The opening and closing of stomata is affected by the following factors:
- Light: In most of the plants, stomata open in the presence of light and closes in darkness. The blue and red wave lengths of light are most effective. Stomata close in the infrared and ultraviolet lights. In succulents or CAM plants, the stomata remains closed during the daytime. They open only during the dark, for example, Opuntia, Agave and pineapple.
- Temperature: Increase in the temperature causes stomata to open in the dark. This is due to increased activity of the hydrolyzing enzymes. Very high temperature leads to the closure of stomata. Reduction in the temperature below 30 degree centigrade results in the opening of the stomata. Usually at 0 and above 35 degree centigrade, stomata remain closed.
- Carbon dioxide concentration: Decreased concentration of carbon dioxide leads to the opening of stomata. When concentration of carbon dioxide increases, more amount of carbonic acid is formed. Carbonic acid decreases the osmotic pressure and the diffusion pressure deficit of the cell sap, which results in the stomatal closure.
- pH of the guard cells: Rise in pH of the guard cells results in the opening of stomata whereas decrease in the pH of guard cells causes closure of stomata.
- Growth hormone: Cytokinin is essential for the opening of stomata while abscisic acid takes part in stomatal closure.
- Water: If the availability of water is poor, the stomata are partially or completely closed. Water stress brings about stomatal closure due to abscisic acid and rise in diffusion pressure deficit of the epidermal cells.
- Oxygen: Oxygen is essential for the opening of stomata.
- Atmospheric humidity: Stomata remain open for a longer period of time in humid conditions whereas in dryer environment, the stomata remain close for longer period of time.
- Mechanical shock: Mechanical shocks lead to the closure of stomata.
- Minerals: Opening of stomata depends upon the availability of K+ ions from the adjacent epidermal cells. Other minerals, for example, Phosphorous, nitrogen, magnesium, calcium etc., are also required for the stomatal movements.
The process of exudation of liquid droplets from the margin and apex of the leaves of herbaceous plants is called guttation. Guttation takes place through special structures called, hydathodes, which are present on the margins and tips of the leaves. Each hydathode consists of a group of loosely arranged achlorophyllous or colourless parenchymatous cells called epithem. The epithem cells, also known as transfer cells, are specialized cells which help in absorbing inorganic and organic solute present in the exudate. The epithem cells are present above the vascular strand and are connected to the outside through a permanent pore in the epidermis, called water pore or water stoma. The development of a positive pressure in the xylem present in the vein ending causes exudation of liquids from the water pore or stoma. The pressure forces the liquid out through the hydathode.
Guttation generally occurs when transpiration is less and the rate of water absorption is high. Under these conditions, root pressure is maximum which forces water upwards to xylem elements and eventually exudes in the form of drops from the hydathode in the leaf margins. Guttation may also occur during daytime if the plants are growing in moist or humid condition which increases root pressure. An incrustation of salts is formed on the surface of the leaves after the guttation liquid evaporates. Guttation fluid is a dilute solution of both inorganic and organic substances .Poorly aerated soils, heavily salted or mineral deficient soils, dry soils and the atmospheric conditions promoting transpiration inhibit guttation. Some common plants which show guttation are wheat, barley, rye, mustard, tomato, nasturtium etc.
DIFFERENCE BETWEEN TRANSPIRATION AND GUTTATION
|1. Loss of water as water vapour through aerial surfaces of plants is called transpiration
|1. Exudation of liquid droplets from the margins and leaf apex is called guttation.
|2. It occurs through stomata, cuticle and lenticels.
|2. It occurs through hydathodes, scars and lenticels.
|3. Water is lost in pure form.
|3. Guttated water is a dilute solution of both inorganic and organic substances.
|4. It does not leave anything on the surface of the plant.
|4. An incrustation of salts is formed on the surface after the guttation liquid evaporates.
|5. Amount of water transpired is regulated by stomatal movement.
|5. It has no regulation.
|6. It usually occurs in the presence of light.
|6. It usually occurs during the dark.
|7. It occurs during dry periods.
|7. Guttation takes place during humid periods.
|8. It does not depend on the pressure of xylary sap.
|8. It depends on the pressure of xylary sap.
|9. It keeps the plant cool by dissipating excess of heat.
|9. It has no such effect.
|10. Environmental factors such as heat, light, wind velocity, etc., affect the rate of transpiration.
|10. It is not affected by environmental factors.
|11. Stomata are usually confined on the dorsal and ventral surface of the leaves.
|11. Hydathodes are found in the apex of the leaves.
|12. Opening and closing of stomata are regulated by the guard cells.
|12. Hydathodes always remain open.
|13. Photosynthesis has an indirect role on stomatal regulation and hence, on transpiration.
|13. Photosynthesis has no role.
|14. Transpiration continues even when the plant is under water stress.
|14. It does not occur under conditions of water deficiency.
|15. Excessive transpiration causes wilting.
|15. Excessive guttation does not cause loss of turgidity.