and position of the leaf. It is constant for  species.

● Depending upon the relative distribution  of stomata on the two surfaces of leaf, five categories of plants have been recognized  : 

(i) Apple Mulberry  type:Stomata present only on the lower or under  surface e.g., apple, mulberry.

(ii) Potato Type:Comparatively  more stomata on under surface than on the upper surface e.g., potato.

(iii) Oat type : The number of stomata are approximately equal on both the surfaces of the leaf e.g., oat. 

(iv) Nymphaea or water lily type:Stomata are found  only on the upper surface e.g., Nymphaea and other floating  leaved aquatic plants.

(v) Potamogeton type :Stomata are either vestigial  or absent e.g., Potamogeton  and other submerged aquatic plants. 

● Types of Stomatal movement  : They fall into following  five  categories:

(i) Poto_ active movements: These  are     controlled directly or indirectly by light. 

(ii) Scoto_ active movements: The stomata of succulents  open during dark and close in light. 

(iii) Hydroactive movements:When  the epidermal  cells are more turgid than the guard cells, the stomata close. They open at the time when epidermal cells suddenly  lose more water as happens during  midday in some cases. 

(iv)  Passive and active movements:Active movement is the opening  of stomata. It is caused by  turgor changes in the guard cells. The passive movement is the closing of stomata. It is due to the turgor changes in the epidermal or subsidiary cells. 

(v) Diurnal or autonomous movements: There is a diurnal rhythmic movement of opening  and closing  of stomata. It is believed  that these movements are autonomous.  

● Transpiration  from the two leaf surfaces is compared by dry paper strips previously soaked in solution of  Cobalt Chloride. Cobalt Chloride  is blue in the anhydrous condition  but becomes pink in contact with water. The change of colour  of cobalt chloride strips from blue to pink indicates that the paper has received water from the surface of leaf. 


Potometers: It is an instrument  designed  to measure  the rate of absorption  of water by a cut shoot and hence, indirectly,  the rate of transpiration  by the shoot in a particular set of conditions. Detaching  the shoots is an abnormal  treatment.  It affects the rate of transpiration . Further, principle of equating transpiration  with absorption  is wrong since transpiration is greater than water absorption during the day time. Nevertheless, the potometers are easier to handle  as compared to other methods. They are of four types: Student's  potometer, Farmer's  potometer, Bose potometer and Ganong's potometer. 

● Porometer : It is a device to measure  Stomatal movement. It is of two types : flow promoter  and diffusion  porometer. 

Factors Affecting  Stomatal  Movements 

1. Light : In the majority  of plants the stomata open in light and close in darkness. Both red and blue parts of light are effective though  the latter is slightly  more effective. Light  is absorbed  by a flavin which on activation may enhance respiration  or PEP carboxylase activity. 

2: Temperature  : Q10 for stomatal  opening is two. At 38°C__  40°C, Stomata can open on complete darkness. In some plants stomata remain closed even under continuous  light at 0°C. At temperatures higher than 30°C, there is a decline in stomatal opening  in some species. 

3: Atmospheric  Humidity: In humid environment the stomata remain opened for longer periods and to a  greater degree while in dry environment they remain closed for longer periods. Stomata remain open at 70% or higher humidity. In arid areas, a sudden humid air is harmful since it causes opening  of stomata and allows transpiration.  

4: Water Content of leaves : Water deficit in the leaves causes stomatal closure due to rise in DPD of the epidermal cells. It is triggered by the formation  of abscussic acid.

5: Mechanical Shock : It causes closure of stomata. The closure in high wind is also due to  shock  effect. 


6: CO2 Concentration: Low CO2 concentration usually  induces opening  of stomata while high CO2 concentration  closes the same. It is the internal leaf CO2 concentration  rather than atmospheric  CO2 tha controls stomatal  opening. If plants are transferred  to carbon dioxide _ free  environment, but kept in darkness, the stomata will still remain closed. This means that, since the internal carbon dioxide is not utilized due to absence of photosynthesis  in the dark, it influences  the stomata to remain closed. If these plants are exposed to light, photosynthesis will utilize  the CO2, permitting  the stomata to open. 

