Mechanism of stomatal transpiration

Students, this ppt give the details about the mechanism involved in the stomatal transpiration.

1. Mechanism of stomatal Transpiration Dr. K. GANESH KUMARI ASSISTANT PROFESSOR DEPARTMENT OF BOTANY SRIMAD ANDAVAN ARTS AND SCIENCE COLLEGE (AUTONOMOUS) TRICHY - 5

2. Introduction Loss of water vapour through stomata of leaves is called stomatal transpiration. It accounts for about 90% water loss from the surface of a plant. Stomata are microscopic pores found in the upper and lower epidermis of leaves. The pore is surrounded by two guard cells which in turn are surrounded by subsidiary cells.

3. Cont … The opening and closing of stomata are due to the activity of guard cells. When the stomatal pore is open (in day time), trans piration occurs. When it is closed (in night) transpiration does not occur. The mechanism of stomatal transpiration involves three steps. Diffusion of water from lead xylem to intercellular spaces. Opening and closing of stomata. Diffusion of water vapour from intercellular spaces to atmosphere.

4. Diffusion of water from leaf xylem to intercellular spaces Leaf xylem, in the veins, supplies water to all the leaf tissues. In the leaves, vascular bundles are surrounded by mesophyll tissue which in most cases is composed of palisade parenchyma and spongy parenchyma. There are small intercellular spaces between the cells of palisade tissue but in the spongy parenchyma intercellular spaces are larger in size.

5. Cont … Since the mesophyll cells are photosynthetic, they accumulate sugars. So their osmotic pressure is very high. The mesophyll cells draw water from the xylem tubes through the cell walls by osmosis. As a result, the mesophyll cells become turgid and saturated with water.

6. Cont … As the cell wall of mesophyll cells contains cellulose microfibrils and other hydrophilic components, water adheres to the microfibrils and soaks the cell wall. Consequently a thin film of water develops on the surface of mesophyll cells. Water from this thin film, exposed to the intercellular spaces, gets evaporated into water vapour

7. Cont … Then water from the cell wall of interior mesophyll cells is brought to the exposed cell walls and evaporated. Because of the continuous vapourization of water from moist cell walls, the intercellular spaces become surround with water vapour. The heat demand for this vapourization is met by solar radiation. Heat from the sunlight, thermal radiations and warm air current are used by the leaves to vaporize water Nearly 500cal of energy is utilized for the vaporization of 1g of water.

8. Opening and closing of stomata After vapourization of water in the intercellular spaces, the stomata must open for the diffusion of the vapour into the outer environment. The stomata open when the guard cells swell, turgid and increase in length. The stomata close when the guard cells become flaccid. Stomata of most plants remain open during the day time and closed in the night. This daily periodicity of stomatal opening and closing is called stomatal march.

9. Cont … The guard cells intake ions by active absorption from other cells. Metabolic products are also accumulated in the guard cells. Consequently, the osmotic pressure (OP) and diffusion pressure deficit (DPD) of guard cells become higher. Water from surrounding cells moves into the guard cells. Hence they become turgid. As the turgidity increases, the guard cells increase in width and curve at the middle due to differential stretching of walls. So the stomatal pores opens.

10. Cont … The turgidity of guard cells decreases when osmotically active substances are transported to the surrounding cells. As a consequences, the guard cells become flaccid and straight because of low OP and DPD. So, the inner membranes of the sister guard cells come close to each other and the stomatal pore is closed. The opening and closing of stomata are controlled by several factors such as light intensity and quality, temperature, relative humidity and intracellular CO 2 concentrations.

11. Theories to explain the mechanism of closing and opening of stomata Von Mohl (1856) suggested that guard cells become turgid and open due to synthesis of osmotically active substances by photosynthesis. Llyod (1906) stated that conversion of starch into glucose and vice versa are responsible for the turgidity of guard cells to open and close the stomata. Linsbauer (1916) proposed that permeability change in guard cells during day and night as the reason for turgidity change in the guard cells.

12. Cont … Mouravieff (1959) noticed that blue light induces turgidity in guard cells and red light reduces the turgidity. According the walker (1966), dark fixation of CO 2 in guard cells is associated with turgidity and flaccidity of guard cells. According to Zelitch (1969) increase in potassium concentration in guard cells is responsible for turgidity.

13. Diffusion of water vapour from intercellular spaces to the atmosphere The water vapour goes out to the atmosphere through the stomatal pore by simple diffusion. The diffusion of water vapour is determined by the concentration gradient of water vapour between the substomatal cavity and outer atmosphere. Diffusion of water vapour is more rapid through small pores in unit area than free water space of equal size.

14. Cont … The average time needed for a water molecule to escape the leaf is approximately 0.042 seconds. It is given by the equation – T = L 2 / D 2 Where L is the distance of diffusion (less than 1mm = 10 -3 m) and D is the diffusion coefficient of water in air (2.4 x 10 -5 m 2 s -1 ).

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