TRANSPIRATION

  S.No. Transpiration                                                                             Evaporation 1. It is a modified physical phenomenon found                 It is a physical process taking place  in plants.                                                                                              any free surface. 2. It is regulated by the activity of guard cells.                     No such mechanism is found in evaporation. 3. In the proc...

  S.No.  Transpiration                                                                                      Guttation 1. It occurs during day time                                                  It usually occurs at the night 2. The water is given out in the form of vapor.             The water is given out in the form of liquid. 3. The transpired water is pure                                     Guttated water contains dissolved salts and                      ...

 Following four methods are usually used for measuring transpiration in plants : a. Weight method The method consists of weighing the potted plants at regular intervals. The loss of weight over a the prescribed period of time represents the weight of transpired water. b. Cobalt Chloride method This method is based on the fact that the dry cobalt chloride is blue in colour while the moist cobalt chloride is prink in clour. Actually, in this method, the weight of transpired water is not measured. But, it measures only the rate of transpiration. A small piece of filter paper is soaked in3% cobalt chloride solution and thoroughly dried. A single cobalt chloride paper is clipped under a glass coverslip along with an ordinary standard pink and ordinary standard blue coloured paper on the surface of the transpiring leaf. The colour of the cobalt chloride will gradually change from blue to pink. The time taken for this change is noted. The rate of colour change is an indication of the rate...

 In some plants, watery drops ooze out from the injured margins of the leaves where the main vein ends. This is called guttation . It takes place usually early in the morning when the rate of water absorption and the root pressure is higher while the transpiration is very low. The watery drops consist of water in which many organic and inorganic substances are dissolved. The phenomenon of guttation is associated with the presence of special types of stomata at the margins of the leaves, which are called water stomata or hydathodes . Each hydathode consists of a water pore. (Fig.8), which remains permanently open. Below this, there is a small cavity followed by a loose tissue called an epithem. The epithem is in close contact with the ends of vascular elements (tracheids) of veins. Fig.8. Hydathodes: A-Entire leaf showing the position B-V.S. of the leaf through hydathode Under higher root pressure, water is given to the epithem by the xylem of the veins. From epithem, the water is r...

 The antitranspirants are any materials applied to the plants for the purpose of retarding transpiration. In the extreme cases of transpiration, about 98% of water absorbed by the plants is lost and only an insignificant amount is being utilized by the plant for its own purpose. If by any chance, this enormous loss of water can be reduced, it will be an asset to nature and also the agriculturists to reduce the transpiration loss and increase the crop productivity in the rainfed and semi-arid regions. The antitranspiratnts are of three types. They are discussed below : a. Stomata closing type : (eg. Phenyl Mercuric Acetate, ABA, Atrazine, CO2 at high conc., Decenyl succinic acid, Xanthoxin) These chemicals will induce stomatal closure and thus transpiration will be reduced. The the problem with using these chemicals is the photosynthesis will also be affected because of reduced entry of CO2. b. Thin film forming type : (eg. Cetyl alcohol, silicone oils, low viscosity waxes, rice gr...

Certain plants are adapted to reduce the rate of transpiration, eg., by reducing leaf size, the transpiration surface is reduced. Some xerophytic plants have needle-like leaves to reduce transpiration. The presence of thick deposition of caution or wax-like substances on the leaf surface also reduces transpiration. The presence of sunken stomata (as in Calotropis, Nerium, and Cycas ) helps in reducing stomatal transpiration. These sunken stomata are found in cavities surrounded by hairs. These depressions accumulate more water vapor reducing the diffusion rate of water through this pore. (Fig.7). The presence of hydrophilic compounds such as gums, mucilage, etc. helps in retarding the rate of transpiration. Several factors such as leaf area, amount of spongy tissue, the volume of intercellular spaces, the orientation of leaf, and extent of root system also affect transpiration rate.

 The factors affecting the rate of transpiration can be categorized into two groups : A. External or Environmental factors (eg. atmospheric humidity, temperature, wind velocity, light, water supply, atmospheric pressure, sprays, and dust and vital activities) B. Internal or Structural factors (eg. stomatal apparatus and its frequency, the water content of mesophyll cells, and structural peculiarities of the leaf). A. External Factors All these factors affect the steepness of the DPD gradient and thus influence the rate of transpiration. 1. Atmospheric Humidity : The rate of transpiration increases with the decrease in the humidity of the external atmosphere. because the humidity influences the DPD gradient between the intercellular spaces and the outside atmosphere. 2. Temperature A high level of temperature increases the rate of transpiration by increasing the rate of evaporation of water from the cell surface and thus decreasing the humidity of the external atmosphere. 3. Wind Ve...

 The last step in the mechanism of transpiration is the simple diffusion of water vapors from the intercellular spaces to the outer atmosphere through open stomata. This is because the intercellular spaces are more saturated with moisture in comparison to the outer atmosphere in the vicinity of the stomata. According to this theory, Potassium ions (K+) have been found to play a crucial role in stomatal movements. The opening and closing of stomata are the results of the active transport of potassium ions into the guard cells and out of them. The adjacent epidermal cell acts as ion storage cells for guard cells. (dark) (Light) The closure mechanism involves the participation of an inhibitor hormone, Abscisic acid (ABA) which functions in the presence of CO2. In dark, ABA inhibits potassium uptake by changing the diffusion and permeability of the guard cells. The potassium moves out of the subsidiary cells. Then, ABA induces the process of acidification in the guard cells, which resu...

 i. Osmotic diffusion In the first stage, water gets evaporated from the surface of turgid cells and collects in the intercellular spaces increasing the water vapor pressure and lowering its DPD. Fig.5. V.S. of dicot leaf. The arrows show the movement of water during transpiration Inside the leaf, the mesophyll cells are in contact with the xylem, and on the other hand with intercellular spaces above the stomata. When the mesophyll cells draw water from the xylem, they become turgid and their DPD and OP decrease with the result that they release water in the form of vapor into intercellular spaces close to stomata by osmotic diffusion. In turn, the OP and DPD of the mesophyll cells becomes higher and hence, they drew water from the xylem by osmotic diffusion (Fig.5) ii. Mechanism of stomatal opening and closing (Steward’s scheme of stomatal movements) Microscopic examination of open and closed stomata reveals that open stomata have turgid cells while closed stomata have flaccid gua...

 Although large quantities of water are absorbed by the plants from the soil, only a small quantity alone is utilized by them. Thus, a considerable amount of water absorbed by the roots are carried to the top of the plants and lost by aerial parts in the form of water vapour or rarely in the form of liquid. About 80% of the absorbed water is lost leaving only 20% of the absorbed water for various metabolic processes. Definition The loss of water from the living tissues of aerial parts of the plants in the form of vapour is called transpiration and that in the form of liquid is known as guttation. Types of Transpiration There are three types of transpiration observed in the plant system. They are: 1. Stomatal Transpiration Most of the transpiration takes place through stomata. Stomata are usually confined in more numbers on the lower sides of the leaves. In monocots, e.g., grasses, they are equally distributed on all sides. While in aquatic plants with floating leaves, they are pre...