7: PH : Rise in PH is known to be required  for opening  of stomata while a fall in pH induces closure of stomata.

8: Growth Hormones : Cytokinins are  essential  for opening of stomata while abscissic acid takes part in stomatal closure.

9: Minerals : Stomatal opening  depends upon availability of K+ ions from adjacent epidermal cells. A  number of other minerals are also essential  for stomatal   movements, e.g., P, N, Mg,Ca, etc. 

10: Oxygen:It is necessary  for stomatal opening. Closure of stomata can be expedited when oxygen is deficient  though  complete anaerobic  conditions prevent stomatal closure. 

Factors Affecting  Transpiration  

External Factors 

1: Relatively  Humidity  : It is the percentage  of water vapour present in the air at a given time and temperatures  relative to the amount required to make the air saturated  at that temperature.  The rate of transpiration  is inversely proportional  to the relative humidity. It is because the leaf interior has a nearly saturated  air in its intercellular  spaces. Relative humidity  of the atmospheric  air  governs its vapour pressure deficit  or DPD or water potential.  Since DPD of atmospheric air is higher at low relative humidity,  more of water vapours will diffuse out of the  leaf interior as compared to high RH when  DPD is lower. 

2: Atmospheric  Temperature: Transpiration  increases  with the increase in temperature. A high temperatures opens stomata even in darkness. It  lowers the relative humidity  of the air and increases th vapour pressure  of saturated  air inside the leaf. The rate of transpiration increases. Very low temperature  closes the stomata and decreases  the rate of transpiration. 

3: Light : In the majority  of plants stomata open in the presence  of light and close in darkness. A strong light increases  transpiration due to its heating effect also. 

4: Air Movements: Transpiration  is lower in the still air because  water vapours accumulate around the transpiring organs and reduce the DPD of the air. The movement of the air increases the rate of transpiration  by  removing   the saturated  air around the leaves. Air currents also cause bending movements in the leaves. It compresses the intercellular  spaces and forces out saturated  air through the stomata by mass flow. Upto 20_ 30 km/ hr, the rate of transportation  increases with  the wind velocity. A wind velocity  of 40_ 50 km/ hr decreases  transpiration  by closing the stomata  due to mechanical effect, drying and cooling of the transpiring organs. 

5: Atmospheric  Pressure  : Low atmospheric  pressure  enhances evaporation, produces air currents and increases the rate of transpiration. 

6: Availability  of Water :The rate of transpiration depends upon the rate of absorption  of soil water by roots.  This is further influenced  by a number of soil factors like soil water, soil particles, soil temperature, soil air etc. A decrease  in water uptake by the root causes partial dehydration of the leaf cells resulting  in closure of stomata and wilting. Thus decreased water absorption  by roots, decreases the rate of transpiration.

Wilting :It is the loss of turgidity of leaves and other soft aerial parts of a plant causing their drooping,  folding and rolling. The symptoms are not shown by thick_ walled tissues. They are less conspicuous  in sclerophyllous plants. It is of  three types: 

(i) Incipient Wilting : There are no external symptoms but the mesophyll cells have lost sufficient  water due to transpiration  being than the availability  of water.It occurs during midday for a brief period on almost all plants even when  sufficient  water is present in the soil. 

(ii) Temporary Wilting ( Transient Wilting): It is the  temporary drooping  down due to loss of turgidity during  noon. At this time transpiration  is maximum . The water absorption is less due to shrinkage of roots and depletion of water around the root hairs. Lower leaves show wilting  earlier than the upper ones. ( Older leaves of herbaceous plants usually wilt before younger leaves, due to the tendency  for the water remaining  in a plant to be redistributed when water stress develops. The younger, more actively growing leaves will hold and attract the available  water within the plant more readily than older leaves and therefore  the latter are likely to wilt first. The newly formed leaves are smaller and therefore   have lower transpiration  rates).It is corrected only after the transpiration  decreases  in the afternoon  accompanied  by replenishment of water around the root hairs. 

(iii) Permanent  Wilting: It is that state in the  loss of turgidity of leaves when do not regain their turgidity  even on being placed in  a saturated atmosphere.  It occurs when the soil is unable to meet the requirement of plant for transpiration . Water is present in the soil largely in unavailable  form. At permanent wilting percentage ( PWP) or coefficient  ( PWC) the soil contains 1_ 15% water depending upon its texture ( about 10% in loam soil). After permanent  wilting the plant dies. 

Internal or Plant Factors : 

1. Leaf Area (Transpiring Area): A plant with large leaf area will show more transpiration. The rate of transpiration per unit leaf area decreases  with the increase in number  of leaves and the density  of foliage  due to shading effect and decrease of air movement  inside the canopy. 

2. Leaf Structure 

(a) Thickness of Cuticle: Cuticular transpiration decreases  with the thickness  of cuticle. 

(b) Number of Stomata : Because  most of the transpiration  takes place through the stomata, their number, size of  aperture, distribution  & spacing etc. influence  the rate of transpiration. 

(c) Sunken Stomata : The sunken stomata are device to reduce the rate of transpiration  by providing  an area where little air movement  occurs. 

(d) Hair : The hair insulates the surface  of the leaf from air currents and air temperature.  

(e) Mesophyll : Compact mesophyll  reduces transpiration  while a lose mesophyll  increases transpiration.  

(f) Leaf modifications : Formation  of prickles, leaf spines, scaly leaves, phyllodes, phylloclades, are all modifications found in xerophtes to reduce transpiration. In xerophytes  the leaves are also smaller ( to reduce the effect of heating) and leathery ( to prevent wilting). 

3. Root/ Shoot Ratio: A low root/ shoot ratio decreases the rate of transpiration  while a high ration increases  the  rate of transpiration because  of greater  availability  of water due to more extensive root system. This  is the reason that smaller plants ( high R/ S ratio) lose more water per unit of leaf area as compared to the large plants ( with less R/ S ration) . 

4. Mucilage and Solutes : They decrease the rate of transpiration  by holding  water tenaciously.  

5. Diseases: The rate of transpiration  is generally  higher in the diseased plants as compared to that in the healthy ones. 

6. Leaf Orientation  : Solar radiations cause more heating  when the flat surface  of the leaf lies perpendicular to the incident  light. The effect is minimum  when it lies parallel  to it. The leaves of Eucalyptus  hang downwardly  to avoid over_ heating  the hot periods of the day. 

Significanc  of Transpiration: 

Advantages 

(1) Ascent of Sap : It mostly occurs due to pull exerted  by transpiration  of water. 

(2) Removal of Excess Water : Plants  absorb far more amount of water than is actually  required . Transpiration, therefore,  removes the excess of water. 

(3) Cooling Effect : Radiant heat falling   on the plants increases their temperature, which may be dangerous  for them. Transpiration, by evaporating   water, lowers down their temperature.  

(4) Mechanical Tissue: The development  of mechanical tissue, which is essential  for providing  rigidity and strength  to the plant, is favored by the increase  in transpiration.  

(5) Distribution  of Mineral Salts: Mineral salts are mostly distributed  by rising column of sap. 

(6) Increasing Comcentration of Mineral Salts : The sap absorbed from the soil contains low concentration  of mineral salts. The loss of water through transpiration increases the concentration  of mineral salts in the plant. 

(7) Root System : Transpiration  helps in extensive development  of root system. 

(8) Quality of Fruits : The ash and sugar content of the fruit increases  with the increase in transpiration. 

(9) Resistance : Excessive transpiration  induces hardening and resistance to moderate drought.  

(10) Assimilation  products : Transpiration  enhances the formation  of assimilation  products like latex, resins,  alkaloids etc. Plants grown for economic  exploitation  of these products are exposed to periodic deficiency  of water. 
 
(11) Drainage of soil water : Rain  increases the water content of soil, reduce aeration and mineral content. Transpiration  reduces the water content of the soil and increases  the soil aeration. 

Disadvantages  


(1) Wilting : Wilting is quite common during noon due to transpiration.Wilting  reduces  photosynthesis and other metabolic activities.  

(2) Reduced  Growth : Transpiration reduces availability  of water inside the plant. Water deficit  decreases growth.

(3) Reduced Yield : A single wilting reduces growth by 50%. It is because decreased availability  of water inside the plant check meristematic activity and hence the formation  of flowers, fruits and seeds. 

(4) Abscissic Acid : Water stress produces abscissic acid. Abscissic acid prevents several plant processes and promotes abscission of leaves, flowers and fruits. 

(5)  Wastage of Energy: Since 98_99% of absorbed  water is lost through transpiration, the energy used in absorption  and conduction of water goes waste. 

(6)  Modification : In order to reduce transpiration  during critical  periods, the plants have to produce  several  types of modifications _ thick cuticle, hair, prickles, spines, thorns, sunken stomata, phylliclades,cladodes, etc. 

(7) Water deficiency  due to excessive transpiration may lead to stomatal closure and hence non_ availability of CO2. It decreases  the rate of photosynthesis.  

 Transpiration  cannot be checked. Stomatal transpiration will always  occur whenever  stomata are open for gaseous exchange. Most of the plant physiologist hold  the view that transpiration  is a price which the plant pays for photosynthesis. As the stomata open in the light for gaseous exchange ( CO2 for photosynthesis  and O2 for respiration) water vapours escape side by side. Cuticular and lenticular  types of transpiration  cannot be checked  as there is  no method of their control. Transpiration  is regarded as a necessary  evil or  unavoidable evil. 



Water Requirement

The amount  of water transpired per unit of dry matter manufactured  by a plant is called water requirement, transpiration  ration or efficiency  of transpiration. It is minimum  for crassulacean or succulent plants, moderate in C4 plants and maximum  in C3 plants. The common value is 300 _ 600. In CAM plants, it is about 50. C3 plants transpire the maximum. Humidity  of rain forests is due to large scale water lost by plants. It results in condensation  and local rain. C3 plants lose 600 _ 900 grams of water for one gram of dry matter  synthesized. It is because  they have high sponginess of their mesophyll tissue due to presence of fine  spaces in between the palisade cells and  larger spaces in between   the spongy parenchyma cells. 

    C4 plants have evolved  a strategy  to maximize  availability  of CO2 and minimize  the water loss. Their transpiration  ratio is 300. C4 plants are twice  as efficient as C3 plants in terms  of fixing carbon. A C4 plant loses only half as much water as a C3 or plant for the same amount of CO2 fixed. CAM plants do not open their  stomata during the day. They also store water. 


Antitranspirants 

Substances  that reduce the rate of transpiration  are called  antitranspirants. They allow farmers to grow crops profitably in unirrigated  areas and help foresters to plant trees even  in extrems arid or desert areas.  They  are of two types,  metabolic  inhibitors  and surface  films. 

  Metabolic  inhibitors reduce transpiration  by reducing  the stomatal  opening  for a period of two or more weeks  without influencing  other metabolic  processes. The most promising  of these inhibitors is phenyl mercuric acetate ( PMA, 10-⁴M). Another  is abscisic acid. Its effect persists only for a few hours. Slight excess of carbondioxide  also acts  as  antitranspirant. 

 Film  forming  chemicals check transpiration  by forming a thin film on the transpiring  surface. They are sufficiently  permeable  to carbon  dioxide and oxygen to allow  photosynthesis and respiration  but prevent movement  of water vapours through them. The important  chemicals of this group are  silicon emulsions, colourless plastic resins and low viscosity waxes. Antitranspirants are still in experimental  stage. 





















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